Unexpected 29″ differences : #1, Braking
June 18th, 2006 by CloxxkiFirst in what may become a series : Braking
When changing the wheelsize on a bike, nearly everything changes. Ideally, only the rider’s fit is not altered from what’s “perfect”. Changes you might not expect when going from 26″ to 29″, are those concerning braking.
Using a given size disc brake rotors, the relatively larger wheels have greater leverage, meaning that you’ll need more lever pressure to obtain the same stopping power. If you were on the maximum of what your disc brakes could do on a 26″ bike, it’s smart to go to a larger rotor. For instance 160->180mm, or 180->200. This compensates for the roughly 10% larger wheels.
Heat build-up on long descends is about the same for a given rotor size but different wheelsizes.
But it’s not all “bad”, au contraire!
Supposing identical front center (BB to front axle), chainstay length, and COG, surprisingly, the 29″ will keep the rear wheel on the trail better under hard braking. It’s not the slightly heavier rear wheel keeping itself down, you need to look at the other end of the bike…
Due to the 1-1/4″ (31.5mm) taller front axle, when endo’ing, the COG will be pivoting around a higher placed pivot. So, at the balance point where you enter the face plant phase, your COG is 31.5mm higher up than it would be with small wheels. It takes more decelleration to deliver energy for that lift. You can decellerate harder before lifting the rear wheel, and when it happens : it’s easier to control.
As a result, on steep technical downhills you may notice you’re not feeling the urge to hang behind the seat as much as with 26″ wheels.
29″ers feel like they have a lower COG, which is actually incorrect, but under hard braking, and also the opposite : steep seated climbing, you do get the traits a lower BB would offer, without the pedal strikes. 29″er cost more effort to get into a wheelie (COG above rear axle)
Another way to explain it : To reach the tip-over point with a 29″ bike, the COG will have to travel more degrees relative to the wheel axle that functions as pivot.
If someone has a beter quality diagram than my crappy Paint sketch, please email!
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Hay Clox!
Does that mean you get more hard-core credibility for endoing on a 29er? Cause if so I’m what they call Bas ASS!!!!!! (or maybe the other)
With the 29er you also get a few more seconds in that faceplant phase to think to your self, “I’m ok, I can pull it out. This will be ok. OH CRAP I’M Gonna STAAAAAAAACKKK!!!”
-M
Yeah, you’re badass alright!
Endo’ing is much rarer now that I ride the big wheels, but when it does happen, it hurts…
I recently managed to endo at the start of a sandy downhill (a corner at the top of a dune, really). I took the 29″ grip and stability for granted, hardly held on to the handlebar. So it got ripped from my hands as the wheel got caught in a sand hole. I endo’d down the dune, and it was a HARD fall. You fall from higher up, after all, but possibly don’t fly as far horizontally. I don’t know, and no further endo experiments planned any time soon.
Change in the height of COG because of wheelsize/axle height does not exist. The dynamics of a bike does not pivot around the wheel axles. The pivot point is the contact point of the tire on the ground
If a bike had two front wheels inline, mounted on a rocker with the fork mounted on axle-height in the middle, endo’ing would happen in the same manner, right? That leads me to believe we should look as the axle as the pivot.
Even if you do use the contact patch, the distance between COG and contact patch increases through the endo, so the “virtual” pivot point comes down to the front axle anyways.
When the front brake is locked, I agree that the COG pivots (rolls) around the contact patch.
Please educate me where I’m wrong this time, thanks for contributing here Shiggy!
This is a question not comment. I’m looking to install front disc brake on redline mono 29. Any suggestions on rotor size and brand. Thanks in advance.
although you’re gonna spend more time in the air endo-ing a 29er, you also get to admire the view for longer as you fly through the air. maybe you’ll even find a softer place to land?…
Hey Guys
I’m considering a 29er and at the moment ride a 26er gary fisher marlin. If I was to buy a new bike, I’d maybe want to spend around £5-600, but the cheapest 29er that I can find is £700. Is it really worth the last wee bit. I can’t quite tell whether you think they’re quality, or not worth the bother. By the way I’m 6 ft 2 and am only 16 so may still grow a few more inches.
Thanks very much, Roger
Oh yeah, can you also possibly recommend a model or two?