Jolly

Eyal,
(obviously) increasing brake bias, or decreasing traction will cause lockup.

If your rear wheels have less traction (wet/cold track, worn/colder tires, less sticky rubber, soft front suspension, braking downhill...etc.) more likely to lock the rears.

70-80% of the braking is done in the front. So IMHO&E there is NO need for the 928 bias valve unless you like doing donuts.

Waterguy

Eyal: Different pads front/rear? Weight removal in rear of car?

Jolly: What year is your 951? As I stated before, the 951S with the 928S4 front brakes have inherently higher front brake bias than the older 951s. I still think the change (upgrade?) makes sense for my car, but haven't tried it yet.

Alpine951

Waterguy,

Are you sure? My understanding is all 951's came with the same bias. The 928 valve moves some of the hydraulic pressure away from the fronts and puts it to the rears. The fronts will still have more buts its a shift from front to back compared to the stock 951 and stock 928.

Eyal 951

I have new rotors and Hawk HP plus all around thats going in soon, so I'll see what if that fixes it. I'll rebleed the calipers, since when i did my SS lines, i could have done a bad job on the fronts.
I can get rear lockup in any situation. Even in gear I can lock em up and stall the motor. the fronts will lock up as well as the rears if i just nail the pedal, actually they all lockup real easy, which is going to make for some interesting braking at the track, if I ever have it track ready, cause it seems to enjoy jackstands.
~Eyal

Chris Prack

HUH?? That may be true for YOU in YOUR car but this is NOT true for everyone.
I have done this to many cars (including my 89) and with the correct set up it is a significant improvment.

A back to back comparision only changing the valve will almost never give you the results you want or expect. It's no differant than anything else upgraded on your car. You cannot expect to just install your new Garrett turbo all by itself and expect the thing to be perfect and make a ton of power. Ain't gonna happen and it's no different with your brakes or suspension or any number of items. Think big picture. But I guess you qualified your statement with "IMHO&E", right?

Jon Moeller

Jolly,
Reduced traction from wet or cold conditions actually increases the available traction of the rear. In these conditions, you want more rear brake bias, as there is not as much weight transfer to the front of the car. I experienced this first hand, as my brakes were great in the wet at LRP, but started locking in the back, when I ran in the dry at WGI.

-Jon

Chris Prack

Eyal, the set up on the car is the problem. With the nonABS cars you have to; as I stated above; tune the brakes so to speak. In my experience, the problem will probably get worse with the Hawk's. They are very aggressive. One way to lessen rear lockup would be to install a new pair of OE brake hoses. The stainless hoses are, obviously, much stiffer than the rubber and will not swell or balloon under braking. This is the reason you get improved pedal height and feel after installing them but also part of the reason your get lockup. Changing the rears to rubber will lessen the force applied to the calipers due to the rubber hoses "ballooning" under pressure. Due to the fact that the rear runs so much cooler you can get away with this safely. If you are determined to keep the stainless in the rear, as some of my customers have, try using a less aggressive pad in the rear. Try a new set of OE pads. I know this sounds strange but it will all help in getting the most out of your brakes without going into suspension set up and tire componds, etc.

Oddjob

Correct, from the factory, all the turbos including TS cars came with the 5/18 valve. Except I believe he was referring to the larger front brakes and larger caliper pistons having a greater front braking capacity relative to the equal/same rear brakes.

And it doesnt take pressure from the front and move it to the rear. What the valve does is reduce the increase of pressure to the rear after the pressure passes the designated set pressure. The 5 is for 50% reduction and the 18 is for 18 bar. So the valve will decrease pressure to the rear brakes by 50% once pressure has passed 18 bar.

For example, You stand on the brakes and put out 30 bar pressure from the master cylinder - the front brakes see 30 bar at the pistons, the rear brakes see 24 bar (30-18/2+18).

If you had installed a 5/33 valve, its not moving any pressure from the front, its increasing the set pressure before it starts the 50% reduction. So for the above example at 30 bar, both your front and rear brakes would see a full 30 bar pressure. So therefore you get a little more grab in the rear, which can also make your car a little rear end happy.

Waterguy

All 951s came with the same 5/18 brake bias valve. (For some reason, all 944S's came with the 5/33.) However, the pistons in the front calipers kept getting larger relative to the pistons in the rear calipers.

MY 1986 951: 38/36 mm front, 30/28 mm rear
MY 1987/88 951: 40/36 mm front, 30/28 mm rear
MY 1988 951S/89-91 951: 44/36 mm front, 30/28 mm rear

At any given braking system hydraulic pressure, the braking force is proportional to the surface area of the caliper pistons, which increases with the SQUARE of the piston diameters. Since they all use (essentially) the same diameter rotor, this means that they all (like all cars) have an inherent front brake bias, independent of bias valve used. However, the increasing size of the front caliper pistons relative to the rear means that the 1986 951 has the least inherent front brake bias, and the 951S has the most front bias.

Eyal, you didn't switch front and rear calipers, did you?

Eyal 951

I have yet to touch them in the ownership of my car, but i have a feeling that this may be a possibility.
~Eyal

Alpine951

Thanks for the clarification Jim and Don. My first track day won't be till mid August and I put in the 33/5 valve on my 86 a few monthsd ago. Its a two day event so I will probably bring along the 18# valve just in case.

Jolly

I humbly disagree. One there is no such thing as a "correct" setup for there are too many factors you can't possibly factor in: driving style, track characteristics, suspension setups, tire character, brake pads.... At best its all a compromise. How exactly are you defining improvement? Just because it seems like a good concept as another "modification' may not make it so- it does have to work for ME. Rear ABS ('89 ?) makes this discussion moot, by the way.

Two Your solution for fixing ( Eyal's) lockup is adding up to 2 more unknowns to a problem that may not have existed in the first place with the stock bias valve. And it also involves throwing more money at the problem. My first suggestion would be to put the originals in and see if the behavior stops, before creating more issues. I'm only trying to pass along my experience tainted advice (IMHO&E); I am not selling anything.

My car is an '86 951 358 RWHP, no ABS, coilovers + 600 lbs. springs , PFC97 race pads on stock calipers/rotors, stock rubber brake lines, and 928 bias valve. IMHE, I can easily lock up my rears before my fronts on every turn if I don't practice threshold braking with prejudice! But I will gladly take one of your many used stock 5/18 valves, if you have one to give away-I'll even pay for shipping.

Jolly

Jolly

I'm not sure I understand how. On my cars with ABS, its much easier to engage ABS (ie lockup) when the road is slippery. The corollary: its much harder to engage ABS (lockup) on track tires than street tires.

LRP and WGI are 2 different tracks, yes? Can you account for any other differences that may have resulted in your experience (tires, suspension, pads, vehicle weight, driving style, etc)?

Thanks,
Jolly

Jolly

I've been on tracks where I've followed VW Rabbits around corners. I laugh out loud to see one rear wheel go up in each corner, and the cars corner on 3 wheels. You can tell when they trailbrake, the wheel that's up stops turning in mid turn.

BTW Rabbits with turbo engines and weigh 2200# can be very fast!

Jolly

ninefiveone

Jolly - I assume you're running the same track pad at all four corners?

I had to play with different compounds in the rear to get the balance I liked best after swapping bias valves. I also have an '86 non-abs 951 I track frequently.

In the end I settled on SBS enduro track pads in front, and metalmasters in the rear. I'd like a little more rear brake than I get with this setup but am happy enough with it to stop experimenting. Right now it feels stable under braking and I can trail brake carefully.

Picking up the inside rear is a pretty consistent VW trait. Even new golf's with coilovers and high spring rates will pick up the inside rear.