Friday, December 08, 2006

It's All About Momentum

Many times when discussing the benefits of going with the 29 inch wheel format we talk about the differences in wheel weight, where that weight is or isn't, and acceleration. One of the things not discussed is the decelleration or loss of momentum that smaller and/or lighter wheels have.

I am aware of a study that Mavic had done concerning climbing speeds and wheel weight. The hypothesis going into the study was that a lighter wheelset would climb at a consistently higher speed than a heavier wheelset and thus would cut overall climbing times. What they actually observed was something very surprising.

The study was done on a long mountain climb on a paved road using road tires on road bikes. The test pool was a large group of cyclotourists numbering into the hundreds. (Some of these details are cloudy in my memory, but the overall point is not) At any rate, Mavic officials saw that overall speeds were not increasing as they had expected they would. What they did see was that the extremely light wheels caused the riders to lose their momentum between pedal strokes! The riders basically were having to re-accelerate the wheel with every downward push on the pedals, while the riders on the heavier wheels were able to escape this plague because their wheels held their momentum through the cyclists pedaling "dead spots". In the final results, the heavier wheelsets, (to a point) were actually faster than the lighter weight ones!

Now I see that this phenomenon has been observed again by testers concerning a lightweight set of Fulcrum carbon clinchers. Here is the pertinent paragraph:

"On the flipside, I did notice that these wheels exhibited less of a flywheel effect. Once spun up, slightly more effort was required to keep them spinning, unlike heavier wheels, whose rotational inertia tends to keep them going. An extension of this phenomenon was that the RacingLights were a little flighty on fast (60+km/h) descents, especially if the road surface was rough. This was not a nice feeling the first time I experienced it - it reminded me of skateboard death wobbles. Heavier wheels do provide more stability through the gyroscopic effect. Speaking of descending, the modestly aero spokes were also faster than I expected - noticeable faster than my conventionally spoked low profile clinchers. "

So, as you can see, having momentum, or a "flywheel effect" is not a bad thing, necessarily, and if you know your wheels have this effect, then you can use it to your advantage. This is one of the nice things about 29 inch wheels. Yes, they may be a bit harder to spin off a dead, or nearly dead start, but once up to speed, they tend to keep that speed. Couple that with the higher degree of traction and stability over 26 inch wheels and you might find that you are braking less and getting through the corners faster which will carry more momentum and save you more energy. This is also the reason why 29"ers tend to "walk away" from 26"ers on downhills.

It's all about the momentum. Get yer self some!

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