Brakes Bias? Godzilla sized brakes

[garrittpwl]

I'm back, but this time for some real technical information. what I dont need to hear because I already know is "you shouldnt run the same size front and rear rotors/calipers, the rears should be smaller"

Now that we've got that out of the way lets talk what I actually have going on.

I've acquired through various horse trades, a full TBM F5 caliper setup with bracketry for my car that utilizes a 14 inch rotor on all 4 corners.

I've done the fluid volume vs travel math and the OEM master cylinder has plenty of volume to push the calipers. Especially considering they have minimal travel with the 1.24 inch thick rotors.

My car is a 96 NON ABS hydroboost car and Im running a single wilwood proportioning valve to the rear (along with the oem non abs proportioning valve as well).

I made a hydraulic gauge setup that will allow me to measure the front/rear brake pressure output when you clamp down on the brakes. That said, what should I be looking for front/rear bias percentage wise? Everywhere I read it looks like anywhere between 69/31 to 75/25.

In my mind, the math here is even easier for me since all 4 corners are the same size. I plan on taking a measurement with the aftermarket rear proportioning valve wide open (to get the OEM valve's pressures). And then dialing out the rear from there.

Does this sound like it'll theoretically work?

 

[cobrajeff96]

Nah, prolly won't work.

 

[lwarrior1016]

I’m wondering if rear pressure will have to be lowered more than normal to accommodate the larger rear rotor.

It may end up being a drive it and adjust kind of scenario.

 

[ttocs]

take it to a dusty/dirty parking lot and see which wheels lock up first under hard braking and then make adjustments from there. Test those on the street after of course but otherwise I am not sure what kind of math you would do to figure out how much the valve should be open, and then get the valve to that point.

 

[garrittpwl]

I’m wondering if rear pressure will have to be lowered more than normal to accommodate the larger rear rotor.

It may end up being a drive it and adjust kind of scenario.

my thoughts on this as well. I figured the OEM was 75/25 but setup for smaller rear rotors and now I'm going to be applying too much pressure to the back. So I put the bias valve in the rear lines so I can take away pressure even more than the OEM does.

take it to a dusty/dirty parking lot and see which wheels lock up first under hard braking and then make adjustments from there. Test those on the street after of course but otherwise I am not sure what kind of math you would do to figure out how much the valve should be open, and then get the valve to that point.

I've seen a few videos showing how to do it on stands with help of someone modulating the pedal. I made hydraulic gauges that screw into the bleeder ports. the gauges have bleeders in them so you get the bias dialed in using the pressure outputs on the gauges. Then tweak from there.

 

[cobrajeff96]

Just curious... what's the OEM m/c bore size for your car? Is it 15/16" or 1" or something else?

 

[garrittpwl]

Just curious... what's the OEM m/c bore size for your car? Is it 15/16" or 1" or something else?

1 inch

 

[cobrajeff96]

Well, I hope that it does move the amount of juice you say it does. I just remember some short years ago that my only option was a 1-1/8" bore m/c in order to move enough fluid for the 4 pots I have at all four corners. And the fronts are freaking massive ones too, they basically were created for the 05 Ford GT supercar. The rears are a Wilwood forging and I actually had to step those down from what I originally bought because the piston area would've been too large and not produced enough force even with a maxed out adjustable prop valve that came with the m/c. It was math heavy back then but fast forward and it all worked out perfectly. I put the car on a brake dyno a few times over the years and the force proportions have been laser perfect, just now with much more brake torque than factory obviously.

 

[Mustang5L5]

70/30 is roughly what is considered "ideal" for the Fox/SN95 cars. There is some wiggleroom based on your actual weight distribution.

The problem is that aftermarket PV's don't bias as much as adjust the knee point curve which is the point at which reduced pressure to the rear starts to be seen. What that means it your initial press of the pedal will have equal pressure front and rear, up until that knee point is reached and then you get your reduction in pressure. Problem is you can't vary what that actual reduction is. So if the reduction is 80/20 (just using a random number here), you can't actually adjust that. You only change the point of pedal pressure in which you go from 100/100 to that 80/20.

Here's a good example. You can see this knee-pint curve of a wilwood valve. Notice the only changible variable is the point where pressure reduction occurs, but you can't change the actual amount it's reduced.


Typically in cases where front rotor sizes are the same or near similar, the caliper piston size it reduced to lower the braking force to the rear. I know Baer does this with their 6P/T4 caliper series in which it's the same caliper, but the rears have much smaller piston bores.

Bias calcs are typically a combination of brake pressure, in addition to rotor diameter, piston diameter and coefficient of friction of the pads. Even if the pressure works out to an ideal 70/30 split, that's not the only variable that needs to be considered due to weight shift when braking. I know you didn't want to hear that, but that's the reality of the situation.

Here's a pretty good bias calc i've used in the past.

Bias Calculator

Do those TBM calipers use the same size pistons in the caliper front and rear? Or are the rears reduced?

 

[garrittpwl]

70/30 is roughly what is considered "ideal" for the Fox/SN95 cars. There is some wiggleroom based on your actual weight distribution.

The problem is that aftermarket PV's don't bias as much as adjust the knee point curve which is the point at which reduced pressure to the rear starts to be seen. What that means it your initial press of the pedal will have equal pressure front and rear, up until that knee point is reached and then you get your reduction in pressure. Problem is you can't vary what that actual reduction is. So if the reduction is 80/20 (just using a random number here), you can't actually adjust that. You only change the point of pedal pressure in which you go from 100/100 to that 80/20.

Here's a good example. You can see this knee-pint curve of a wilwood valve. Notice the only changible variable is the point where pressure reduction occurs, but you can't change the actual amount it's reduced.
View attachment 33748

Typically in cases where front rotor sizes are the same or near similar, the caliper piston size it reduced to lower the braking force to the rear. I know Baer does this with their 6P/T4 caliper series in which it's the same caliper, but the rears have much smaller piston bores.

Bias calcs are typically a combination of brake pressure, in addition to rotor diameter, piston diameter and coefficient of friction of the pads. Even if the pressure works out to an ideal 70/30 split, that's not the only variable that needs to be considered due to weight shift when braking. I know you didn't want to hear that, but that's the reality of the situation.

Here's a pretty good bias calc i've used in the past.

Bias Calculator

Do those TBM calipers use the same size pistons in the caliper front and rear? Or are the rears reduced?

Same size. 1.5 inch.

 

[garrittpwl]

Well, I hope that it does move the amount of juice you say it does. I just remember some short years ago that my only option was a 1-1/8" bore m/c in order to move enough fluid for the 4 pots I have at all four corners. And the fronts are freaking massive ones too, they basically were created for the 05 Ford GT supercar. The rears are a Wilwood forging and I actually had to step those down from what I originally bought because the piston area would've been too large and not produced enough force even with a maxed out adjustable prop valve that came with the m/c. It was math heavy back then but fast forward and it all worked out perfectly. I put the car on a brake dyno a few times over the years and the force proportions have been laser perfect, just now with much more brake torque than factory obviously.

Per motion raceworks calculator, a 1 inch master cylinder with a travel distance of 1.41 inches has the available fluid volume of 1.11 cubic inches. The travel distance on my pads should be tighter than this, but guestimating high at 0.025. My calipers have 1.5in pistons x4 each. Thats 0.18ci of fluid per caliper, I should be at 0.72inches of fluid required. I should be able to move enough fluid.

 

[Mustang5L5]

Per motion raceworks calculator, a 1 inch master cylinder with a travel distance of 1.41 inches has the available fluid volume of 1.11 cubic inches. The travel distance on my pads should be tighter than this, but guestimating high at 0.025. My calipers have 1.5in pistons x4 each. Thats 0.18ci of fluid per caliper, I should be at 0.72inches of fluid required. I should be able to move enough fluid.

I do my MC calcs based on surface area of all pistons and base it off ratios.

For example. Your stock brake setup with the 1" bore MC used a 66mm front piston and 38mm rear piston. Works out to 6842mm^2 for front brake piston surface area, and 2268mm^2 for rear for total piston surface area of 9110mm^2. The 1" Master has a piston surface are of 507mm^2. That works out to a slave/master ratio of 17.9:1. Actual distance traveled doesn't really matter much because the pistons will always be pushing the pads up to the surface of the rotor.

With your proposed setup of 4 piston calipers with 1.5" pistons, you take the surface area of one half of the caliper. So 8 pistons at 1.5" (38mm) comes out to 9072mm^2 with the same 507mm^2 MC. That works out to 17.89:1. SO essentially this system is identical in terms of hydraulic ratio. You should essentially feel no difference in pedal feel when pressing the pedal down. So i concur with you that the MC providing enough fluid is not a concern

You will notice a difference in responsiveness of course, due to leverage of the larger rotors and pad surface area/coefficient of friction.

 

[garrittpwl]

I see your knee chart but mine won't directly reflect the wilwood chart as I'm still going to try and run the Ford non-abs valve but then also run the single line wilwood adjuster to the rears. The way I'm looking at that knee chart is even on its lowest setting, it only gets about a 63/37 split in pressure.

My thought there is that instead of adjusting the full pressure that the front is getting and dialing back from there. That I'm going to be taking the stock rear pressure and then dialing back from that. its not the best, nor ideal but I do still think I can get it streetable and then dial in the feel in on local hpde days. (if I ever attend any).

This is irrelevant kinda but it should be known I'm not an inexperienced driver, nor am I inexperienced to high rear brake bias. I've road raced karts for 25 years primarily on karts that don't even have front brakes. As long as I can get a decent baseline as to not lock the rears up early on the street, I should be fine for what this car spends 95% of its life doing.

 

[ttocs]

cart racing always looked like a good time for sure.

 

[Mustang5L5]

I see your knee chart but mine won't directly reflect the wilwood chart as I'm still going to try and run the Ford non-abs valve but then also run the single line wilwood adjuster to the rears. The way I'm looking at that knee chart is even on its lowest setting, it only gets about a 63/37 split in pressure.

The Ford valve also does the same thing. I just don't know what the values for it's setting would be though. So really you have two knee-point valves in series. I agree that you'll likely have the Wilwood set to full reduction.

If you can get measured pressure values, look for a different brake bias calc online that will let you manually enter caliper pressure and add in the rotor dims alone with caliper/pad info, which should be easy to do since they are the same. That should give you your answer in terms of what sort of bias to expect. If you start dipping down into the 60/40 range, it might be of some concern but ultimately you seem like you kow what you are getting into.

 

[garrittpwl]

cart racing always looked like a good time for sure.

Its sacrilege to spell it with a "c" Here's a link to a damn good race we had through a local park a few years ago. race starts about 4:45.

https://www.facebook.com/video.php?v=755731572440463

 

[Mustang5L5]

BTW,

With regards to your initial question, Not sure if you ever stumbled across this and gave it a read. CLick the link and then read the "fitment" tab. Keep in mind this was written specific to the Fox mustang but not much is different between that and the SN95.

Hydroboost Conversion Kit, 1996-1998 Hydroboost in 1979-1993 Mustang

(you'll have to click and read the "fitment" tab)

 

[garrittpwl]

BTW,

With regards to your initial question, Not sure if you ever stumbled across this and gave it a read. CLick the link and then read the "fitment" tab. Keep in mind this was written specific to the Fox mustang but not much is different between that and the SN95.

Hydroboost Conversion Kit, 1996-1998 Hydroboost in 1979-1993 Mustang

(you'll have to click and read the "fitment" tab)

very useful information!

 

[TrickVert]

Great thread with a lot of good information, but I have to ask ... Why? Especially since a few posts back you say the car will be 95% street-driven. 14-inch top hats at 1.25 in thick will add a ton of unsprung weight and rotational mass, and unless you plan to run 345's on all four corners (yes, I know you can't), your brakes will likely far exceed the ability of the tires to stay stuck to the pavement.

(If you say "because it'll be cool," that's fine with me.)

 

[95opal]

Great thread with a lot of good information, but I have to ask ... Why? Especially since a few posts back you say the car will be 95% street-driven. 14-inch top hats at 1.25 in thick will add a ton of unsprung weight and rotational mass, and unless you plan to run 345's on all four corners (yes, I know you can't), your brakes will likely far exceed the ability of the tires to stay stuck to the pavement.

(If you say "because it'll be cool," that's fine with me.)

Not always true.
My 14" Wilwood 6 pots with 1.25 rotors are way lighter than the 13" cobra set up i took off.