Rear brake bias options
11-06-2012, 03:49 PM
#1
Rennlist Member
Thread Starter
Rear brake bias options
So a buddy and I were recently chatting about options for modifying brake bias to the rear of the car.
The standard route that grassroots racers take is simply running a different pad compound in the rear.
I've never liked this; if possible I've always preferred to install some kind of bias adjustment.
Question 1: What is the stock brake line routing on the 951? I'm assuming it's not front-rear, like with race cars, but rather cross-connected (i.e., LF<-->RR, RF<-->LR).
Regardless, I've been toying with installing a SS needle valve to adjust flow to the rear. What am I missing here? Obviously, if the brake routing is x-ways and not Front-Rear, then this is a more annoying undertaking.
Just thinking out loud.
Or maybe I'll just say "to hell with it" and go full Tilton.
The standard route that grassroots racers take is simply running a different pad compound in the rear.
I've never liked this; if possible I've always preferred to install some kind of bias adjustment.
Question 1: What is the stock brake line routing on the 951? I'm assuming it's not front-rear, like with race cars, but rather cross-connected (i.e., LF<-->RR, RF<-->LR).
Regardless, I've been toying with installing a SS needle valve to adjust flow to the rear. What am I missing here? Obviously, if the brake routing is x-ways and not Front-Rear, then this is a more annoying undertaking.
Just thinking out loud.
Or maybe I'll just say "to hell with it" and go full Tilton.
11-07-2012, 08:32 AM
#2
Drifting
While I'm not sure if the '86 non-ABS cars have it, the ABS cars have a rear proportioning valve that sits on top of the ABS pump. Apparently it's interchangable with valves from 928, 911 turbos, etc., which were offered in various levels of force...
11-07-2012, 09:51 AM
#3
Rennlist Member
Line routing off the master cylinder - 3 channels, LF, RF, and rear. The rear T's near the torsion bar carrier and splits to the LR and RR.
944 Turbos used an 18 bar rear brake bias/proportioning valve, right on the master cylinder for non-ABS cars. If you want more rear bias, you can use the 944S 33 bar valve. Don't go to a higher rear bias than that if using stock brake calipers on all 4 corners. Rears will start to lock first, which is bad...
If you want to play w/ adjustable bias, buy an actual adjustable brake bias valve (willwood, tilton, etc). Brake hydraulics work on line pressure, not flow. You do not want to use a needle valve, as those do not control pressure. An adjustable brake bias valve is actually a rather complicated decreasing rate pressure regulator, where it has a proportional decrease output to a rising pressure input. For instance, the valve can be bought w/ a given reduction rate, say 50%. And the adjustment allows increasing or decreasing the set point or knee point, which is a set limit at which the reduction begins.
The factory non-adjustable bias valves all have a 50% reduction rate, and the set point or knee point is how the valve is rated, e.g. 18 bar, 33 bar, 45 bar. For instance - W/ an 18 bar valve, equal line pressure front to rear up to 18 bar, then above 18 bar the rear line pressure is reduced at a 50% proportional rate to the front line pressure, so if the front brake pressure is 50 bar, the rear would be 34 bar. W/ a 33 bar valve, when the front pressure is 50 bar, the rear would be 41.5 bar.
944 Turbos used an 18 bar rear brake bias/proportioning valve, right on the master cylinder for non-ABS cars. If you want more rear bias, you can use the 944S 33 bar valve. Don't go to a higher rear bias than that if using stock brake calipers on all 4 corners. Rears will start to lock first, which is bad...
If you want to play w/ adjustable bias, buy an actual adjustable brake bias valve (willwood, tilton, etc). Brake hydraulics work on line pressure, not flow. You do not want to use a needle valve, as those do not control pressure. An adjustable brake bias valve is actually a rather complicated decreasing rate pressure regulator, where it has a proportional decrease output to a rising pressure input. For instance, the valve can be bought w/ a given reduction rate, say 50%. And the adjustment allows increasing or decreasing the set point or knee point, which is a set limit at which the reduction begins.
The factory non-adjustable bias valves all have a 50% reduction rate, and the set point or knee point is how the valve is rated, e.g. 18 bar, 33 bar, 45 bar. For instance - W/ an 18 bar valve, equal line pressure front to rear up to 18 bar, then above 18 bar the rear line pressure is reduced at a 50% proportional rate to the front line pressure, so if the front brake pressure is 50 bar, the rear would be 34 bar. W/ a 33 bar valve, when the front pressure is 50 bar, the rear would be 41.5 bar.
11-07-2012, 10:15 AM
#4
Rennlist Junkie Forever
So a buddy and I were recently chatting about options for modifying brake bias to the rear of the car.
The standard route that grassroots racers take is simply running a different pad compound in the rear.
I've never liked this; if possible I've always preferred to install some kind of bias adjustment.
Question 1: What is the stock brake line routing on the 951? I'm assuming it's not front-rear, like with race cars, but rather cross-connected (i.e., LF<-->RR, RF<-->LR).
Regardless, I've been toying with installing a SS needle valve to adjust flow to the rear. What am I missing here? Obviously, if the brake routing is x-ways and not Front-Rear, then this is a more annoying undertaking.
Just thinking out loud.
Or maybe I'll just say "to hell with it" and go full Tilton.
The standard route that grassroots racers take is simply running a different pad compound in the rear.
I've never liked this; if possible I've always preferred to install some kind of bias adjustment.
Question 1: What is the stock brake line routing on the 951? I'm assuming it's not front-rear, like with race cars, but rather cross-connected (i.e., LF<-->RR, RF<-->LR).
Regardless, I've been toying with installing a SS needle valve to adjust flow to the rear. What am I missing here? Obviously, if the brake routing is x-ways and not Front-Rear, then this is a more annoying undertaking.
Just thinking out loud.
Or maybe I'll just say "to hell with it" and go full Tilton.
If this is a street car, you do not want to use an adjustable bias valve. The reason being is that a adjustable bias valve will always be cutting pressure to the rear brake circuit no matter the pedal pressure. In other words... you will never get 100% pressure to the rear brakes.
The stock setup on the other hand, as mentioned above, has a bias valve (really should be called something else BTW) that gives full pressure to the rear brake circuit until a x amount of pressure is reached. At that point, it cuts pressure at an approximate 50% ratio. The difference in the factory valves at what pressure the valve kicks in.
On a race car, pretty much every time you use the brakes it's close to 100% braking with hot sticky tires on a track with rubber on it.
On a street car you almost never brake at 100%, and when you do brake it's on a very wide variety of road surface conditions .
You put a bias valve in the street car and set it up so that you don't get rear lock at 100% braking and you will be getting front wheel lock in low/medium traction conditions for sure.
TonyG
11-07-2012, 10:30 AM
#5
Rennlist Member
Thread Starter
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.
Line routing off the master cylinder - 3 channels, LF, RF, and rear. The rear T's near the torsion bar carrier and splits to the LR and RR.
944 Turbos used an 18 bar rear brake bias/proportioning valve, right on the master cylinder for non-ABS cars. If you want more rear bias, you can use the 944S 33 bar valve. Don't go to a higher rear bias than that if using stock brake calipers on all 4 corners. Rears will start to lock first, which is bad...
If you want to play w/ adjustable bias, buy an actual adjustable brake bias valve (willwood, tilton, etc). Brake hydraulics work on line pressure, not flow. You do not want to use a needle valve, as those do not control pressure. An adjustable brake bias valve is actually a rather complicated decreasing rate pressure regulator, where it has a proportional decrease output to a rising pressure input. For instance, the valve can be bought w/ a given reduction rate, say 50%. And the adjustment allows increasing or decreasing the set point or knee point, which is a set limit at which the reduction begins.
The factory non-adjustable bias valves all have a 50% reduction rate, and the set point or knee point is how the valve is rated, e.g. 18 bar, 33 bar, 45 bar. For instance - W/ an 18 bar valve, equal line pressure front to rear up to 18 bar, then above 18 bar the rear line pressure is reduced at a 50% proportional rate to the front line pressure, so if the front brake pressure is 50 bar, the rear would be 34 bar. W/ a 33 bar valve, when the front pressure is 50 bar, the rear would be 41.5 bar.
944 Turbos used an 18 bar rear brake bias/proportioning valve, right on the master cylinder for non-ABS cars. If you want more rear bias, you can use the 944S 33 bar valve. Don't go to a higher rear bias than that if using stock brake calipers on all 4 corners. Rears will start to lock first, which is bad...
If you want to play w/ adjustable bias, buy an actual adjustable brake bias valve (willwood, tilton, etc). Brake hydraulics work on line pressure, not flow. You do not want to use a needle valve, as those do not control pressure. An adjustable brake bias valve is actually a rather complicated decreasing rate pressure regulator, where it has a proportional decrease output to a rising pressure input. For instance, the valve can be bought w/ a given reduction rate, say 50%. And the adjustment allows increasing or decreasing the set point or knee point, which is a set limit at which the reduction begins.
The factory non-adjustable bias valves all have a 50% reduction rate, and the set point or knee point is how the valve is rated, e.g. 18 bar, 33 bar, 45 bar. For instance - W/ an 18 bar valve, equal line pressure front to rear up to 18 bar, then above 18 bar the rear line pressure is reduced at a 50% proportional rate to the front line pressure, so if the front brake pressure is 50 bar, the rear would be 34 bar. W/ a 33 bar valve, when the front pressure is 50 bar, the rear would be 41.5 bar.
Tilton Adjustable Proportioning Valve (****-type)
If this is a street car, you do not want to use an adjustable bias valve. The reason being is that a adjustable bias valve will always be cutting pressure to the rear brake circuit no matter the pedal pressure. In other words... you will never get 100% pressure to the rear brakes.
The stock setup on the other hand, as mentioned above, has a bias valve (really should be called something else BTW) that gives full pressure to the rear brake circuit until a x amount of pressure is reached. At that point, it cuts pressure at an approximate 50% ratio. The difference in the factory valves at what pressure the valve kicks in.
On a race car, pretty much every time you use the brakes it's close to 100% braking with hot sticky tires on a track with rubber on it.
On a street car you almost never brake at 100%, and when you do brake it's on a very wide variety of road surface conditions .
You put a bias valve in the street car and set it up so that you don't get rear lock at 100% braking and you will be getting front wheel lock in low/medium traction conditions for sure.
TonyG
The stock setup on the other hand, as mentioned above, has a bias valve (really should be called something else BTW) that gives full pressure to the rear brake circuit until a x amount of pressure is reached. At that point, it cuts pressure at an approximate 50% ratio. The difference in the factory valves at what pressure the valve kicks in.
On a race car, pretty much every time you use the brakes it's close to 100% braking with hot sticky tires on a track with rubber on it.
On a street car you almost never brake at 100%, and when you do brake it's on a very wide variety of road surface conditions .
You put a bias valve in the street car and set it up so that you don't get rear lock at 100% braking and you will be getting front wheel lock in low/medium traction conditions for sure.
TonyG
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, 10:43 AM
#6
Drifting
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.
Actually, I'm surprised you don't overwhelm those skinny front tires when trail braking and understeer before the rears lock...
11-07-2012, 11:07 AM
#7
Rennlist Member
Thread Starter
Hmm, sounds opposite to what my track friends have experienced, which is too little rear bias (in the ABS cars, at least). I don't seem to have that problem trail braking and actually would like more rear bias to help the car rotate.
Actually, I'm surprised you don't overwhelm those skinny front tires when trail braking and understeer before the rears lock...
Actually, I'm surprised you don't overwhelm those skinny front tires when trail braking and understeer before the rears lock...
One possibility is that the previous owner installed a larger bias valve. How does one tell? For the record, the car is an '86 non-ABS.
Suspension setup is 350# front springs and 30mm rear T-bars with Koni yellows all around.
Don't get me wrong, I do understeer -- and I regularly use the rear bias to help rotate the car. That said, relying on brake "unbalance" to rotate a car is a bandaid, not a solution. I would prefer to increase the front tires to 245's and/or stiffening my rear bar and/or providing a little more toe-out in the rear.
The benefit of an adjustable valve is that when I make these changes elsewhere in the car, I can compensate for any brake imbalance that may result.
Gets a bit disconcerting flying into T1 at Summit and being concerned the rear will come out. I could use a little more balance through T3 as well. Or, the turn before the Bridge Straight at Shenandoah -- rotating a bit too much and have to be careful.
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11-07-2012, 11:48 AM
#8
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.
11-07-2012, 11:50 AM
#9
Rennlist Member
Thread Starter
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.
...So what your'e saying is...become a better drover.
11-07-2012, 11:53 AM
#10
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I ran a 5/33 valve on stock turbo brakes on my 86 951 track car and the brake bias was excellent. Simple and easy, no reason to over complicate things.
11-07-2012, 12:12 PM
#12
Nordschleife Master
If interested, i have a brand new 5/33 valve i never installed that i dont need anymore.
I will be using a Tilton bias valve with my LS1, Ford Hydroboost setup.
PM me and ill figure out a reasonable price shipped.
I will be using a Tilton bias valve with my LS1, Ford Hydroboost setup.
PM me and ill figure out a reasonable price shipped.
11-07-2012, 12:26 PM
#13
Rennlist Member
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.
http://www.wilwood.com/MasterCylinde...derValves.aspx
His point is that these aftermarket adjustable bias valves are not a good application for a street car. And should only be considered for use on a dedicated track car.
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).
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).
The only reason pad thickness should have an impact on “bite” is when they are thinned to the point that they overheat quicker (due to loss of material mass to absorb/dissipate heat) - the coefficient of friction of the pad material drops off as the temp increases. What are you using for brake cooling?
Suspension setup is 350# front springs and 30mm rear T-bars with Koni yellows all around.
Don't get me wrong, I do understeer -- and I regularly use the rear bias to help rotate the car. That said, relying on brake "unbalance" to rotate a car is a bandaid, not a solution. I would prefer to increase the front tires to 245's and/or stiffening my rear bar and/or providing a little more toe-out in the rear.
Gets a bit disconcerting flying into T1 at Summit and being concerned the rear will come out. I could use a little more balance through T3 as well. Or, the turn before the Bridge Straight at Shenandoah -- rotating a bit too much and have to be careful.
You do NOT want toe-out in the rear, that will cause all sorts of braking instability problems.
11-07-2012, 12:46 PM
#14
Rennlist Member
Thread Starter
The pistons are only moving hundredths, maybe only thousandths of an inch when the brakes are applied. So that will generate maybe only an inch or so (?) of back and forth fluid movement w/in a brake line. That is not a practical application for a needle valve (or any type of flow control valve). And for arguments sake, if there was sufficient flow to be controlled by a flow control valve, all that would accomplish would be to delay the system response. It would take longer for pressure to build in the rear line, but it would still get to the same pressure as the front. And when the brake pedal was released, it would take longer for the rear line pressure to drop – this would be bad. But again, in reality, there is not enough flow to control by that method.
As Tony mentioned, apparently these aftermarket valves (like the tilton you posted or the wilwood linked below) do just adjust the reduction rate, they do not offer an initial set point (which is a feature of the factory prop valve).
http://www.wilwood.com/MasterCylinde...derValves.aspx
His point is that these aftermarket adjustable bias valves are not a good application for a street car. And should only be considered for use on a dedicated track car.
http://www.wilwood.com/MasterCylinde...derValves.aspx
His point is that these aftermarket adjustable bias valves are not a good application for a street car. And should only be considered for use on a dedicated track car.
A 951 should seldom lock the rear brakes first w/ stock calipers and a stock bias valve. Same as Doc, I ran the higher 33 bar valve on my stock class 951 for many years w/o a rear lockup issue, so I am surprised you would have much problem w/ a stock 18 bar valve. Do check what valve is installed on the car – located on the master cylinder. They are stamped with a number set, 5/18, 5/33.
Furthermore, I'll be replacing the master cylinder, which is definitely nearing the end of its life.
Based on everyone's input, I'll hold off on installing any kind of valving until after I've testing the car with a new MC.
The only reason pad thickness should have an impact on “bite” is when they are thinned to the point that they overheat quicker (due to loss of material mass to absorb/dissipate heat) - the coefficient of friction of the pad material drops off as the temp increases. What are you using for brake cooling?
Cooling is full brake hoses via custom ducts that pull air from the factory duct location. 968 hose adapters on on the rotor. Never had a problem with brakes overheating with this setup, though as you mentioned when at the end of my pads' life they do get squishier.
In addition to a lower absolute heat capacity, the pad chemistry actually changes.
Are you actually smoking or flat spotting rear tires? When you are talking about “balance through” a turn, that sounds more like an oversteer issue, not a brake bias problem. And your 350 lb/in w/ 30mm t-bars is potentially a tail happy setup.
You do NOT want toe-out in the rear, that will cause all sorts of braking instability problems.
You do NOT want toe-out in the rear, that will cause all sorts of braking instability problems.
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.
Thanks for all the input --- really appreciate it. I'm still not convinced I shouldn't install an adjustable valve. If I had the funds, everything chassis-wise would be adjustable. I'm slowly getting there.
11-07-2012, 12:49 PM
#15
Rennlist Member
Thread Starter
Good luck with the "new" car -- that thing should be awesome.