"Ed Huntress" wrote in message
...
On Sat, 8 Jun 2013 19:33:00 -0700, "PrecisionmachinisT"
wrote:


"PrecisionmachinisT" wrote in message
...

"Ed Huntress" wrote in message
...
On Sat, 8 Jun 2013 18:07:13 -0700, "PrecisionmachinisT"
wrote:


"Ed Huntress" wrote in message
om...
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.


Yes I know....

And thanks for clarifying.

--what you had written earlier had the potential to make lot of people
even more clueless that they had been before.

Although, reading your reply again, I have to say that the reason for
biasing to the front is NOT so that they will "lock up first"....in fact,
if
any wheels lock up it is way better for them to be the rear....

Oh, no. Never, ever.

Front-wheel lockup occurring first causes the car to lose steering but
to plow straight ahead. Rear-wheel lockup occurring first causes the
rear wheel adhesion to drop to a value less than the moment of the
rear end attempting to go forward (around the front wheels as a
"pivot") while the front end is being braked by the front wheels.

In other words, a simple spinout, or an oscillating fishtail, and
complete loss of control.

Take it from an old road racer. g Seriously, you could look this up
in any discussion of tire adhesion, cornering, and braking. Front
wheels must lock up first. In a race car, particularly a road-racing
car, you want the differential to be very small, but never for the
rear to lock first. That's deadly.


In other words, the reason brakes are biased towards the front is simply
because they can be, due to the increased traction that results when the
vehicle weight shifts forward.

Well, that's most of it. But when you're proportioning front verus
rear, you proportion them so the fronts lock first. Or you should.



Aww well I guess I'll stand corrected then.