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Dual brake master cylinders as in 2 anyone do it?
- Thread starter Toyanvil
- Start date
Barry
Diamond Level Sponsor
As I see it, your front end will have 10" rotors and 3.55 square inches of piston area and the back end will have 10" rotors with 1.47 square inches of piston area. At the same hydraulic pressure and with the same pad compound, that would work out to about 70% front / 30% rear braking force.
That is a pretty high front-to-rear bias, so I am not sure you will need to reduce the rear hydraulic pressure / braking force even during maximum braking on a dry surface.
Having said that, the only way to be sure is to try it with your car / suspension / tires / roads and see what happens.
Please let us know the results.
That is a pretty high front-to-rear bias, so I am not sure you will need to reduce the rear hydraulic pressure / braking force even during maximum braking on a dry surface.
Having said that, the only way to be sure is to try it with your car / suspension / tires / roads and see what happens.
Please let us know the results.
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Barry
Diamond Level Sponsor
Thought this might be interesting to some forum participants.
Achieving optimum brake balance has always been a challenge because the braking system requirements are vastly different for various conditions.
One extreme is a maximum effort stop on a high traction surface with “sticky” tires. A lot of weight is transferred to the front tires and very little rear brake force is needed (or can be tolerated). The optimum braking force distribution is probably close to 80% front / 20% rear.
The other extreme is light braking, especially on a low traction surface. There is very little weight transfer and the braking force should ideally be only a little more in the front than in the rear. The optimum braking force distribution is something like 55% front / 45% rear.
Front lock-up makes the car go straight ahead and rear lock-up makes the car go out of control. Both are bad, but out of control is worse, so the normal goal (for a non-ABS system) is to have the front brakes lock-up a little before the rears.
ABS is a very effective way to prevent brake lock-up under almost any situation, but it is very complex and very expensive (a basic Bosch Motorsport aftermarket ABS package starts at about $7,000).
Other than ABS, the best approach is for the brake system to inherently generate about 55-60% front braking force versus 40-45 % rear braking force during light braking and do something to reduce overall rear braking force (and rear lock-up) during hard stops. Various methods have been used, but the only feasible solution for the typical car builder is an “adjustable proportioning valve” which does very little / nothing during a light to moderate stop (low hydraulic pressure), but reduces the hydraulic pressure to the rear brakes during a hard stop (high hydraulic pressure). Most of the suppliers of adjustable proportioning valves say something like “up to XX% pressure reduction.” The odds of any given proportioning valve exactly matching the needs of a given car under all braking conditions are slim-to-none, but it is the best game in town. The proportioning valve adjustment can only be optimum for one condition and that is typically a maximum effort stop from highway speed.
If you are determined to "improve" the Alpine brake system, start by selecting calipers and rotors that will result in 55-60% of the total braking effort at the front end. Relative hydraulic pressure, relative piston area, relative rotor diameter and relative pad compound influence front-to-rear balance, but relative pad area does not. Bigger pads typically last longer and may have greater fade resistance, but they do not inherently generate more braking force. Then use an adjustable proportioning valve to reduce rear braking force during hard braking. Testing and adjusting the brake balance during hard (wheel / tire lock-up) braking is part of the process.
Just my experience and opinion, YMMV.
Achieving optimum brake balance has always been a challenge because the braking system requirements are vastly different for various conditions.
One extreme is a maximum effort stop on a high traction surface with “sticky” tires. A lot of weight is transferred to the front tires and very little rear brake force is needed (or can be tolerated). The optimum braking force distribution is probably close to 80% front / 20% rear.
The other extreme is light braking, especially on a low traction surface. There is very little weight transfer and the braking force should ideally be only a little more in the front than in the rear. The optimum braking force distribution is something like 55% front / 45% rear.
Front lock-up makes the car go straight ahead and rear lock-up makes the car go out of control. Both are bad, but out of control is worse, so the normal goal (for a non-ABS system) is to have the front brakes lock-up a little before the rears.
ABS is a very effective way to prevent brake lock-up under almost any situation, but it is very complex and very expensive (a basic Bosch Motorsport aftermarket ABS package starts at about $7,000).
Other than ABS, the best approach is for the brake system to inherently generate about 55-60% front braking force versus 40-45 % rear braking force during light braking and do something to reduce overall rear braking force (and rear lock-up) during hard stops. Various methods have been used, but the only feasible solution for the typical car builder is an “adjustable proportioning valve” which does very little / nothing during a light to moderate stop (low hydraulic pressure), but reduces the hydraulic pressure to the rear brakes during a hard stop (high hydraulic pressure). Most of the suppliers of adjustable proportioning valves say something like “up to XX% pressure reduction.” The odds of any given proportioning valve exactly matching the needs of a given car under all braking conditions are slim-to-none, but it is the best game in town. The proportioning valve adjustment can only be optimum for one condition and that is typically a maximum effort stop from highway speed.
If you are determined to "improve" the Alpine brake system, start by selecting calipers and rotors that will result in 55-60% of the total braking effort at the front end. Relative hydraulic pressure, relative piston area, relative rotor diameter and relative pad compound influence front-to-rear balance, but relative pad area does not. Bigger pads typically last longer and may have greater fade resistance, but they do not inherently generate more braking force. Then use an adjustable proportioning valve to reduce rear braking force during hard braking. Testing and adjusting the brake balance during hard (wheel / tire lock-up) braking is part of the process.
Just my experience and opinion, YMMV.
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Barry, it seems to me your very accurate explanation make the case for a pressure regulator instead of proportioning valve. Design the brakes components so the rears are oversized. Adjust the regulator to limit rear pressure so the rears never lock up. The regulator would do nothing until pressures became high enough to lock the rear brakes. That would allow increased rear braking under all conditions except maximum braking conditions, which would be equal to braking using a conventional proportioning valve.
I read about such a setup 20 years ago and see nothing wrong with this approach. Do you? Also, do you know where such a regulator could be found?
Bill
I read about such a setup 20 years ago and see nothing wrong with this approach. Do you? Also, do you know where such a regulator could be found?
Bill
I think I'd be taking Berry's advise (from what I read here) put it together and see what you have , here's some info as to what we did front calipers 1.75x2 rear calipers 1.375x2 10.250 discs all around master cylinder 1" single channel this combo was picked by Wilwood way back when we converted the original Tiger brake system, on the new car we're building were using the std Ford calipers and 11.3 Rotors (on the rear) and a 4 piston (1.75) front caliper with an 11.75 rotor and single channel master 1.062 bore we have used this before on two cars and it works just fine. as to the duel master not much gained there except piece of mind, jmho. again I think I'd put it together and see what you get. and I suppose if you really want to get trick get some pressure gauges an see what you really have.
Barry
Diamond Level Sponsor
Bill,
Before addressing pressure regulators, we need to discuss hydraulic pressure in brake systems. On a reasonably modern car, the maximum hydraulic pressure at the master cylinder probably never exceeds 1000 PSI and even a normal hard stop probably does not exceed 500 PSI. With modern power brakes, 500 PSI line pressure might require a pedal force of only 25 lbs. On a non-boosted Alpine with a 4:1 ratio brake pedal lever and a 0.75" master cylinder, it takes a pedal force of about 55 lbs. to get 500 PSI line pressure which is why drivers used to modern cars say the Alpine brakes feel "hard."
In order to have reasonably balanced braking effort for light / moderate braking, the braking effort distribution needs to be something like 60% front / 40% rear. For the same diameter rotors / pad compounds, that means the front caliper piston area needs to be about 50% greater than the rear caliper piston area. This relative caliper piston area works fine for light braking, but will almost certainly cause rear brake lock-up during hard braking.
For hard braking which results in significant weight transfer, the braking effort distribution needs to be something like 75% front / 25% rear. For the same diameter rotors / pad compounds, that would require the front caliper piston area to be about three times the rear caliper piston area. A 75 / 25 setup is probably OK for a race car where hard braking is normal and light braking is not a concern. During light braking with a 75 / 25 setup, the rear brakes would be doing almost nothing.
There is no way to balance the brake system for both light and heavy braking with relative caliper piston area alone. Something must be done to make the braking effort distribution different during light and heavy braking. That "something" is usually a proportioning valve which reduces the pressure to the rear brakes during hard braking to prevent rear brake lock-up.
Car makers have used proportioning valves for a long time under the name combination valve. A combination valve is simply a fixed proportioning valve combined with a differential pressure switch that turns on a warning light if the front / rear hydraulic pressure is wacky. The car makers use fixed proportioning valves to eliminate potential legal liability resulting from improper adjustment. They do a LOT of testing / development and manufacture a different combination valve for just about every platform. For "one-off" street rods built by individuals without the resources to test / develop / manufacture a vehicle specific fixed proportioning valve, the solution is an adjustable proportioning valve.
An Adjustable Proportioning Valve (APV) is a pressure regulator, but unlike a typical air or water pressure regulator which is preset to a fixed output pressure, the output pressure is a relatively constant proportion (aka, percentage) of the input pressure. The pressure reduction is typically about 50% (+/- about 10%); what the adjustment does is change the input pressure at which the output pressure starts to be reduced. Independent measurements for Wilwood APV's indicate that the "knee of the curve" can be as low as about 100 PSI and as high as about 1,000 PSI.
It seems to me that a single output value pressure regulator could be used to limit maximum rear brake effort, but it would only be optimum for one braking condition (presumably a maximum effort stop on dry pavement ???). The advantage of the proportional pressure regulator is that it can provide near optimum rear brake effort over a relatively wide range of braking conditions.
Just my opinion and you know the saying about opinions; YMMV.
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Barry
Diamond Level Sponsor
The setup you described with 1.75" x 2 piston front calipers, 1.375" x 2 piston rear calipers and 10.25" disks all around has a calculated braking force distribution of 61.5% front and 38.5% rear. Pretty durn close to 60 / 40.
That combination should be about right for light to moderate braking, but I suspect that it would result in rear brake lock-up during a really hard stop. Seems to me that an adjustable proportioning valve would be a good idea, but that's just my opinion and YMMV.
Barry, I think this statement is what I am having trouble with:
The advantage of the proportional pressure regulator is that it can provide near optimum rear brake effort over a relatively wide range of braking conditions.
I Guess I don't understand the meaning of "optimum rear brake effort". If it has been determined the rears can accept, say, 500 lbs of rear pressure (set regulator pressure) under the worst conditions, and the driver creates 500lbs braking pressure on the front brakes, what is gained by limiting it to say, 400lbs with a proportioning valve?
If 500 lbs won't create lock up, isn't 500lbs more optimum than 400 lbs?
Bill
The advantage of the proportional pressure regulator is that it can provide near optimum rear brake effort over a relatively wide range of braking conditions.
I Guess I don't understand the meaning of "optimum rear brake effort". If it has been determined the rears can accept, say, 500 lbs of rear pressure (set regulator pressure) under the worst conditions, and the driver creates 500lbs braking pressure on the front brakes, what is gained by limiting it to say, 400lbs with a proportioning valve?
If 500 lbs won't create lock up, isn't 500lbs more optimum than 400 lbs?
Bill
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Barry
Diamond Level Sponsor
Bill,
Been doing some hard thinking about your "rear brake pressure limiting regulator" concept.
For the sake of discussion, let's assume that:
It is clear that the rear braking effort must be reduced to prevent rear lock-up during hard braking; the question is how.
I have always believed that a basic 60/40 braking effort distribution plus an adjustable proportioning valve was the best available combination for a high performance street car. This discussion forces me to conclude that a basic 70/30 braking effort distribution plus an adjustable proportioning valve would be a better mousetrap. Certainly not optimum during light / moderate braking, but probably "as good as it gets" for a non-ABS system during extremely hard braking.
It seems to me that the ideal system would provide input / output PSI values of something like 100/100, 200/200, 300/300, 400/300, 500/250 and 600/200. I have no idea how those completely non-linear input / output values could be achieved with a reasonably simple mechanical system.
Not sure where to go from here.
Been doing some hard thinking about your "rear brake pressure limiting regulator" concept.
For the sake of discussion, let's assume that:
The front and rear rotors are the same diameter.
The front caliper piston area is 4.8 square inches.
The rear caliper piston area is 3.2 square inches.
The hydraulic pressure required to lock-up the front brakes during a hard stop is 600 PSI.
The rear brakes lock-up at 225 PSI during a hard stop.
The front caliper piston area is 4.8 square inches.
The rear caliper piston area is 3.2 square inches.
The hydraulic pressure required to lock-up the front brakes during a hard stop is 600 PSI.
The rear brakes lock-up at 225 PSI during a hard stop.
It is clear that the rear braking effort must be reduced to prevent rear lock-up during hard braking; the question is how.
The simplest approach would be to reduce the rear caliper piston area to about 1.0 square inch. That should prevent rear brake lock-up during any conditions, but would result in rear brakes that don't contribute much during light / medium braking.
Another approach would be to keep the rear caliper with a piston area of 3.2 square inches and use your proposed "rear brake pressure limiting regulator" device that would limit the rear hydraulic pressure to about 200 PSI. That should also prevent rear brake lock-up under any conditions, but would also result in rear brakes that don't contribute much during light / medium braking.
Another approach would be to keep the rear caliper with a piston area of 3.2 square inches and use your proposed "rear brake pressure limiting regulator" device that would limit the rear hydraulic pressure to about 200 PSI. That should also prevent rear brake lock-up under any conditions, but would also result in rear brakes that don't contribute much during light / medium braking.
The third approach would be to use an adjustable proportioning valve that would limit the rear hydraulic pressure to about 200 PSI during a hard stop when the system pressure was 600 PSI. The available data suggest that an adjustable Wilwood (or Tilton, SSBC, Summit, etc.) proportioning valve cannot be set to reduce an input pressure of 600 PSI to an output pressure of 200 PSI. It appears that the best that can be achieved is input / output pressure values of about 100/100, 200/150, 300/200, 400/233, 500/266 and 600/300. Those output values would not prevent rear brake lock-up during hard braking and would still result in rear brakes that don't contribute much during light / medium braking.
My conclusions are that:
All of the obvious approaches to reducing rear braking force / preventing rear brake lock-up result in rear braking force that is less than optimum during light / medium braking.
Your proposed "rear brake pressure limiting regulator" device would be at least as good as the available "adjustable proportioning valves" and would probably be somewhat better during light / moderate braking. A suitable "rear brake pressure limiting regulator" would have to be compatible with DOT 3/4/5.1 brake fluid, able to handle at least 1,000 PSI input pressure and be "internally self-relieving." It would ideally be adjustable. I don't remember ever seeing such a gizmo and don't know where to start looking.
Your proposed "rear brake pressure limiting regulator" device would be at least as good as the available "adjustable proportioning valves" and would probably be somewhat better during light / moderate braking. A suitable "rear brake pressure limiting regulator" would have to be compatible with DOT 3/4/5.1 brake fluid, able to handle at least 1,000 PSI input pressure and be "internally self-relieving." It would ideally be adjustable. I don't remember ever seeing such a gizmo and don't know where to start looking.
I have always believed that a basic 60/40 braking effort distribution plus an adjustable proportioning valve was the best available combination for a high performance street car. This discussion forces me to conclude that a basic 70/30 braking effort distribution plus an adjustable proportioning valve would be a better mousetrap. Certainly not optimum during light / moderate braking, but probably "as good as it gets" for a non-ABS system during extremely hard braking.
It seems to me that the ideal system would provide input / output PSI values of something like 100/100, 200/200, 300/300, 400/300, 500/250 and 600/200. I have no idea how those completely non-linear input / output values could be achieved with a reasonably simple mechanical system.
Not sure where to go from here.
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Toyanvil
Gold Level Sponsor
Wow that's a lot of math, this is why I let Wilwood do it 
I am still waiting to see if my F10 M/C gets here, not in stock at the factory. Based on the Wilwood tech, if I run one tandem M/C with the Sunbeam pedal 3/4" M/C is good for the front but to much pressure for the back and needs the proportioning valve or the back will lock up first. If I run two M/Cs, I need a 3/4 for the front and a 7/8 in the rear and a balance bar to adjust bias. It's all in the size of the M/C smaller bore has more pressure less volume, larger bore has less pressure more volume. Tire size, wheel base, vehicle weight and vehicle weight distribution also had to be factor in. I am hoping to install the F10 M/C with proportioning valve and go from there.
I am still waiting to see if my F10 M/C gets here, not in stock at the factory. Based on the Wilwood tech, if I run one tandem M/C with the Sunbeam pedal 3/4" M/C is good for the front but to much pressure for the back and needs the proportioning valve or the back will lock up first. If I run two M/Cs, I need a 3/4 for the front and a 7/8 in the rear and a balance bar to adjust bias. It's all in the size of the M/C smaller bore has more pressure less volume, larger bore has less pressure more volume. Tire size, wheel base, vehicle weight and vehicle weight distribution also had to be factor in. I am hoping to install the F10 M/C with proportioning valve and go from there.
Ken Ellis
Donation Time
Barry wrote: "I have no idea how those completely non-linear input / output values could be achieved with a reasonably simple mechanical system."
I wrote and erased two possibilities for at least the first four cases, because they were either doable with a basic regulator, or too sketchy to put on a brake system. Electronically-driven systems are easier, if brake-rated assemblies can be procured with appropriate failure modes. Reliable electronics would be the remaining challenge.
I wonder if vacuum-based power brake boosters could be modified to drive the rear circuit, based on modulated vacuum...
Or perhaps a second adjustable proportioning valve powering a cylinder that slides the mounting location of the rear master cylinder enough to effectively decrease the pressure on that cylinder when the main pressure gets to the target number. Still awfully squirrely for a braking system, though.
I wrote and erased two possibilities for at least the first four cases, because they were either doable with a basic regulator, or too sketchy to put on a brake system. Electronically-driven systems are easier, if brake-rated assemblies can be procured with appropriate failure modes. Reliable electronics would be the remaining challenge.
I wonder if vacuum-based power brake boosters could be modified to drive the rear circuit, based on modulated vacuum...
Or perhaps a second adjustable proportioning valve powering a cylinder that slides the mounting location of the rear master cylinder enough to effectively decrease the pressure on that cylinder when the main pressure gets to the target number. Still awfully squirrely for a braking system, though.
Barry
Diamond Level Sponsor
Wow that's a lot of math, this is why I let Wilwood do it.
The math is the easy part; the hard part is the assumptions about traction, deceleration rates, center of gravity, weight transfer, etc. Like all assumptions, they can be very misleading (or worse).
Even the car makers start with assumptions when they need a brake system for a new platform. The difference is that they have lots of historical data and computer modeling to narrow the assumptions and then they invest a HUGE amount of time, resources and money to test and refine the systems.
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Not sure where to go from here.
I did a quick search on "brake pressure regulators". A few items popped up, one even appeared to be adjustable. The problem is that "pressure regulator" and "proportioning valve" are often used interchangeably. I also began to think that some of the OEM proportioning valves are really pressure regulators. It does help that I am not familiar with OEM braking systems.
I think I will try to find the paper Barry (from Texas) sent to me about 20 years ago. I'm pretty sure that's where I read this. Who knows, I still may have it.
Bill
I did a quick search on "brake pressure regulators". A few items popped up, one even appeared to be adjustable. The problem is that "pressure regulator" and "proportioning valve" are often used interchangeably. I also began to think that some of the OEM proportioning valves are really pressure regulators. It does help that I am not familiar with OEM braking systems.
I think I will try to find the paper Barry (from Texas) sent to me about 20 years ago. I'm pretty sure that's where I read this. Who knows, I still may have it.
Bill
Barry
Diamond Level Sponsor
You have a good memory!
GM used load sensing (actually rear suspension height sensing) proportioning valves on a lot of vehicles (not just pick-up trucks) in the 1980's and early 1990's. I had a 1982 Buck Century T-type with that system and it worked very well to adjust rear braking force on dry roads, but not so much on slick surfaces where there was not much weight transfer. IIRC correctly, they disappeared about the mid-1990's when ABS started to become mainstream. I suspect that the relatively stiff springs and limited suspension travel in a small sports car might make it hard to use suspension position sensing.
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I found one source, some pages from Fred Puhn's "Brake Handbook". He describes the pressure regulating valve, but dismisses it as "working well only on extremely nose heavy cars". No explanation why. There is something about me that won't allow me to take flat statements with no explanation as the truth. He also talks about ideal pressure to g force relationship curves that can only be achieved with proportioning valves and that every car needs them, even ones with dual master cylinders. Makes no sense to me. Seems to me the curves would be the inverse to what a proportioning valve produces. Just call me hardheaded Bill.
I'm pretty sure that at one time I had another reprint that went into some detail about pressure regulating valves and cars that had them. I think they were early disc brake Chevys, but I can't find that reprint.
Hardhead
I'm pretty sure that at one time I had another reprint that went into some detail about pressure regulating valves and cars that had them. I think they were early disc brake Chevys, but I can't find that reprint.
Hardhead
Barry
Diamond Level Sponsor
IIRC, the first Chevy disk brakes were on the '65 Corvette followed by full-size cars with performance packages in '67. The pressure regulating valve sounds like something that might have been tried on the Corvette.