Rear brake bias options
11-07-2012, 01:14 PM
#16
Rennlist Member
Just to throw this out there, because I was surprised when I heard it....
At the Road America PCA club race this past September, I spent over an hour talking with David Murry. He related some stories from back when he was racing 944 Turbos. He said that what HE ultimately wound up doing was removing the bias valve altogether! Said that once he got used to it, it was faster.
Most of his suggestions for me were technique changes. The one item he said I should change on the car: get more rear bias in there.
At the Road America PCA club race this past September, I spent over an hour talking with David Murry. He related some stories from back when he was racing 944 Turbos. He said that what HE ultimately wound up doing was removing the bias valve altogether! Said that once he got used to it, it was faster.
Most of his suggestions for me were technique changes. The one item he said I should change on the car: get more rear bias in there.
This would be worth further discussion w/ David before removing the bias valve altogether on a stock non-ABS turbo. David drove S2 & 968 Clubsports in the Firestone Firehawk series. All those cars had the big M030 front brakes (more front brake balance), with ABS. Running no prop valve on a stock caliper 86-87 951 w/o ABS will lock the rears under straight line braking and the car will try to swap ends.
11-07-2012, 01:30 PM
#17
Rennlist Member
As for the balance, its actually a combination of factors. The susp setup is very neutral, as a matter of fact. It's under heavy trail braking that the car likes to get loose -- it is easily correctable, but I do lose time doing so. I know that's an obvious statement, but I do know there's more there to be had -- based on the current setup, would like to have a bit more bite on the front and a bit less bite on the rear.
11-07-2012, 02:15 PM
#18
Previous owner on my car had a Wilwood adjustable bias installed right after the master cylinder to decrease the rear bias. (Wanted less rears.) I was going to take a picture last night; but I got caught up in cleaning the garage after the wheel bearing change.
11-07-2012, 03:48 PM
#19
Drifting
I didn't read past "Hawk Blues" on the rear. The easy way out is to just run a less aggressive pad on the rear. Once you find the right combo your done. No messing with the bias.
11-07-2012, 04:19 PM
#20
Burning Brakes
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I don't think you can definitively say one way or another whether it's a good idea. It depends on a lot of variables: rear tires, rear brake pads, rear rotors.
I've got 285's in the back with stock pads and cheaper rotors, and i've noticed i can add a lot of bias to the back and still break the front tires loose long before the rears.
FWIW, i have an original ABS car (ABS disabled) and i found it easier to gut the stock valve than to remove it.
I've got 285's in the back with stock pads and cheaper rotors, and i've noticed i can add a lot of bias to the back and still break the front tires loose long before the rears.
FWIW, i have an original ABS car (ABS disabled) and i found it easier to gut the stock valve than to remove it.
11-07-2012, 04:21 PM
#21
Rennlist Member
Thread Starter
Getting loose under trail braking is not necessarily a brake bias issue. I would say, a car should be setup differently for a driver that heavily trail brakes vs. one that does nearly all straightline braking. You would want more neutral push for a driver that trail brakes. Also, what type of rear diff setup do you have? Open, stock LSD, other?
Thanks for the comments everyone.
11-07-2012, 07:42 PM
#22
Drifting
Colin,
You mentioned some instability under straight line braking, too. Are you locking the rears then or is the rear just dancing around under heavy braking?
I get a wagging rear under heavy braking and oversteery trail braking when the rear tire pressures get too high. Why? I dunno, but I bleed them back to the proper point and the wagging disappears. I notice this when I get ~3psi higher or more than desired.
If you're just locking the inside rear under trail braking, you've developed the poor man's version of Porsche Torque Vectoring and should love it!!! ;-)
You mentioned some instability under straight line braking, too. Are you locking the rears then or is the rear just dancing around under heavy braking?
I get a wagging rear under heavy braking and oversteery trail braking when the rear tire pressures get too high. Why? I dunno, but I bleed them back to the proper point and the wagging disappears. I notice this when I get ~3psi higher or more than desired.
If you're just locking the inside rear under trail braking, you've developed the poor man's version of Porsche Torque Vectoring and should love it!!! ;-)
11-07-2012, 09:12 PM
#23
Rennlist Junkie Forever
No. I said it's (an adjustable proportioning valve) not a great idea for the street because it is always cutting brake pressure to the rear circuit.
On the street, you want full brake pressure up to a point, then you want it to taper down (which is what the stock bias valve does).
TonyG
On the street, you want full brake pressure up to a point, then you want it to taper down (which is what the stock bias valve does).
TonyG
Thanks for the responses, guys.
Yeah, I had thought about it, but I really would like something adjustable. If I had dual master cylinders I could just use a bias bar like the big boys...
Interesting to hear, thanks. I'm not sure I follow your logic that it's not flow we are regulating -- brake fluid is incompressible and the fluid is physically displacing the piston. Once displaced, it becomes a pressure game (force measured at the pad face). Regardless, I think I get what you're getting at. Was thinking something like this:
Tilton Adjustable Proportioning Valve (****-type)
Hmmm, my car is mainly a track car. Why not use for the track? Not sure I follow.
I'm actually looking to decrease the amount of pressure on the rear, given that I usually am causing the rears to lock up before the fronts on the track -- especially when trail braking (which these cars love to do). Stock 951 brakes with Hawk Blue pads. Small rubber: 225R16 Front, 245R16 Rear.
The brakes are quite good; I really don't want the pressure much less than what it currently is. Also, having a prop valve will enable me to adjust on-the-fly as my pads near the end of their life (the fronts tend to bite less as the pads wear).
What did you do with your old 951? Didn't think you had dual MC's.
Yeah, I had thought about it, but I really would like something adjustable. If I had dual master cylinders I could just use a bias bar like the big boys...
Interesting to hear, thanks. I'm not sure I follow your logic that it's not flow we are regulating -- brake fluid is incompressible and the fluid is physically displacing the piston. Once displaced, it becomes a pressure game (force measured at the pad face). Regardless, I think I get what you're getting at. Was thinking something like this:
Tilton Adjustable Proportioning Valve (****-type)
Hmmm, my car is mainly a track car. Why not use for the track? Not sure I follow.
I'm actually looking to decrease the amount of pressure on the rear, given that I usually am causing the rears to lock up before the fronts on the track -- especially when trail braking (which these cars love to do). Stock 951 brakes with Hawk Blue pads. Small rubber: 225R16 Front, 245R16 Rear.
The brakes are quite good; I really don't want the pressure much less than what it currently is. Also, having a prop valve will enable me to adjust on-the-fly as my pads near the end of their life (the fronts tend to bite less as the pads wear).
What did you do with your old 951? Didn't think you had dual MC's.
11-07-2012, 10:40 PM
#24
Rennlist Member
Thread Starter
Hmmm, good question. Under braking there is definitely instability, but the rears will sometimes lock up a bit if I'm not careful (especially into bumpier braking zones).
I'll try toying with tire pressures next time I'm out there. Recently switched tires as well, so I haven't quite nailed them down. For what it's worth I do know my shocks are under-damping; unfortunately koni yellows aren't quite enough for 30mm t-bars. Again not terrible, but I do need more damping.
And maybe your right-- perhaps I DO have PTV!
I'll try toying with tire pressures next time I'm out there. Recently switched tires as well, so I haven't quite nailed them down. For what it's worth I do know my shocks are under-damping; unfortunately koni yellows aren't quite enough for 30mm t-bars. Again not terrible, but I do need more damping.
And maybe your right-- perhaps I DO have PTV!
Colin,
You mentioned some instability under straight line braking, too. Are you locking the rears then or is the rear just dancing around under heavy braking?
I get a wagging rear under heavy braking and oversteery trail braking when the rear tire pressures get too high. Why? I dunno, but I bleed them back to the proper point and the wagging disappears. I notice this when I get ~3psi higher or more than desired.
If you're just locking the inside rear under trail braking, you've developed the poor man's version of Porsche Torque Vectoring and should love it!!! ;-)
You mentioned some instability under straight line braking, too. Are you locking the rears then or is the rear just dancing around under heavy braking?
I get a wagging rear under heavy braking and oversteery trail braking when the rear tire pressures get too high. Why? I dunno, but I bleed them back to the proper point and the wagging disappears. I notice this when I get ~3psi higher or more than desired.
If you're just locking the inside rear under trail braking, you've developed the poor man's version of Porsche Torque Vectoring and should love it!!! ;-)
11-08-2012, 06:40 PM
#26
Burning Brakes
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11-09-2012, 11:02 AM
#27
Rennlist Member
Thread Starter
Thanks for the picture -- good to know it fits there.
Agreed that it will only remove flow, but that's my goal here. Would prefer to put a big a$$ prop valve in the rear and then meter the flow with an adjustable valve downstream.
Looks like I'll hold off on installing the adjustable valve until after I replace the MC. Also, for what it's worth, my I do have the stock prop valve (5/18) -- meaning there's definitely something going on with the flow of fluid to my front brakes. Considering everything else in the system is new/replaced, looks like that's the culprit.
11-10-2012, 01:24 AM
#28
Pro
Are you in fact locking both rears simultaneously or only the inside rear? I believe I have seen people lock the inside rear on multiple occasions and have done this before with my car particularly when my car was stock with street tires (lots of front weight transfer/nose dive under braking and low grip.) More recently when I have had rear lockup of the inside tire under braking (with the stock bias valve) I have blamed my open diff - and poor technique. I guess I ask because I would see locking both rears as more of an indication of a bias problem and locking just the inside wheel as an indication that something else may be in play.
11-10-2012, 09:22 AM
#29
Addict
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my experience with an 86 944 turbo non-ABS with Big Red front brakes, stock rears brakes, stock LSD
5/33 proportion valve - too much rear bias, would lock the rears and try to swap ends
5/18 proportion valve - less tendency to lock the rears
LS1 swap with hydroboost, no proportion valve - way too much rears. Rears lock quickly and feel loose
LS1 swap with hydroboost, Tilton adj valve - 1 notch from closed, bias now feels great
5/33 proportion valve - too much rear bias, would lock the rears and try to swap ends
5/18 proportion valve - less tendency to lock the rears
LS1 swap with hydroboost, no proportion valve - way too much rears. Rears lock quickly and feel loose
LS1 swap with hydroboost, Tilton adj valve - 1 notch from closed, bias now feels great
02-15-2014, 06:22 PM
#30
