Dependency of friction on contact area
Dependency of friction on contact area
(OP)
Folks-
I've always wondered how first-order statics and dynamics calculations allow that the friction force F=uN where N is the normal force and u is the coefficient of friction. It's obvious that the contact area plays a role, for example, a wide tire vs. a narrow tire provides better grip. Am I overlooking something?
I've always wondered how first-order statics and dynamics calculations allow that the friction force F=uN where N is the normal force and u is the coefficient of friction. It's obvious that the contact area plays a role, for example, a wide tire vs. a narrow tire provides better grip. Am I overlooking something?
Tunalover





RE: Dependency of friction on contact area
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RE: Dependency of friction on contact area
Now this is off the top of my head...
Tobalcane
"If you avoid failure, you also avoid success."
RE: Dependency of friction on contact area
It seems intuitive to see surface area as playing a proportional role, but I don't believe this is the case (apart from the exceptions above, which aren't truly exceptions).
Jeff Mowry
www.industrialdesignhaus.com
Reason trumps all. And awe transcends reason.
RE: Dependency of friction on contact area
Check out this discussion that has already taken place on this forum:
http://www
-Reidh
RE: Dependency of friction on contact area
Perhaps it is not, really, if N is considered as the product of pressure and area, N = P*A. Then, for the normal force the contact area is implicit, or for given N, P*A = constant.
This does not address something like the contact patch of the tire mentioned in tunalover's original post, whose behavior is complex.
My best guess, I probably need to dig out the old physics book.
Regards,
Mike
RE: Dependency of friction on contact area
In no particular order there is chemical adhesion (micro welds), cogging, the work required to overcome damping in the rubber as it drags over the surface, and one which I have forgotten or never knew.
It is by no means necessarily true that a wider tire develops more grip. Usually it is, since a wider tire can be made of softer rubber and still have a reasonable life. Soft rubber tends to have more chemical adhesion, whether that is because it squooges into the declivities of the road surface, and so has more bonding sites in proximity, I am not sure. Typically it has less damping, but if the slip velocities are higher (due to the squooging) then more work may be performed, even though the damping per unit volume is less.
Static friction is even more complicated. I do not know much about it.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Dependency of friction on contact area
RE: Dependency of friction on contact area
That is, for the same friction material, with the same normal force, friction changed a little, but not a lot, with a change in area.
However, if you are disapating the same energy, through a smaller contact patch, the temperature will increase more than a large contact area.
Friction is highly dependant on temperature.
RE: Dependency of friction on contact area
*The question of tyres is something I'll never tire of.
corus
RE: Dependency of friction on contact area
Assume you have a force of 100N acting on 4 feet of a chair. These feet have a small surface area, of 100mm^2. The total pressure is 1N/mm^2. If you increase the area, you DECREASE the pressure, since the force is constant. There is a tradeoff between contact area and the pressure which is generating the friction.
Obviusly, thick heavy tires have higher friction force that thin and light tires :)
RE: Dependency of friction on contact area
Incidentally, what sort of friction is involved when you 'ring' two measuring blocks together?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Dependency of friction on contact area
Oil lubrication is another example. People often refer to the friction in a bearing. With a complete oil film there is viscous drag which will increase with velocity. This is not friction as defined in Physics which does not depend on velocity.
The real problem is just that the word friction is very commonly misused and this leads to a lot of confusion. Similar to the distinction between force and pressure which have very precise and different definitions in Physics but are often interchanged speech (as occured above).
RE: Dependency of friction on contact area
RE: Dependency of friction on contact area
RE: Dependency of friction on contact area
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Dependency of friction on contact area
I do know this. In A level physics (last 2 years of high school) we did some experiments that showed it to be true that F=uR and changes in surface area had no measurable effect.
RE: Dependency of friction on contact area
This is why your mechanic can pressurize your tires when your car is up on a pneumatic lift (with no weight on the tires) to 35 psi each and when you let it down onto the ground, the tire pressures will still be 35 psi, even with the weight of the car on them.
Don
Kansas City
RE: Dependency of friction on contact area
RE: Dependency of friction on contact area
Tunalover
RE: Dependency of friction on contact area
The normal force has nothing to do with the contact area. If it were the case then high heels would be sold as a way of losing weight in an instant. Then again the idea would probably sell.
corus
RE: Dependency of friction on contact area
Keeping constant mass, and hence R, but adjusting the surface area had no noticable effect on friction force.
Keeping constant area and adjusting mass (R) did have an effect on frictional force.
Hence F=uR.
I seem to recall being told that the great width of race tires was at least partly to dissipate heat but can't recall the details.
RE: Dependency of friction on contact area
A tire is a very complex suspension and power transmission component that should not be used as an example to demosntrate or discuss the charateristics of friction. Friction has very little to do with tire/track interaction for drag racing (and other high-perf racing) since these tires are designed to "adhere" to the track through their "gooey", sticky surface compound.
I hope this helps out and makes sense.
Cheers,
Kaz62
RE: Dependency of friction on contact area
RE: Dependency of friction on contact area
Regards,
Mike