Master Cylinder Size ??

Hi guys, I need some help selecting the correct master cylinders for my setup. The car is a race/time attack dedicated Mazda RX7 FD3S. Weight 1300Kg with me in it and very close to 50:50 weight distribution running 18" wheels with 635mm tyre diameter. Stock aero. Pedal box is a Tilton 72-856 (Ratios from 4,8:1 to 6.1:1) and front brake area is 10020mm squared (CP9660 6pot with 345x32mm discs) and rear is approx 3800mm squared (stock rear single piston caliper with 335x20mm discs). I would like to have decent padel feel and travel without having the balance bar completely offset to compensate for the front/rear disparity in stopping power. If you need anymore info let me know :)

Thanks a lot for your help.

These calipers - https://apracing.com/race-car/brake-calipers/pro-5000-r-range/6-piston-cp9660-2-3s4l-180mm-centres-18mm-thick-pad ?

Given piston area is 50.1cm^2 - are you adding both sides together? Same with the rears as that would suggest a 6.95cm diameter, whereas 4.9cm might be more reasonable (don't know, just a guess)? It doesn't really matter, as the area ratios are the same, it's just the normal convention.

Three points that would be helpful are the pad(s) co-efficients of friction at the expected operating temperatures, the approximate height of the Centre of Mass, and the tyres' approximate C of F - or at least their type.

4am here, and just got up, will try and respond more usefully when I'm actually awake.

Thanks for replying even if it's early morning on your side of the pond :)

The link is correct for the front caliper. My area is total for front and rear axle. Tyres will be slicks which i think have a Cf of 1.1/1.2. Centre of mass from info i found is 430mm approx. Brake pads are still up for debate but I think a 0.42 rear and 0.48 front. I am open to changing pads if it means I have a more balanced setup.

Looking forward to your reply. Thanks

Anyone care to chime in?

Plugging all those numbers into my spreadsheet, I end up with something around a 5/8 MC for the front and a 7/8 MC for the rear.

It's very important to understand that there are a lot of different combo's you could make work here, and also what you like for pedal effort etc etc.

but that could be a good starting point.

Having a larger rear MC is a result of your front and rear discs being such a similar size, and this will result in your bias bar tilting as it travels.

For a more balanced hydraulic system you might need to address the size of the mechanical parts or even a less aggressive pad in the back.

This does open up a new can of worms with temperatures etc.

So again those MC sizes could be a good starting point but expect some tuning from there.

Hope that helps

Hi Connor thanks for your feedback. Few questions doesn't the pad area make a bigger difference on master size than disc size? Also what friction co-ef would you suggest front and rear to have a more balanced setup? Thanks

Hey Henry, I'd really recommend checking out our brake system course we cover everything being discussed here.

Pad area makes almost no difference to the braking force or any relation to MC size. (Basic friction equation F = N.Mu Notice how area doesn't factor into this)

More pad area is mostly for more thermal capacity and reducing pad wear.

Pad Coefficient of friction front to rear is quite a complex subject if we really want to get into it. It's not just as simple as the relative friction front to rear for bias - but also how the friction of the pad changes with heat and therefore the bias can change through a single brake application - this is a powerful tuning tool, but not where you should start.

If you can design your brake package around using the same pad front to rear that can be a good place to start (0.4 front and back with the same pad compound) but expect to change this as you dial in the setup. Brake temperatures play a huge part in what pads we use and i'd guess from your setup your rear brakes run much colder than your fronts.

If you are stuck with the calliper/disc/MC setup you have then you can change the pads as a bit of a bandaid as well.