[CAUT] Friction (was restrung D)

[Jeff Tanner]

On Apr 17, 2007, at 9:30 AM, Steve Fujan wrote:

> I asked the "big tire" question way back in engineering school, and  
> the explanation was that when a tire slips on concrete it is not a  
> matter of exceeding a friction factor, it is actually tearing the  
> material.  Since larger tires have more surface contact area, there  
> is more material to tear which takes higher forces and results in  
> more traction.  This is not the classical "friction" scenario.

Ok, then obviously, if the rubber is being torn, there is an  
indication that some sort of friction bond was created between the  
rubber and the asphalt stronger than the makeup of the rubber material.

So then, the screeching we hear is the sound of rubber tearing?  Does  
that mean the singing sound produced by my wet finger massaging the  
edge of a crystal glass is the sound of skin coming off my finger?   
No.  Certainly there is a degree of actual slippage rather than  
tearing when you hear the screeching tires.  But that would be the  
part of the equation that is not building a friction bond.


> Any farmboy or stevedore who has worked a load on a rope is  
> familiar with
> this phenomenon, where more contact equals more friction.

And any South Georgia redneck understands about tires getting  
traction on a racetrack.

Ahem, or back country road :-)

John Delacour wrote:

> No amount of polishing is going to reduce friction.

I'm no physicist, nor engineer.  But I know a brand new polished  
string will render across the capo better than the old ratty corroded  
string all other factors being the same.  Do we confuse friction with  
some other physical force?

> Big trucks achieve greater stopping power by the application of  
> greater _force_, using compressed air.

I think air brakes are more a product of function than  
"greater_force".  Air is easily transferred to trailer brakes.  Air  
pressure is also easier to regulate via release valves.  I believe my  
father explained to me years ago that air brakes work backwards from  
hydraulic brakes.  When the air pressure is off air brakes, the  
brakes are locked.  Air pressure is required to release the brake  
rather than apply it.  That is why big trucks have to sit a while  
after start up.  They're not just warming the engine.  They can't go  
anywhere until the air pressure builds.  And that's why you won't see  
a truckless trailer rolling around.  The brakes lock the trailer down  
when it is detached from the truck.  But I'm thinking hydraulic  
systems are actually more powerful or you would see air systems on  
farm tractors rather than the hydraulic systems used to raise and  
lower heavy implements.  Same would apply to industrial loaders and  
dump trucks.


> It is the increased normal force that results in greater friction  
> and the parts are made big in order to distribute wear and spread  
> the heat produced (which causes a reduction in the coefficient of  
> friction, and thus brake fade) over a larger area.
>

Actually, brake pads work just like the racing tires.  They are made  
of composite material that "tears" as the surface builds a friction  
bond with the rotor.  It would be the strength of that composition  
applied over a larger area that would be required to perform the same  
stopping function as the vehicle produces a higher inertia force.  A  
smaller brake pad would disintegrate in no time.



Jeff Tanner, RPT
University of South Carolina



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