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Wheel size comparison, data based

Discussion in 'Downhill & Freeride' started by mtg, Dec 15, 2012.

  1. mtg Member

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    So, this doesn't really seem to exist, and I had a couple spare minutes. With a sketch in CAD, I drew what is basically a 26" wheel, 27.5" and 29" hitting a square edge feature to put some numbers behind "rolling over rocks".

    A quick tangent about load transfer: the neck of the woods that I was involved in with racecars used the terms of "elastic" and "geometric" load transfer to describe the difference between loads that came from the tires and either went into the springs & dampers, or directly through the chassis. In the mtb world, this would be equivalent to pivot height in the rear suspension. A BB-concentric pivot location has poor square edge performance because a significant portion of the tire load goes directly into the frame (high geometric load transfer) and feels harsh, or "gets hung up". A super high pivot, conversely, has higher elastic load transfer, and much less of the tire load goes directly into the frame without first going through the spring & damper.

    Returning from the tangent, the below graphic shows the effective approach angle of different wheel sizes. With that, geometric vs elastic load transfer can be calculated. That will also vary directly with head angle and square edge height. I'll get to those calcs soon.

    Discuss.

    Wheel size approach angle comparison.jpg
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  2. Sandwich just shake your rump

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    Did you take into account actual rim sizes? I know 650b is not 27.5", but I was kind of under the impression that neither is 26 actually 26" nor 29er exactly 29"...just for the sake of accurate comparison.

    Norco presented some interesting stuff here: http://reviews.mtbr.com/2013-norco-range-and-sight-650b-bikes but some of the mathification is more "Rolls gooder" vs. "17% increase in tactical rollover"

    I think head angle will have to be "re-figured out" in that the holy grail 63* HA may not work for bigger wheels...but you bring up another very interesting point. If you have better rollover due to bigger wheels, could you make the pivot lower, giving better cornering response but not dealing with the harshness of a lower pivot?
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  3. mtg Member

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    Those diameters in the sketch are nominal sizes. If we can get measured diameters of equivalent setups in different wheel sizes, I can update the sketch.

    That video was interesting, but...
    Quote from that video that sums it up nicely: "No real facts, here". Scroll to 6:41, it's where there's a graph where the Y axis values are "pretty good", "noticeably better" and "mega awesome". Lol. That Y axis should have labels of Watts.

    And, the whole "bigger wheels have moar inertia, and that's good for rolling" is complete BS. If that were true, everybody would run DH tires on their 26" wheels and be winning at everything. Also, I'd like to see a detailed speed trace while riding at "steady speed". I highly doubt that even the TdF road riders can spin a smooth enough circle to keep the speed completely constant. There's bound to be a cyclical nature in speed, which means that you are constantly providing pulses of power to the wheels, even when riding at a "constant speed". Thus, you're constantly accelerating the wheels while pedaling.
    It's been a bit since I've done this calc (I'll have to redo it to make sure it's correct), but I recall calculating the rotational inertia difference between a 26" and 29" setup with the same wheel and tire combo. If memory serves me correctly, the 29" setup had 25% more rotational inertia, which is a big deal.
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  4. kidwoo Celebrating No-Pants Day

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    That sounds kind of contradictory to me. If there's 25% more rotational inertia, that's where the moar gooder rolling comes from. Talking coasting here, not acceleration, where obviously it's the exact opposite.

    Neither of which are news really though. That's kind of the conventional wisdom anyway. Continuity is great, any kind of acceleration/deceleration suffers the bigger the wheel.
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  5. OGRipper Active Member

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    Inertia plays a part but so does the physical properties of the bigger wheels relative to obstacles. That's how it's been explained to me anyway.
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  6. Sandwich just shake your rump

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    I always thought it was approach angle and had nothing to do with inertia. I didn't sense a large increase in rolling mass with lightweight tires and 29s vs lightweight tires and 26. There seemed to be a small decrease in acceleration but I attributed it to moar leverage on the wagon wheels. That's why I think 29ers COULD have merit on a DH bike. I don't think they've been done right yet though. It's all about geometry and 29ers have their challenges. 650b solves most of em. I also wonder about contact patch and what that could do on a big wheel. As has been discussed, it gets longer on niners, so I wonder if you could have a more specific contact patch which could provide for lower rolling resistance due to lack of contact of shoulder knobs when vertical, or something like that.
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  7. buckoW Active Member

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    My favorite part about the middle sized wheels is the increased directionality when drifting. They seem to hold a better line and almost feel like little "rails" (similar to skiing). I think the difference in roll over is slightly compromised by the new relation of the main pivot to rear axle. The bigger the wheel size the more forward the axle path for all systems without idler pulleys. There is an increase in rollover but if all things were equal (only wheel size changes) it would be even greater.
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  8. Sandwich just shake your rump

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    Interesting about the tires/drifting!

    I hadn't thought about the effect on wheelpath via the increase in axle height....I don't have the CAD experience to try and determine pivot height vs. approach angle but I think it would be interesting. Is there enough of an improvement in approach angle to lower the pivot, and therefore get better handling? Is there really a good way to calculate that?
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  9. joeg Member

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    I am going to build a 100% rearward axle path bike. If only one person never says rearward axle path again, it will have been worth it.
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  10. Sandwich just shake your rump

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    I'll keep saying it just to annoy you. Also because I think the bikes ride better.
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  11. Huck Banzai Active Member

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    Edge of reality
  12. rosenamedpoop New Member

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    You should try a V10... super rearward
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  13. joeg Member

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    drops to flat would suck
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  14. rosenamedpoop New Member

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    Not if you're pointed straight at the ground.
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  15. dilzy New Member

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    Rotational inertia of a wheel will not really effect the it's ability to 'roll betterer' over things, especially since an increase in rotational inertia usually comes with an increase in unsprung weight. The linear inertia of the bike+rider however sure does.

    You want to roll over things better, eat some pies/build your main frame (not your swingarm) out of lead.

    Personal opinion after damaging many 26" wheels, f-29". It's just silly, so's 650b. The force vector difference when encountering an obstacle with these different size wheels is bugger all (as noted by the drawing on the op) and the whole thing smells of i-phone.
    Last edited: Dec 18, 2012
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  16. dilzy New Member

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    You seem to build 29'ers, which do the same thing, reduce the angle of incidence on the oncoming obstacle, so I wouldn't be so hasty to bag the rear axle path crowd.
    Last edited: Dec 18, 2012
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  17. rosenamedpoop New Member

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    I like how my phone smells. It has a vibrator app.
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  18. Sandwich just shake your rump

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    Yeah, hitting a 4" obstacle doesn't make that huge of a difference, but nobody truly expects big wheels to flatten a 4" rock. I look to them for support in 1-2" roots and rocks, and use my suspension for larger impacts.

    Could you repeat the drawring with a 2" rock?
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  19. SuspectDevice New Member

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    this- it's one of the mostest-funnest things. I had to start gluing the tubeless beads onto the rims for the xc racers because they started(wasting so much energy) slap-chopping and slashing stuff.
    #19
  20. General Lee New Member

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    Show me a fancy graph with lots of vecterz or I don't believe you
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  21. mtg Member

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    Yes, that's no problem. I can tell you that the difference gets smaller on a 1"-2" step than 4", though. Before I posted the original graphic, I played around with differing step heights. I'll pull it up and post it when I get a chance.
    Last edited: Dec 18, 2012
    #21
  22. atrokz Active Member

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    Results look pretty minimal. What's the % dif on a 2" rock?
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  23. Sandwich just shake your rump

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    sure, may as well have all the data out there.
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  24. SuspectDevice New Member

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    I know you're pulling my chain but:
    Give me a $20k grant, wait until next spring and I'll give you a peer-reviewed scientific study on funnnnnn v. wheelsize.
    I can think of a couple of sport psychologists that need thesis projects(publish or perish).

    In regard to hard science:

    It's not hard to design a study to evaluate things that are relatively irrelevant(parse that one) or marginally significant-
    The current president of USAC got some fat grants from both ProFlex and Specialized in the '90s when he was at the University of Utah to publish somewhat dubious claims about suspension efficiency, for use in marketing materials, for example.

    As far as the reality of science and 650b stuff-

    The vaults of the French Cycling Federation contains a study they did in the run up to Atlanta(there was a huge arms race going on in cycling then) with Girard, Bossard, the Université de Lyon, Hutchinson,Mavic and Michelin over the winter 1995/spring of 1996(w/all the data they could collect(expired gas, power, blood lactate, kinematics telemetry) that identified 650b as the right tool for the job to win the XC race at the Atlanta Olympics. They were even spotted in Conyers over the winter poaching the course in February '96 on funny looking bikes covered with electronic crap and dudes with stopwatches and big noses.

    Clearly, they never rolled that stuff into the public sphere, but it was common knowledge at the time that they were working on it and what the findings were. The speculation is that they were afraid of the IOC and the manufacturers wanted to make sure they could sell what they were winning on.

    '96 inspired the Treaty of Lugano, and maybe we're all lucky that the French didn't race on custom wheels, otherwise we might be stuck with some of the same stupid technical rules that were implemented for road and track after Atlanta.

    I'm quite sure that the testing that Scott did in the leadup to this year's Olympics was scientifically valid as well.

    Science isn't hard and winning races is the most important thing in the whole darn world. I bet the DH testing Scott has done was also scientifically valid. It's Switzerland ferchrissakes!

    At the most elite level of competition even the most marginal gains make a huge difference.
    #24
  25. Sandwich just shake your rump

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    I get the sciencey ****, but I feel like preference is as much of an issue here. People may like or not like what big wheels do. I feel like 18" chainstays on a trail bike handle like ****, but turner freaking loves them, and there are plenty of homers to agree with them. The frontward axle pathed, 18.3" chainstayed 29er FS bike I had was one of the worst performing and handling bikes I've ridden, and that's saying a lot.

    Just saying, even if lord Science tells us that 650b is only marginally better, or that 29ers can't corner as well in theory, if people go faster on them, isn't that telling enough? Even if the faster is because a rider does more better on them?

    I need to pick up a second lefty hub and nevegal, then I can do my own comparisons with 650b and 26" back to back.
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  26. mtg Member

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    Here's some more graphs, with a 1" and 2" tall square edge:





    The tangent lines are nearly indistinguishable on a 1" tall bump.
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  27. HardtailHack used an iron once

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    If you make new standards that make bad riders faster in a straight line you would surely just end up with trails with blown out corners.

    Big wheels should be in different race categories just like BMX.

    Change is for hipsters:tinfoil:
    #27
  28. Sandwich just shake your rump

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    Come on dude, that's a pretty tired argument, especially in a thread that's supposed to cut to the facts.

    Besides, the thing that really blows out corners is toolbag pinkbikers and their "skid into every berm" roost-throwing mentality. If you want to prevent the ruining of trails, I assure you limiting people out of wheels that provide more traction while allowing them to just skid constantly is not going to solve it.
    #28
  29. Sandwich just shake your rump

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  30. Owennn New Member

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    Apart from the lack of detail on effort put into each lap, the bike setup for each wheel size, wheel build etc. Also reads as if they used one rider for the whole test, do the same thing with many more people and they may have numbers more useful.
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  31. DMdh Member

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    uphill? xc bikes?
    Is this the DH forum?
    #31
  32. Sandwich just shake your rump

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    sorry, the actual article mentioned more specifics, those were the only scans available.

    "of three identical weight carbon hardtails, 26"/650B/29er tired, on the same course, using the powertap to keep the power output of the rider in check as he does laps on a course. The bikes were a Felt Six Pro, Felt Nine Team, and a KHS SixFifty 609. They used water bottle ballast to get the bikes to the identical scale weights, and all ran Schwalbe Hans Damf 2.35 tires pumped to 25psi.

    The course has a 3.3km uphill followed by a 3.7km downhill for a 7km total lap. The climbs were done with the power kept to an average of 227 wats (+/- 2 watts) and the downhill with no restriction on power, just on tire grip.

    On the uphill section, the 26er scored 13:54, the 650B 13:41 and the 29er 13:39, and the downhill they were 7:58, 7:47 and 7:44 respectively. So overall the 26er was 21:52 for a lap, the 650B was 21:28, and the 29er was 21:23. "
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  33. sundaydoug Member

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    Someone test that same comparison with similar full-suspension bikes and then I'll care what it says. You also need a lot more than 1 rider for the stats to be significant.
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  34. marshalolson Active Member

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    30 seconds over 20 minutes eh? 2.5% difference could easily be explained within (the many) testing errors....

    get 50 riders to each ride the same course 20 times on each bike on different days in different conditions and then you might, maybe, get something useful. but realistically all you will get is some riders are faster on one type of bike and some riders are faster on a different bike.

    just go have fun. who cares about the rest of it.
    #34
  35. Sandwich just shake your rump

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    A 10-20 second improvement on the downhill section alone, which averaged about 7.5 minutes and 2.2 miles, repeated over 4 rounds, while maintaining the same levels of power, by switching wheel size while keeping bike weight the same.

    You've got your repeatability, you've got a predominantly controlled study, and you've got an improvement in 10s of seconds with the same rider on the same course....and you're debating that there's no value in the world of DH, where races are run by hundredths of seconds, because you've said so.

    You people are worse than derp o'slurp when it comes to gun control. You're going to tell me that if somebody came up to you and said "I can give you 7 seconds improvement on your 1 mile downhill run, just by switching wheels" you'd say no?
    #35
  36. HardtailHack used an iron once

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    I would, however if you gave me a free ticket to a skills clinic to make ride faster I'd be all over that.

    Since we are on numbers, what would people estimate as a 29/26er ratio at XC races? I know they are taking over the world but are there many dinosaurs that refuse to conform to the new standard?
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  37. mattmatt86 Active Member

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    I remember lining up at collegiate XC races in 08/09 and out of 30-40 guys in my race I was one of maybe 5 guys on a 29er. At XC races today it's probably the exact opposite, with a few 650b's sprinkled in.
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  38. marshalolson Active Member

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    you are telling me you would not be skeptical at all, and just hand them a blank check, especially based on a sample-size of one?

    of course i have a set of 650b enve rims sitting in my garage waiting for non-horsesh!t tires to come out to put on them, so yeah. i find out for my damn self ;)
    Last edited: Dec 19, 2012
    #38
  39. mtg Member

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    Sandwich, thanks for posting the MBUK test data. I think that's a great start, but there isn't enough data to draw a conclusion.

    I looked up a wheel and tire combo, (Sun Ringle Charger Pro SL & Schwalbe Hans Dampf 2.35) in 26" vs 650B. The weight difference is a claimed 160g between wheels and tires, for reference. That's a little smaller than I was expecting, but it's still a weight gain.
    #39
  40. Hacktastic Active Member

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    Right. Plus from the sounds of it, he got the larger wheels later in the test, when he knew the course better. Of course he's going to shave a few seconds off lap after lap just on his cornering setups.
    #40