# Bike Rotational Weight

Q:
I have a two-part question concerning rotational vs. static weight on a bicycle; sorry if some or all of this is already on your site, but I didn’t find it. I’ve heard (and experienced) that changing your wheels for lighter ones makes more of a difference on a bike than lightening parts on the frame, since wheels are rotational weight. So I’m wondering 1)if there is a ratio of rotational to static weight change as a rule of thumb; 2)and is all static weight equal--in other words if I buy a two-pound lighter frame, is it the same as just losing two pounds myself? Thanks in advance, and I hope this gives you physics guys something to think about. Oh, I don’t speak "science" so something in translation will be appreciated!
- mark bloom (age 50)
chico, ca usa
A:
Nice questions.
Yes, the wheel weight counts more than the frame weight. The reaon is that when you accelerate the bike everything moves forward together but the wheels also rotate. The average velocity over the whole wheel is the same as the rest of the bike, because the different parts move all the different directions in a circle. however, energy goes as the square of the velocity, and the rotational energy of the wheel just adds to the energy of the average motion. So you have to pump more energy into the bike to get a pound of wheel moving than to get a pound of frame moving. If all the weight of the wheel were out at the rim (which isn’t too far from true) the total wheel energy per pound would be twice the frame energy per pound, i.e. equal rotational and static weight. The real rotational number would be a little less.
Of course if you’re going uphill, the extra work to lift the bike against gravity only depends on the ordinary weight, not how it’s distributed.
As for weight on you or the bike, I guess they count the same, except that the weight on you has to get lugged around even when you’re not on the bike.

Mike W.

(published on 10/22/2007)

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