On Oct 17, 7:43*pm, whit3rd wrote:
On Oct 17, 12:07*pm, Jeff Wisnia wrote:

Someone asked me why their car's front brakes always seem to need
replacing long before the rear brakes do.

At rest, or at moderate speeds, the front and rear tires
bear the same load (that's why you use the same, or
nearly the same, tire pressures front and rear).

When braking, the nonrotation of the car means the torque (by
the wheel/road friction) and countertorque (by imbalance of
front wheel/rear wheel load force) are equal. *That
means the front wheels bear more load during the braking
of forward motion than at rest.

Since the front wheels bear more load during braking,
they can safely apply more friction force (and are sized and
proportionally engaged to do so). *Higher friction force means
more wear on the front brake parts than on the rear.

Phrases like 'throws weight forward' are suggestive of
the car center-of-mass shifting with respect to the wheelbase.
That doesn't happen. * Compression of the front springs
(the hood dips when you brake) is easy to see happening,
and should indicate (to folk who don't do force diagrams)
the front-tire-load situation.

The front wheels need bigger brakes for two, maybe three
reasons: 1. The front is often heavier. 2. The rear end tends to get a
bit light as the car rotates around the center of mass as the braking
forces are applied. 3. You DON'T want the rear end breaking loose. In
the days before ABS, rear brakes that were as strong as the front
could cause the rear wheels to lock up, and if you've ever done the
park-brake-skid thing, you'll know that once the rear wheels are
locked you might as well have a skid plate back there. There's no
particular direction the wheels will want to go, and so the car will
try to swap ends. Dangerous. Much better to make the rear brakes
weaker than to risk breakaway.

Dan