Front /back pad wear etc , was pads/disks for 90 long-ish - reply just a long...

[Larry C Leung]

You are essentially correct. The rear weight bias of the 911 is
pronounced enough that it applies tractive effort to both front and rear
axles, enought that ther rears are actually effective and used in
braking. Rear weight bias also does mean that the CG is also further to
the rear, hence, weight distribution IS a fairly good indicator of the
relative usage of front/rear brakes. But, as you have indicated, the
height of the CG also plays a significant role in weight transfer. To use
basic design start point, that of maximum traction for non
aerodynamically aided acceleration (braking is just this in reverse), the
perfect design would put the entire weight of the car on the drive wheels
at max acceleration. For a RIGID suspension, based upon torques about the
CG, the ideal design says that the fore and aft CG should be the same
distance from the rear wheels as is the height of the CG (hence the
location of the engine in rail dragsters). Of course compliance of the
suspension, tires and rates of damping somewhat alter this location, but
for the sake of argument, lets just go with the generallization. Now,
consider they typical front engined passenger car (which for the most
part is what we all drive, unless we are quite IMHO, lucky), with a say
typical 60/40 weight distribution, the fore and aft location of the CG is
about 1/3 back from the front wheels. Fortunately, the CG is NOT the same
distance up from the ground, or under maximum braking (i.e. tires
approaching 1 g of braking grip, which is pretty close to what modern
tires of the street performance catagory can reliably achieve under ideal
conditions), the car would be doing braking nose "standies" as opposed to
wheelies under acceleration. Since the CG is considerably lower, we just
end up with slightly less overall braking traction, and we begin to slide
somewhere around 0.8 - 0.9 g's of braking rate. At any rate, the use of
rear brakes, as you can see, is pretty minimal. 

So, as Rob suggested, WHAT IF we could lower the CG so that it BELOW the
axle line? The answer would be to reverse the torques, and you'd actually
have a car that would squat under braking, thereby utilizing all the
contact patches. Only problem is, at least so far, no one has come up
with a way to lower the CG to below, or even level with the axle line, at
least for a practical car. In some respects, I would thing a convertible,
traditionally reinforced low in the frame/body would approach the ideal
lowered CG. It would be curious to do a comparisson of similar weight and
braked cars, hardtop vs. soft top.....

LL - NY

On Wed, 11 Jul 2001 19:20:57 +0100 "rob hod" <rob3 at hod3.fsnet.co.uk>
writes:
>
>    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)
>>
>
>