Thursday, February 08, 2007


RANT: When an ounce is not an ounce

I have been reading bike forums again. I know. I know. but the misunderstanding about reducing weight on a bicycle is astounding. Folks are weighing every part, as if speed was all that mattered.

It is rotating mass and frontal area that limits speed. Reducing either or both will increase speed if nothing else changes. In all cases.

Rotating mass (pounds if you don't remember mass from high school physics) is much more important than just pounds hanging on the frame. Otherwise small people would go faster and the rest of us would finally see that big is only worthwhile in stoopid sports like basketball, football, ad nauseam.
While that is true it is beside the point.

Without getting into the detail the reason is that rotating mass must be multiplied by the radius (half the diameter of a circle) around the center of the rotating mass. Consider the radius as a weighting factor (pun intended) that just plain old mass, like the fat on your ass, does not have, fortunately.

Simply put, if you want to go faster (believe me there will be another rant on this later), reduce the weight on the wheels, crank, and shoes. Oh for slim and svelte calves and dainty feet. Maybe you ought to try an aerodynamic profile first, but that addresses the frontal area. Most people don't know that racers don't use skinny tire just because of reduced rolling resistance (a minor issue) it is the frontal area and the rotating mass they are after.

If speed matters, look at the rotating parts. I see all kinds of blather about how small wheels are faster. Sure they are. They have a lower moment of momentum, i.e. the mass times the radius (mr). Which is larger 500gm X 20 or 400 X 26. You do the math and the result demonstrates the obvious. Just do a bit of looking and see how much a 400 gram tire costs these days.

In closing, if you want to reduce weight--forget the titanium seat post and go on a diet instead. Remember an ounce is not always equal to an ounce.

by WrencherWOAC

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