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Discussion Starter · #1 ·
Watching 'fail' video compilations of MTB jumps, following a Martyn Ashton comment at GMBN, I noticed that many otherwise unexplained nose dives on jumps are caused by the rider putting weight back and stopping the rear wheel. This rapidly rotates the front end down while the rear has not left the jump point and results in a face plant. The wheels are 27.5/29 diameter. Jeff Lenosky may have missed this as cause for a fail example in his excellent tutorial on how to jump video. And modern DH runs have the riders' rear end scraping the tire. Rider height, weight, bike wheelbase would affect how this all fits together I guess. Such that I could see looking for a low BB 26er and adding a 27.5 fork to it, my bike seems to handle better with a larger tire up front.

We also climb, and rotating weight is higher with a larger diameter wheel. We can't all afford carbon rims. Triathletes used 650C for better aerodynamics, they don't turn much, and they gave up the better control of 700c/29er. The flatter tangent of ground to 27.5/29 definitely improves traction in off road turns and lowers rolling resistance. But are these trade offs? Where is the balance? What do you think?
 

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Discussion Starter · #4 ·
Some riders may not realize the danger of getting low and back over a large diameter rear wheel. On my severe descents, I get close to the 26er rear but do not touch it - and I've noticed more crashes caused by stopping the rear wheel on the fail videos. And there is a rotating weight penalty for climbing.

Forum->Classic Mountain Bike Forums->27.5->27.5" rear x 29er front touches on this for handling. It looks to be a better design to have a smaller rear wheel. I'm asking for thoughts and comments on this subject as this is a feature I'd look for in my next bike.
 

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WillWorkForTrail
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I've never touched my butt to the back tire, not once, ever, especially not trying to "get my weight back" over a jump. I've also ridden a 29er single speed with cheap, heavy wheels that have cheap heavy wire bead tires on them. It didn't stop me. I didn't particularly care or notice. The biggest thing I noticed about upgrading to non-wire bead tires when riding was they had much better traction. But that was because they had a real rubber compound and weren't cheap tires.

Question for you: Does the average rider need to care about the efficiency gained from using 650 tires on a TT bike vs 700? Will that much power conservation or extra speed from the aero gains make any difference to someone who isn't racing at a high enough level to have a team of people who can help them sort these things out?

I think some of the answer may lie in World Cup racing. If you look at the XC crowd, they seem to be largely done experimenting with wheel size. It looks like most of them like 29ers, and a few of the really short ones dig the 650b. On the other hand, Downhill seems to have just really started playing with wheel size. They ran 26 forever, changing to 650b was nearly a scandal, and people gasped that there was no way a 29er could make a full run. The wheel would taco, or the rider wouldn't be able to make turns with them, or nature would simply strike them down for being obtuse. Yet, there are clearly riders who prefer, and seem to do better on 29ers. Of course, to your point, there are now riders on "97.5" bikes with 29 front and 650b rear. Maybe if you're pointed down hill all the time, this starts to make some sense. I'm not certain though, that the average rider wants to, for instance, carry two "just in case" backup tubes on every ride rather than just one. I'm certainly content with my garage full of 29ers, both with and without carbon wheels.
 

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Cycologist
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Triathletes using 650C was a long time ago. Tri bikes went back to 700c.
 

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I've tried many times to stop the front wheel during a wheelie -never had an issue with it not rotating. I guess because I'm going slow enough that the physics are small scale and it won't drop my front end, or loop out, with change in wheel speeds.

I don't get enough air to stop the rear wheel to test.


The scenario outlined by tapping the brake for the rear is dirt bike racing 101 to lower the front.
 

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I don't think the issue you're looking at has anything to do with wheel size.
Yup. Sounds like bad form to me.

Why would you be throwing your weight so far down and back on the takeoff of a jump that you hit your tire with your ass (unless maybe you're going for a backflip)?
 

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I think the OP is referring to drops rather then jumps. I have seen a few fails and bails where someone hits their rear tire with their butt causing them to crash off the end of the drop. But that seems like more of a form problem although I could see if being an issue for shorter riders.
 

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Cycologist
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Yup. Sounds like bad form to me.

Why would you be throwing your weight so far down and back on the takeoff of a jump that you hit your tire with your ass (unless maybe you're going for a backflip)?
Maybe he was nose heavy?

Bicycle tire Bicycle wheel rim Wheel Bicycle wheel Bicycle frame
 

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Yes you can drop the front end a bit by grabbing rear brake while in the air. However to gyro force on a MTB wheel is kind of small (even as a %) so it probably won't do nearly as much as it does on a dirt bike (which is very noticeable).
 

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Yes you can drop the front end a bit by grabbing rear brake while in the air. However to gyro force on a MTB wheel is kind of small (even as a %) so it probably won't do nearly as much as it does on a dirt bike (which is very noticeable).
in the OP, the rear wheel has not left the ground. doesn't take a lot of gyro force for the front end to drop HARD in this scenario.

besides, with a mountain bike being much smaller/lighter than a dirt bike, it doesn't require the same gyroscopic forces as a dirt bike to make you hurt.
 
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