# Wicht bore do i need for G60 brakes/ clutch



## Tjeerd (Dec 14, 2009)

Hi,
I have a question about the bore size of the master cylinders for the brakes and clutch wich will be builded on my mk1 Jetta.
I'm going to run brakes from the golf mk2 G60 and also the hydraulic clutch form a Golf mk2 G60.
The pedal box wicht i wille be using is a Wilwood:








Whit this extension kit:








The extension kit can be ordered in various 'bore sizes' wich one will i need?


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## 91JETTAboy (Mar 16, 2008)

im pretty sure the bore you want is 22mm, but that is with the stock booster setup


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## Tjeerd (Dec 14, 2009)

I think you haven't read my post verry good. I'm going to run the wilwood system with 3 separate cylinders. 
I know i need one with aprox. the same content as the clutch slave cylinder of the g60 gearbox. But i don't know the content.
Next to that i need one bigger and one smaller resevoir for the front/ rear brakes.
I thought maybe someone here knows this kind of system (i bought it from the US) and the bore sizes of the cylinders i need.


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## Tjeerd61 (Feb 14, 2005)

Nobody any idea?


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## Tjeerd (Dec 14, 2009)

?


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## o2bad455 (Aug 22, 2003)

*Re: (Tjeerd)*

The clutch is easy. Use a stock slave and duplicate or approximate the bore of the stock master.
The brakes are a lot trickier. You will need some data and some formulas to figure the bores of the two master cylinders and minimum strokes. There are whole books on the subject. I had one, but can't seem to find it right now (frustrating). You might try searching in the racing forum. 
I'll assume you're converting from the stock dual-diagonal split to a front-rear split, since the Wilwood pictures you posted have separate master cylinders for front and rear, and that you will not be running any form of brake booster. By itself, switching to front/rear puts more of a draw on the front MC, which must now operate two big calipers rather than one big and one little. The Wilwood system you're getting probably includes a brake balance/bias bar for front to rear. If you were to keep the dual-diagonal setup, you would NOT be able to use the Wilwood balance bar, and would have to install a brake proportioning valve in both circuits. Let them know that it's for a FWD application in case they have a specially offset balance bar for FWD. 
Since you already know which calipers you want to use, the next step is to make sure you have enough displacement from a given master cylinder to operate both front brakes safely. You can also use the same size for the rear, where it will be overkill. There are guidelines on this, and there may also be rules or laws. You'll have to determine that for yourself. 
Although smaller bore master cylinders reduce the required pedal force, they require more travel/stroke to actuate the brake calipers. This can be an issue without a booster on anything weighing as much as a typical street car, even if lightened for racing. I can't tell you what to use, but I can tell you that the force needed to operate dual unboosted 22mm bore/diameter MCs would be quite exhausting, unless you happen to have a bionic leg. Dual 17mm MCs would be somewhat easier to operate. Whatever you get, make sure that they have sufficient total displacement (bore times stroke) to meet your safety needs. 
Here are some formulas to get you started:
master cylinder piston area = (master cylinder bore) x (master cylinder bore) x 1/4 x 3.14159
brake fluid pressure = (pedal force) x (mechanical force ratio) x 1/(master cylinder piston area)
caliper piston area = (caliper bore) x (caliper bore) x 1/4 x 3.14159
brake caliper force = (brake fluid pressure) x (caliper piston area)
stopping force = (brake caliper force) x (brake disc application radius) x 1/(tire radius)
desired ratio of front to rear stopping force = {(front weight) + [(total weight) x (front tire coefficient of friction) x (height of center of gravity) x 1/(wheelbase)]} / {(rear weight) + [(total weight) x (rear tire coefficient of friction) x (height of center of gravity) x 1/(wheelbase)]} -- NOTE: this can be easily measured for static, but would change under heavy braking as more weight is transferred to the front.


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