Brake Bias Valves
02-17-2011, 04:49 AM
#16
Racer
Thanks for the documentation. I should have see it in mine, but did not thought it was out there.
So it is now very clear. Then If you mount a 55/5 bias valve, the braking pressure will grow equally front to rear until it reach 55 bars.....
Question, what is the max pressure the brake system is able to?
Puyi
So it is now very clear. Then If you mount a 55/5 bias valve, the braking pressure will grow equally front to rear until it reach 55 bars.....
Question, what is the max pressure the brake system is able to?
Puyi
02-17-2011, 12:35 PM
#17
Rennlist Member
how is your rear locking up, if it is in gear and the engine is running?
Between shifts?
It would be reall interesting to see some in car video of the problem to see shift points and the problem with the greater rear bias settings . I dont think rebound settings will change the effects of the problem, because you are not changing weight transfer, that is based on braking force overall. However, you do change the rise of CG which effects that braking transfer slightly in your case, but not enough to solve this kind of problem.
what are finding out to happen under trail braking?
Between shifts?
It would be reall interesting to see some in car video of the problem to see shift points and the problem with the greater rear bias settings . I dont think rebound settings will change the effects of the problem, because you are not changing weight transfer, that is based on braking force overall. However, you do change the rise of CG which effects that braking transfer slightly in your case, but not enough to solve this kind of problem.
what are finding out to happen under trail braking?
02-17-2011, 12:59 PM
#18
Rennlist Member
Another thing, if you are looking at truly rear lockup, its probably out of gear when it happens, so I ask, what are you doing with your fuel cut off , Idle switch still live? I would recommend disabling it, expecially in a autox use. reason, when you push in the clutch, and you ever let the RPM down, or dont aggresively blip throttle, it can give extra compression braking that can give a lock up feel, not locking up, but increased slip, which can unsettle the car. same as too much brake bias to the rear, which in many cases is not locking up, but feels like it, as the rear wheels are going slower than the fronts under braking.
02-17-2011, 02:06 PM
#19
Drifting
If your rear were locking up that heavily the ABS is not functioning correctly.
02-17-2011, 02:13 PM
#20
Rennlist Member
and the engine would be stalling. 
If you blip from 100mph under hard breaking, or wait and blip, you really cant get lockup in any way, due to the rear wheels being connected to the engine.
. Its like built in "ABS". and ABS will work on the wheel that is lifted that can overcome the LSD and lock up when its unweighted or in the air in the rear.
If you blip from 100mph under hard breaking, or wait and blip, you really cant get lockup in any way, due to the rear wheels being connected to the engine.
. Its like built in "ABS". and ABS will work on the wheel that is lifted that can overcome the LSD and lock up when its unweighted or in the air in the rear.
02-17-2011, 03:26 PM
#21
Rennlist Member
Thread Starter
Okay, here are more details...
Due to accidentally disconnected accelerometer both the ABS AND PSD were off during the event.
My times were not anywhere near my regular time because I had problems both accelerating with only one wheel drive (no PSD) and braking (no ABS). Yes I have become a lazy driver and use ABS as the signal that I have reached the braking limit and start my threshold braking. There were many times where I locked up the front wheels and not the rear. The rear brake lockup I am describing started happening once I got more tuned to not having ABS was was not locking up the fronts.
Where I noticed the rear smoking was in the relatively long braking zone at the end of the fastest straight into the slowest corner that I was seeing tire smoke in the rear view mirror. I was not turning or trail braking when I noticed the smoke. It was in the straight line braking zone. I was downshifting 2 gears durning the braking. Downshifting during braking is something I normally do. The car did not unsettle ie., the rear did not get loose, but there evidentially was enough differential in braking for the rear tires to smoke. At turn-in the steering was still very crisp so my deduction is that the rears were causing the smoke and that maybe my 5/55 rear bias valve may have been the culprit.
Also, as far as engine braking my GTS is stock and does not engine brake very much on throttle lift. I would rather it did. How do I disconnect the idle switch? I am very smooth doing the blip downshift while braking. And I do know there is a difference as there are instances where I use the engine speed differential on downshifts to purposefully unsettle the rear, like negotiating turn-in on a tight chicane. That was not the case here.
What I am questioning is whether the rear bias valve is too big and if that would prematurely initial ABS. Sometimes *when the ABS is functioning* I feel that car is not braking nearly was well as it should when the ABS kicks in.
Due to accidentally disconnected accelerometer both the ABS AND PSD were off during the event.
My times were not anywhere near my regular time because I had problems both accelerating with only one wheel drive (no PSD) and braking (no ABS). Yes I have become a lazy driver and use ABS as the signal that I have reached the braking limit and start my threshold braking. There were many times where I locked up the front wheels and not the rear. The rear brake lockup I am describing started happening once I got more tuned to not having ABS was was not locking up the fronts.
Where I noticed the rear smoking was in the relatively long braking zone at the end of the fastest straight into the slowest corner that I was seeing tire smoke in the rear view mirror. I was not turning or trail braking when I noticed the smoke. It was in the straight line braking zone. I was downshifting 2 gears durning the braking. Downshifting during braking is something I normally do. The car did not unsettle ie., the rear did not get loose, but there evidentially was enough differential in braking for the rear tires to smoke. At turn-in the steering was still very crisp so my deduction is that the rears were causing the smoke and that maybe my 5/55 rear bias valve may have been the culprit.
Also, as far as engine braking my GTS is stock and does not engine brake very much on throttle lift. I would rather it did. How do I disconnect the idle switch? I am very smooth doing the blip downshift while braking. And I do know there is a difference as there are instances where I use the engine speed differential on downshifts to purposefully unsettle the rear, like negotiating turn-in on a tight chicane. That was not the case here.
What I am questioning is whether the rear bias valve is too big and if that would prematurely initial ABS. Sometimes *when the ABS is functioning* I feel that car is not braking nearly was well as it should when the ABS kicks in.
02-17-2011, 03:54 PM
#22
Rennlist Member
It would be real interesting to get some of this on video. Im surprised you havent any incar footage. Its very useful for all sorts of things like this.
next, engine braking is substantial. Im able to run a 1:45 at laguna, with absolutly no brakes!
it sounds like the the differential braking of the rears is what you are talking about , not lock up.
smoke from the rear could be fronts too, unless someone saw it was coming from the rears only.
I dont even know what break bias I have now. its been weaker, because Ive been constantly adding more brake potential up front with larger rotors and more agressive pads.
next, engine braking is substantial. Im able to run a 1:45 at laguna, with absolutly no brakes!
it sounds like the the differential braking of the rears is what you are talking about , not lock up.
smoke from the rear could be fronts too, unless someone saw it was coming from the rears only.
I dont even know what break bias I have now. its been weaker, because Ive been constantly adding more brake potential up front with larger rotors and more agressive pads.
02-17-2011, 07:47 PM
#23
Rennlist Member
Thread Starter
Yep spectators told me the smoke was coming from the rear tires. Couldn't have seen that on in-car video anyway.
Until driving a specific car I would not make broad stroke comments about engine braking. On both my 88 Turbo S and 90 GT engine braking was significant. Drifting a corner at drift maintaining throttle, when it drifts out a little too far just lift a little and the engine braking would tuck the front end and rotate a little and get back on the throttle. This made drifts very easy to control. With my 01 BoxsterS, 89 944 turbo, and the current 94 928 GTS not so much. The boxster S was the worst. Lifting the throttle had less deceleration than pushing in the clutch. On the GTS I have to blip the throttle then lift completely off to get the front to tuck and rotate the car in a little more. Then it is more breaking the rear loose than maintaining and throttle steering a 4 wheel drift.
I LOVED the engine braking on the 90 GT for Highway cruising. I could floor it in the gaps between groups of cars then just lift to slow to pass them. With ALL the other cars I've driven I had to tap the brakes to not make the deceleration distances ridiculously long. However it was not something you ever want to experience icy roads with.
Until driving a specific car I would not make broad stroke comments about engine braking. On both my 88 Turbo S and 90 GT engine braking was significant. Drifting a corner at drift maintaining throttle, when it drifts out a little too far just lift a little and the engine braking would tuck the front end and rotate a little and get back on the throttle. This made drifts very easy to control. With my 01 BoxsterS, 89 944 turbo, and the current 94 928 GTS not so much. The boxster S was the worst. Lifting the throttle had less deceleration than pushing in the clutch. On the GTS I have to blip the throttle then lift completely off to get the front to tuck and rotate the car in a little more. Then it is more breaking the rear loose than maintaining and throttle steering a 4 wheel drift.
I LOVED the engine braking on the 90 GT for Highway cruising. I could floor it in the gaps between groups of cars then just lift to slow to pass them. With ALL the other cars I've driven I had to tap the brakes to not make the deceleration distances ridiculously long. However it was not something you ever want to experience icy roads with.
02-18-2011, 10:22 AM
#24
Rennlist Member
RKD,
I had the 55 bar bias valve on my '88 S4 that I was driving on the tracks in the east. There was one section of one track (Mid-Ohio), where there is a significant rise the downslope in the track in a braking zone. I found under moderately hard braking in that zone - due to the unweighting of the rear as I crossed the rise - that the rear end of the car was getting VERY loose.
I changed to the 45 bar for that car (and my subsequent '89), and I felt the car was a good bit more controllable (and safer) in that braking zone. Probably did reduce the rear braking a bit in other zones, but not enough that I could tell it by the "seat of my pants".
Gary Knox
I had the 55 bar bias valve on my '88 S4 that I was driving on the tracks in the east. There was one section of one track (Mid-Ohio), where there is a significant rise the downslope in the track in a braking zone. I found under moderately hard braking in that zone - due to the unweighting of the rear as I crossed the rise - that the rear end of the car was getting VERY loose.
I changed to the 45 bar for that car (and my subsequent '89), and I felt the car was a good bit more controllable (and safer) in that braking zone. Probably did reduce the rear braking a bit in other zones, but not enough that I could tell it by the "seat of my pants".
Gary Knox
