On Sat, 8 Jun 2013 18:07:13 -0700, "PrecisionmachinisT"
wrote:


"Ed Huntress" wrote in message
.. .
On Sat, 08 Jun 2013 14:56:23 -0500, Ignoramus20041
wrote:

On 2013-06-08, Bob Engelhardt wrote:
On 6/8/2013 12:05 AM, Ignoramus29060 wrote:

... I just replaced the rear brake pads for the
first time in my pick-up truck, at 88,000 miles. The front brake pads
were less than half [worn] and did not need replacing.
...

That's interesting: on every vehicle that I (or my wife) have ever had,
the front brakes wear 2 or 3 times as fast as the back. 'Cause that's
where the weight is - especially on stopping, with the inertial weight
transfer. Do you generally have a lot of weight in the back?

Not really. It is a pick-up, most weight is in front. Actually what
you said does not make sense.

In most vehicles, front brakes wear far faster than rear brakes. And
it's because the weight shifts to the front upon braking. It applies
even to rear-engined cars.


Brakes not bear any vehicle weight; that is what axle bearings are for.

Brake wear is due solely to torque, which is going to identical on all four
wheels until one of them locks up.

Then let's put this another way. Because rear wheels are so lightly
loaded in braking, maintaining balance (near balance; you want front
brakes to lock up first) means that the brake bias runs from 70% front
up to 90% front.

Rear wheels will skid first if you apply more brake friction to them
than that. That's bad. So the friction you need on the rear brakes is
a fraction of the friction you need on the fronts. Because either
wheel diameter or brake weight limits the size of the front brakes
(but not the rear; the demand for braking effort there is so much
less), the brakes on the front wear faster.

The ability of the tires to produce that braking effort without
skidding -- again, 70% - 90% on the front -- determines how much
friction you can apply to the brakes at each end. That's a direct
product of the weight shift to the front upon braking.

You can measure the weight shift by measuring the load on the
suspension. It's not exactly equal ro suspension travel, because of a
geometric suspension feature called "anti-dive." . But the load is
still there, even if the actual travel is less than the weight shift
would indicate.

--
Ed Huntress