Which rear brakes for Coupe?

[s2_wot]

Paul,
Information below blatently robbed from s2central. What would you think of changing the brake pressure regulator from cars with upgraded fronts to allow more brake pressure to the rear.

If we say in round figures that there's a 10% gain in front axle braking then would we not gain the same 10% to the rear by sourcing a different regulator.

I see from the figures below it's not linear (if that's the correct term), is there any further information on this? I might have a trawl through a bendix design manual and see if I can find any information.

Brake Pressure Regulator

A non adjustable brake pressure regulator is fitted to the S2 in a rather unusual place - alongside the electro-hydraulic unit belonging to the ABS.

The function of the brake pressure regulator is to reduce the amount of hydraulic pressure that is applied to the rear brakes. This ensures the rear brakes do not lock up prematurely under heavy braking as load over the rear wheels is greatly reduced. Furthermore, failure of the front brake circuit activates a stop-piston which enables full brake pressure to be passed to the rear.

To check the function of the brake pressure regulator, specialised brake pressure guages (VAG1310) need to be fitted to the bleed screw points on the calipers. The correct specification on front and rear brake pressure is dependent on model year and is shown in the table below.

For cars built from MY92, (chassis PA 000 001 onwards) there is a different brake pressure regulator that is marked with a blue dot and the number '35'.

Note that higher rear brake pressure are provided on the later system. My unproven theory on this, is that Audi figured that some of its problems with rear caliper seizures across the earlier 80 series was because the rear calipers endured such little work and so would be prone to seizure.


Front pressure(bar) 50 ------------ 100

Rear Pressure
Up to MY92 (bar) 30 - 35 ----------- 45 - 50

Rear Pressure
From MY92 (bar) 37.5 - 41.5 ---------- 51.5 - 57.5

[S2central.net]

The right thing to do is replace the fixed BPR valve with a variable one - as per rally cars - then adjust until balance is correct for best overall braking effort on whatever setup exists on the car.

[s2_wot]

The right thing to do is replace the fixed BPR valve with a variable one - as per rally cars - then adjust until balance is correct for best overall braking effort on whatever setup exists on the car.

I don't think so now after looking at how it's piped in more detail. That regulator is a complicated little thing.

The ABS in the s2 is a 4S/3M meaning 4 Sensors and 3 Modulators. 1 sensor per wheel however there is 1 modulator for each of the front wheels and one modulator for the back axle.

This would give rise to a nasty situation in the event of rear axle lock up because instead of releasing the brake belonging to the wheel that's skidding it's releasing the brake for the 2 wheels at the back axle. Without being able to control each wheel individually the safest thing to do was put in a pressure regulator I guess.

A quick look under the bonnet of my S6 and the ABS is a 4S/4M but the earlier S4 had the same 4S/3M setup as the S2. I wonder has the urS4 got the brake pressure regulator??

Sorry HPmuirt if my ramblings are dragging your thread off topic.