Master Cylinder Sizes

[Aussie Mike]

Originally Posted by jon@harrison.ne

As a mechanical engineer I appreciate Mikes thorough explanation or brake line mechanics. I would only add that we should be more precise about terms like force and pressure especially regarding units. "Force" has the units of pounds , "pressure" has the units of pounds per square inch, and "torque" is in "inch pounds" or, dividing by 12, foot pounds. Braking is achieved by applying a force to the pedal which then produces a different force (probably larger due to the pedal leverage and balance bar) to the master cylinder piston. This creates a pressure in the brake fluid which in turn creates a force on the wheel cylinder piston. This force results in a torque on the wheel (torque=caliper force X pad coefficient of friction(dimensionless,i.e. no units) X rotor radius.

Thanks Jon,

I'm no mechanical engineer, just a layperson. I've no formal training in this stuff but an interest in all things mechnical I've always been fairly good at looking at a system and visuallising how it works. Probably because I pulled everything apart when I was a kid.

I've gone back through my post and fixed up some of my incorrect terms. Hope it makes more sense. most of the time I'm thinking faster than I can type.

Cheers

[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

[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.

[stephen low]

This may lead to some tweaking when I'm back on the road as I'm positive my brake master which had come with the kit, had come off a falcon wagon. This was the descriptor my brake man used when he had to replace the brittle fluid reservoir in my early build days.

Knowing now one should at least chase a ute brake master for the fairly neutral bias balance, I have some work in front of me I suspect. Time will tell.

Thanks again Prof Mike!

[Maurice Butler]

Try checking this article http://www.autospeed.co.nz/A_110601/cms/article.html for what effect the crack point has on brake bias

[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.

[NASSTY]

There is also another option that I have seen on only one cobra, use an ABS module from I think an EL Falcon. It's a stand alone system and all you need to do is sort out the sensors. The owner of this particular cobra said that the brake bias issues disappeared after he installed it.