Master cylinder sizing

[Bill Musarra]

I tried consulting with both Wilwood and Tilton for the proper sizing of the master cylinders and got two different answers. Wilwood said to go with two 3/4" masters and get the pedal ratio to 6:1. Tilton says 3/4'' and 7/8'' with no alteration of the pedals. I have at least two of each size including a couple of 1" for the slave cykinder. I am running with the Wilwood Dynalite calipers, 6 piston fronts, 4 rear on 11.75 rotors. Tires are for now Nittos,225 front and 325s in rear. Circuferences are about 192 and 227cm. I have the Tilton dual pedal floor mount with a ratio of 5:1 and of course a balance bar. I don't mind experimenting to get them right, but it sure would make life a little easier if I could do it once. Any suggestions or advice?

Bill

 

[Dean Lampe]

Bill, I run 1 inch in the rear and 7/8 in the front with a 3/4 clutch. This works great with my pedals but I think I am closer to 6:1. With a 5:1 you might want to go down one size for a little easier brake pedal.

 

[Bill Musarra]

I just checked the Tilton site. They list the pedals at 5.5:1. That makes it interesting. May follow your advice anyway as I don't want a hard pedal.

Bill

 

[Bob Putnam]

6 piston Dynalite? Are you sure?

Knowing the number and the diameter of the pistons in each caliper is crucial to determining the master cylinder size.

 

[Bill Musarra]

I may be off on he Dynalites as it has been a while since they were obtained. I did pull the calipers this morning and measured the pistons as best as I could. The rears are the same size, about 40mm. The fronts are different sizes 1, 35mm and 2, 25mm. Hope that helps.

Bill

 

[Dean Lampe]

I am using the same calipers. Alan Nichols from Wilwood did the setup research for Fran at RCR with those calipers and the masters that I mentioned above. They work very well as a combination.

 

[Bob Putnam]

Given your piston diameters, the results come out a little odd, since the piston area of your rears is larger than the fronts. Assuming that you want approximately a 60% front/40% rear brake-force balance, your master cylinder diameters should have a ratio of about 1:1.4 (front and rear, respectively). So... I would start out with a 5/8" front, 7/8" rear combo.

 

[Randy V]

Those would be Superlite calipers. Same ones on my RCR.

Bill - Although I know you're still in mock-up mode - I think you may have your front calipers on the wrong sides.
Make sure that the larger of the 3 pistons is at the trailing side of the caliper rather than the leading side.

 

[Julian]

Bill,

See attached spreadsheet for what I used with my DRB. Short answer is, use 5/8" master cyl if you don't want a hard pedal and don't mind some pedal travel. Mine does not have a hard pedal, and is very easy to modulate, albeit with a fair amount of pedal travel. Got to bleed brakes up with proper race fluid for first competition event this Sunday, along with a dozen other things to attend to!

Spreadsheet is good to work out some 'what ifs' based on different front/rear static weight distributions, dynamic load transfer under braking and tyre diameters. Some of the figures I've used in the spreadsheet are assumptions based on experience, others known values. But will give you a good starting point. You will have to modfiy formulas a bit to account for 6 piston front calipers - it is based on 4 piston front and rear, which is what I have.

Cheers,

Julian