suspension guys-traction??

[dhkid]

i am confused on how some bikes grip amazingly and some not so... (rear traction.)

=>basic physics tells us that F(friction/traction)=U(coefficient of friction)*R(reaction force)

neglecting how the rear suspension moves your body weight around, assuming same coeficient of friction, neglecting geometry, assuming on all bikes that the reaction from the riders weight,R is constant. and assuming on flat ground(no bumps) and the tire doesn't skip, the suspension would sag and transfer that R to the tires.

=>so does that mean all bikes (within reason and following the assumptions) will have the same amount of traction on flat surfaces with out the rear wheel skipping.

but then traction on dirt surfaces is different, just for simplicity i'll assume that dirt gives a constant coefficient of friction. so it comes down to the bumps of the surface.... and how well the rear suspension reacts to the surface and how it applies the Reaction force R.

=>so i have come to the conclution that rear suspension that are firmer in the beggening of the stroke when compared to bikes which are very supple in the begining stroke will grip better.

i come to that conclution from when the suspension hits a bump, (assuming constant rebound damping on all the bikes shox) the firmer suspension designs will rebound as a faster speed because it is stiffer and given the rate is higher then the supple design and a constant rebound damping, it would rebound faster. so when a bike goes through a non perfectly smooth surface, the bike with the faster effective rebound will apply a greater total reaction force to the ground.

does this apply to when the rear tire skips in a drift too??

discuss, are my assumptions valid?

sorry if their gramma/spelling mistakes, i tend to make alot when i am focused on trying to put my point forward....

[Hannu]

Where it goes wrong is when you assume that all shocks have same amount of rebound damping, because that is not the case. What you need to do is assume that rebound speed on all shocks is equal, in other words rebound is tuned correctly according to spring rate. There is no point to just pick one rebound damping rate and compare different spring rates with that.

[dhkid]

yea, i didn't say that i assume same rider weight, suspension sag. so when i say same rebound danmping i mean same rebound speed at the shox. independent of spring rate.

[Honus]

In a world that works only on assumptions I don't discount what you're saying. According to your assumptions, ideally the bike that has a faster effective rebound will keep the tire contacted with the ground better- however, neglecting how the rear suspension moves your body weight around and assuming on all bikes that the reaction from the riders weight is constant changes everything. Weight transfer plays a huge role in how a suspension bike works.

So it doesn't really answer your original question.....

It's just not that simple- wheel path plays a big role here (since dampers are speed sensitive the shape of the bump to be absorbed can be more important than the size of the bump) as does frame geometry and frame rigidity (lateral as well as torsional.)

There are just too many variables here to try and negate some of them so that you can arrive at a conclusion. I think you have to look at the whole dynamics of the bike as a system. The reason why one bike grips and another doesn't can't be broken down into little individual parts and analyzed- you have to look at how the entire system works as a cohesive whole.

[Inclag]

i am confused on how some bikes grip amazingly and some not so... (rear traction.)

=>basic physics tells us that F(friction/traction)=U(coefficient of friction)*R(reaction force)

neglecting how the rear suspension moves your body weight around, assuming same coeficient of friction, neglecting geometry, assuming on all bikes that the reaction from the riders weight,R is constant. and assuming on flat ground(no bumps) and the tire doesn't skip, the suspension would sag and transfer that R to the tires.

=>so does that mean all bikes (within reason and following the assumptions) will have the same amount of traction on flat surfaces with out the rear wheel skipping.

but then traction on dirt surfaces is different, just for simplicity i'll assume that dirt gives a constant coefficient of friction. so it comes down to the bumps of the surface.... and how well the rear suspension reacts to the surface and how it applies the Reaction force R.

=>so i have come to the conclution that rear suspension that are firmer in the beggening of the stroke when compared to bikes which are very supple in the begining stroke will grip better.

i come to that conclution from when the suspension hits a bump, (assuming constant rebound damping on all the bikes shox) the firmer suspension designs will rebound as a faster speed because it is stiffer and given the rate is higher then the supple design and a constant rebound damping, it would rebound faster. so when a bike goes through a non perfectly smooth surface, the bike with the faster effective rebound will apply a greater total reaction force to the ground.

does this apply to when the rear tire skips in a drift too??

discuss, are my assumptions valid?

sorry if their gramma/spelling mistakes, i tend to make alot when i am focused on trying to put my point forward....


My brain just filled up with candy.

[dsotm]

The shock with the faster rebound will apply a greater force to the ground at specific moments, but the shock that applies a more constant reaction force (the shock that allows the tires to better conform to the ground) will provide the most traction and even more so, the more consistent traction (even more importan). The same principle applies to tires. Tires that conform to the ground better, grip better, and so less air pressure (to a point) provide better traction.

[mandown]

The shock with the faster rebound will apply a greater force to the ground at specific moments, but the shock that applies a more constant reaction force (the shock that allows the tires to better conform to the ground) will provide the most traction and even more so, the more consistent traction (even more importan). The same principle applies to tires. Tires that conform to the ground better, grip better, and so less air pressure (to a point) provide better traction.

good thoughts. at some point rebound that is too fast will "kick" and bounce the wheel off the ground again, the opposite of rebound that is too slow and packs up. it is about balance.

[Sandwich]

I think wheel path, tires, chain torque and air pressure are the most important factors regarding traction. I think your argument is physically correct, but I think you'd find that with a firmer bike, as you're saying, you'll find the rear wheel will start skipping over bumps rather than absorbing them, which further reduces traction.

Chain torque is a big deal too, pulling the wheel up (on some very low pivot designs) reduces it and pulling it down (on high pivots) helps increase traction...to a point.

Ideally, the wheel will follow every bump, up and down, keeping the tire in contact with the surface at all times. I think the goal of a bike designer is to isolate as many variables as possible (wheel path, braking, pedalling) so that the wheel can naturally follow the terrain with no outside forces.

[manhattanprjkt83]

this should put an end to all this discussion:

http://www.turnertech.co.uk/Images/cad3.gif

:banghead:

[ChrisKring]

A suspension that is critically damped will have the best traction. Excessive rebound force due to not enough rebound damping will cause the reaction force to put the sprung mass in motion enough to take the wheel off the ground.

To expain this to you better, take a blown out rear shock and go for a ride.

[ChrisKring]

See these links for an introduction to vibrations:

http://en.wikipedia.org/wiki/Damping http://www.abdn.ac.uk/physics/vpl/pendulum/underdamped.html

[dhkid]

honus, when i say i neglect the momevent of body weight its just to make it simpler, coz some may argue that low single pivots will move your body weight backward under corneing loads ect... its just gets too complicated after a while

so what other major things should i not neglect?

sandwich, would chain torque apply since when conering its most likely you are not pedaling.

if wheel path comes in it would make it too complicated agian....

chris king, i find with modern shoxs, even with the fastest rebound you dont get much of a pogo stick effect as just using a blown shox. might be different for hevier riders tho.

good to know that everyone agrees that the suspension design that will keep the the most total reaction force on the tires through out the corner will grip the best, that already clears up alot.

then i guess it just how a suspension will be able to achive this^^

thanks for the replies so far.:thumb:

[Whoops]

for bikes, on dirt, with knobbly (whhoorrr) tyres friction is not the same as grip.

the tyre bites into the ground, giving a mechanical interface (as opposed to a fritional one).

[bullcrew]

for bikes, on dirt, with knobbly (whhoorrr) tyres friction is not the same as grip.

the tyre bites into the ground, giving a mechanical interface (as opposed to a fritional one).

Thus advancing the madula oblingata and breaking the interface equasion to the semi chromatic induction theory that applies to the overly tuned linear driven device the equals a flux capacitor. :D

Start twisten knobs! :D

[dsotm]

good thoughts. at some point rebound that is too fast will "kick" and bounce the wheel off the ground again, the opposite of rebound that is too slow and packs up. it is about balance.
Exactly, the shock that allows the wheel to conform to the ground best will allow for the best traction.

[dhkid]

for bikes, on dirt, with knobbly (whhoorrr) tyres friction is not the same as grip.

the tyre bites into the ground, giving a mechanical interface (as opposed to a fritional one).

not entirely true, otherwise softer compound tires wouldn't give more grip on dirt. roots and rocks of course, but they do help on dirt...

[TheInedibleHulk]

Ive always thought that the ideal suspension setup was the one that allows your tires to remain in contact with the ground for the highest possible amount of time. So an overly stiff supension that is bouncing over bumps would not be ideal. On the other hand, and overly soft suspension will wallow in its travel will also not adequately maintain tire contact, as the travel is not being utilized. I think you will find, as you examine setups from different levels of riders, that in general springs get stiffer and damping gets heavier as you move up the field. Stiffer springs gives a better feel for exactly what the tires are doing, not to mention that faster riders are hitting stuff faster and harder and therefore need more resistance to bottoming than the guy who is just putzing along. I know a couple of pros who ride absurdly stiff supension for the feel reason. I tend to fall somewhere in the middle.

On other side note, the stronger you get, the less you need your suspension. If you can take the hits and stay in control, a stiffer setup should be faster due to better effieciency for pedaling, pumping, ect.

[dhkid]

i have heard about the siffer set ups too, but i though it was stiffer and faster rebound...

of course when i mean stiff set up i still mean 25-30% sag, when compared to say a v10 which has 40% sag and very supple initial travel.

[Superdeft]

I'd see if you can find a book on motorcycle suspension, the book I've been looking at for racecar suspension generally deals with the same equations, but it comes down to keeping the wheel firmly planted on the ground as much of the time as is possible, which is why designs like the v10 often work so well people describe them as 'dead.' There's a lot of internet engineering/marketing crap around here that's just people throwing around buzzwords, so I'd go to the source of the knowledge and find a good book if you're actually looking to gain some knowledge about suspension.

[heikkihall]

I'd see if you can find a book on motorcycle suspension, the book I've been looking at for racecar suspension generally deals with the same equations, but it comes down to keeping the wheel firmly planted on the ground as much of the time as is possible, which is why designs like the v10 often work so well people describe them as 'dead.' There's a lot of internet engineering/marketing crap around here that's just people throwing around buzzwords, so I'd go to the source of the knowledge and find a good book if you're actually looking to gain some knowledge about suspension.


It is true that there is aot of internet engineering and marketing crap out there, but there is also alot more to downhill than cornering, braking, and the traction characteristics that are experienced during those things. As with everything there are a lot of give and takes.

I will say that the V-10 gets good traction. But it is also hard to argue that the bike does not feel dead. A bike with that much travel that is designed to be run with that much sag will be much harder to get off the ground than your typical DH bike with 8" of travel and 30% sag. But that is exactly the reason that the V-10 gets good traction, without that sag the wheel would be bouncing off the ground and losing traction more often. That is just something that the engineers at SantaCruz decided was an ok sacrafice. Some like it, some dont, and in the end it all comes down to riding style and personal preferance.

[RaID]

we also have to remember that sometimes getting the most traction doesnt lead to the quickest way to get down the mountain
eg skimming over stutters rather then making your bike to stick every ground part of the stutters

or take the rally car analogy, the controlled slide turns they make, not the most amount of traction yet the fastest way to travel around that corner

[dhkid]

dammit, its got to the point where you have to name bikes. i suspect this tread will go downhill fast...

the v10 getting good traction? i think that its less grippy then say a sunday or commencal....

[Whoops]

not entirely true, otherwise softer compound tires wouldn't give more grip on dirt. roots and rocks of course, but they do help on dirt...

That's why I said "on dirt" :clapping: :lighten:

[Banshee Rider]

the v10 getting good traction? i think that its less grippy then say a sunday or commencal....


You've owned one or have had extensive enough time to dial the shock for yourself and still find that it doesnt track well?.........Interesting

[Sandwich]

sandwich, would chain torque apply since when conering its most likely you are not pedaling.

if wheel path comes in it would make it too complicated agian....


chain torque won't matter much in a corner, you are correct, except if you have a high pivot design. Imagine if your feet/cranks are locked, the rear wheel has two options, it can lock out, or it can roll forward, neither of which is really beneficial to traction.

wheel path will make the conversation more complicated, but I think it's more important than ramping up rebound damping. In fact, I think bikes track better with faster rebound damping.

[Honus]

If you want a really good book on the dynamics of two wheeled vehicles read "Motorcycle Handlig and Chassis Design- The Art and Science" by Tony Foale. I've read this book cover to cover and it's probably the best book of its kind. His suspension analysis software looks pretty neat too.
http://www.tonyfoale.com/

[Socket]

A suspension that is critically damped will have the best traction. Excessive rebound force due to not enough rebound damping will cause the reaction force to put the sprung mass in motion enough to take the wheel off the ground.

To expain this to you better, take a blown out rear shock and go for a ride.


I disagree. For a sinusoidal input at a constant speed blah blah, assuming you have a linear damper constant (which no shocks do) that might be the case, but what is considered "underdamped" for a rider's mass will invariably follow rough terrain the best. Underdamping will not "take the wheel off the ground" unless it's REALLY extreme given the spring rate to mass ratio (ie you're running bugger all sag and are topping out harshly), but overdamping most certainly will if the wheel can't follow the ground properly. For something like braking bumps (similar size/spacing hits) then yes there is arguably a critical point at which the suspension will be able to follow the terrain the best (where its resonant frequency is the same as that of the braking bumps) but otherwise I would most definitely err on the side of underdamping rather than overdamping.

dhkid: friction on a flat surface should be roughly the same between any bikes with the same tyres with the exeption of pitching moments and g-out compression caused by cornering, which alters your geometry and introduces greater dynamic input laterally into your tyres. If you were just riding around in a circle (say down some big spiral thing) at a constant speed in a constant position then yeah all bikes would be the same, but that's not at all analogous to a dh course.

Bikes that follow the ground well often have good grip in a vehicular sense, but the fact is that the rider is a far bigger component of the suspension/mass on a mountain bike than any other suspended vehicle, and we need good feedback to be able to make use of that grip. The problem with overly soft bikes (which do tend to follow the terrain very well, like V10s) is that you don't get that feedback to the same degree that you do with stiffer setups (to an extent). Predictability and geometric stability are, in my opinion, hugely important to the rider's sense of grip. Generally speaking, stiffer setups (or those with more damping) mean that the bike dives/rakes out less, and this means that you are able to achieve (or get closer to) dynamic stability in a corner quicker than you are with a softer setup. The sooner the bike's geometry stabilises (or if its movement is slower) the easier it is to feel when it's going to break loose. Again, if the suspension does anything sudden or especially variable through its stroke, you lose some of that predictability.

All that has to be balanced against bump absorption (incl axle path) and chassis stiffness, and a bike that is stable AND able to track the ground with the minimum necessary change in normal reaction force (which may be "low" if it's only 1000%!) will give you very good grip in corners.

[dhkid]

banshee, i have riden many v10s. but not mine, although the one which i have done runs on has a rider of similar weight. of course thats not the best way to comment on it, i found that it rode very similar to a vp, which i owned at the time. and it wasn't that i had problems with traction or anything, its just that the bikes that i said grip well are at a whole new level.... i had to get used to my new bike coz of that, i have had similar comments from a my mate who went from a dhr to a sunday...

sandwich, its agreed that bikes with faster rebound will track better... i get it what you mean by how the chain torque come in now, but then again most high pivot designs have pullies. but i get what you mean...

socket(tffm), i agree that the perfect situation in cornering doesn't work in real life, i was just building up my argument.


so, are they any comment from other ppl who feel the opposite to what i have felt on testing these two different suspension designs? i am a bit boggeled since a bike that i think doesn't grip as well when compared to other designs has been mentioned twice to have good corering grip....:huh:

when talking to ppl who ride those bike, they would comment on how it eats bumps and pedals through anything, but never how well it grips, with those bikes that i think grip well, other owners of these bikes have agreed how noticable the change was....

edit:forgot to add that maybe the siffer suspension designs get better grip just for the simple reason that you can pump them better? but i dont think thats the whole case tho, since they still grip as well on long high speed corners.

[Whoops]

Here's a question.
How often do you crash because you ran out of grip?

For me, the answers is... almost never. I crash because I run into a tree, or get bucked off by a rock I didn't see, or something else that doesn't involve lowsiding the bike (under or over steer).

To me this says that 'grip' isn't really the issue most of the time...

[dhkid]

but with more grip, you would be able to corner faster,tighter and avoid those trees and rocks...

whatever, i am not going to debate on how dh races are won, i just wanna know more about how rear sus works.:monkeydance:

[Udi]

I think there was a fair bit of useful information right here that you should try to read and understand if you didn't already.. somehow I don't think you did?

Bikes that follow the ground well often have good grip in a vehicular sense, but the fact is that the rider is a far bigger component of the suspension/mass on a mountain bike than any other suspended vehicle, and we need good feedback to be able to make use of that grip. The problem with overly soft bikes (which do tend to follow the terrain very well, like V10s) is that you don't get that feedback to the same degree that you do with stiffer setups (to an extent). Predictability and geometric stability are, in my opinion, hugely important to the rider's sense of grip. Generally speaking, stiffer setups (or those with more damping) mean that the bike dives/rakes out less, and this means that you are able to achieve (or get closer to) dynamic stability in a corner quicker than you are with a softer setup. The sooner the bike's geometry stabilises (or if its movement is slower) the easier it is to feel when it's going to break loose. Again, if the suspension does anything sudden or especially variable through its stroke, you lose some of that predictability.

All that has to be balanced against bump absorption (incl axle path) and chassis stiffness, and a bike that is stable AND able to track the ground with the minimum necessary change in normal reaction force (which may be "low" if it's only 1000%!) will give you very good grip in corners.

[LaharDesign]

If you want a really good book on the dynamics of two wheeled vehicles read "Motorcycle Handlig and Chassis Design- The Art and Science" by Tony Foale. I've read this book cover to cover and it's probably the best book of its kind. His suspension analysis software looks pretty neat too.
http://www.tonyfoale.com/

I second your advice. That books like the bible of two wheeled performance chassis design and was one of my inspirations when drawing my first gearbox designs in the early 90's.

Sprung to unsprung weight ratio is the biggest influence on traction. Even on tarseal racetracks a 5% improvement is considered a race winning traction advantage. The lighter the mainframe of the bike the more its kicked upwards as the suspension compresses. As the suspension rebounds the heavier the wheel is relative to the mainframe, the more the mainframe rebounds upwards rather than the wheel rebounding down.
Too much compression damping - like platform systems- causes more upward energy transfered to the main frame, to little for the mass of the wheel will cause it to continue upwards past the top of the bump.
Too much rebound damping stops the wheel getting back on the ground after the bump, too little for the mass of the wheel will cause the wheels downward inertia to compress the tire like a spring and bounce the wheel off the ground.
With a gearbox configuration you can run much faster compression and rebound due to these effects and issues with rebound bucking are eliminated by the combination of high pivots and gearbox mass distribution. eg/ Lahar pros run fox dhx5.0 with propedal (initial compression damping) turned off hard against the stop and no more than 1 or two clicks of rebound. This doesn't feel at all "dead" but highly responsive as the natural frequency is very fast, enhancing the riders inputs.
more rearward wheelpaths give allow the suspension to deal with large bumps over a much longer elapsed time and with less wheelspeed, getting a lot more out of what you have. more vertical wheelpaths should have a small advantage in traction on smoother surfaces due to less distance travelled on rebound but this appears compromised by weight distribution shifting rearward.
Suspension enhances traction even on smooth surfaces because the breakaway and hookup dynamics are much gentler.
rally cars get max traction in a drift from the mass of gravel and dirt that is scooped up and packed against the outside of the tyres. I'm don't think this effect is common in bikes though.

[spacemanspiff06]

=>so i have come to the conclution that rear suspension that are firmer in the beggening of the stroke when compared to bikes which are very supple in the begining stroke will grip better.

i come to that conclution from when the suspension hits a bump, (assuming constant rebound damping on all the bikes shox) the firmer suspension designs will rebound as a faster speed because it is stiffer and given the rate is higher then the supple design and a constant rebound damping, it would rebound faster. so when a bike goes through a non perfectly smooth surface, the bike with the faster effective rebound will apply a greater total reaction force to the ground.



when a firmer and a softer suspension hit the same bump, they both recive the same amount of force and give the same amount of force back to the ground, the softer one just uses more travel and takes a little more time to give that force back. true the firmer one will give you more traction but it does it for less time. in a real world situation, the softer one with give more useable traction.

[dhkid]

lahardesign, wouldn't you have to account that the rider has some effect on the main frame too? and in that case the weight of the rear wheel is much lower then the combined mass of the main frame and whatever effect the rider has on it? but i get what you mean by a lighter rear wheel and rear triangle will grip better as it will react to the ground better.

udi, i read through it agian. so grip in corners is a combination how the rear sus reacts to the ground and how well it keeps the bike and rider in a position for cornering? hence giving an explanation of why softer set ups give the impression of less grip??

[ChrisKring]

I disagree. For a sinusoidal input at a constant speed blah blah, assuming you have a linear damper constant (which no shocks do) that might be the case, but what is considered "underdamped" for a rider's mass will invariably follow rough terrain the best. Underdamping will not "take the wheel off the ground" unless it's REALLY extreme given the spring rate to mass ratio (ie you're running bugger all sag and are topping out harshly), but overdamping most certainly will if the wheel can't follow the ground properly. For something like braking bumps (similar size/spacing hits) then yes there is arguably a critical point at which the suspension will be able to follow the terrain the best (where its resonant frequency is the same as that of the braking bumps) but otherwise I would most definitely err on the side of underdamping rather than overdamping.

I agree with what you are saying. However, I answered his ultra hypothetical question with the assumption of an equally hypothetical sine wave input.

I personally have always run underdamped set ups that are much faster than most people like. It works great in a straight line. However, in an off camber loose corner is where you need a little more rebound damping to prevent the rear wheel from drifting out. As I have gotten faster and have more experience on tuning the MX bike suspension, I have started to dial in more rebound damping and tend to run the front stiffer to keep the chassis level.

[Honus]

lahardesign, wouldn't you have to account that the rider has some effect on the main frame too? and in that case the weight of the rear wheel is much lower then the combined mass of the main frame and whatever effect the rider has on it? but i get what you mean by a lighter rear wheel and rear triangle will grip better as it will react to the ground better.

udi, i read through it agian. so grip in corners is a combination how the rear sus reacts to the ground and how well it keeps the bike and rider in a position for cornering? hence giving an explanation of why softer set ups give the impression of less grip??

Bingo! The rider's mass is so much greater than that of the bike that it has to be factored into how the bike will react when the rider's CG is shifted- especially during accelerating and braking forces. All things being equal, the less the unsprung mass the better the suspension will work. Damping rates are probably ultimately something that is realised through development and testing. It is interesting that Lahar brings up the topic of frequency as that's a pretty big deal in racecar damper design. I was just reading an issue of RaceCar Engineering where there was an article about Koni's latest develpoments with what they call frequency selective damping.......

When designing a suspension system you have to look at the big picture- how it all works as a system. Then you prioritize what is most important to you and try to figure out the packaging of the system to achieve your goals.

A few other decent books are "Competition Car Suspension" by Allan Staniforth and "Motorcycle Chassis: Tuning" by John Robertson.

[LaharDesign]

lahardesign, wouldn't you have to account that the rider has some effect on the main frame too? and in that case the weight of the rear wheel is much lower then the combined mass of the main frame and whatever effect the rider has on it? but i get what you mean by a lighter rear wheel and rear triangle will grip better as it will react to the ground better.

udi, i read through it agian. so grip in corners is a combination how the rear sus reacts to the ground and how well it keeps the bike and rider in a position for cornering? hence giving an explanation of why softer set ups give the impression of less grip??

Bikes are indeed a complex system with the rider the sprung weight and the whole bike the swingarm of a secondary suspension system. your legs are the spring and damper when standing, your ass a shorttravel elastomer when seated.
How much the riders weight can contribute to sprung mass of the primary suspension is dependant on frequency, but at the speed required to keep the wheel evenly weighted on choppy ground the sack of jelly that is a human being has little effect. Every person or company that has built a gearbox configuration know that the increases in traction are remarkable and obvious to every rider or spectator. The first national champion (junior dh and ds) to ride the Lahar mk1 proto in 1997 tried all the way down a 600ft switchback hill to break the rear-ends traction and came back raving about how everything he tried failed to do this. (this was with a position sensitive homemade damper that had very fatst response in the beginning of travel). Lahars have won mud races without losing traction once on the course while every other bike slid out every 2-3 seconds. There have been a number of 30 sec win in 3 minute mud races and one 50 sec one. 12 national champions have been involved in racetesting lahars and every one of them and dozens of other pro racers have found the traction enhancements of textbook chassis design to be overwhelmingly obvious. I'm afraid that anyone that now resists the results that me and dozens of other performance bike designers have found from basicly working with the Law of conservation of momentum is just looking for an excuse to no shell out on a gearbox bike. So build one yourself. I'll sell you a rohloff with all the mounting plates and chaingear. I'll call it the L-Box (for L-Men, the G-Whizzers can use the G-boxx).

suspension that rapidly stiffens as it goes through travel doesn't answer the max traction requirement of keeping the tire as evenly weighted on the ground as possible. It weights it too much on bumps and too little on hollows. Ideally the springs restoring force wouldn't change and some fancy computers would work out the required ride height and achieve it through dynamic damper adjustment. I hope we never go there!

[big-ted]

when a firmer and a softer suspension hit the same bump, they both recive the same amount of force and give the same amount of force back to the ground, the softer one just uses more travel and takes a little more time to give that force back. true the firmer one will give you more traction but it does it for less time. in a real world situation, the softer one with give more useable traction.

You're getting confused between the terms "force" and "impulse".

[Whoops]

Aaron = nail+hammer+head.

Spot on.

[LaharDesign]

Aaron = nail+hammer+head.

Spot on.

Cheers whoops:cheers:

Further to the "rider is sprung weight " debate:
On a conventional bike that has chain extension under compression where the rider is standing:
-the only part of the riders body at all rigidly connected to the bike is his feet and lower leg. As lashback through the pedal is the result of suspension actuation, this mass must be accelerated to activate the suspension.
Therefore the riders mass that is involved in the primary suspension system is unsprung not sprung weight!

[Honus]

Further to the "rider is sprung weight " debate:
On a conventional bike that has chain extension under compression where the rider is standing:
-the only part of the riders body at all rigidly connected to the bike is his feet and lower leg. As lashback through the pedal is the result of suspension actuation, this mass must be accelerated to activate the suspension.
Therefore the riders mass that is involved in the primary suspension system is unsprung not sprung weight!

OK- I can swing with that. The mass that is accelerated (rider's legs) is a result of pedal feedback. However, this is only a percentage of the rider's total weight and the degree to which the mass is accelerated is in direct correlation to chain growth. A similar mass acceleration happens in a URT design, except that the degree to which the mass is moved is a result of the pivot position- specifically in relation to the CG of the rider. Either way it does effectively add to the amount of unsprung weight but I wouldn't go so far as to say that the the rider's mass is entirely unsprung weight- the worst case scenario would be a forward pivot URT and the best case scenario conventional design would be one with minimal chain growth. A floating BB design would be somewhere in between....

So in a properly designed gearbox design the rider's mass is isolated from the suspension system, making the rider's mass entirely sprung weight so the suspension system is more effective. As always, the lower the unsprung weight the better the suspension system can function and ultimately improve grip. :)

And of course I would be interested in an "L-box." :) Who wouldn't....

[LaharDesign]

OK- I can swing with that. The mass that is accelerated (rider's legs) is a result of pedal feedback. However, this is only a percentage of the rider's total weight and the degree to which the mass is accelerated is in direct correlation to chain growth. A similar mass acceleration happens in a URT design, except that the degree to which the mass is moved is a result of the pivot position- specifically in relation to the CG of the rider. Either way it does effectively add to the amount of unsprung weight but I wouldn't go so far as to say that the the rider's mass is entirely unsprung weight- the worst case scenario would be a forward pivot URT and the best case scenario conventional design would be one with minimal chain growth. A floating BB design would be somewhere in between....

So in a properly designed gearbox design the rider's mass is isolated from the suspension system, making the rider's mass entirely sprung weight so the suspension system is more effective. As always, the lower the unsprung weight the better the suspension system can function and ultimately improve grip. :)

And of course I would be interested in an "L-box." :) Who wouldn't....

Good spotting. I was going to admit that if no-one bit.
a pivot or virtual pivot around the top of the chainring can move the pedal about 40% of suspension travel on quick bumps, so you could assume 40% of the mass of the lower legs as unsprung. Simular to standing on the swingarm 4/10 from the pivot to the axle.
Actually I'm not clear on whether the other 60% can be considered sprung. Anyone? It could in the swingarm analogy but its so long since I've seriously considered non-independant drive that I haven't pondered this in years.
Hondas running low pivot independant drive. They need an electric "Big Red" lockout button for this, but they wouldn't compromise their spr/unspr ratio by chain extension- or use an icky artificial stiction platform system.

[Whoops]

Hmm, what does this mean in practice? Is it also more than just the mass of the unsprung leg (or part thereof)? If the rider is exerting some force on the pedal that counts as 'mass' too.

So = dead weight of leg (shin/knee whatever, must get a hacksaw and find out) + force

That force could be muscular (ie above that required to hold the riders weight up... the dead weight), or accelerative (likey if the bike is compressing).

So let's say it's 40% of the leg mass. What's that? 10kg? Let's put a 3g accel in there - so roughly 10x9.81x3=300N-ish of instantaneous load. How much muscular force can a rider exert on one pedal (assuming disco slipper use)? A couple of hundred Newtons? Anyone got a good guess? My brain is a little hampered by the alcohol.

How much chain lash could occur? over what time?


edit to ask - hang on, what was the original question for this thread? traction? Sorry. some tyres grip well, some not so well. usually ALL tyres work better in dry conditions on smooth (but not too smooth), flat surfaces. the suspension is the to make the ride comfortable and cushion my haemaroids.

edit # 2 to add bibliographic references - "Race Car Engineering & Mechanics" by Paul van Valkensomething. and anything by Carroll Smith ("Engineer to win" etc). Both a bit dated now and looking at 4 wheels (bad) not 2 wheels (good) - but still worth a read - even if just from a race prep point of view.

edit #3 to appologise to any orwell fans. I meant 4 wheels good, 2 wheels bad. Long live the revolution!

[Socket]

Cheers whoops:cheers:

Further to the "rider is sprung weight " debate:
On a conventional bike that has chain extension under compression where the rider is standing:
-the only part of the riders body at all rigidly connected to the bike is his feet and lower leg. As lashback through the pedal is the result of suspension actuation, this mass must be accelerated to activate the suspension.
Therefore the riders mass that is involved in the primary suspension system is unsprung not sprung weight!


This is, of course, only if the rate of chain extension is great enough to exceed the potential chain slack of the wheel's rotational speed (when coasting - obviously irrelevant when pedalling).

By the way, a pivot height around the top of the chainring will NOT produce 40% of the wheel's travel as pedal kickback. By my calculations, it would be about 16.5% which, for the record is about 0.8" total pedal movement if you bottom out an 8" travel bike fully, from the sag point (assuming about 37.5% sag), and that's only if the rear wheel is NOT rotating at all relative to the cassette (eg you're pedalling or at a standstill). Once the wheel rotation speed picks up (assuming coasting) this number decreases greatly, especially as shaft speed is directly linked to wheel speed for a given bump. In other words, standard bike industry marketing hyperbole :)

[Honus]

I'm going to have to go with Socket on this one- I haven't crunched the numbers but just quickly thinking about it there's no way it's going to be as high as 40%- the chain growth under load just isn't that great given pivot placement at the top of the chainring.

Ultimately, the best design for all around DH performance (and by that I mean not having the chain influence suspension one way or the other) is to have the chain force parallel to the swingarm, which is different from zero chain growth. This can be argued against of course if you specifically wanted to generate a squat/anti-squat force. In motorcycle design they shift the positions of swingarm pivot and engine output sprocket to achieve a desired result- as in pro-squat, anti-squat, etc. Even if a bicycle had only one chainring and one rear sprocket it still becomes difficult to completely model a system- it's like having a motorcycle that has a huge poorly balanced parallel twin engine with a constantly moving CG. And on a bicycle the ratio between rider/bike mass is so much greater, further complicating things....

Which is why we have platform valving- it has more to do with abrubt weight transfer than it does with chain torque reaction. So we're getting off topic but ultimately it does all affect traction.... :)

[dhkid]

This is, of course, only if the rate of chain extension is great enough to exceed the potential chain slack of the wheel's rotational speed (when coasting - obviously irrelevant when pedalling).



yea, i was just gonna say that. the speed of the cassette, or freehub will have to be the same as the wheel, aka the hub engaging. so it would depend on how fast you hit a bump and how much travel you use..

keep the disicusion going guys... great stuff!!:thumb:

[Whoops]

... to have the chain force parallel to the swingarm, ...

You mean parallel to an imaginary line between the (swingarm/instantaneous) pivot and the axle, right?

btw - sorry for my previous post. Came home a little boozed. Kids, don't drink and surf!:shocked:

[dhkid]

edit, drunken sillyness

[Honus]

You mean parallel to an imaginary line between the (swingarm/instantaneous) pivot and the axle, right?

Yep- sorry, I should have been more specific.

[Socket]

Ultimately, the best design for all around DH performance (and by that I mean not having the chain influence suspension one way or the other) is to have the chain force parallel to the swingarm, which is different from zero chain growth.


If the chain is parallel to the swingarm then there will be no "chain growth" in the sense that you will never tension/slacken the chain by moving the suspension even though the BB-to-axle distance changes (and conversely, as you say, the chain itself will not have any input to the suspension) but in my opinion it is more useful to have a small amount of chain-induced extension force (ie chain-component of anti-squat) than it is to totally eliminate the possibility of ANY amount of pedal feedback. I know LaharDesign disagrees on this point (believing that it is more important to be able to pedal through the rough at any cost), but my own experience is that if it's that rough, you can't pedal efficiently anyway and sometimes you're simply better off being smooth and coasting than trying to crank hard over choppy ground.

[Honus]

If the chain is parallel to the swingarm then there will be no "chain growth" in the sense that you will never tension/slacken the chain by moving the suspension even though the BB-to-axle distance changes (and conversely, as you say, the chain itself will not have any input to the suspension) but in my opinion it is more useful to have a small amount of chain-induced extension force (ie chain-component of anti-squat) than it is to totally eliminate the possibility of ANY amount of pedal feedback. I know LaharDesign disagrees on this point (believing that it is more important to be able to pedal through the rough at any cost), but my own experience is that if it's that rough, you can't pedal efficiently anyway and sometimes you're simply better off being smooth and coasting than trying to crank hard over choppy ground.

There are definitely mixed feelings on the subject of chain-induced extension force. If it's a small amount I really don't see it as that big a problem- it's not an absolutely optimal situation but a large percentage of riders will probably never feel it and if it makes the design of the bike more effective in another area then I think it's fine. There are plenty of successful bike designs out there that have it to a small degree.

With the chain being parallel to the swingarm there won't be any chain growth BUT conversely it is possible to not have chain growth and and still not have the chain force parallel to the swingarm. Many bikes that pivot around the BB shell have this issue- the original Rotec bikes come to mind.

[Socket]

There are definitely mixed feelings on the subject of chain-induced extension force. If it's a small amount I really don't see it as that big a problem- it's not an absolutely optimal situation but a large percentage of riders will probably never feel it and if it makes the design of the bike more effective in another area then I think it's fine. There are plenty of successful bike designs out there that have it to a small degree.

With the chain being parallel to the swingarm there won't be any chain growth BUT conversely it is possible to not have chain growth and and still not have the chain force parallel to the swingarm. Many bikes that pivot around the BB shell have this issue- the original Rotec bikes come to mind.

A large percentage (I'd say ~100%) of riders will most certainly feel the difference between a bike with 0% anti-squat (ie no chain- or tractive-force-induced response to acceleration, only weight shift) and a bike that uses that small amount of chain extension to generate ~100% anti-squat. If I've misinterpreted that, and you actually meant that most riders won't feel a small amount of (theoretical) feedback, then yes I agree. There can be a certain amount of chain pull before it will be sufficient to overcome the speed of the spinning wheel, and then another small amount before it becomes noticeable to the rider. Subtract the amount required to overcome the speed of the wheel, and you've got the maximum amount of chain pull (I use the term "pull" separate to the term "growth" intentionally btw, I'll get to that in a sec) that you can "ideally" have.

I propose that we use the term "chain growth" as referring to the change in distance from axle to BB (or centre of drive sprocket for a jackshaft/idler setup), and "chain pull" as the radial (axial to the chain) distance that the chain is actually pulled by suspension compression. These are obviously not the same; if you imagine that you had a BB-centric pivot like a Rotec with a 38t on the front and a 19t on the back, no matter what happened there'd never be any chain growth because the BB-axle distance is constant. However, if you imagine rotating the swingarm around the BB in complete circles, the amount of chain unwound from the front chainring would be 38 links (well 19 links if you're gonna be technical, 38 half-links) per revolution, yet the amount wound onto the rear sprocket would only be 19 half links (9.5 full links), so clearly slack would generate in the chain. Therefore, a BB-centric bike with positive gearing will actually have negative chain pull, yet zero chain growth. Since chain pull (not chain growth) is a direct component of the extensive force on the axle (used to counter the rider/bike weight shift to the rear wheel under acceleration), one can summarise that chain pull is directly related to the pedalling efficiency of a bike (though it is not the ONLY thing... tractive force is the other component, whole 'nuther story I don't have time for now).

Edit: not that I don't think you understand this stuff... just for clarity's sake, because your statements were a bit ambiguous.

[Honus]

A large percentage (I'd say ~100%) of riders will most certainly feel the difference between a bike with 0% anti-squat (ie no chain- or tractive-force-induced response to acceleration, only weight shift) and a bike that uses that small amount of chain extension to generate ~100% anti-squat. If I've misinterpreted that, and you actually meant that most riders won't feel a small amount of (theoretical) feedback, then yes I agree. There can be a certain amount of chain pull before it will be sufficient to overcome the speed of the spinning wheel, and then another small amount before it becomes noticeable to the rider. Subtract the amount required to overcome the speed of the wheel, and you've got the maximum amount of chain pull (I use the term "pull" separate to the term "growth" intentionally btw, I'll get to that in a sec) that you can "ideally" have.

I propose that we use the term "chain growth" as referring to the change in distance from axle to BB (or centre of drive sprocket for a jackshaft/idler setup), and "chain pull" as the radial (axial to the chain) distance that the chain is actually pulled by suspension compression. These are obviously not the same; if you imagine that you had a BB-centric pivot like a Rotec with a 38t on the front and a 19t on the back, no matter what happened there'd never be any chain growth because the BB-axle distance is constant. However, if you imagine rotating the swingarm around the BB in complete circles, the amount of chain unwound from the front chainring would be 38 links (well 19 links if you're gonna be technical, 38 half-links) per revolution, yet the amount wound onto the rear sprocket would only be 19 half links (9.5 full links), so clearly slack would generate in the chain. Therefore, a BB-centric bike with positive gearing will actually have negative chain pull, yet zero chain growth. Since chain pull (not chain growth) is a direct component of the extensive force on the axle (used to counter the rider/bike weight shift to the rear wheel under acceleration), one can summarise that chain pull is directly related to the pedalling efficiency of a bike (though it is not the ONLY thing... tractive force is the other component, whole 'nuther story I don't have time for now).

Edit: not that I don't think you understand this stuff... just for clarity's sake, because your statements were a bit ambiguous.

Yes, you are absolutely correct and I concur on your use of terminology. I do have a tendency to over simplify in my explanations. I shall endeavour to describe with a greater degree of clarity in the future :) - I was going to go into an identical explanation of a BB-centric design but I had to put my older son to bed- kids come before bike talk! :)

And yes I did mean that most riders probably wouldn't feel the small amount of feedback....

[LaharDesign]

over 8 inches travel a pivot at the top of the chainring gives 40mm extension of the distance bb to axle.

[LaharDesign]

hmmm...
If positive chaingrowth means that you have to split the riders weight between say 30% unsprung and 70% sprung when standing...
Does it follow that negative chain growth means that the split may be -30% unsprung and 130% sprung mass?

[mech]

Grip is mostly determined by damping in combination with the wheelrate. Kicking of the bike has for 95% to do with compression damping not rebound. Your compression damping (Low speed)needs to be quite soft at the inital stage (about 35-40% with the sag set at 25-30%) this will provide mechanical grip, grip is also affected by tire choice and pressure. Problem with bikes is the better the grip the worse the pedaling wil be. Then you want to get your compression damping as stiff as possible in a progressive build-up. Spring rate is also of great effect for grip, the softer the spring the more grip, but also more kicking will happen. Most kicking occurs when you have the compression damping in combination with the spring to soft. The bike will ride to deep in the stroke and can't handle highspeed impacts, this causes kicking.
As for rebound you want the rebound damping as fast as possible until the bike start to feel bouncy, like when you hit a rock at a slower speed you feel the bike pushes you backwards and slows you down.

[dhkid]

If the chain is parallel to the swingarm then there will be no "chain growth" in the sense that you will never tension/slacken the chain by moving the suspension even though the BB-to-axle distance changes (and conversely, as you say, the chain itself will not have any input to the suspension) but in my opinion it is more useful to have a small amount of chain-induced extension force (ie chain-component of anti-squat) than it is to totally eliminate the possibility of ANY amount of pedal feedback. I know LaharDesign disagrees on this point (believing that it is more important to be able to pedal through the rough at any cost), but my own experience is that if it's that rough, you can't pedal efficiently anyway and sometimes you're simply better off being smooth and coasting than trying to crank hard over choppy ground.

umm, i am a bit confused by the 'chain is parellel to the swingarm' statement... wouldn't the chain slacken under compression? and tug under rebound. there still would be "chain growth" right?

lahardesign, i do not follow what you saying anymore.. :huh: :redface:

[Honus]

umm, i am a bit confused by the 'chain is parellel to the swingarm' statement... wouldn't the chain slacken under compression? and tug under rebound. there still would be "chain growth" right?


What we should really be saying is chain force..... Imagine a swingarm where the drive sprocket (chainring) is concentric to the swingarm pivot. Now imagine that the drive sprocket (chainring) and the drive sprocket at the rear wheel are the exact same size. In this case the chain would always run directly parallel to the swingarm and wouldn't influence suspension action.

Given the same situation, if the drive sprocket (chainring) is larger than the rear sprocket then the suspension would tend to compress under a pedaling load- the chain force wants to pull the rear wheel up. If the drive sprocket (chainring) is smaller than the rear sprocket then the suspension would tend to extend under a pedaling load. In either case there is no chain growth. This is why single swingarm bikes like the Rotec that had the swingarm pivoting around the BB shell pedaled poorly.

Make sense?

[Honus]

hmmm...
If positive chaingrowth means that you have to split the riders weight between say 30% unsprung and 70% sprung when standing...
Does it follow that negative chain growth means that the split may be -30% unsprung and 130% sprung mass?

Hmmm.... well I don't think you can have more than 100% of the rider's mass as sprung. I don't really think it's possible to divide the rider's mass percentages, at least with any degree of accuracy.

[Honus]

Grip is mostly determined by damping in combination with the wheelrate. Kicking of the bike has for 95% to do with compression damping not rebound. Your compression damping (Low speed)needs to be quite soft at the inital stage (about 35-40% with the sag set at 25-30%) this will provide mechanical grip, grip is also affected by tire choice and pressure. Problem with bikes is the better the grip the worse the pedaling wil be. Then you want to get your compression damping as stiff as possible in a progressive build-up. Spring rate is also of great effect for grip, the softer the spring the more grip, but also more kicking will happen. Most kicking occurs when you have the compression damping in combination with the spring to soft. The bike will ride to deep in the stroke and can't handle highspeed impacts, this causes kicking.
As for rebound you want the rebound damping as fast as possible until the bike start to feel bouncy, like when you hit a rock at a slower speed you feel the bike pushes you backwards and slows you down.

Given that everyone has more or less the same damper technology (unless you're a pro running double top secret stuff) a damper wouldn't determine why one bike had more grip than another- which I believe was the original question. Dialing in a damper to work with a given bike design and rider preference plays a tremendous role here of course. What I mean to say is that dampers do play a large part in how much grip is generated but they are far from the only thing to consider.

But since we're on the topic of damping.....
All suspension settings are a compromise between having as much compliance as possible and having as much chassis stability as possible. Rebound damping unloads the tire whenever the suspension has an extension velocity, resulting in reduced grip on the downhill side of a bump- too much rebound damping and the tire loses grip. Too little rebound and the suspension extends too rapidly, causing loss of grip and chassis instability.

Compression damping also does this because when the suspension resists compression, the sprung mass (rider and frame) has a greater upward velocity on the uphill side of the bump. Too much compression damping and the tire loses contact with the ground at the crest of the bump. It's always a balance ....

As a general rule, the response to large bumps is controlled by high speed damping (meaning high damper piston velocity) while the overall attitude of the bike (chassis stability) is controlled by low speed damping (meaning low damper piston velocity.) So, bump inputs are at high velocities and rider inputs are at low velocities. So if your bike design is able to minimise rider inputs due to chain/weight transfer forces (braking/accelerating) then you have greater overall chassis stability. What platform dampers try to do is discriminate between rider inputs and bump inputs to give better levels of chassis control.

When designing a bike, you also have to take into account the amount of mechanical friction in the suspension sytem. This is known as Coulomb damping. Coulomb damping is independent of velocity and is digressive at low velocities. A bike design with multiple pivots will inherently have more of this than a simple three pivot swingarm bike. Detail design (bearing systems, etc.) is very important here and can greatly affect damping.

[Socket]

over 8 inches travel a pivot at the top of the chainring gives 40mm extension of the distance bb to axle.

Which as you know means approximately f**k all unless you specify gearing. If your gearing was high enough you could STILL have negative chainpull even with that increase in BB-axle distance. Stop pulling out irrelevant figures.

hmmm...
If positive chaingrowth means that you have to split the riders weight between say 30% unsprung and 70% sprung when standing...
Does it follow that negative chain growth means that the split may be -30% unsprung and 130% sprung mass?

Again, assuming we're talking whilst pedalling here, because coasting as we have seen is not affected unless the chainpull is sufficient to overcome the wheel's speed... If you had so much negative chain growth that your foot actually DROPPED when you hit a bump, and I mean dropped relative to the ground not the bike frame, then yes you could effectively have negative unsprung mass. A notable side effect would be that your bike would tend to bob like crazy because every time you pedalled, you'd compress the suspension a loooong way.

[Socket]

umm, i am a bit confused by the 'chain is parellel to the swingarm' statement... wouldn't the chain slacken under compression? and tug under rebound. there still would be "chain growth" right?

lahardesign, i do not follow what you saying anymore.. :huh: :redface:

If the chain is parallel to the swingarm (ie the chainline being parallel to an imaginary line drawn between the pivot and the axle), then there will be no component of the axle's motion that is parallel to the chainline. In other words, the only movement of the chain will be to rotate it, not to pull on it.

[Whoops]

over 8 inches travel a pivot at the top of the chainring gives 40mm extension of the distance bb to axle.

So - just measured mine (whoooor)

Axle to BB dimension

uncompressed length = 545mm
compressed length = 585mm

all dimensions approximate (using a builders tape measure).

[Socket]

Yeah VERY approximate I'd say... unless you really do have 21.5" static chainstays. Don't spose you actually meant 445 and 485mm?

[dhkid]

If the chain is parallel to the swingarm (ie the chainline being parallel to an imaginary line drawn between the pivot and the axle), then there will be no component of the axle's motion that is parallel to the chainline. In other words, the only movement of the chain will be to rotate it, not to pull on it.

i get that, but are you talking about an instant in the bikes travel? coz the only way this can happen is when a consentric pivot about the bb. or are you thinking about a single pivot that is designed so that in its travel when traction is most important it was designed so there the chain will have the least amount of effect on it within that region of travel?

[Whoops]

Yeah VERY approximate I'd say... unless you really do have 21.5" static chainstays. Don't spose you actually meant 445 and 485mm?

Yep. my bad. the 40mm was right though.

[Socket]

i get that, but are you talking about an instant in the bikes travel? coz the only way this can happen is when a consentric pivot about the bb. or are you thinking about a single pivot that is designed so that in its travel when traction is most important it was designed so there the chain will have the least amount of effect on it within that region of travel?

At any point or region yeah. You could fairly easily design a linkage (well, if you only had a single chainline, ie gearbox bike) that moved the CC in such a way that the chainline and swingarm line were always kept parallel however. The only time that could happen with a concentric pivot is if you had a 1:1 gear ratio (which would require a gearbox, unless you happen to be on a trials bike or something).

[LaharDesign]

.. If you had so much negative chain growth that your foot actually DROPPED when you hit a bump, and I mean dropped relative to the ground not the bike frame, then yes you could effectively have negative unsprung mass. A notable side effect would be that your bike would tend to bob like crazy because every time you pedalled, you'd compress the suspension a loooong way.

Not neccesariry so grasshopper.
neutral inertial pitch behaviour is with a pivot ~200mm above the bb and close to the wheel. with sag ~40% and zero chain extension at 50% travel the m9 is mostly in negative extension in even small bumps. Guys like Nico V rated my highest 25cm ovr bb pivot bike as the most promising design with even more negative chain extension.
The possibility of infinite sprung/unsprung mass ratio is an intriguing scenario for high pivot bikes under pedalling. (or other vehicles with counter leveraged masses to achieve the same effect):clapping:

[Socket]

Not neccesariry so grasshopper.
neutral inertial pitch behaviour is with a pivot ~200mm above the bb and close to the wheel. with sag ~40% and zero chain extension at 50% travel the m9 is mostly in negative extension in even small bumps. Guys like Nico V rated my highest 25cm ovr bb pivot bike as the most promising design with even more negative chain extension.
The possibility of infinite sprung/unsprung mass ratio is an intriguing scenario for high pivot bikes under pedalling. (or other vehicles with counter leveraged masses to achieve the same effect):clapping:

Yes, it is necessarily so. If your foot/lower leg moves in the same direction as the bump force then its inertia will resist the force, regardless of all else (since F = ma, if a is positive then so must be F). If it moves in the opposite direction then you could say that A is negative and thus so is the force required to accelerate it. Having negative feedback may decrease the effective unsprung mass but it will not create an actual negative effective unsprung mass until such time as your foot actually drops whilst pedalling, when you hit a bump. That requires that the amplitude of the movement transmitted to the frame be lower than the negative feedback.

Edit: this is based on your theory of "more travel at the pedal" which of course assumes you are weighting the front pedal heavily at the time of the impact, and ignores the possibility that your cranks are at 12 and 6 instead of 3 and 9, in which case the effect obviously shifts to fore/aft motion. If the force you're applying through your legs is still sufficient to accelerate at a great enough rate that the chain never slackens then you can still decrease the effective unsprung mass... however personally I think it's just a wank. I can pedal my low-pivot, positive feedback bike over 6" high bumps easily enough, but regardless of whether I'm pedalling or not, it's simply a rough ride, and when I'm pedalling I can't smooth things out like I can when I'm coasting.

[LaharDesign]

Yes, it is necessarily soreferring to "wallowing under pedalling" I was. If your foot/lower leg moves in the same direction as the bump force then its inertia will resist the force, regardless of all else (since F = ma, if a is positive then so must be F). If it moves in the opposite direction then you could say that A is negative and thus so is the force required to accelerate it. Having negative feedback may decrease the effective unsprung mass but it will not create an actual negative effective unsprung mass until such time as your foot actually drops whilst pedalling, when you hit a bump.better than kicking up when you hit a bump is That requires that the amplitude of the movement transmitted to the frame be lower than the negative feedback.

Edit: this is based on your theory of "more travel at the pedal" which of course assumes you are weighting the front pedal heavily at the time of the impact,in a power stand I am and ignores the possibility that your cranks are at 12 and 6 instead of 3 and 9, in which case the effect obviously shifts to fore/aft motion.fully sprung mass % I think Personally I think it's just a wank.ngahngahferrousPonyfan

[Socket]

haha I was editing while you were posting... re-read.

[Whoops]

So when will active suspension be trialled? (a la Williams F1)

I can't see 'fully active' (ie driving the linkages) suspension due to weight penalties, but I can imagine active valving on dampers and (if air sprung) perhaps active chamber dimensions (to alter 'spring' rates).

Perhaps Honda could do it and get some more patents?

[LaharDesign]

So when will active suspension be trialled? (a la Williams F1)

I can't see 'fully active' (ie driving the linkages) suspension due to weight penalties, but I can imagine active valving on dampers and (if air sprung) perhaps active chamber dimensions (to alter 'spring' rates).

Perhaps Honda could do it and get some more patents?
Quick lets talk about it and get our priors in!
Socket you nerd I was going to qualify my comment about 12 and 6 oclock cranks with comment on how pedal force may overcome inertial force or not dependant on frequency but you beat me too it.
sockets issues with more sag when you point things uphill would be eliminated with active pressure adjustments. saw a german bike with that stuff in a mag about 10 years ago. you pitched it forward or back and reset the attitude with a button. Bulky and klutzy but thats just the detail.

[Honus]

So when will active suspension be trialled? (a la Williams F1)

I can't see 'fully active' (ie driving the linkages) suspension due to weight penalties, but I can imagine active valving on dampers and (if air sprung) perhaps active chamber dimensions (to alter 'spring' rates).

Perhaps Honda could do it and get some more patents?

As if bikes aren't expensive/complex enough as it is.....

Have they even used active suspension in motocross yet? I know it's been used in GP bikes....

[Whoops]

So instead of sensors reading the ground ahead... perhaps an interim system might 'just' use a pot on the forks to feed data for controlling the rear.

Add inputs from the brake levers, and maybe a strain gauge on the cranks...

Easy.

Mr Honda, pm me and I'll send my CV and renumeration requirements immediately.

Edit to say: we can put the flux capacitor under the seat tower.

[Honus]

So instead of sensors reading the ground ahead... perhaps an interim system might 'just' use a pot on the forks to feed data for controlling the rear.

Add inputs from the brake levers, and maybe a strain gauge on the cranks...

Easy.

Mr Honda, pm me and I'll send my CV and renumeration requirements immediately.

Edit to say: we can put the flux capacitor under the seat tower.

I know a guy that's a data acquisition engineer for one of the top CART teams- I'm sure he could hook something up. Now trying to take that and effectively control damper valving using a solenoid or ERM fluid and software- that's something else. Didn't Noleen make an electronic brain shock several years ago that kind of adjusted itself? I know Cannondale played around with some electronic valving control on their team DH bikes in the Fulcrum era.... If I remember correctly everyone hated this stuff.

And using a flux capacitor would be cheating- no bending of the space time continuum to finsh first. :nerd:

[LaharDesign]

I can't see on the fly attitude control should add more than a couple of bucks. Given that both forks and rear shocks have air pressure spring boosting commonly all you need is systems designed to have simular pressure at a good pitch attitude on flat ground. And some hose connecting them both to a push button valve on the bar.

As for terrain sensors and active control, why not add quantum computing to instantly evaluate all possible outcomes and pick the best.
our brains probably already do that though.

[Honus]

I can't see on the fly attitude control should add more than a couple of bucks. Given that both forks and rear shocks have air pressure spring boosting commonly all you need is systems designed to have simular pressure at a good pitch attitude on flat ground. And some hose connecting them both to a push button valve on the bar.

As for terrain sensors and active control, why not add quantum computing to instantly evaluate all possible outcomes and pick the best.
our brains probably already do that though.

All you need to do is get a super MRI scan of Vouilloz's brain to model your computer......

[LaharDesign]

All you need to do is get a super MRI scan of Vouilloz's brain to model your computer......

I've always thought the super reaction speed and 3D processing abilities of the common houseflys brain to hold enormous promise as a downhill pilot.
Australian flys are particularly tenacious, chasing you along a trail as fast as you can pedal.

[Whoops]

I can't see on the fly attitude control should add more than a couple of bucks. Given that both forks and rear shocks have air pressure spring boosting commonly all you need is systems designed to have simular pressure at a good pitch attitude on flat ground. And some hose connecting them both to a push button valve on the bar....

We could nick the old hydrolastic suspension out of a Morrie Minor eh? Or if we wanted some french speed would could go for the Citroen version...(americans - use Google to work out what the hell I'm talking about)

[Whoops]

I've always thought the super reaction speed and 3D processing abilities of the common houseflys brain to hold enormous promise as a downhill pilot.
Australian flys are particularly tenacious, chasing you along a trail as fast as you can pedal.

Got to watch out for those corks-on-a-string though.. that'll put them right off, aye Bruce?


rule number 1, no poofters
etc etc

[LaharDesign]

The mountain flys at mt buller were nasty. By the time you finished a swat they would have come back and landed again, probably on a pimple or the sweatiest bit of exposed skin.
I asked an Aussie how they could stand them.
He told me after you were there a while they stopped landing on you- while 6 or so crawled around his face! (thats traction)

[Honus]

We could nick the old hydrolastic suspension out of a Morrie Minor eh? Or if we wanted some french speed would could go for the Citroen version...(americans - use Google to work out what the hell I'm talking about)

I'm an American and I know what the hell you're talking about (although I was born in England......) but I'm not sure you guys from NZ are making much sense! Fly brains controlling bikes....:crazy: lol....

Moulton bikes use hydrolastic suspension (not quite in the sense you're talking about) but that would make sense since he invented it.

Soooo.... have we efffectively answered the original question to any degree?

Lahar, I'd be curious to know your thoughts on floating BB designs- Mongoose, Maverick, etc.

[LaharDesign]

I'm an American and I know what the hell you're talking about (although I was born in England......) but I'm not sure you guys from NZ are making much sense! Fly brains controlling bikes....:crazy: lol....

Moulton bikes use hydrolastic suspension (not quite in the sense you're talking about) but that would make sense since he invented it.

Soooo.... have we efffectively answered the original question to any degree?

Lahar, I'd be curious to know your thoughts on floating BB designs- Mongoose, Maverick, etc.

Apart from the unsprung mass considerations fl BB's can work real well. The hybrid systems with (bb on lower rocker and GTs over complex and overweight i-drive) are possibly the best way to make a susp bike with chain bb-axle. Chain extension is drasticly reduced for good antisquat, unsprung mass only affected a little and pivots can go up towards where they should be.

If we superglued our flies to microswitches on wands infront of our wheels they would react to the bumps coming towards them and the data could raise and lower the suspension accordingly. Given the short lifespan of flies you'd need a simulator to train a supply of fresh flies.

[profro]

In college I worked with a ME professor to create a computer program that attempted to solve vibration equations to find optimal damping coefficients for a 4 wheeled lunar buggy. It was a sound set of assumptions and equations and program, but basically it never reached a solution.

All that aside, its a pretty complicated subject matter lmuch ike fluid dynamics and lots and lots of assumptions have to made to reach any analytical solutions.

But I think most of the traction equation is solved by any good quality damper. The final 10-15% that makes the real difference is rider input.

[Whoops]

ok - how about a transducer on the bb that looks down to the ground below. That would give a better 'read' of the ground profile than the forks.

Make it a binary system at first - if it picks up some huge spike threshold it zaps the damper to open the floodgates.... then once that works, refine in incremental stages.

[LaharDesign]

:lighten: ok - how about a transducer on the bb that looks down to the ground below. That would give a better 'read' of the ground profile than the forks.

Make it a binary system at first - if it picks up some huge spike threshold it zaps the damper to open the floodgates.... then once that works, refine in incremental stages.

Definately sounds doable and promising. On the fly quantum computing could instantly solve the problems that conventional linear algorithms find so difficult however. An N bit quantum computer can solve an N variable simultaneous equation in one operation.
I'm seeing bike shops competing to breed and train the best flys for local tracks. They'd be a great moneyspinner.

[Honus]

Apart from the unsprung mass considerations fl BB's can work real well. The hybrid systems with (bb on lower rocker and GTs over complex and overweight i-drive) are possibly the best way to make a susp bike with chain bb-axle. Chain extension is drasticly reduced for good antisquat, unsprung mass only affected a little and pivots can go up towards where they should be.


That's pretty much what I've been thinking for some time now...... I might just have to weld one up. I don't think it's the greatest DH design ever but on my rides I have to pedal up the hills.

I do have a couple of DH designs but one of them is just sooooo crazy different- I still have a few things to work out on it.

[Socket]

Apart from the unsprung mass considerations fl BB's can work real well. The hybrid systems with (bb on lower rocker and GTs over complex and overweight i-drive) are possibly the best way to make a susp bike with chain bb-axle. Chain extension is drasticly reduced for good antisquat, unsprung mass only affected a little and pivots can go up towards where they should be.

The problem with those is that to move the suspension, it also has to move the front triangle relative to the rider mass (even if only the lower legs), which will mean either pushing the front triangle forwards, pulling the rider backwards, or a combination of the two. The rearwards-looking axle path starts to become irrelevant because it's not actually very rearwards compared to the centre of mass, and in the end that's the critical measure of just how rearwards an axle path is. In the end they're effectively much the same as a normal fixed-BB bike.

[Honus]

The problem with those is that to move the suspension, it also has to move the front triangle relative to the rider mass (even if only the lower legs), which will mean either pushing the front triangle forwards, pulling the rider backwards, or a combination of the two. The rearwards-looking axle path starts to become irrelevant because it's not actually very rearwards compared to the centre of mass, and in the end that's the critical measure of just how rearwards an axle path is. In the end they're effectively much the same as a normal fixed-BB bike.

Thanks for your input- this is why I brought it up. I haven't ridden any of the newer floating BB designs but I have built a couple of URT frames that have the pivot about 9" directly above the BB and they work pretty well as a XC bike. Everything changes as the travel gets longer though.......