Someone please explain brake bias pedal boxes to me

[Mieran]

How do they work in comparison to a servo?

Why are they better than the standard servo and master cylinder?

How do you choose master cylinder sizes (is it based on calliper size?) and are the 2 reservoirs for front and rear?

And lastly why do people use a brake bias valve when they have a pedal box installed?

Feel free to add any more info.

[mowgli]

ok.. a servo is simply a means of assisting with the braking effort of the single master cylinder.. but people insist on calling the whole thing a servo.

[Edd]

i'd like to know the answers to the above questions to :)

[mowgli]

the twin cylinder setup is all about adjustability, if a driver prefers it to stand on its nose, or maybe even it out towards the rear, then its a way better setup for competition use.

as for the cylinder sizes, peope who sell upgrades will have that sort of info in tables, i wouldn't dare to calculate them myself..but trial & error is also a big part of getting a setup right, thats why they call it testing...

if someone has twin m/cylinders & a bias valve, ... they must be into serious left foot braking.....

[Stuart]

rough rule of thumb... massive cylinder for the rears and small one for the fronts.

OBP have a reasonable page on bias boxes on their website

[novarally]

A servo is simply there to reduce the amount of effort your leg has to exert onto the brake pedal for a given level of stopping. It has nothing at all to do with how well the car stops, or the level of bias (distribution of front to rear braking effort).

The only benefit of a bias pedal box is that it gives you the ability to alter the ratio of front to rear braking force. It is not better than a standard set-up, but has a place in cars used for motorsport. For a road car they are generally a massive pain as the pedal needs pushing so much harder (unless you fit a remote servo as well).

The ability to alter braking force between front and rear is beneficial for many reasons, e.g. in wet weather you want less braking on the front to avoid lock-up, differnet drivers have a different preference and so on.

Many people (myself included) also have a bias valve, which typically is used to reduce the pressure still further going to the rear brakes (talking FWD cars here).

The two fluid reservoirs are one for each brake circuit (front and rear).

The whole subject of braking is massively complex, I have a couple of books on the subject.

[novarally]

As Stuart mentioned, OBP make a reasonable stab at explaining these things, try some of their 'Helpsheet' downloads at;

http://www.obpltd.com/help/category/5-help-sheets

[Mieran]

Thanks for the posts

Colin, what size cylinders are you running for front/rear?

[scott.parker]

This thread could be worth being "stickied" so others can see it without asking again, as Colin has put in some good replies thats helped me to understand it better.

[Stuart]

Its worth noting that a LOT of the bias valves simply dont/wont work very well for light FWD cars due to the way they limit the pressure.
They are linear to a point (lets say 100psi) as in they dont reduce the pressure coming in, and then when they hit 100psi the output is less than the input at a varable rate depending on the adjuster. SOME light fwd cars would lock up at less than 100psi, so the adjuster did naff all to help lol

NOTE 100psi is just as an example number and NOT to be taken as gospel.

[novarally]

Thanks for the posts

Colin, what size cylinders are you running for front/rear?

Good question, it's impossible to see the size markings on my master cylinders when they are installed in the car, so I'll have to work from memory for the time being.

I think it's a 0.625" on the front, and a 1" on the rear.

When I got the car initially, it had a 0.7" on the rear circuit, but it was still doing too much rear braking for my liking, so we went up on the cylinder size (bigger cylinder = less pressure).

The typical size you will find on a FWD car is 0.625" front, 0.7" rear (e.g. the Nova rally car I have on EBAY runs with these sizes).

[Dayle_]

Agree with Colin on this, Most FWD small cars you see will have "The typical size you will find on a FWD car is 0.625" front, 0.7" rear" We used to run this.

The two reservoirs as stated are for front and back cylinders. They also allow you to run different types of brake fluid. Some that warm up quicker than others, It does get rather complicated when you start to look at things in that depth!

Compbrake do some reasonably priced pedal boxes like OBD but pedal boxes aren't the greatest on the road its like setting your foot in concrete but you get a greater deal of adjust ability also gets rid of the X pattern found on most cars braking systems.

[bazil]

I ran .625 and .7

Here's a article I posted a wee while back

http://www.pngclub.com/forum/showthread.php?176197-*-INFO-*-Pedal-box-set-up-and-adjustment-and-bedding-in-procedures