Brake bias issues
11-26-2010, 08:45 PM
#1
Brake bias issues
I updated my brakes to turbo brakes all the way around and am wanting some input on the best way to increase bias to the rear.
I ask because during heavy breaking I will lock up the front wheels momentarily before the ABS kicks in. So I figure I can move some of the bias to the rear to have a more balanced braking experience.
At this point I'm thinking that hopefully I can swap out the proportioning valves on the rear brake lines to allow more pressure to the back.
All comments and suggestions welcomed. Thanks, Albert.
I ask because during heavy breaking I will lock up the front wheels momentarily before the ABS kicks in. So I figure I can move some of the bias to the rear to have a more balanced braking experience.
At this point I'm thinking that hopefully I can swap out the proportioning valves on the rear brake lines to allow more pressure to the back.
All comments and suggestions welcomed. Thanks, Albert.
11-26-2010, 10:01 PM
#2
Race Car
One option is to put a more aggressive pad in the rear. I know of guys that use Pagid Yellows in the front (RS19 or 29) and then use a Pagid Black in the rear (RS14)
11-26-2010, 11:51 PM
#3
Rennlist Member
I'm going to think "out loud" here...If you increased your brakes on all four corners, the system should be as balanced as it was before. Keep in mind that your fronts should always bear the heavier load than the rears. Before swapping proportioning valves, be sure your ABS is functioning properly.
11-27-2010, 12:05 AM
#4
RL Technical Advisor
The other good one is to install a pair of RS rear calipers,..this works VERY well, too.
11-27-2010, 11:14 AM
#5
I updated my brakes to turbo brakes all the way around and am wanting some input on the best way to increase bias to the rear.
I ask because during heavy breaking I will lock up the front wheels momentarily before the ABS kicks in. So I figure I can move some of the bias to the rear to have a more balanced braking experience.
At this point I'm thinking that hopefully I can swap out the proportioning valves on the rear brake lines to allow more pressure to the back.
All comments and suggestions welcomed. Thanks, Albert.
I ask because during heavy breaking I will lock up the front wheels momentarily before the ABS kicks in. So I figure I can move some of the bias to the rear to have a more balanced braking experience.
At this point I'm thinking that hopefully I can swap out the proportioning valves on the rear brake lines to allow more pressure to the back.
All comments and suggestions welcomed. Thanks, Albert.
as noted use a pad combination w/ moe rear such as Pagid yellow front black rear or install RS rear calipers.
RS rears are great but they do move bias way to the back, to take full advantage you will want a low, stiffly sprung car w/ effective lsd.
11-27-2010, 03:55 PM
#6
Rennlist Member
From what I know there is no "proportioning valve" on a 993, the bias is built into the master cylinder. There are limiter (blowoff) valves that limit ultimate pressure in a panic stop, but from what I have read the pressure limit is plenty high and they do not need to be removed (unlike the 964).
Question though: Anyone know what happens in an ABS car if the rears lock up first? In a non-ABS car things can get hairy real quick… but how does our ABS handle it? Is it smart enough to recognized and correct for it or does the same danger still apply?
Question though: Anyone know what happens in an ABS car if the rears lock up first? In a non-ABS car things can get hairy real quick… but how does our ABS handle it? Is it smart enough to recognized and correct for it or does the same danger still apply?
11-27-2010, 04:43 PM
#7
From what I know there is no "proportioning valve" on a 993, the bias is built into the master cylinder. There are limiter (blowoff) valves that limit ultimate pressure in a panic stop, but from what I have read the pressure limit is plenty high and they do not need to be removed (unlike the 964).
Question though: Anyone know what happens in an ABS car if the rears lock up first? In a non-ABS car things can get hairy real quick… but how does our ABS handle it? Is it smart enough to recognized and correct for it or does the same danger still apply?
Question though: Anyone know what happens in an ABS car if the rears lock up first? In a non-ABS car things can get hairy real quick… but how does our ABS handle it? Is it smart enough to recognized and correct for it or does the same danger still apply?
cars w/ awd and cars w/ M220 option(lsd& abd) have 2. These cars have 4 channel abs others have 3channel(1 for each front wheel & 1 for the rear axle)
the p/v(s) is on the back of the abs pump. the 4 channel cars will have 2 p/v's(1 for each rear wheel) and 4 separate lines, others will have 1 p/v on the single rear circuit.
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11-27-2010, 05:06 PM
#8
Rennlist Member
Agree with everything you stated... except I believe they are blow off valves rather than proportioning valves... Here is the answer I recieved from Joel Reiser when I ask him several years ago...
-------------------------------------------------------------
Thanks for great questions. If we gave out coffee mugs for great ones, you would get one for this. I've been wondering when someone would ask this. The brake pressure regulator is different from a bias valve. This is a common confusion. What Porsche has done here is put in a valve that does nothing, unless and until the pressure (to the rear brakes) reaches some defined large value. At that point it will cut off any further pressure. So it does not set the bias for normal braking, it sets the bias for a panic stop. Normal braking includes light, medium and heavy, while the pressure limiter (or regulator) only kicks in at the max. The rear pressure regulator in the 964 cars was too limiting for track use when the front brakes were upgraded. So what Porsche did in the Carrera Cup cars was to remove and bypass it, with a plain pipe (a longer one to skip over the distance previously spanned by the regulator). This was part of a set of changes which included bigger black front calipers, rotors, and pads. Since more braking capacity was added up front, the idea was to try to add a little more at the limit by removing the rear limiter valve. A lot of people like to change them in the 964 to use a different one, such as the C2 Turbo one, that is a better solution for a driver ed and street use car than just discarding it, as you would for a trailered race car on slicks. The pressure regulators in the 993 are OK. I would not mess with that. Actually I had a 1995, and did the big red brake upgrade, and didn't mess with the regulators, and the car and the brakes were great. I also used Pagid orange. They can squeal and throw a lot of rustwater up in big rainstorms, but on the track they are outstanding. And normally on the street they are tolerable, apart from that. Though it is always a good idea to check your rearview mirror before braking hard, since you can stop a lot faster than they can. I say "regulators" in the 993 versus "regulator" in the 964 because there was one in the 964 for the rear brakes, while there were two in most 993's. The first 993's had only one rear brake circuit, but all the turbos, and C4's, and cars with ABD had two rear brake circuits, therefore they had two regulators installed, one for the left rear line, and one in the right rear line. All of the regulators are installed at the ABS hydraulic unit in the trunk. They are a few inches long, a round cartridge sort of looking thing. Joel Reiser - PCA WebSite - 2/25/2005
-------------------------------------------------------------
Thanks for great questions. If we gave out coffee mugs for great ones, you would get one for this. I've been wondering when someone would ask this. The brake pressure regulator is different from a bias valve. This is a common confusion. What Porsche has done here is put in a valve that does nothing, unless and until the pressure (to the rear brakes) reaches some defined large value. At that point it will cut off any further pressure. So it does not set the bias for normal braking, it sets the bias for a panic stop. Normal braking includes light, medium and heavy, while the pressure limiter (or regulator) only kicks in at the max. The rear pressure regulator in the 964 cars was too limiting for track use when the front brakes were upgraded. So what Porsche did in the Carrera Cup cars was to remove and bypass it, with a plain pipe (a longer one to skip over the distance previously spanned by the regulator). This was part of a set of changes which included bigger black front calipers, rotors, and pads. Since more braking capacity was added up front, the idea was to try to add a little more at the limit by removing the rear limiter valve. A lot of people like to change them in the 964 to use a different one, such as the C2 Turbo one, that is a better solution for a driver ed and street use car than just discarding it, as you would for a trailered race car on slicks. The pressure regulators in the 993 are OK. I would not mess with that. Actually I had a 1995, and did the big red brake upgrade, and didn't mess with the regulators, and the car and the brakes were great. I also used Pagid orange. They can squeal and throw a lot of rustwater up in big rainstorms, but on the track they are outstanding. And normally on the street they are tolerable, apart from that. Though it is always a good idea to check your rearview mirror before braking hard, since you can stop a lot faster than they can. I say "regulators" in the 993 versus "regulator" in the 964 because there was one in the 964 for the rear brakes, while there were two in most 993's. The first 993's had only one rear brake circuit, but all the turbos, and C4's, and cars with ABD had two rear brake circuits, therefore they had two regulators installed, one for the left rear line, and one in the right rear line. All of the regulators are installed at the ABS hydraulic unit in the trunk. They are a few inches long, a round cartridge sort of looking thing. Joel Reiser - PCA WebSite - 2/25/2005
Last edited by jscott82; 11-27-2010 at 05:21 PM.
11-27-2010, 06:14 PM
#9
Rennlist Member
This will provide my segue into another related topic.... I've always wondered how to potentially improve the the ABS in a 993 without spending $10,000 on a Bosch M4 kit. With slicks, stiff suspension and various racing conditions, it sure would be nice to take advantage of a more performance oriented ABS program (or perhaps even different programs for different conditions). I don't think I've ever heard of anyone "chipping" or re-programming a stock Bosch ABS controller.
I guess if it was possible, someone would have done it... but then again, the obvious consequences of an error in this department would be quite dire, and maybe the liability associated with messing with the ABS has prevented any industrious soul from attempting to market it or make available a "motorsport tune" for our ABS controllers. I know some guys have installed 996 cup ABS units in their cars, but that too would be pretty pricey.
I guess if it was possible, someone would have done it... but then again, the obvious consequences of an error in this department would be quite dire, and maybe the liability associated with messing with the ABS has prevented any industrious soul from attempting to market it or make available a "motorsport tune" for our ABS controllers. I know some guys have installed 996 cup ABS units in their cars, but that too would be pretty pricey.
11-27-2010, 06:31 PM
#10
Agree with everything you stated... except I believe they are blow off valves rather than proportioning valves... Here is the answer I recieved from Joel Reiser when I ask him several years ago...
-------------------------------------------------------------
Thanks for great questions. If we gave out coffee mugs for great ones, you would get one for this. I've been wondering when someone would ask this. The brake pressure regulator is different from a bias valve. This is a common confusion. What Porsche has done here is put in a valve that does nothing, unless and until the pressure (to the rear brakes) reaches some defined large value. At that point it will cut off any further pressure. So it does not set the bias for normal braking, it sets the bias for a panic stop. Normal braking includes light, medium and heavy, while the pressure limiter (or regulator) only kicks in at the max. The rear pressure regulator in the 964 cars was too limiting for track use when the front brakes were upgraded. So what Porsche did in the Carrera Cup cars was to remove and bypass it, with a plain pipe (a longer one to skip over the distance previously spanned by the regulator). This was part of a set of changes which included bigger black front calipers, rotors, and pads. Since more braking capacity was added up front, the idea was to try to add a little more at the limit by removing the rear limiter valve. A lot of people like to change them in the 964 to use a different one, such as the C2 Turbo one, that is a better solution for a driver ed and street use car than just discarding it, as you would for a trailered race car on slicks. The pressure regulators in the 993 are OK. I would not mess with that. Actually I had a 1995, and did the big red brake upgrade, and didn't mess with the regulators, and the car and the brakes were great. I also used Pagid orange. They can squeal and throw a lot of rustwater up in big rainstorms, but on the track they are outstanding. And normally on the street they are tolerable, apart from that. Though it is always a good idea to check your rearview mirror before braking hard, since you can stop a lot faster than they can. I say "regulators" in the 993 versus "regulator" in the 964 because there was one in the 964 for the rear brakes, while there were two in most 993's. The first 993's had only one rear brake circuit, but all the turbos, and C4's, and cars with ABD had two rear brake circuits, therefore they had two regulators installed, one for the left rear line, and one in the right rear line. All of the regulators are installed at the ABS hydraulic unit in the trunk. They are a few inches long, a round cartridge sort of looking thing. Joel Reiser - PCA WebSite - 2/25/2005
-------------------------------------------------------------
Thanks for great questions. If we gave out coffee mugs for great ones, you would get one for this. I've been wondering when someone would ask this. The brake pressure regulator is different from a bias valve. This is a common confusion. What Porsche has done here is put in a valve that does nothing, unless and until the pressure (to the rear brakes) reaches some defined large value. At that point it will cut off any further pressure. So it does not set the bias for normal braking, it sets the bias for a panic stop. Normal braking includes light, medium and heavy, while the pressure limiter (or regulator) only kicks in at the max. The rear pressure regulator in the 964 cars was too limiting for track use when the front brakes were upgraded. So what Porsche did in the Carrera Cup cars was to remove and bypass it, with a plain pipe (a longer one to skip over the distance previously spanned by the regulator). This was part of a set of changes which included bigger black front calipers, rotors, and pads. Since more braking capacity was added up front, the idea was to try to add a little more at the limit by removing the rear limiter valve. A lot of people like to change them in the 964 to use a different one, such as the C2 Turbo one, that is a better solution for a driver ed and street use car than just discarding it, as you would for a trailered race car on slicks. The pressure regulators in the 993 are OK. I would not mess with that. Actually I had a 1995, and did the big red brake upgrade, and didn't mess with the regulators, and the car and the brakes were great. I also used Pagid orange. They can squeal and throw a lot of rustwater up in big rainstorms, but on the track they are outstanding. And normally on the street they are tolerable, apart from that. Though it is always a good idea to check your rearview mirror before braking hard, since you can stop a lot faster than they can. I say "regulators" in the 993 versus "regulator" in the 964 because there was one in the 964 for the rear brakes, while there were two in most 993's. The first 993's had only one rear brake circuit, but all the turbos, and C4's, and cars with ABD had two rear brake circuits, therefore they had two regulators installed, one for the left rear line, and one in the right rear line. All of the regulators are installed at the ABS hydraulic unit in the trunk. They are a few inches long, a round cartridge sort of looking thing. Joel Reiser - PCA WebSite - 2/25/2005
When max braking occurs weight is transfered to the front so you need less line pressure in the back, the pressure curves when a p/v is used can more closely correlate to the curve needed than an open rear line.
A 964RS has too much front bias due to differential rotor size and caliper piston sizing, they need to remove the p/v to get enough rear bias even though the rear pressure curves are less than ideal. Similarly early 911 have close to ideal native bias but can profit from fine tuning the shape of the rear line pressure curves, though most don't experiment in this area
a 993 has pretty good base bias but still profits from the p/v, a 993tt is way worse than a 964RS, and can similarly profit from removing it's 2 p/v's, a 993RS on the other hand has a lot of static or base rear bias, it approaches the max in this area and profits greatly from reducing front weight transfer under braking and stabilizing braking w/ an effective high lock up lsd.
the only devises in the m/c area that affect bias are the twin master setups that are used on RSR and late Cup cars. Here different sized m/c for the front and rear are used as well as a mechanical bias bar. Both of these mechanisms affect equally bias at all line pressure levels and are not proportionally effective at all. There are some front engine cars that are equipped w/ stepped m/c, these are functionally the same as a twin master setup w/ 2 different m/c
Some people do not like a p/v among the reasons are the time that it takes to engage and release line pressures, it takes a very finely trained foot to detect this response lag but some do take issue.
11-27-2010, 06:35 PM
#11
This will provide my segue into another related topic.... I've always wondered how to potentially improve the the ABS in a 993 without spending $10,000 on a Bosch M4 kit. With slicks, stiff suspension and various racing conditions, it sure would be nice to take advantage of a more performance oriented ABS program (or perhaps even different programs for different conditions). I don't think I've ever heard of anyone "chipping" or re-programming a stock Bosch ABS controller.
I guess if it was possible, someone would have done it... but then again, the obvious consequences of an error in this department would be quite dire, and maybe the liability associated with messing with the ABS has prevented any industrious soul from attempting to market it or make available a "motorsport tune" for our ABS controllers. I know some guys have installed 996 cup ABS units in their cars, but that too would be pretty pricey.
I guess if it was possible, someone would have done it... but then again, the obvious consequences of an error in this department would be quite dire, and maybe the liability associated with messing with the ABS has prevented any industrious soul from attempting to market it or make available a "motorsport tune" for our ABS controllers. I know some guys have installed 996 cup ABS units in their cars, but that too would be pretty pricey.
11-27-2010, 08:50 PM
#12
Does anyone want to buy a pair of almost new 993tt rear brake calipers so I can buy some RS calipers??
Does anyone here run EBC pads? My car is my dd and is performing more weekend duty at the track so I was trying to find a good balance of street/track pads. Is my thinking silly and I should have a set of pads I swap in for track use?
Thanks!
11-28-2010, 01:10 PM
#13
Race Car
This will provide my segue into another related topic.... I've always wondered how to potentially improve the the ABS in a 993 without spending $10,000 on a Bosch M4 kit. With slicks, stiff suspension and various racing conditions, it sure would be nice to take advantage of a more performance oriented ABS program (or perhaps even different programs for different conditions). I don't think I've ever heard of anyone "chipping" or re-programming a stock Bosch ABS controller.
https://rennlist.com/forums/964-foru...e-upgrade.html
https://rennlist.com/forums/964-foru...ke-system.html
11-28-2010, 01:36 PM
#14
Rennlist Member
I also switched to Big Reds and have just lived with the slightly front-biasing of this setup. You can do the math, but IIRC it shifts the brake bias around 7% forward (roughly). Sure RS calipers would be better but is it worth the price? Perhaps, but not for me.
I actually bought new higher Pressure Limiting valves (for the TT) for like $80 each(?) from Porsche, but I'm not bothered enough to even install them. Maybe someday.
I might be seeing higher brake pad wear in the front now, but it's not too significant.
I actually bought new higher Pressure Limiting valves (for the TT) for like $80 each(?) from Porsche, but I'm not bothered enough to even install them. Maybe someday.
I might be seeing higher brake pad wear in the front now, but it's not too significant.
11-29-2010, 12:02 PM
#15
I also switched to Big Reds and have just lived with the slightly front-biasing of this setup. You can do the math, but IIRC it shifts the brake bias around 7% forward (roughly). Sure RS calipers would be better but is it worth the price? Perhaps, but not for me.
I actually bought new higher Pressure Limiting valves (for the TT) for like $80 each(?) from Porsche, but I'm not bothered enough to even install them. Maybe someday.
I might be seeing higher brake pad wear in the front now, but it's not too significant.
I actually bought new higher Pressure Limiting valves (for the TT) for like $80 each(?) from Porsche, but I'm not bothered enough to even install them. Maybe someday.
I might be seeing higher brake pad wear in the front now, but it's not too significant.
compare a n/a to RS 1.572 to 1.472 is ~6.7% more rear in an RS, whicj is why youwant to limit forward weight transfer and use an effective lsd
compare a tt to RS 2.061 to 1.472, there is ~40% more front in a tt
the 993 n/a actually has a little more front bias due to the slightly larger front rotor compared to rear rotor diameter. the tt and RS have the same rotor diameter f/r so.
W/ any of these the big issue for track guys is rotor cracking, the best available solutions all involve non drilled 322mm rotors