Balance Bar

I took the pedal box out of the SLC to make some changes to the MC fittings to allow me to move the PB forward an extra inch. While I was at it I noticed the MCs were seeping a bit of fluid so took the opportunity to rebuild them. I see in some of the Tilton videos they recommend lengthening the front MC clevis an extra 1/4" to give the front brake some additional bias. I've got both the front and rear clevis set the same and the balance bar in the center of its adjustment and was wondering what other folks have theirs set at? Is anyone adding in the additional 1/4" as the Tilton video suggests or are you running the front/rear MC rods/clevis the same?

Thanks in advance for the help. Scott
 

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Terry Oxandale

Skinny Man
Mine are equal length on the pushrods, and the bias maybe is a turn or so off of center. I guess I need to look at Tilton's video. I don't understand the purpose unless the assumption is that there is more piston stroke on the front, so a full pressure, the front started "behind" the rear, at mid-pressure, is even with the rear, and at full pressure, has passed by the rear. In my case, I have the exact same calipers and rotors front and rear, and am very close to a centered balance bar with that, so again, not sure why they suggest that.
 
You need to have enough clearance between the front and rear Clevis and the center section so that the bias bar can rotate the full extent. If you loose fluid on one side, you need to have the bar rotate all the way until it hits a stop. Then the other side can produce pressure. If you don’t have any clearance, then the front and rear master cylinders will have the same stroke and will not bias pressure. In order to bias pressure, the bar must be free to rotate to have different strokes of the master cylinders. You should check if the bias bar can rotate the full amount before you connect the push rods. The mentioned 1/4” might be about right. The length of the push rods is not what determines the brake pressure bias.

-Bob Woods
 
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Terry Oxandale

Skinny Man
As a side note, I haven't got many miles on my project, yet I had to change out both Tilton MCs this year due to seals going bad (very slight leaking, but enough to soften the pedal). I'm concerned that with perhaps no more than 5,000 miles in the years I've had this car, that I'm dealing with this. The first one I attributed to having the pushrods non-parallel (if you drew a line through the centers of both pushrods, the lines would intersect several feet behind the driver). It wasn't much of an angle, and with the pivot on the pushrod, I would have assumed any minor angle of the pushrod would have been inconsequential, but when I examined the bore of the MC, it had abnormal wear on only one side of the bore. The other MC appeared to have no abnormal wear, and the seals looked good, so it was hard to determine why it was leaking.
 

Howard Jones

Supporter
I have a Tilton balance bar on my SLC. Running one of these is really not necessary on a street car. A tandem master with an adjustable brake proportioning valve in the rear brake circuit would be better IMHO.

Now having said that, balance bars take some time to work out. First, the master cylinder sizes must be pretty close to optimum. The balance bar systems are not intended to be a simple way to accommodate and compensate for the wrong size masters, just finely tune balance front to rear.
Second, they must be checked over a lot. I look at mine every time I get into the car. Little rocks and other debris can and will jam things up. This can be dangerous. Something stuck in the moving bits can and will prevent one or more of the masters from releasing pressure when you take your foot off the brakes.

So I would set the balance bar on the bench to completely centered, then lock it down to that position. Now pre/bench bleed the masters and plug the fluid outlets for both front and rear. Play with the balance bar on the bench and see how it works with pressure it it. Become familiar with the idiosyncrasy of these systems. They can be finicky. especially if they are adjusted into the overtravel range.

Since you are building the car you might try setting up the system on the bench with just one front and one rear caliper (block of wood in the place of the rotor). Now you can see how it all works. A lot can be learned from this, You will have a clear understanding of the adjustment linkage and its limitations.

Now once you have the entire system on the car you can work on the brake balance front to rear by experimenting with master sizes, brake pad materials. and even caliper sizes until the brakes are VERY good. Only ow you can use the balance bar to finely tune balance.

That's my story and I'm sticking to it.




 
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Terry Oxandale

Skinny Man
Okay, my assumption was correct in that "typically the front master cylinder is usually smaller than the rear, and feeds larger front calipers..." explains why they recommend the longer 1/4" pushrod length for the front. Being the optimal setup (according to a calculator I used when building this thing) calls for the same exact setup on all four wheels, the equal-length assembly would be fine.
 

Howard Jones

Supporter
Sorry, Terry, the front master bore size and thus its volume given an equal stroke is usually larger so that it CAN fill the larger front caliper's piston size versus the rear. The front calipers typically have a larger piston area so that they can produce a higher clamping force and thus a proportionally greater stopping force to the front of the car.

A good rule of thumb would be about 55-65% F versus 35-40% R of the total piston area of all the calipers will be in the front calipers for a mid-engined car.

Here's my SLC. Front 6 piston caliper currently 1.62,1.12,1.12 and the rear are 4 piston 1.25 1.25 for a piston area of F= 4.04 sq/in R=2.46 Sq/in or 62.2% on the front. The masters are F=.7 and the rear =.7 with about 10% of the total dialed out of the rears and balanced to the front. So effectively something like 60-40 F-R

I think I am going to try changing the fronts to 4-piston calipers 1.88, 1.62. That would be 4.84sq/in and taking out the dialed-out amount in the rear. This would result in 66.3% on the front. This may require a .75 front master but we will see.

Why? that is a much longer post.

But you can see that the front of the car requires the most brake effort and thus everything is bigger. In my case even the rotors with F13.06X1.25 R12.88X1.25


One last thing. Be careful with the push rod lengths. Remember no matter what you do you must allow the piston in a given master to retract back far enough to uncover the return port in the bore. Otherwise, the brakes will not release for that master. This is very easy to F up if you start screwing with pushrod lengths in the car and on the fly. I think for what it's worth it is better to keep push rods the same geometry and use piston sizes to set a good baseline balance with the balance bar mechanism set to a near-neutral configuration.

If you want to play with numbers then this calculator is very useful

 
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Neil

Supporter
Sorry, Terry, the front master bore size and thus its volume given an equal stroke is usually larger so that it CAN fill the larger front caliper's piston size versus the rear. The front calipers typically have a larger piston area so that they can produce a higher clamping force and thus a proportionally greater stopping force to the front of the car.

A good rule of thumb would be about 55-60% F versus 35-40% of the total piston area of all the calipers will be in the front calipers.

Here's my SLC front 6 piston caliper currently 1.62,1.12,1.12 and the rear are 4 piston 1.23 1.25 for a piston area of F= 4.04 sq/in R=2.46 Sq/in or 62.2% on the front. The masters are F=.7 and the rear =.7 with about 10-12% of the total dialed out of the rears and balanced to the front. So effectively something like 70-30 F-R

I think I am going to try changing the fronts to 4-piston caliper 1.88. 1,75. That would be 5.18sq/in and taking out the dialed-out amount in the rear. This would result in 67.18% on the front. This may require a .75 front master but we will see.

Why? that is a much longer post.

But you can see that the front of the car requires the most brake effort and thus everything is bigger. In my case even the rotors with F13.06X1.25 R12.88X1.25

Just to carry Howard's point a bit further... (probably to the point of tedium), a mid-engine car is a different animal from a conventional layout. It probably has 60- 65% of its total weight on its rear wheels so one might think it will need more brake pressure on the rear (given equal rotor sizes, brake pads, & caliper sizes). The weight distribution changes dramatically under deceleration, since the car CG is located above the ground level, and there is a weight transfer to the front tires (vectors can show this but it's fairly intuitive). For optimum braking, it is advantageous to distribute the braking 50% front & 50% rear under heavy braking. Generally, to achieve this, a static front bias is needed. Getting everything "dialed in" can take some effort as there are many factors that can be juggled to achieve the proper balance. As already mentioned, the variables can be M/C bore sizes, rotor diameters, caliper bore sizes,brake pad friction coefficient, brake balance bars, & a rear proportioning valve. It isn't a straightforward exercise.
 
Doesn't the smaller front MC diameter create more caliper pressure for the same foot pressure but requires a longer foot stroke. Installing a larger rear MC dia reduces rear braking, can be further reduced by brake bias bar, or did I miss something.
Andrew.
 

Howard Jones

Supporter
"smaller front MC diameter create more caliper pressure for the same foot pressure but requires a longer foot stroke" This is in fact true.

"Installing a larger rear MC dia reduces rear braking" is also true but only if the force applied is equal in both examples.

The problem becomes keeping exactly the same foot pressure and the exact same foot travel. This is why I believe it is best to start with the same size master's front and rear and a centered balance bar. A good starting point for both the masters would be .75. This will allow for varying available sizes of both larger and smaller if needed.

Then continue to design the rest of the system. Next caliper types will need to be selected as well as rotor diameters, This is done together. The rotor diameters more than likely will be as big as will fit in the wheels and then select a caliper that is designed to work with that diameter. The C6 corvette wheels on my SLC will accommodate 14-inch rotors in the front and 12.88 in the rear keeping the standard SLC lower A-Arms.

Now on to the caliper piston area. Here's where trial and error will become a factor. However, a ratio of piston area of about 5 for the fronts and about 3 for the rear is where I would start. That's about 63% on the front and about 27% on the rear. From here you can vary the balance bar by 3-5 % and still be within the balance bar's normal range of movement.

Lastly is the pad friction coefficient. Cheap street pads are about, .35, and the highest friction race pads I know of are about .675. There is a huge difference between the two. I am using the Wilwood B pads at about .625

My experience is with mid-engined cars at about 2500 total pounds and 60F/40R weight distribution 6 to 1 peddle ratio without power brakes and 4 to 1 with PB's.
 

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Neil

Supporter
"smaller front MC diameter create more caliper pressure for the same foot pressure but requires a longer foot stroke" This in fact true.

"Installing a larger rear MC dia reduces rear braking" is also true but only if the force applied is equal in both examples.

The problem becomes keeping exactly the same foot pressure and the exact same foot travel. This is why I believe it is best to start with the same size master's front and rear and a centered balance bar. A good starting point for both the masters would be .75. This will allow for varying available sizes of both larger and smaller if needed.

Then continue to design the rest of the system. Next caliper types will need to be selected as well as rotor diameters, This is done together. The rotor diameters more than likely will be as big as will fit in the wheels and then select a caliper that is designed to work with that diameter. The C6 corvette wheels on my SLC will accommodate 14-inch rotors in the front and 12.88 in the rear keeping the standard SLC lower A-Arms.

Now on to the caliper piston area. Here's where trial and error will become a factor. However, a ratio of piston area of about 5 for the fronts and about 3 for the rear is where I would start. That's about 63% on the front and about 27% on the rear. From here you can vary the balance bar by 3-5 % and still be within the balance bar's normal range of movement.

Lastly is pad friction coefficient. Cheap street pads are about, .35, and the highest I know of is about .675. There is a huge difference in the two. I am using the Wilwood B pads at about .625

My experience is with mid-engined cars at about 2500 total pounds and 60F/40R weight distribution 6 to 1 peddle ratio without power brakes and 4 to 1 with PB's.
For street use, the Performance Friction "CarbonMetallic" pads have an unusually high coefficient of friction and they are easy on your rotors.
 

Howard Jones

Supporter
I should have added that in reality this whole thing really starts with the tires. A light car on narrow tires like a lotus Europa doesn't need 15-inch diameter rotors and massive calipers, especially on street tires. Nor will stock brakes work on a modified 3200 lb Camaro with 600hp and wide wheels sporting DOT radials. My comments are related to what I have owned and run on the track with both high-performance summer-only tires and DOT slicks. Nearly all were V8 2500 lb midengined prototypes with 10-13 inch wide tires.

Taking a home-built car to the track and developing it to not only be quick but reliable is not for the person who easily gets distracted. But if you are persistent and willing to learn It certainly can be done. I was at COTA the time before last and there was a guy with a new GT3 Porsche with Hoosiers on it. We turned out to be in the same run group which really means we are more or less equally capable drivers. During the day we ran a lot of laps together and had a great time. His 250K Porsche and my 100K SLC were running nose to tail with nothing to show between them. Until......we got run down and passed by an Instructor with a student onboard in his race-prepped Miata. The Miata had a rotary in it but just saying.............. it's really all in the driver.

Back to brakes. This is pretty much the hardest system to get just right. It will take swapping out some parts and some trial and error. But if you do the labor then it's really not that expensive.................compared to having Porsche do it. I have some information on an older race-prepped Porsche somewhere and it was surprisingly close to the same rotor and caliper sizes as I have alluded to above. The difference was in the caliper total piston area but not really in the ratios between the front and rear. The Porsches calipers were generally larger in piston area as are a lot of real racecars with manual braking systems.

As to swapping parts. You are going to have to pick something and set a baseline to tune the rest of the system to. I think it is the front calipers/rotors. Because they are the most expensive parts. If I had to do it all again I would put these on the front ( 5.18 piston area, 1.25 rotor thickness, 14-inch diameter rotors) and tune the rest of the car to them. These are not the most-greatest-latest-bling but they are also not the most expensive. Note that the calipers have the "Thermolock" pistons in them. I think this is a requirement for track cars but maybe not street cars. At least on the front of the car. This piston is very stable and does not grow in diameter or distort under extreme heat. It is a lot more money but I think it is worth it on a race car.

So there ya go: what I know about brakes.

 
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Terry Oxandale

Skinny Man
Adding, with ultra-high performance tires, my bias with identical corners for my car is almost a perfect balance. Put some R7s on the wheels instead, allows a higher deceleration, and consequently a further forward shift in needed bias at the front. My calcs of using the same setup for all four corners was the CG, tire sizes, etc,. Then I worked from there to get a MC size that was appropriate (which was .75"), and then a caliper piston area (greater than 5"). At ~1 g, this put all 4 corners within 5% of each other (F/R), which was well within my personal tolerance for balance bar adjustment.
 
Yes, 2X Brian. Thank you all for your input and experience. As an update, I decided to go ahead and put the pedal box back in the car the way it came out with the balance bar centered and the MC clevis/rods set the same. I figure I can start with this setup and then adjust from there. FYI, this is the SLC built by Grant/Deadshot. I've been doing some maintenance over the winter and making a few adjustments to try and make it my own. I have been learning a lot about the car and working in the foot boxes is teaching me some new skills in the art of contortion! Thanks again for all the help. Scott
 

Howard Jones

Supporter
Here's one last idea for a new (haven't bought anything yet) build. Start with a 3/4 and a .7 master cylinder then to change bias all you would need to do is swap output hyd hoses from front to rear. I would start with the .75 in the front, and if you wanted to later you would only need to switch the two hoses in the foot box to put the 3/4 on the rear circuit and the .7 on the front. These could be any two close sizes. .7 and 5/8 would be another pair you might try. Just an Idea.


 

Neil

Supporter
I should have added that in reality this whole thing really starts with the tires. A light car on narrow tires like a lotus Europa doesn't need 15-inch diameter rotors and massive calipers, especially on street tires. Nor will stock brakes work on a modified 3200 lb Camaro with 600hp and wide wheels sporting DOT radials. My comments are related to what I have owned and run on the track with both high-performance summer-only tires and DOT slicks. Nearly all were V8 2500 lb midengined prototypes with 10-13 inch wide tires.

Taking a home-built car to the track and developing it to not only be quick but reliable is not for the person who easily gets distracted. But if you are persistent and willing to learn It certainly can be done. I was at COTA the time before last and there was a guy with a new GT3 Porsche with Hoosiers on it. We turned out to be in the same run group which really means we are more or less equally capable drivers. During the day we ran a lot of laps together and had a great time. His 250K Porsche and my 100K SLC were running nose to tail with nothing to show between them. Until......we got run down and passed by an Instructor with a student onboard in his race-prepped Miata. The Miata had a rotary in it but just saying.............. it's really all in the driver.

Back to brakes. This is pretty much the hardest system to get just right. It will take swapping out some parts and some trial and error. But if you do the labor then it's really not that expensive.................compared to having Porsche do it. I have some information on an older race-prepped Porsche somewhere and it was surprisingly close to the same rotor and caliper sizes as I have alluded to above. The difference was in the caliper total piston area but not really in the ratios between the front and rear. The Porsches calipers were generally larger in piston area as are a lot of real racecars with manual braking systems.

As to swapping parts. You are going to have to pick something and set a baseline to tune the rest of the system to. I think it is the front calipers/rotors. Because they are the most expensive parts. If I had to do it all again I would put these on the front ( 5.18 piston area, 1.25 rotor thickness, 14-inch diameter rotors) and tune the rest of the car to them. These are not the most-greatest-latest-bling but they are also not the most expensive. Note that the calipers have the "Thermolock" pistons in them. I think this is a requirement for track cars but maybe not street cars. At least on the front of the car. This piston is very stable and does not grow in diameter or distort under extreme heat. It is a lot more money but I think it is worth it on a race car.

So there ya go: what I know about brakes.


Howard has posted some very valuable advice. "Getting F/R braking right is not easy and it will require some time and testing."

To illustrate the lengths some go to get the right mix of parts to give optimum performance, I'll give my rear brake setup as a harebrained example :). Since I used a Porsche G50/01 transaxle and Porsche 996 rear uprights & axles, the hubs had a standard Porsche metric bolt pattern. I spent some time researching possibilities for a brake system that did not cost an arm and a leg but one that would be really effective. It turned out that a Porsche rotor and Porsche (Brembo) aluminum caliper from a Porsche 928 S4 FRONT fit right on the 996 rear hub! With the right M/C and pad choice, it worked well with my Corvette front brakes. I do not use a balance bar, only a rear brake proportioning valve.

Let me stress that my setup is for a scratch-built "homebuilt" rather than a GT40. Your setup should probably start with what your kit provider recommends.

RR Brake Caliper.jpg
 
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