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.

The m/c and rear calipers are both from the donor E34, the only different parts are the sierra front calipers. I still have the donor front calipers, perhaps I could look into fitting them instead of changing the m/c? Would need bigger discs though.

im not sure greenstuff are that great either.... mintex 1144 and 1155 always seem to come much more highly rated on my vx220turbo.

I have used EBC black, green and reds. Reds were good with a bit of warmth in them. Green not impressed with, black were good for a road pad.

It seems that the brake system get over engineered thinking that it will be better than the original and we tend to lose the plot with large calipers and discs only to find exactly what this thread is about.

The donor whatever it is weighs possibly three times the Roadster and the brakes are made accordingly.

The clamping force is paramount and size is secondary,but larger is better with regard to heat dissipation.

Pad material again is a consideration as you need vehicle mass and pedal pressure to generate heat and the Roadster is light.

I go back to my post and still think its the mixture of parts that is mismatched with regard to m/cyl bore and caliper piston sizes.

Larger discs will require bigger carriers.

Yeah i understood, and that's why i suggested I might look into the possibility of fitting the donor E34 BMW front calipers so it would then be the entire system as taken from the donor, with the omission of the hydraulic servo.

At the moment, the only mismatch is the sierra front discs/calipers.

Sam
dont under estimate the amount of assist the servo gives
just think how crap the brakes are on your road car without the engine running .
as soon as you pump the brakes and use up any stored vacuum in the servo the brakes are non-existant.
ie. the servo was doing all the work
i think you need to increase the line pressure as the brakes are probably more than adequate .
as said by someone before a reduction in m/cyl bore size would increase line pressure
cheers les g

Good call Les,I used to think the opposite that larger gave more force untill proven wrong.

Good call Les,I used to think the opposite that larger gave more force untill proven wrong.

The race car I have built in conjunction with Saturn has used a G20 Ginetta as a donor and the pedal box has two cylinders and the front is .70 and the rear .75.

May be the answer to the problem Sam fitting the Bee Em parts,hope you get it sorted.

Does anyone know the dimensions of a sierra master cylinder and servo assembly?

I'll have a look at it tonight and see if I can fit it in. The problem with running a big V8 is it takes up most of the room in the engine bay.

Sam
why why not fit a remote servo it dont even need to be close to your master cylinder
Check out Car Builder Solutions i,m sure they do them
cheers les g

Hi Les,

yeah I've spotted the remote servo's, unfortunately I would need to run 2 as I have dual circuit brakes. As they are nearly £200 each it's not exactly something i can stretch my budget to.

Good idea though.

Hi aerosam, there are a number of articles on the web regarding mcyl sizing and pedal ratios which should help, basicaly a smaller diameter mcyl will give you more brake pressure with slightly more pedal travel and you shold have a pedal ratio of about 6 to 1. Here are a couple of interesting articals; http://www.hotrodheaven.com/tech/brakes/ and http://www.markwilliams.com/braketech.aspx . Cheers Brendan