[Phys-l] rolling friction

[John Denker <jsd@av8n.com>]

It appears there are multiple layers of confusion about the definition
of "rolling friction".

For starters, there are at least _four_ different notions of friction
that apply:
 a) friction transverse to the rolling, which prevents the tire
  from skidding sideways.  This is important during cornering.
 b) friction along the direction of roll, which prevents the
  tire from spinning freely.  This is particularly important
  during acceleration from a stop.
 c) friction that causes dissipation during rolling.
  This includes internal friction in the sidewall as
  it flexes, plus some sliding friction as different parts of the
  footprint squirm relative to each other and therefore relative
  to the road.
 d) Dynamic friction during outright skidding.

"Good" tires will have large values of (a), (b), and (d), but small
values of (c).

Normally (a) and (b) are comparable.  They are called quasi-static
rolling friction, and have much in common with static friction,
even though the tire is in motion.
  http://www.av8n.com/physics/car-go.htm#sec-rolling-friction

Without additional context, it is impossible to know which of these
quantities is intended when people talk about "the" coefficient
of rolling friction.

==================

As always, keep in mind that for all types of static and quasi-static
friction, the relevant law is not an equation, but rather an
inequality.  It therefore cannot be used to predict "the"
frictional force, just an upper bound on the force.
  http://www.av8n.com/physics/car-go.htm#sec-static-friction

====================================================

R. McDermott wrote:
> >   Firstly, isn't the model of 
> > "rolling friction" simply a means of making sense of a mechanism that 
> > results in a loss of energy?  

That's part of it;  see item (c) above.

> > Isn't this really a compression-expansion of 
> > the flexible tire material that gradually results in a loss of energy to the 
> > surroundings?  

That's one contribution to item (c).

> > A rolling tire is stationary at the point where it contacts 
> > the roadway, so is classical "friction" acting? 

Sure it's friction.  I call it quasi-static rolling friction.

> > As to "a whole range of 
> > accelerations", this is limited by the available static friction of tire on 
> > roadway, is it not?  Generating a torque exceeding that available due to 
> > static friction results in the tire "peeling out".  

Outright skidding is dynamic friction, not static or quasi-static.

========================

If that doesn't answer the questions, please re-ask.