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  #1  
Old 15th May 2011, 11:07 AM
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aerosam aerosam is offline
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Default 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?
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Old 15th May 2011, 11:17 AM
michael92 michael92 is offline
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By no means am i braking expert 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" ?
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Old 15th May 2011, 11:51 AM
MarkB MarkB is offline
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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!!!
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Old 15th May 2011, 01:20 PM
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davidimurray davidimurray is offline
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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?
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Old 15th May 2011, 03:43 PM
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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!
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Old 15th May 2011, 04:01 PM
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Ok - after lots of digging through BMW websites, I've found the master cylinder is 25mm diameter.
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Old 15th May 2011, 06:38 PM
flyerncle flyerncle is offline
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No expert either but would suspect its the mixture of components that dont match and cylinder/piston and m/cyl bore sizes are incompatable.
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Old 15th May 2011, 07:13 PM
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davidimurray davidimurray is offline
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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.
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Old 15th May 2011, 08:43 PM
flyerncle flyerncle is offline
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Makes a lot of sense David,add the size of the rear brakes to the mix and Bobs your uncle.
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  #10  
Old 15th May 2011, 11:18 PM
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The rear calipers are sliding units with a 60mm piston.
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