Race Pedal Box Master Cylinder Bore Dia. Selection

[mat205125]

Hi all,

Does anyone have a useful guide to follow for selecting brake master cylinder bore diameters, for use in a competition pedal box, for different brake caliper types front and rear.

I'd prefer not to guess and get wrong

[GreenV8S]

The slave cylinder effective piston area needs to be balanced front/rear based on the physical geometry and weight distribution of the vehicle.

The total slave cylinder effective area needs to be balanced against the master cylinder area. This needs to take account of whether it is dual master, tandem or single m/c. The balance needs to give acceptable pedal weight and travel.

[mat205125]

GreenV8S said:

The slave cylinder effective piston area needs to be balanced front/rear based on the physical geometry and weight distribution of the vehicle.

The total slave cylinder effective area needs to be balanced against the master cylinder area. This needs to take account of whether it is dual master, tandem or single m/c. The balance needs to give acceptable pedal weight and travel.

So, for arguments sake, the car were 60:40 weight split from front to back, we'd ideally want to have at least 60% of the braking piston area at the front of the car?

Furthermore, the master cylinder area's would then be aligned to the total piston area in a similar proportion ....... My pedal box with have a separate cylinder for the front wheels, and one for the rears.

For the same caliper piston area, a larger master cylinder could give more pressure for a given travel of pedal, albeit requiring more effort by the driver's foot????

[OldDuffer]

Truth is there is absolutley no way you'll get this right even with a best guess. All you can do is make a wild stab at it and get a bias valve to adjust this by trail and error. The only way to get it spot-on is to do what a manufacturer would do, run numerous varaitions on a mule until it' sits in parameters. With all due respect I doubt you are prepared to do this, or have the time, budget or inclination. You won't have the means to test yoru results anyway.

Look to the nearest stock set-up and go from there.

Edited by OldDuffer on Monday 9th September 20:37

[GreenV8S]

It's not piston area; to get the static balance right you need to consider the total mechanical leverage from the pedal to the tyre contact patch. If it's a single m/c system or equal pressure split system that makes the calculation easier, but you still need to multiply up the effective piston area and the ratio of the effective radius of the caliper versus the radius of the tyre to get the total leverage. The ratio front/rear should match the static weight distribution to get the static balance right.

If the static balance is right then when you drive the car in a circle close to the limit of grip and brake lightly, the car remains neutral.

Then you need to use the bias valve to get the dynamic balance right. This part is very subjective and is likely to involve trial and error to suit your driving preferences, the amount of engine braking you use, the suspension/handling characteristics and so on. If you get the dynamic balance right then the car will remain neutral under heavy braking.

[RLK500]

The below book is a must for that kind of stuff.

https://www.ebay.co.uk/itm/Brake-Handbook-by-Fred-...

[PaulKemp]

You said a competition pedal box so I would assume it’s a bias pedal and therefore you would have adjustment available so the remaining question is what size master cylinders for front and rear.
You don’t say what car but I would imagine that there are others who race them therefore you need to dig in to the collective knowledge of others who race them.

[anonymous-user]

mat205125 said:

For the same caliper piston area, a larger master cylinder could give more pressure for a given travel of pedal, albeit requiring more effort by the driver's foot????

A smaller bore master cylinder gives more brake line pressure for the same load on the pedal.

If you want to increase front braking effort you put a smaller diameter cylinder on the front.

The smaller the cylinder diameter, the longer the pedal throw will be.

Drivers react to pedal pressure, not travel. There is always a balance to be made between acceptable pedal pressure required against acceptable pedal travel required.

The pedal ratio, master cylinder size and brake component size at the wheel all play a big part in creating a working brake system, as does the basic car design.

Modern road cars have quite sophisticated brake systems. My own shifts the brake bias automatically depending on how much G it is pulling. If you change the brake system on a car like mine you have to maintain the OEM mechanical balance or it won't work properly.