Master Cylinder Sizes

[Aussie Mike]

Hydraulics is all about leverage. The difference in sizes between the size of the master cylinder bore and the caliper piston bore will give you an increase in leverage.
Here's a rough example:

You apply 100 pounds of force to your brake pedal. There is a 6:1 ratio in your pedal so that equals 600 pounds of force against the master cylinder.

The master cylinder bore is 3/4" which if you work out the piston area is 0.4418 square inches.

Pi x Radius squared: 3.14159 x (0.375x0.375) =0.4418

We want to work out the pressure in square inches so we divide 600 pounds by the area we are pushing against (0.4418 square inches) and we get 1358 Pounds per square inch. This is our line pressure.

We then feed that down a steel brake line to the caliper where it has a piston 1.5" in diameter. This has a surface area of 1.7671 square inches. We are applying 1358 pounds per square inch of pressure so we multiply that by the number of square inches of piston and that gives us 2400 pounds of clamping force.

So with these calculations we can work out how much clamping force we generate for the front and rear brakes.

When working out the clamping force you find opposed pistons cancel each other out. If you have a pair of 1.5" pistons working against each other for the calculated area you use only one piston.

So the rear has a single piston caliper and for the front example we'll assume 4 piston calipers with 1.25" pistons (2.454" square).

With a 3/4" master and our 1358 PSI of line pressure that equates to 3332 pounds of clamping force on our front brakes.

The front and the rear of the car require different amounts of braking force to stay in balance. By that I mean we want the front and rear tyres to lock up about the same time or preferably the fronts before the rears.

There are a bunch of different factors that come into play here, the main one being weight transfer. The weight of the vehicle moves to the front of the car as it decelerates forcing the front tyres onto the road and making them grip better. The front to rear weight bias of the car, and suspension setup are a factors that control weight transfer.

The diameter of the brake rotors affects how much leverage the calaper has on the disc. A larger diameter disc has more leverage than a smaller disc and requires less clamping force. The friction coeficient of the pad material comes into effect too. some pads are grippier than others. The tyres and their level of grip is a factor

So what we have with our Cobra is a car with a rearward weight bias and low center of gravity. Add that to firm suspension and you have a car that won't transfer nearly as much weight as say your average Ford Falcon. This means a Cobra can use more rear brake and less front brake in comparison with that family sedan.

What the net result of having the same size master cylinders front and rear is that we may not get the right bias between front and rear braking forces. You may end up with your balance bar wound right accross to one side. You dont have enough adjustment to get the bias right where fronts and rears are close to locking at the same time. The balance bar should really a tuning aid and you need the system balance to be pretty close in the first place.

dropping the rear master cylinder size to 5/8" gives us 0.3067 square inches. This makes 1956 PSI of line pressure for the same pedal force.

If you already have 2x 3/4" master cylinders give them a try, they may work OK. If you find you need more rear bias you can switch to the smaller rear cylinder.

A proportioning valve will only take pressure out of the rear circuit. I don't think it's legal to run one in the front circuit.

Hope this helps.

Cheers

[hsv300]

Great work Mike you really know your stuff

[Aussie Mike]

Originally Posted by hsv300

Great work Mike you really know your stuff

Thanks Greg,

The theory is pretty simple once you get your head round it. The trick is getting it to work in the real world. Understanding the theory a little at lest makes it easy to understand what's happening when you make changes.

Cheers

[gjkrv8]

Originally Posted by Aussie Mike

So what we have with our Cobra is a car with a rearward weight bias and low center of gravity. Add that to firm suspension and you have a car that won't transfer nearly as much weight as say your average ford falcon. This means a Cobra can use more rear brake and less front brake in comparison with that family sedan.

What the net result of having the same size master cylinders front and rear is that we may not bet the right bias between front and rear braking forces. You may end up with your balance bar wound right accross to one side and have enough adjustment to get the bias right where fronts and rears are close to locking at the same time.

Hey Mike, I've got dual 3/4" in my CR and have to have my brake balance bar wound all the way to the left (front) to get what I believe is a front bias. Although I think my problem is that the Front Discs are std VZ and the rear Std VE. I think maybe if I had higher performance fronts I might get a better balance gived the weight transfer dynamic you mentioned in the Cobras.

What do you reckon professor Mike?

[Aussie Mike]

Originally Posted by gjkrv8

Hey Mike, I've got dual 3/4" in my CR and have to have my brake balance bar wound all the way to the left (front) to get what I believe is a front bias. Although I think my problem is that the Front Discs are std VZ and the rear Std VE. I think maybe if I had higher performance fronts I might get a better balance gived the weight transfer dynamic you mentioned in the Cobras.

What do you reckon professor Mike?

So your rears are locking way before the fronts? What size are the rear rotors and calipers?

Slowy,

If your fronts are locking way before your rears you need to add more line pressure to the rear circuit or take it out of the front circuit. That means an increase in crack pressure (that's if the valve is in the rear circuit).

The problem with Tandem master cylinders that most regular cars run is they have the same piston bore for front and rear. This means the caliper piston sizes need to be dialed in to suit. Easy done if you are a big manufacturer as you can ring up PBR and give them the spec of what you want. When we are taking parts from these production cars and adapting them to ours we end up with missmatched systems. Proportioning valves are used as a band-aid fix when they should be a tuning aid.

Cheers

[gjkrv8]

Originally Posted by Aussie Mike

So your rears are locking way before the fronts? What size are the rear rotors and calipers?

Mike

I am pretty sure my balance is ok now, but my problem is that the Balance bar is wound all the way to one side (Front bias) to get them to lock up first. So my adjustment range is gone. I thought maybe down the track if I put in higher perf Front Brake I might get some range back. Works ok for now.

Rgds

Gregg

[Aussie Mike]

I'd try some grippier front pads first. Might be a cheap and easy fix.

Cheers

[gjkrv8]

Originally Posted by Aussie Mike

I'd try some grippier front pads first. Might be a cheap and easy fix.

Cheers

Yes not a bad idea.

Whats those red pads that people seem to get for better braking ?

I'd need them to fit VZ standard Calipers.

Rgds

Gregg

[NASSTY]

I had my rear end rebuilt by a jag specialist when I built my kit so the rear disks work reasonably well but as this discussion has stated it's pretty hard to adapt donor brakes to cobras and get the balance right. It's certainly not right in my cobra. Given our jag diffs are 30 odd years old I wanted to make sure they were working properly from the start.

I know of a fair few cobras, with jag diffs and falcon master cylinders, who have removed the proportioning valve altogether to improve the brake balance and their fronts still lock up first. I'll plan to do this mod myself when I next change the brake fuild.

[trularin]

A little late but well done Aussie Mike.

A lot of guys have a mix of calipers front and rear, so a little work with the caliper volume may be in order to get the balance you are looking for. I know I changed the rear master when I swapped out the Lincoln setup for the GM setup.