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Discussion Starter · #1 ·
So after reading that these cars frequently have too much front brake bias when using the factory prop valve this is what I was thinking.

I have a 95 cobra master cylinder which I believe is directly proportioned at the outlets 1:1 front to rear. Normally the stock prop valve would then adjust the bias front to rear. So my plan is to delete the factory prop valve altogether and run 1:1 front to rear bias with the 95 cobra dual piston 13" brakes up front and 11.65 single piston rears.

If that results in too much rear brake bias, I can add a prop valve to bring down the rear a touch. But I'd rather do that then add a prop valve to the front and the factory prop valve.

Thoughts on this setup appreciated.
 

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With what you have, add HP+ pads to the rear. You may or not need an adjustable prop valve then.
 

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Along with the better pads as recommended make sure you corner balance the car.

George
 

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Why not keep the ABS system from the 95 Cobra? With the exception of a professional driver on a known surface (none of which apply here), the ABS system will brake more quickly and safer. I plan to keep the ABS in my build.

It doesn't apply to you, but for those builds using 99+ GT brakes all the way around: The 94-98 cars got a different front brake caliper with less piston area. Why not install these calipers instead of the PBR calipers? That would effectively reduce the front bais, and could still allow you to fine tune the bais with different pads.
 

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Discussion Starter · #8 ·
Seems like most people replying here, missed the part about ditching the factory prop valve resulting in 50/50 bias.

This should result in too much rear brake bias... how would adding grippier rear pads help??
 

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Ryan: The original prop valve set up was for a nose heavy Mustang. Your car is much better balanced and you do not need the stock prop valve to compensate for this. I added a prop valve to my build for the rear circuit but have never used it, some guys that run spec racers actually install the prop valve on the front circuit (not recommended for street use). I have the same brake set on my Coupe, manual Jeep MC, stock front pads (NAPA)brakes and switched the rears to Hawk HP+ plus pads. This made a BIG improvement on the rear under braking, my Coupe was always twitchy under hard braking. These pads have a high grip, wear fairly fast, can be hard on rotors and are dusty, so expect some brake dust on your wheels after you drive but they bite like crazy. I'll add this for what it is worth. One of my great finds while reading this forum was a more efficient search function so here's what you do:
1) Go to google or use the link below:
2)Enter site:ffcars.com then what you want info about
like site:ffcars.com prop valve or site:ffcars.com brake bias, bump steer,caster camber etc.
This was a great help to me when AFTER I had built my Coupe and needed to make changes to impove it.
Happy Holidays:
HTH
CB
 

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I had the factory p-valve on my donor build. Even with the rear drums,I needed more rear brake. Gutted out the valve and now enjoy eye-sucking braking!
 

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Yup, the same . . .

setup as you [almost] . . . 13" up front, 11.65 in the rear with the Jeep master. Adjustable proportioning valve in the rear line adjusted for maximum braking pressure. Stock pads for the PBR calipers both front & rear. Stops on a dime and gives 5¢ change . . . albeit, it took some serious testing to get to this point with the adjustments. Next step, corner balance the car and see if it gets even better.

Just my 2¢

Doc :beerchug:
 

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Discussion Starter · #14 ·
Hmmm, I would think going from a 70:30 split front to rear to a 50:50 split is more compensation that the difference in weight balance between a mustang and a FF roadster?

I guess I'll plan on playing with things.

It's funny that the MKIV manual says to use the factory prop valve and put an adjustable prop valve on the front lines. This seems like a bad idea IMO.
 

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Hmmm, I would think going from a 70:30 split front to rear to a 50:50 split is more compensation that the difference in weight balance between a mustang and a FF roadster?
Think through this one step at a time.

1- The MCs you're talking about deliver equal pressure to front and rear brake lines.

2- The pistons on the front calipers are twice the size of the pistons on the rear calipers.

3- The front rotors are larger diameter than the rear (and the brake pads engage them at a greater diameter).

4- You took a pressure limiting valve out of the back system.


Quick, what's your new bias? (it's not 50:50)
 

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When I was running my DSR I did extensive brake testing. It had a similar weight distribution of 47% front and 53% rear but had the same rotors and calipers on the front and the rear. There is a very large difference between static weight distribution and dynamic weight distribution. When you hit the brakes there is a great deal of weight transferred to the front so you do need more braking in front than in the rear even though there is more static weight on the rear. I had sensors on all 4 shocks to check weight transfer, pressure gauges on the brake lines, and made tools to check the torque split needed between front and rear. Because there are many different combinations of master cylinder bores, caliper bores, number of pistons, sizes of rotors, and pads, it would be hard to cover all combinations but the way I found worked for me and should work of others is by checking the torque split between the front and the rear. I used a bending beam type of torque wrench and made adaptors that bolted to the hubs. I used a small board square on one end and tapered on the other that I could wedge between the frame and the brake peddle so I could hold the brake peddle on with about 100 ft lb’s of torque required to turn the front wheel at a slow steady rate. I would then adjust the bias so that it took around 55-60 ft lb’s of torque to turn the rear wheels at a slow and steady rate. No matter what combination of master cylinder, caliper, rotor, or pad you have this would be a good starting point to set the brake bias. Just as a point of interest when my brake bias was set correctly I would end up with about twice as much pressure to the front brakes as to the rear.

Bill Lomenick
 

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Discussion Starter · #17 ·
Think through this one step at a time.

1- The MCs you're talking about deliver equal pressure to front and rear brake lines.

2- The pistons on the front calipers are twice the size of the pistons on the rear calipers.

3- The front rotors are larger diameter than the rear (and the brake pads engage them at a greater diameter).

4- You took a pressure limiting valve out of the back system.


Quick, what's your new bias? (it's not 50:50)
The bias I am referring to is at the master cylinder. :001_rolleyes:

Removing the biased prop valve IMO should overcompensate for the relatively minor difference in weight balance between the cars.

Does weight bias really effect brake bias significantly? My initial answer would be that it shouldn't. Once a deceleration vector is applied to the car, the majority of the weight is going to shift to the front wheels under hard braking.

But again, I think I'll just need to play with things. I am fully anticipating needing an adjustable prop valve for the rear circuit though.
 

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I have a 95 cobra master cylinder which I believe is directly proportioned at the outlets 1:1 front to rear.
The bias I am referring to is at the master cylinder.
Removing the biased prop valve IMO should overcompensate for the relatively minor difference in weight balance between the cars.
I am fully anticipating needing an adjustable prop valve for the rear circuit though.

You're on your own, I can't help you. good luck.
 

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Not trying to muddy the water too much here, but there are detailed calculations to go through to calculate the ideal brake bais for a car. It is based on rotor diameters, piston caliper areas, weight distribution, center of gravity and many other variables. There are far too many variables just to slap on a number and say 50/50, or 70/30. In my opinion, you are changing to many variables to make a reasonablly good guess at how the car will behave without actually making the calculation.

A few comments though... the effect of weight balance depends on the height of the center of gravity. The cobra has a lower center of gravity and less braking nose dive than the mustang, so less weight will be transfered to the front. Combine this with a rear weight bias, and we are talking about a substaintial amount of additional rear braking that needs to be done.

As for the proportioning valve... the valve in the mustang was only setup to accomodate different drivetrain combinations. A heavy V8 engine in the front changes the weight bias compared to the V6. The factory proportioning valve was there only for small changes in chassis setup, so don't expect a dramatic difference from removing the valve completely.
 

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Ryan I think you are on the right track…or at least close assuming a 1:1 front to back ratio will be close to balancing the braking.

When I was building my car I sent all my specs to Tilton Engineering who makes brake pedal assemblies for all sorts of race cars from NASCR to Indy cars and they came back and told me to use a 15/16” master cyl. for the front and for the rear.

Same Size front and rear.

My setup is a little different than yours.

Front: 10.84” x 1.0" vented front rotors, 99+ V6 & GT 44.5 x 44.5mm dual piston PBR calipers.

Rear: 1994 Lincoln MK VIII 1.730 in single piston 10.5 in rotor

Pedal Assembly Wilwood pedal assembly 6.2 : 1 pedal ratio, use a 15/16" master cylinder both front & rear.

I am very happy with my breaking performance…at least for now on the street.

Jack
 
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