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People who know Bias Pedal Boxes. Please help.

13K views 53 replies 11 participants last post by  Gordo 
#1 ·
Alright guys.. Im abit dim on this side of things so bare with me.

On my corsa I will be running 300mm with 4 pot calipers on the front and GTE rear caliper and disks.

I want a Bias pedal box from CompBrake.

Can some one please educate me on the difference between the mastercylinders (0.625, 0.7 & 0.75) and what one would be a good recomendation for the brake set-up stated above for road and track.

Any info would be much appreciated.

Cheers lads:beer:
 
#4 ·
Ill find out matey.. thank you for the response.

Like I said.. abit dim on this and would like to learn it all. Did'nt realise the piston size determined what cylinder is required.

The piston size meaning the diameter of the piston?

Thanks
 
#6 ·
Thanks buddy. Ill be back on tomoz with diameter sizes.

Do you mind if I pm you kid??

:beer:
 
#8 ·
Piston sizes.....

The front,
4 pots have x4 38.6mm dia pistons per caliper.

The rear,
There are 2 types of valver rear calipers so ill have to go home and scratch the paint off to see what ones mine are.. There numbered 33 and 35.

So..
The piston dia of these are, the 33 is 30mm and the 35 is 33mm.

If anyone could possibly gives me the needed cylinder size for both (4 pots/rear 33 and 4pots/rear 35) then ill know what one ill need when I get home.

Also could someone please explain how you work this out.

Thanks in advance peeps:beer:
 
#10 ·
Cheers pal.. Ill have a look when I get home.. Work wont let me on the pdf file.

Thanks again

Col B
 
#11 ·
That's a heck of a difference in bores - and remember it's the CSA that counts, so it's much more than the simple diameter.
Id first ask you how strong you are, as you'll no longer have brake assistance - unless you plumb in a couple of inline units. The smaller diameter will give a longer pedal but less effort will be needed, whereas the largest diameter will give a good pedal but need a lot of effort.
I'd start with two of the 0.7s as that'll be a reasonable pedal travel without excessive effort needed - it's a light car so that shouldn't be a problem anyway.
If you're fairly strong in the leg, you may even want to go to .75, it being a light car.
I certainly wouldn't stuff around with different bore diameters unless you were SURE you needed to change one. You'll know if you do as the balance bar will be quite a long way towards one end of the settings or the other. I would suggest leaving it if it's in the middle half.
I trust you're getting a driver adjustable setup, as it's a cow having to stop each time you want to adjust it.
 
#12 ·
Yes mate.. im getting driver adjust setup.. I concider myself as strong so not to bothered about how hard the pedal is. I just want a good pedal as the std master cylinder with this caliper setup is very spongey. Bais box is the way farward to solve this.. I just want to learn abit about it before i go ahead and do it. I dont like bolting things to my car I dont understand if you know were im coming from mate. Thanks for the reply:beer:
 
#14 ·
its a shame you are so far away as i have a selection of cylinders in all sizes you could try each combo before buying any. the bolt on pedal box's dont work as good as a proper fabricated job as they use the origional pedal>servo fixing with around 4-1 ratio, my car is using scooby twin pot calipers with 285mm disk and the pots are 45mm each and large single pot ones on the rear and i have .7 front and .75 rear, m y mate's astra mk2 stage car has a rallydeisgn pedal box and wilwood 4 pots 44mm pistons and that has to have a .625 f-.75 r as otherwise you cant stop it.
i keep looking at it to move the linkage point but it seems a lot of work
 
#15 ·
i run .625 on the front and .750 on the rear of my nova, and i have very similar total piston area to your car, works very well ime, and even with the cylinder bore difference you'll probably still want the bias to be toward the front to stop the rear locking under heavy braking, if you had even cylinder size front and rear you'll be locking the rear all day long even with the bias fully forward
 
#19 ·
I take it back in this instance, the chap's run that setup although I'd be inclined to stop the rears locking up using the bias and disc/caliper size, rather than master cylinder size, as it'd usually mean the brakes are more evenly balanced in running temperatures.

However, theory is one thing, what works is often another.:cool:
 
#16 ·
Thanks for the offer there jim.. I appreciate that mate

Thanks alot guys.. You have all been a big help:beer::beer::beer::beer:

Going to do a .625 front and .750 rear.
Now to pass all this info on to the mrs for my crimbo box. hope she gets it all right:confused:lmaolmaolmaolmao
 
#17 ·
oh and avoid buying the cylinders from compbrake.. we have had a few now that just dont work and thats really not good for brakes.. they did change them no probs but still not on.
i have an excell spreadsheet that was done by someone else , you can put in caliper size disk size and all sorts of stuff and the weight of the car and it will calculate what you need etc, i dont know to upload it to anywhere but i can email it if you want to pm your email addy
 
#20 ·
i have rear drums, so not really an option to adjust the rear disc/caliper size, and you dont have enough bias on the bar to get the rears biased out enough with equal cylinder sizes ime, the rear of a FWD car like a nova or corsa weighs so little it needs next to no braking
 
#21 ·
Hmm, so master cylinders 'may' not apply as much with the different setup.

Lee, back in the day a common mod was to use larger wheel cylinders where available for stronger braking- in your case, have you tried to find smaller wheel cylinders - even sleeving with suitable pistons?
 
#22 ·
no as it seemed like entirely too much hassle considering i have a setup that works very well and i can see no benefit to running equal size m/s in my application and ime if you run rear discs instead of drums you need a larger m/s again and even less bias to the rear, hence why i dont see the point of a rear disc conversion unless you go rallying and need abetter handbrake
 
#26 ·
that can be a problem with running a pedal box on a stock pedal, the pedal ratio is set up for a servo assisted system, i have a tilton pedal set which give 9 - 1 ratio for the brake pedal, worth checking to see what a stock pedal gives i reckon...
 
#36 ·
i run a pedal box that replaces the master cylinder and with the standard piviot point on the brake arm the brakes were just shocking it was like pushing a brick wall, i measured the ratio and it was 3:1 far to little for a servoless system

i removed the bolt and drilled a hole just above it when measured this gave me around 6:1 and the difference is night and day !!!

that would be my first point of call for anyone on a standard pedal it take 3 mins and makes a huge difference




are you sure about that as the most i have seen is around 6:1 some details if it is 9 would be very much appreciated
 
#30 ·
very very good read this.. and i know i keep saying it but THANKS.

Ill go for .625 front and .725 back with it being a light car.
 
#31 · (Edited)
#33 ·
I agree with you, for the most part, however there are people who don't have the physical strength to apply the brakes with some cars and setups. I mean, small enough cylinders may not displace enough fluid with some big brake setups to give an acceptable pedal - especially if pad knock back is a problem.

I knew one guy who had an old leg injury and couldn't apply the brakes hard enough without the boosters assisting him. Damned good driver, though!
 
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