Front /back pad wear etc , was pads/disks for 90 long-ish

[rob hod]

    Just thought I'd chime in with my thoughts , and see what the list
thinks


    My understanding is that static weight distribution is irrelevant to the
work required to be done front and rear by the brakes. This is  due to
'weight transfer'. I.e. under braking the mass of the vehicle combined with
the centre of gravity of the vehicle causes a torque effect around the front
wheel axis which results in a 'shift' in 'weight' away from the rear wheels
and over the front.

This is also the reason you went over the front handlebars on your bicycle
when as a kid you tried riding into kerbs to see what would happen.

As a result the harder you brake, the less grip the rears have and the more
the front have, which may lead to rear lock up unless you have ABS or in the
case of  Non ABS systems the lovely heath-robinson spring actuated rear
compensator which uses changes in the rear ride height to reduce the
pressure to the rear brakes.

    For this reason brake systems are designed from scratch in terms of
relative piston area so that the fronts do most of the work. This greater
work done is *usually* compensated for by the use of large pad area at the
front and smaller at the rear, but not to the extent that both wear out at
the same time. Usually rear pads are relatively 'bigger' than they need to
be and hence last longer.

    Older cars with no rear compensators almost invariably had pathetic rear
brakes by design, or locked up the rears very easily compared to modern
cars, beacause of weight transfer.

    Having said that, I wonder what happens if you manage to get a vehicle
with the centre of gravity below the front axle line?

    Oh by the way , if you have pads wearing at different rates chances are
you've got crud in the piston /seal assembly, leading to slight binding when
Not using the brakes, - Although there may be no discernable difference in
braking performance, crud may be preventing the pads floating back off the
disc when you take your foot off the brake, thus wearing them out quicker.
Check the temp. of your wheels after a long run with only gentle braking
involved, you might be surprised that ones hotter than the other. The hot
side is the binding side.

rob
----- Original Message -----
> Message: 2
> From: Nate Stuart <nathan.stuart at maine.edu>
> To: Simon Allcorn <Simon.Allcorn at CARTESIAN.co.uk>,
>         "'Michael Dewar - Sun Scotland'" <Mike.Dewar at Sun.COM>,
>         quattro at audifans.com
> Subject: Re: pads/disks for 90
> Date: Wed, 11 Jul 2001 11:14:41 -0400
>
> On Wednesday 11 July 2001 10:40, Simon Allcorn wrote:
> > I guess that's a prime cause but I picked those values (worn to the bone
> > and 1/2 ...) because the rear pads on my 80 went the other day like that
> > and all guide pins were ok (checked them with a hammer :) )
>
> But I bet the E-Brake cam was sticky (as it is on my 90)! This is one of
the
> most common failure modes for the rear brakes. Even some rebuilt calipers
> have the same problem b/c the e-brake cam apparantly isn't removed and
lubed
> by some rebuilders... or so I've heard.
>
> Later,
> -Nate
> '89 90q (getting a replacement RR caliper in 1/2 an hr)
> '86 5kt (donor)
> '87 5ktq (donor)
>