Weak Brakes - Master Cylinder?
 

Hi guys - had my car out on track yesterday, and the brakes were pretty lacklustre. It's all working correctly, stops in a straight line,they're just a bit weak, I really have to stand hard on the pedal.

They are sierra fronts and BMW rears (discs). The rears are huge so power is re-directed forward by a bias control valve. The master cylinder is out of my donor BMW E34 5 series, and it had a hydraulic powered servo, which I didn't use as it was very long and the hydraulics were complex.

So, I'm guessing the unassisted master cylinder simply can't provide enough stopping force. Isn't the roadster normally fitted with a sierra master cylinder without the servo? Is there room for the servo, if required?

I'm thinking of replacing my donor cylinder with a sierra one, with servo, in order to seriously upgrade my braking power. Unless you guys can think of anything else that would be hold the brakes back?

By no means am i braking expert :p but just by reading what you have said i too would assume its the reason of a non-servo system, i also assume that there is no "spongy-ness" ?
:)

You don't want or need a servo on such a light car, what pads are you using as normal road pads won't take much punishment on a track day they will fade!!!

What is the diameter of the E34 master cylinder compared to the sierra one? Have you checked the hydraulic ratio compared to the original sierra setup?

To answer all 3 questions:

1. No there's no spongeyness, brakes all bled nicely.
2. EBC Greenstuff pads all round
3. Don't know the dia of the ATE master cylinder - still trying to discover that. I' slightly embarassed to say I don't know what you mean by hydraulic ratio - could you explain please? Thanks!

Ok - after lots of digging through BMW websites, I've found the master cylinder is 25mm diameter.

No expert either but would suspect its the mixture of components that dont match and cylinder/piston and m/cyl bore sizes are incompatable.

I've been busy with my calculator (well excel to be exact)
Making some simple assumptions -
BMW master Cylinder 25mm diameter
Ford master Cylinder 22.2mm diameter
Ford Caliper Piston 54mm diameter

If you assume that a force of 100lbs (45kg although that is not strictly a force before anyone says anything!) is applied to the master cylinder, then for each combo the force at the pad would be in the region of -

BMW MC 466lbs
Ford MC 591lbs

So basically with the BMW master cylinder you are only seeing about 78% of the pad clamping force you would with the ford unit.

Hope that is some help.

Makes a lot of sense David,add the size of the rear brakes to the mix and Bobs your uncle.

The rear calipers are sliding units with a 60mm piston.