From: Ronald Wong (ronwong@inreach.com)
Date: Sat Jan 29 2000 - 16:08:33 PST
Date: Sat, 29 Jan 2000 16:08:33 -0800 (PST) Message-Id: <l03102800b4b8b78b82a9@[209.209.17.197]> From: Ronald Wong <ronwong@inreach.com> Subject: friction with tires
Sarah said:
>My colleague and I were talking about how to calculate the friction
>coefficient between car tires and a road.
There are three types of friction each with its own set of coefficients.
The first is static friction which must be overcome if one object is to
start moving with respect to another. The second is kinetic friction which
must be taken into acount when one object is sliding over another. The
third is rolling friction which comes into play when one object is rolling
over another.
>We agree that it is probably a coefficient of static friction that we are
>looking for, as the tire and road surface do not slide with respect to each
>other as the car moves.
If the "tire and road surface do not slide with respect to each other as
the car moves" than you are not dealing with either static friction
("...the car moves") or kinetic friction ("... do not slide"). The only
energy loss that we will have under these circumstances is due to the
deformation of the materials at the point of contact as the wheel moves
along - rolling friction.
You probably have discovered that the static and kinetic coefficients of
friction are readily available for rubber on wet/dry concrete.
Unfortunately theses are of no value to you in the present case even if
your tire IS moving over a concrete surface. They don't apply because it's
rolling.
>But my colleague also seems to remember somewhere reading about "rolling
>friction" as distinct from static friction. We can't find it in any of our
>current books. Does anyone else have info about this?
Sort of.
If you want to get some idea of what the differences might be between the
three types of friction, consider the following which represent typical
values for metal on metal when lubricated (whatever that means):
Coefficient of static friction: 0.15
Coefficient of kinetic friction: 0.07
For lubricated metal ball bearings (the inner and outer rings rotate with
respect to one another) the figure might be something like:
Coefficient of static friction: <0.01
Coefficient of rolling friction: <0.01
The reason you don't run across tables of coefficient of friction for
rolling is that too many variables come into play - even in the simplest
situations (the Handbook of Chemistry and Physics doesn't even list such
figures).
Your need to determine the coefficient of friction for a car's wheel as it
rolls along the road is a good example. Here, the degree to which the
rubber tends to regain its original shape after being deformed will
determine the coefficient of rolling friction. This will vary depending on
whether the tire is unmounted and simply rolling along the road by itself,
mounted on a wheel with no/very-little air pressure, mounted on a wheel
with "typical" air pressure, mounted on a wheel with excessive air
pressure, etc. For each one of these examples, there will be a different
coefficient (and we haven't even taken into consideration the elasticity of
the road which also effects the value).
When engineers really need to know the coefficient of rolling friction, it
is my understanding that they just go out and determine the value
experimentally. The figure arrived at is far more useful to them than any
number they might find in a reference. I suspect the same is true for
scientists.
Having said all of that...
...if you happen to have a high school physics book from the 40's and 50's
it will tell you that the coefficient of rolling friction for rubber tires
on concrete is around 0.030. Under what circumstances is anyone's guess. It
may be totally useless for your needs but then again...
Finding the value experimentally might make for a nifty demonstration or
experiment - a simple one can be done with an inclined plane.
Hope this proves useful - ron
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