Selecting a Master Cylinder...gulp!

Hi all,

I came across this website in my search for answers to calculating what master cylinder I need to replace the current setup in my car. I found a couple of posts that taught me something I didn't yet know, and my problem so far is that everywhere else people can't back up their suggestions with logic.

My situation is that I have a braking system that gives no feedback. It is fragile, and the parts are made of unobtanium. I currently have an issue, but the whole system needs to go.

I've done my research and my calculations, but the result doesn't look right to me. I'm humble enough to admit I muse be missing something, but so far no-one can tell me what.

I have a car that weighs 1200lbs on the front axle and 1700 on the rear, and a stock pedal ratio of 6.25:1. Lovely!

4 pots on the front with 2 38.1mm (diameter) pistons and 2 41.3mm (diameter) pistons each.

On the rear I have single pots at 36mm diameter. I've been told to expect 0.25mm of piston travel.

So...

((38.1mm x 0.25mm = 285.13cmm) x 4 = 1140.5ccm

((41.3mm x 0.25mm) = 335.05cmm) x 4 = 1340.2cmm

2480cmm aka 2.480 ccm for the front

((36.0mm x 0.25mm) = 254.57) x 2 = 509.2cmm

0.510 ccm for the rear

That's a total of 2.990 ccm, so let's call it 3ccm. And let's assume we will need some slack for pipe swell and that needing the whole pedal travel is not ideal and double it to 6ccm. Or 0.366cu.

The smallest bore master cylinder I have found (Wilwood TM1 5/8") will displace 6.6ccm or 0.4cu.

Is this correct? It seems odd to me that beefy brakes like that could be stopped by the smallest available bore.

I really hope it is, because 5/8" would require little force and allow me to do without a servo, and it should have a nice high psi too. But somehow I think I've missed something

Brakes work by pressure, not volume.
You need to provide enough MC volume to take up the slack until the pads touch the disk,
without running out of pedal stroke, allowing for a margin of safety.
It's all about the ratio of MC piston area to sum of brake piston area, the coefficient of friction of pads,
and the amount of pressure that you must provide at the pedal to achieve lockup.
A smaller MC diameter yields less pedal pressure, but more pedal movement to take up the slack,
a larger MC diameter gives less pedal movement but more pedal pressure.
Another variable that you can play with is the leverage ratio of the brake pedal lever.

Moderator note: Moved to GRM forum.

Right. As I understand it, you need to get the MC that will move the volume you need using the smallest bore in order to keep the pressure high. Then if you have pressure to spare you can up the bore to reduce pedal travel.

If I'm putting 80lbs on the pedal that will be 500lbs onto the MC push-rod. Divide that by the area (0.31 sqin) and you get 1613psi. This sounds like it's in the "OK but not amazing" bracket.

Be careful with that little Wilwood- it's a single channel master. Meaning, it's a single outlet and you'll be tee'ing off it to feed all four brake calipers. Spring a leak somewhere along the way and you lose all brakes. It's meant to be run in a configuration of two master cylinders with a balance bar. I had one on my manual brakes E30 but got too paranoid, and have since switched to a Porsche 914 master.

erohslc hit on some of the big points. let me go a little further:

on a car with that much static rear weight bias, why aren't you running a larger rear caliper? that weight distribution sounds Corvairy to me, and you could do a lot more brake work at the rear with some more piston area back there.*

*depending on things like CG height, wheelbase, tire size and type, rotor diameter, etc.

also, consider how much of the MC stroke you'll have to use to provide the required fluid volume? multiply that by your pedal ratio and that's how far your pedal will travel to build your target pressure. long travel is easy to modulate but is disconcerting. you also want to have some reserve travel for things like friction decreasing at higher temps, pads becoming more compressible at higher temps, pads becoming tapered with use therefore requiring more fluid volume to get uniform clamping force on the rotor, etc.

Here is a handy little Excel spreadsheet that might be able to help get you into the right ballpark...Brake Components Calculator