Dependency of friction on contact area 3

[Tunalover]

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?


Tunalover

 

[corus]

jdubu,
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

 

[KENAT]

I'm pretty sure the experiments showed (I'm not going to say proved as being high school they were pretty simple and probably not that well controlled etc) that surface area/pressure had little to no effect on the frictional force.

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.

 

[Kaz62]

"GregLocock" made a brief, but important statement about "shearing". Typically people recall seeing very wide tires on drag racers' cars. This makes most everybody think that wider tires must give you more friction and therefore friction must be dependent on area. But what these same poeple don't realize is that these tires (most notably funny car & top fuel drag race cars) need to carry a much larger shear load since it is assumed that the tire completely grips ("hooks-up", or adheres) the track. The tire grips the track due to many reasons - the rubber compound, pressure, temperature, etc. When these tires do hook-up, all the torque that makes its way to the rim gets transmitted to the road through the tire's contact patch. The tire must then create as large a contact patch as possible (this is where "wide" comes in) to handle the shear stress it must carry.

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

 

[KENAT]

Thanks Kaz62 & Greg originally. I'd had the thing with wide tires explained to me in some depth previously and knew it wasn't a simple friction aspect but couldn't recall the details.

 

[SnTMan]

Hello All, for one more whack at that horse, I seem to recall that wide tires are less likely to have (or are likely to have less of) the contact patch disturbed by irregularties in the road and so forth.

Regards,

Mike