Quantcast

Time to build my own DH frame

ZenkiGarage

Monkey
Jan 9, 2007
341
0
Portland, Or
After modifying a few frames, and building a couple hardtails, Im diving in on building my own DH rig. Im working on the jig right now. Should have it done in the next few nights, then I can start in on the fabrication of the frame. Its gonna be a 4130 frame, aluminum swinger, High pivot jackshaft arrangment, 13" bb, 63* HA, 15" CS(static). Should be a fun project.

Here's the jig, as of last night.



 
Last edited:

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
After modifying a few frames, and building a couple hardtails, Im diving in on building my own DH rig. Im working on the jig right now. Should have it done in the next few nights, then I can start in on the fabrication of the frame. Its gonna be a 4130 frame, aluminum swinger, High pivot jackshaft arrangment, 13" bb, 63* HA, 15" CS(static). Should be a fun project.

Here's the jig, as of last night.



i'd recommend swapping those materials. alu front, steel swingarm.

also, i still think jackshaft frames are cool.
 

Tetreault

Monkey
Nov 23, 2005
877
0
SoMeWhErE NoWhErE
typo maybe? i dont think there is any possible way, most dj guys have a hard time going under 15" with 2.2 semi slicks... 15" cs with 2.3+ knobbies is going to be, well, impossible.
 

ZenkiGarage

Monkey
Jan 9, 2007
341
0
Portland, Or
Not a typo, 15" CS is my goal. Being a high pivot, the CS will grow as it goes through its travel, so I want it to be as short as possible at static ride hieght. To accomplish this I will be using a BB of my own design(similar to a spanish BB) to increase clearance.
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
Not a typo, 15" CS is my goal. Being a high pivot, the CS will grow as it goes through its travel, so I want it to be as short as possible at static ride hieght. To accomplish this I will be using a BB of my own design(similar to a spanish BB) to increase clearance.
his point is that 15" CS is gonna create clearance issues with a tire, even with a modified BB.

8". Yeah, it is low, but its something Ive wanted to try for a while now.
advice: add at least 1/4". i used to have a bike with a bb just under 13", and even with 165mm cranks I was constantly smashing pedals.
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
also, don't misinterpret our advice. nobody is trying to give you a hard time, just offering some constructive criticism, and lessons learned from experience.
 

norbar

KESSLER PROBLEM. Just cause
Jun 7, 2007
11,346
1,587
Warsaw :/
Yup. I haven't seen 15'' CS on a dirt hardtail and they try really hard to make make it short (though it is probably possible).
What worries me more is a 63ha and a 15cs combination. Even if it is really rearward at sag it will be noticably shorter than many bikes with the weight distribution being waay out over the back wheel. The only way is to go with a long tt.

13'' bb would be good idea if it was a total push bike for a bikepark but for that I always thought something more popy and less rearward would make more sense.


Still can't wait to see the results. I'm curious to see the results and to be honest even if I would go with different geo I'd gladly try and ride a bike with such geo. Keep us posted.
 

ZenkiGarage

Monkey
Jan 9, 2007
341
0
Portland, Or
his point is that 15" CS is gonna create clearance issues with a tire, even with a modified BB.



advice: add at least 1/4". i used to have a bike with a bb just under 13", and even with 165mm cranks I was constantly smashing pedals.
I've already done a mock up rear end(wood & pvc lol) with a 2.5 minion, and at 15", I still with have .125 of clearance from the BB shell. Once there is any weight at all that clearance will increase. I may bump it out to 15.25, but Im gonna try to get it as short as possible, so when its squated out its not a limo.
I'll definitely keep the bb height in mind.
 

norbar

KESSLER PROBLEM. Just cause
Jun 7, 2007
11,346
1,587
Warsaw :/
I've already done a mock up rear end(wood & pvc lol) with a 2.5 minion, and at 15", I still with have .125 of clearance from the BB shell. Once there is any weight at all that clearance will increase. I may bump it out to 15.25, but Im gonna try to get it as short as possible, so when its squated out its not a limo.
I'll definitely keep the bb height in mind.
I understand that for now it's a bike for you only but Portland is pretty wet. Some kinds of clay really stick to your tires and a 0.125 will be very little (not to mention may not work with wetscreams). I've been on a track where a large % of the riders had their wheels stop from spinning and the clearance was higher. It's a one in a year situation but it's worth to have in mind.
 

HardtailHack

used an iron once
Jan 20, 2009
6,628
5,443
Very cool indeed, I am with the others on the BB height my current DH bike is 13" high at the BB and it's good but I'd think with a nice rearward axle path a bit more sag may help it ride faster through the chop. It's only a gut feeling but 13" is low and I have snapped one of my Bugtec axles running that height.

I was hoping you were going to make a hardtail but this sounds good too;)

How come if I click a smiley I get a new tab to Bike Magazine? Wierd.
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
21,031
5,921
borcester rhymes
FYI BCD got 16.7" chainstays with his deathtrap which had 29" wheels. Subtract the 1.5" of clearance, and you're at 15.2". Any shorter and I'd wager you'll get buzz, but on a DH bike with a high pivot, you'll only get growth, so it should be 16" at sag or so. Mud clearance won't be a problem except at top out or if you design the swingarm poorly.

Personally I'd raise the BB to 13.5-13.8", but you seem keen on pushing the limits, so go for it!
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
I've already done a mock up rear end(wood & pvc lol) with a 2.5 minion, and at 15", I still with have .125 of clearance from the BB shell. Once there is any weight at all that clearance will increase. I may bump it out to 15.25, but Im gonna try to get it as short as possible, so when its squated out its not a limo.
I'll definitely keep the bb height in mind.
word. how much is your CS gonna grow through the travel? ie - how long will it be when the bike is fully bottomed.
 

Tetreault

Monkey
Nov 23, 2005
877
0
SoMeWhErE NoWhErE
Do you have any initial drawings that you could post up? even hand drawn stuff? I'm very curious to see the entire thing planned as it sounds like a very interesting build. it's going to ride very different though thats forsure, thinking about it at 25-30% sag makes me wonder...
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
Make the build super cool and do it as a 29er
JTFC dude stfu about 29'ers. yea, we get it, you have a raging hard-on for them. you pop a boner so loud i can hear it on my side of the internet anytime someone so much as mentions a mountain bike with road wheels. clearly this guy wants to build a bike with his own specs and riding style in mind, not a bike for you to stand on a soap box and play pocket pool.
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
21,031
5,921
borcester rhymes
JTFC dude stfu about 29'ers. yea, we get it, you have a raging hard-on for them. you pop a boner so loud i can hear it on my side of the internet anytime someone so much as mentions a mountain bike with road wheels. clearly this guy wants to build a bike with his own specs and riding style in mind, not a bike for you to stand on a soap box and play pocket pool.
You know what would be about as annoying as somebody who kept talking about how great 29ers were? Somebody who talked about how bad 29ers were EVERY TIME THEY GOT BROUGHT UP. Just a little observation for you. For chrissakes the guy's like 7.5' tall, no wonder he likes big wheels.
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
You know what would be about as annoying as somebody who kept talking about how great 29ers were? Somebody who talked about how bad 29ers were EVERY TIME THEY GOT BROUGHT UP. Just a little observation for you. For chrissakes the guy's like 7.5' tall, no wonder he likes big wheels.
i don't talk about how bad they are, just that they're not the end-all be-all solution for mountain biking. I've ridden plenty of them over the years, and while I can certainly see why they would appeal to some people, they're not my thing (and I've stated this MANY times). I only pretend to hate them because its funny to see how much that attitude pisses off people who act like 29'ers are the second coming of John Tomac.
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
21,031
5,921
borcester rhymes
all hail the hypnotomac

I like my 29er HT but my Canzo 29er was ****! They definitely have their place, but 26ers are the spot for DH, I think.
 

LMC

Monkey
Dec 10, 2006
683
1
If using a spanish BB arrangement then you could have the middle of the bb shell cut away for more tyre clearance, thats the route I would go. Also i'll second the raising the BB slightly advice.
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
If using a spanish BB arrangement then you could have the middle of the bb shell cut away for more tyre clearance, thats the route I would go.
that's a really cool idea, but unfortunately that would give more access to the BB bearings to get clogged with mud/dirt/grime/etc.

But like OP said, high a high pivot, the CS is only going to grow, so the only time it could be a problem when riding is when airborne and the back end isn't under any load.
 

Tetreault

Monkey
Nov 23, 2005
877
0
SoMeWhErE NoWhErE
i can't stop thinking about what this bike is going to be like under sag.... 61* head angle, 12.2" bb height and 16" chainstays? It will be interesting, i keep thinking that it will handle like a wheelbarrow getting pushed down a hill
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
i can't stop thinking about what this bike is going to be like under sag.... 61* head angle, 12.2" bb height and 16" chainstays? It will be interesting, i keep thinking that it will handle like a wheelbarrow getting pushed down a hill
my old bike that i referred to earlier had a 12.625" BB. Unsagged. Stable as all get out. But it murdered many a pedal at the hand of east coast rox.
 

kidwithbike

Monkey
Apr 16, 2007
466
0
Hoboken, NJ
am I missing something here? last time I checked Euro BB's were a smaller diameter than a Spanish shell. I respect your engineering judgement and agree that the pressfit of the spanish is a better interface, it does away with bandaid external bottom brackets, and will give more weld area than a threaded euro...
but if you are looking at solely clearance...
 

marshalolson

Turbo Monkey
May 25, 2006
1,770
519
FYI BCD got 16.7" chainstays with his deathtrap which had 29" wheels. Subtract the 1.5" of clearance, and you're at 15.2". Any shorter and I'd wager you'll get buzz, but on a DH bike with a high pivot, you'll only get growth, so it should be 16" at sag or so. Mud clearance won't be a problem except at top out or if you design the swingarm poorly.

Personally I'd raise the BB to 13.5-13.8", but you seem keen on pushing the limits, so go for it!
my BCD has 15.5 chain stay at static. they are about 16.5 +/- at 35% sag. there is more than plenty of room with a 2.5 DHF to go shorter.
 
Last edited:

LMC

Monkey
Dec 10, 2006
683
1
that's a really cool idea, but unfortunately that would give more access to the BB bearings to get clogged with mud/dirt/grime/etc.

But like OP said, high a high pivot, the CS is only going to grow, so the only time it could be a problem when riding is when airborne and the back end isn't under any load.
I was actually thinking of two different arrangements that may suit, one where the BB shell consists solely of two tabs mated to the down/seat tube to house the bearings, but as you say that leaves the bearings more exposed to a build up of mud.

The second arrangement would be similar but inolve a regular cylindrical BB shell, but with the center being only being wide enough to clear the axle, then stepping up to a shoulder at each side to house the bearings. A bit like if you were to machine an all in one headset and headtube out of the one piece of metal.
 

LMC

Monkey
Dec 10, 2006
683
1
am I missing something here? last time I checked Euro BB's were a smaller diameter than a Spanish shell. I respect your engineering judgement and agree that the pressfit of the spanish is a better interface, it does away with bandaid external bottom brackets, and will give more weld area than a threaded euro...
but if you are looking at solely clearance...
I dont think he is working to the spanish BB standard, just that general layout. Could be two external BB shell bearings pressed into the frame.
 

jonKranked

Detective Dookie
Nov 10, 2005
85,565
24,182
media blackout
I was actually thinking of two different arrangements that may suit, one where the BB shell consists solely of two tabs mated to the down/seat tube to house the bearings, but as you say that leaves the bearings more exposed to a build up of mud.

The second arrangement would be similar but inolve a regular cylindrical BB shell, but with the center being only being wide enough to clear the axle, then stepping up to a shoulder at each side to house the bearings. A bit like if you were to machine an all in one headset and headtube out of the one piece of metal.
i get what you're saying. intriguing.
 

4130biker

PM me about Tantrum Cycles!
May 24, 2007
3,883
447
that's a really cool idea, but unfortunately that would give more access to the BB bearings to get clogged with mud/dirt/grime/etc.
You could notch it, then cap the notch (with a peice of a tube), grind the welds and make it look pretty sweet.

Interested to see what Zenki came up with though.

Anyway, sweet looking project and good luck! I've been stuck in the "research" phase for too long now...
 

sikocycles

Turbo Monkey
Feb 14, 2002
1,530
772
CT
:rofl:
JTFC dude stfu about 29'ers. yea, we get it, you have a raging hard-on for them. you pop a boner so loud i can hear it on my side of the internet anytime someone so much as mentions a mountain bike with road wheels. clearly this guy wants to build a bike with his own specs and riding style in mind, not a bike for you to stand on a soap box and play pocket pool.