Andre Hedrick

After reading this thread, I am confused!

MY 81 with 88s4 brakes tend to lock the fronts on a regular bases.
I just got a pair of 5|18 valves and it sounds like I will not need them.

At this point I may just remove them from the system completely and join them with a block so everybody get the same line pressure. Is this nuts?

I think I have either 5|33 or 5|55 on the car I just do not like the breaking behavior.

dr bob

If the fronts lock well before the rears, you might consider putting in the higher-pressure bias valve to shift some more of the braking effort to the rear. If you already have a 55bar regulator on there, there probaby won't be enough change going to no regulator to make it worth the effort. IMHO anyway.

Andre Hedrick

In the rain, why do I even have back brakes ...
In the dry, very hard braking (95 down to 65) with front lockup and rear starts to come around randomly (not favoring either side).

With a 10-80 build date, is it likely I have 5-33 or 5-55 ?

dr bob

On my later car with ABS, the bias regulator is right at the front of the ABS unit, easy to read. Not sure on yours, but it will need to be in the rear line, most likely near the master cylinder.

Curious: Do the early dual-diagonal-braking cars have bias regulator(s)? Would seem that there would need to be two regulators, one for each rear circuit.

Andre Hedrick

There are a pair of these puppies under the master cylinder.
I know because I have to pull my booster that is leaking and have to dork with the second one to get the BB out.

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I also want to consider undoing the cross diagonal braking profile as it was removed in later years.

Speedtoys

if your rears dont lock up with a 54, its another issue.

Could be temp/pad selection (a huge fun place to get tons of brake performance out of..both dont need to be the same)..could be other things to fix/change..but..I would expect you could lock up rear on the 54.

Andre Hedrick

I assume the pads are correct as they came matched with the rotors from 928intl for the s4 -> OB big black/red upgrade rotor package.

Speedtoys

If that was for me..im not saying theyre the wrong pads for the rotors.
Im saying..

Theres a HUGE field of tuning you can do with brake pads to match temps to friction coefficients, driving style...hell, _weather_ surface condition.

IE: A Raybestos ST43 front, and a Porterfield R41 rear for a spec racer that cant touch bias (Super grip <2800lbs small tire car up front, but a pad with huge friction numbers at temps <500d when bias limited) as one example. Another driver with a different braking style may go up to an ST41 up front because he doesnt brake as hard/deep as someone else, but uses braking to rotate more, earlier. Same car another driver, may swap that front down to a Hawk Blue for more instant grab, but little else above that (with increased pedal pressure) as an instant-on threshold braker..

Good tuning can make a better package on OEM hardware than on thrown together (no offense to anyone) big brake packages.

Its not hard to _double_ your pad braking efficiency for..say...the track. If you wanted to on most any car. But at a cost. (swap for track/street, use different rotors per set of pads, etc, etc, etc).

Just work out the right compound for your car/tires combination..and well, having some insight on front/rear bias helps too...or how the rear reacts to high grab brakes (like a Camaro that will axle hop)...

That was my weekend business for 12yrs.

Andre Hedrick

ST: data noted.

Front 1005.3 mm squared of piston surface
Rear 753.9 mm squared of piston surface

There is a about 33% more in the front wrt back or 25% less in the back wrt to front. I also don't have the weight of the 88s4 in the front where these Brembo's were harvested.

RKD in OKC

The accelerometer under the driver's seat came unplugged at an autocross event. On the GTS that means NO PSD and NO ABS. Under heavy braking from 100mph to 40mhp with 235 front and 285 rear and the 5/55 bias valve the rears would lock up before the fronts.

I may be nitpicking but my thinking is that this behavior means I am NOT getting maximum braking even when the ABS is working. There is still front braking left when the rears cause the ABS to kick in.

jheis

Andre:

I may be wrong, but as far as I know, the two units screwed into the bottom of your master cylinder are the switches for your brake lights and not bias valves. (not sure where the bias valve is located on your car.)

James

Scratch that. Your bias regulators are indeed mounted below the MC (but they do not screw into the MC - those are the brake light switches).

James