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Re: Rolling friction (again)



Gary Turner wrote:

> Just an idea, please comment:
>
> The rolling friction of a car tire

I still find the terminology ambiguous.
What means "rolling friction"?
What would it mean to have no "rolling friction" ...
-- pure sliding, like eels on teflon, with no traction?
-- ideal rolling, like an ideal cog-wheel, with 100% traction?

In this case I will assume the intent was to
talk about the _dissipation_ due to rolling.

- this is caused by the flexing of the
rubber, which is a result of the normal and (probably to a much lesser
extent), the static friction.

If static friction means what I think it means,
it can't contribute to the dissipation.
You've got F dot dx, where static means dx=0.

As the tire is compressed at the front, the road is acting on the tire,
pushing it up and back**. As the tire recovers, the road is pushing up and
forwards**. All that is required for a rolling friction force is that int
{F dt} over the compression/recovery be non-zero.

This depends on all sorts of details about
the shape and design of the tire. See below.

Unless the tires are perfectly elastic, I would expect this to be the
case. In this event, "rolling friction" does not appear to be friction at
all, but the effect of a variable normal force.

I still say the terminology is deeply troubled.
If it is even remotely possible to say
> "rolling friction" does not appear to be friction at all

then we've got a problem with the terminology.

And it's a little obscure to talk about a variable
normal force. I assume that means it's variable
in the frame comoving with a particular patch of
rubber? It's not variable in the frame comoving
with the car chassis.


**Does the back/forwards automatically imply friction?

Consider a very wide tire, so that the effect
of the sidewall is relatively insignificant.

Then it's not even true that there's any forward
and back motion. You can flatten a cylinder
without stretching it. Also imagine a caterpillar
tractor tread -- it flexes without stretching.

This is in contrast to a rolling sphere. You
cannot make a flat spot on a sphere without
stretching it.

Can a rolling tire flex on a frictionless surface?

Huh? If you smash a tire against a surface,
it will deform, whether or not it's rolling,
whether or not the surface is slippery.

Is the plane of contact (at the front and
rear contact points) parallel to the road surface?

If the road is planar and the tire is in
contact with it, surely the area of contact
must be planar.