Tom Sherman <
[email protected]> wrote in
news:[email protected]:
>
> a&b wrote:
>>
>> I think Steve Delaire recently posted that one thing they found in coast downs and fastness tests
>> was that the longer bikes were faster but they didn't know why.
>
> All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
> distance between the center of mass of the bicycle/rider combination and the center of mass of the
> earth will be less, and of course gravitational attraction varies inversely with the square of the
> distance between the masses in question.
>
> Tom Sherman - Various HPV's Quad Cities USA (Illinois side)
Technically true, but in practice it's meaningless. A human being would not be able to tell the
difference without _very_ precise measuring systems.
Perhaps the longer wheelbase has less induced rotation due to irregularities in the road surface
(cracks, holes, etc). A short wheelbase will rotate more about its cross axis due to these
irregularities. This increased rotation means that more load is placed on the contact surfaces due
to torque, so the tires will deform more. Deformed tires mean more kinetic energy lost until the
tires go back to their proper shape.
Or, to look at it differently. You have a long board with a fixed hinge at one end, a weight in the
middle, and you holding it toward the other end. The farther out you go, the less force you have to
exert to raise your end of the board one inch because you get more torque and the distance you raise
the weight is less. On a bicycle, the hinge is the rear tire contact point, the weight is you, and
the person lifting is the front tire contact point when it hits a ridge in the road. The farther
forward the front tire is, the less force is necessary to raise the front of the bike over the
obstacle. All of that force is applied through the tire, deforming it. The process of deforming and
returning to its correct shape absorbs kinetic energy from the bicycle, slowing it down.
That, at least, is my best guess.
-Bill