26 vs. 27.5 vs. 29 Down Hills

This will be too much information for most of you, but this settles, yes settles, the debate: 26″,  27.5″, and 29″ wheels will reach the bottom of a slope AT THE SAME TIME (regardless of the mass of rider and/or bike). Tire pressure and tire selection would be the only factor to change this reality (friction) (and of course if the rider pedals and their suspension is different).

@ about 9-12 minutes the meat of the above is summarized for solid cylinders (hollow @ 10:11) (Lec 24: Rolling Motion, Gyroscopes | 8.01 Classical Mechanics, Fall 1999, Walter Lewin):

In essence, the acceleration down hills is the same regardless of mass and wheel size. The same principle as having a golf ball and bowling ball drop vertically at the same rate, only now a slope gets in the way, but not in gravity’s way.

See the other video below for Walter Lewin’s demonstration of the video math above. He shows us on an inclined plane the same as the video above: that wheel size DOESN’T matter. I’ll put my faith in Lewin, or better, nature’s laws, not bike industry pimps– no matter how pimp your bike. Nature’s physical laws settle the debate. The bike industry sells some lies, or at least fails to mention some key physical facts about the world that they can’t manufacture or design their way out of…but they sure the hell can sell us more shit than we (really) need, including some beliefs. Bike mags have done no better because the industry pays them, believe that. I’m nobody’s fool, and have interest in nothing but the truth no matter how much it hurts my feelings, which are generally fluid if better data comes around.

Lewin shows the math play out: starts at about 6 min, meat at 9 min, and 11 min. Wheel size doesn’t matter down hills. Period.

As far as I know, after researching this as well, tire contact patches (and mass) are independent of the friction coefficient (static or kinetic) so the differences will be minimal between 26, 27.5, and 29– same with the amount of fun we have on whatever wheel size we ride! In other words, hey fat-biker, the contact patch bit means you too!

I’m not here to say one wheel size is better than the other, just one “fact”: they reach the bottom of the same hill at the same time, all else equal. I suppose I am here to replace bike industry hokum and related biker beliefs with facts…please don’t cry.

My big suspicion, or question, in all this comes down to “statistical significant difference.” Does it exist between 26, 27.5, and 29, if so, how much is it, and was the change from 26 to 27.5 really necessary? Does it matter? In my mind no, except for trail design purposes.

I can see 29ers (and maybe 27.5) being faster than 26 wheels in that they have the potential to lower the friction coefficient by rolling over objects more easily. However, is there a statistically significant difference? If its not real chunky, no, as then the friction coefficient will be similar, or provide no significant difference.

To me, find what bike size works for you. You can base your decision on science, witchcraft, intuition, what a buddy says, or your own demos/trials. You can also buy industry hype (or should I say “deception”?).

I personally want numbers, and want to know if there is a statistical difference (not some marketing witchcraft) so I can make an educated choice. The educated choice is whatever choices we make, as long as we know the fact that down hills size doesn’t matter (and lighter bikes will climb more easily).

Here is more info, and more along the lines of what I was looking for in regards to downhill speed:

Why care?

I found these videos because I’ve been interested in trail science, and designing ‘perfect’ reversals where large reversals occur and the rider will not have to pedal to crest the reversal, and/or has the option to sky it…changes with in-run and out-run slopes and reversal angles (in essence height changes), which is basically potential energy (mgh). I’m also interested in minimizing conflict on multi use trails to help keep them open by controlling bike speeds, without sucking the life out of the trail for bikers either. Stopping skidiots is also of interest.

Part of my interest developed after riding Allegrippis, a roller coaster of flow. It inspired me to dream-up a spreadsheet of % grade, or height changes, and coasting speeds– helping the design of big reversals for fun flowy rolling or flying with little pedaling or braking needed. And importantly, a way to control speeds, and squeeze a long descent out of very little elevation change. Knowing the height changes, coasting speeds can be estimated (and controlled to some degree). I was concerned about 26 vs. 29, among other things, in trying to determine these runs. I was worried trail design would be different for each wheel size given the goals above.

As it turns out, my quest revealed that tire size won’t amount to much. There are other factors that certainly will, like drag. Regardless, I am guessing there is a window, or range of speeds, where most riders will fit (if they coast from one roller to the next). Stay tuned for a spreadsheet or calculator of DH runs and angles, and corresponding uphill reversal height changes to provide an awesome riding (and flying) experience.

“Facts don’t cease to exist just because they are ignored.” Aldous Huxley

more info:

Similar, but different bike nerdiness: Why Shorter Cranks are Better (According to Science)

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