Mayf;
Do the same experiment. Drag the weighted rubber pad and measure the force
with a fish scale or something. Of course, you could just lock up your
brakes and see how long it takes to stop on the salt-- then calculate Cf.
I doubt that you'll get a very accurate figure for the Cf of a tire,
though-- that's pretty complex, what with all the dynamics of tread squirm,
tread temperature, tire contact patch deflection, etc going on as it rolls.
Regards, Neil Tucson, AZ
-----Original Message-----
From: DrMayf [mailto:drmayf@teknett.com]
Sent: Wednesday, July 02, 2003 6:37 AM
To: land-speed@autox.team.net
Subject: Salt-Tire Coefficient of Friction
I am contemplating the fabrication of a doohickey that will permit me to
measure the coefficient of friction betwen tire "rubber" and the salt. This
would be a small simple pull type of device. I would use a "pad around 4
inches square with an "eye" hook to attach a cable to. The cable would be
wound through three turning pullies so that I could pour sand into a bucket
as the falling pull weight. I would load the skid with sand also to some
measure weight, hook it up, pour sand into the bucket until the static bond
between the tire rubber and salt is broken and it slides. Then I would
measure the bucket and sand. I could then determine the static coefficient
of friction, or breakaway friction. This number with the weight on a cars
driving wheel(s) wold permit a quick calculation as to the amount of torque
that could be applied (force times rolling radius). This could be a gage on
how to apppply power as you motor down the course. However, it should be
known that best acceleration probably comes when the tires are hazed by
maybe 20% over the breakaway friction.
As an experiment, I would perform the pull test several times, enough to get
a 95% confidence interval, then use the data to find the one sigma force
values. I would repeat the experiment several times during the day to test
the idea of water rising through the salt during the day.
How could I determine the kinetic coefficient of friction?
Anybody have any thoughts on this? Of any value to anyone? Off list please,
no need to pester everybody who does not want to read replies.
mayf, the red necked ignorant desert rat in Pahrump
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