Re: [Phys-l] car skidding and spinning

[John Denker <jsd@av8n.com>]

1) On 12/04/2011 11:14 AM, chuck britton wrote:

> Steering (scuffing) also causes more wear on the front tires.

Indeed.

2) At 9:28 AM -0800 12/4/11, Bernard Cleyet wrote:

> >  When braking the net downward force moves forward.  (A problem 
> >  in Sears and Zemansky.)  Does this mean the front tyres wear more?  

The magnitude of the effect will depend on how abruptly you stop.

=========

3) Front-wheel drive increases the wear on the front tires, whereas 
rear-while drive increases the wear on the rear tires.

4) We must also consider the weight distribution.  The engine is
heavy, and in a passenger car it is common to find the fore/aft 
weight distribution to be something like 60/40.

Obviously on a truck, all bets are off.  Also note that a Porsche 
911 was 40/60 (engine in the rear).

On 12/04/2011 10:31 AM, Anthony Lapinski wrote:
> >  I believe. The front brakes also wear more, which is why they are
> > changed more often. At least for my car this is true.

It is commonly observed that the front brakes wear out quicker,
and one should never argue against the observations ... but I'm 
mystified by the /explanation/ for this observation.  This is
not a rhetorical question;  I'm genuinely confused.

On some cars, the hydraulic cylinders in all four wheels are
identical or nearly so.

a) So, assuming the hydraulic force is all the same and given 
that the rotation rate of the tires is all the same, shouldn't 
the brakes wear the same?

  I'm talking about the brakes here, not the tires.

b) Maybe if the weight distribution is 60/40, they could make
the front brake more powerful than the real by a factor of
60/40, perhaps by making the hydraulic cylinder larger in
diameter, or giving it more leverage.  Has anybody ever seen
something like this?

This would seem to be a safety issue, because if you apply
the same amount of braking torque to all wheels, and there is
less weight on the rear wheels, the rear will skid out first,
in accordance with the laws of friction in the high-school
physics book.  As previously mentioned, a rear-wheel skid is 
relatively dangerous.

c) If they make the front brake more powerful why don't they 
make the brake pad larger in the same proportion, so everything 
wears at the same rate?

For that matter, _no matter what the explanation_, if it is 
foreseeable that the front will wear out sooner, why don't
they design around this somehow?  That would reduce the number
of times the car would need to go into the shop.