RKD in OKC

The bias valve lets the same pressure go to the rear brakes until it reaches a change over pressure, then limits the increase in pressure going to the rear compared to the front by the marked amount where 5 is a 50% reduction and the 18, 33, 45, 55, 65 is the set pressure in bar. The higher number the higher pressure the rear brakes get the same pressure as the rears before starting the 50% of the increased pressure over that amount. This produces an overall increase in rear braking pressure. Example: 5/18 vs. 5/33 at say 50 bar braking force. 50-18 is 32, and half that is 17. 18+17 is 35. So with the 5/18 and front pressure of 50 the rear pressure is 35. 50-33 is 17, half that 8.5. 33+8.5 is 41.5. So with the 5/33 and front pressure of 50 the rear ressure is 41.5.

On my 90 GT it had the stock 5/18, stock tires sizes, eibach springs, Devek front sway bar, and Ott steroid rear drop links. I was having problems with the front brakes getting way too hot autoXing. The brakes getting hot enough to boil the brake fluid and warp the rotors. The first thing I did was to replace the rotors with cryotempered slotted rotors. This helped with the warping and boiling but the front brakes were still getting way too hot. Some temp sensitive paint showed they were getting 800°F. I also noticed the front rotors were getting a blue tint to them while the rears were not. Upgraded bias valve to a 5/33 and only a little better, still getting blue front rotors and rears were staying gray. Switched to the 5/55 and finally started getting some blue on the rear rotors. However, I did have to be careful braking on curves as the rear would get light and try to come around. A quick flick of opposite steering would easily catch it. Did not know there was a 5/45 valve available at the time, think that would have been perfect.

With my GTS I am running a completely different setup. I still have the Devek front sway bar and Ott Steriod rear drop links, but have Hypercoil springs, Koni red adjustable dampers, and 235 front and 285 rear tires. With the bigger brakes on the GTS I am not having the overheating brakes problem. But did notice under heavy braking the fronts were locking to trip the ABS. Thinking back to the 90 GT went to 5/55 bias valve. It does better, but the ABS still seems to come on a little soon. The accelerometer came unplugged during on event this disables the ABS. Noticed in the heaviest braking zone the rears were locking up. While with ABS and adjustable rear rebound I am able to control the rear tendency to rotate, too much rear braking is causing the ABS to come on well before the car reaches maximum braking. Think it needs the 5/45 or even the 5/33 with this setup.

A great way to check your bias setting is to unplug the ABS relay to disable ABS and see whether front or rears are triggering the ABS.

Jim M.

Roger and I both installed 993TT rear calipers and rotors on our GTS's. I installed 45 Bar bias valves in both cars. During spirited driving (with 315 rear tires) I can force rotation of the tail to assist in handling. For those inexperienced drivers I would recommend the max of 33 bar valves. With the stock 18 bar valve you can easily induce the ABS on the front during a minor panic stop. With any of the other valves you will almost never induce the ABS in the front. Without the 993TT rear calipers I was running a 55 Bar valve and IMHO it was perfect.

worf928

+928 To these sentiments. Any folks thinking of adding rear bias that haven't had some training should not go above 33. Even 33 can be a bit much for some folks.

worf928

RDK, so, if I read correctly, with a 5/55 regulator you are able to lock-up 285 rears with 235 fronts? What Hypercoils are you running? (I ask because a) I always enjoy reading your detailed and cogent handling reports and b) because one of these days I will get around to installing my Hypercoils on my '89 with a 5/55 regulator.)

RKD in OKC

Not sure which hypercoils as they were on the car when I got it. I assume because the time Konis, hyper coils, and Devek sway bar were done about all that was available was 400/600. With the stock 18 bar bias valve and the rear koni rebound set to full soft I could pump the brakes and force locking up the rear 285 with 235 fronts. It is how the brakes come on and weight transfers not just the bias valve. That's why I listed the components and tire sizes.

Speedtoys

Abs works in a straight line nicely. And first requires some lockup. Right?

Over aggressive weight transfer followed by a spin is something abs has no knowledge of.

RKD in OKC

If you have the brakes on and any wheel or wheels starts rotating very much slower than the others, ABS.

There are also general wheels speed and accelerometer information factored.

hacker-pschorr

Just the opposite. With ABS you can safely run far more front bias without fear of locking up.

worf928

Absolutely. That's why I wanted to know about the spring rates.

So you think you've got the 600 pound Hypercoils on the front and 400s on the rear? And I assume you've got noticeably less-than-stock dive?

And you're describing straight line breaking, yes?

KenRudd

Timely thread!

Gary Knox

I too have used 33, 55, and 45 bias for my '89 manual S4. Settled on 45, as I found a bit too much rear braking with the 55 when going into corners with a slight rise, then fall just before the corner - specifically, Mid-Ohio, just before the carousel on to the front straight.

45's are no longer available. They were used on 964's, and a lot of those guys have gone to 55's. I've found my 45's by posting a "wanted" on the 964 forum.

Gary--

huskeric

what is involved in swapping this part? I've not yet done my homework... do the brakes need to be bled?

rick

dr bob

Just the opposite.

The higher bias valve restriction point allows more brake pressure to the rears. The question is how much more rear brake pressure is tolerable without risking a snap spin while turning or on an uneven surface. 928 ABS helps in a straight line, but treats the whole rear as one circuit for pressure modulation so less help when rears are locking earlier as you would with a higher bias valve rating.

dr bob

The bias valve sits on the front of the ABS unit on my car, between the ABS unit and the hard pipe to the rear brakes. Right by the power steering fluid reservoir on my LHD car.

You could probably get away with just a "local" bleed after installing the new regulator. So new regulator installed to the ABS unit, hardline installed one thread. Have a helper press slowly on the pedal to half stroke a couple times while you catch the fluid from the loose connection. After a couple half-stroke pumps, have your helper hold the pedal down at the half-stroke position while you snug the hardline down correctly. Top up the fluid and you should be OK.

Gary Knox

I have used the procedure Bob outlines above on a couple of occasions - but using a pressure bleeder rather than a helper. Works fine, but be sure to use paper towels or something very absorbent to keep the bleeding fluid from your paint.

Gary-