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Author: Subject: Quick Question thats bugging me
AntonUK

posted on 11/9/18 at 09:07 PM Reply With Quote
Quick Question thats bugging me

Brake master cylinders....Everyone had differing sized master cylinders to control front and rear brake balance.

Thinking of dual masters (the girling style typically) should the larger of the pair be used for the front or the rear brakes?

I keep changing my mind... my current thought (which means my car is wrong) is the smaller should be used for the front as this creates a higher pressure on the caliper pistons improving performance...

I know it should be simple, but the more I think about it the more I confuse myself.





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tegwin

posted on 11/9/18 at 09:18 PM Reply With Quote
Mmm.

Smaller diameter will exert less pressure than a larger one surely for the same stroke.

Id have said the larger one would be fronts so there is higher pressure and thus the fronts lock first.

The above assumes the slave cylinders are all the same size.

[Edited on 11/9/18 by tegwin]





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adithorp

posted on 11/9/18 at 09:43 PM Reply With Quote
Small on the front and large on the rear.





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StrikerChris

posted on 12/9/18 at 03:29 AM Reply With Quote
got me thinking at the end of a nightshift now,and its a ling time since college! If force =pressure x area, my understanding is, for the same force,ie your leg is applied to a smaller mastercylinder you'll get more pressure in the brake line

i.e 100 leg, = (10area of mastercylinder) x (10pressure on caliper) so for a smaller mastercylinder

100leg = (5area of mastercylinder) x (20pressure on caliper) and

50leg = (5area of mastercylinder) x (10 pressure on caliper)

or I might have that completely arse about face and just added to the confusion,

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snapper

posted on 12/9/18 at 05:56 AM Reply With Quote
Why would you bias the brakes this way when the bias bar is designed to do that anyway?





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JC

posted on 12/9/18 at 06:33 AM Reply With Quote
It also depends on the size of the calipers! On my midi I had M16 front calipers at the front and VW at the rear. I didn’t have a balance bar so did the maths and the fronts would have been way overbraked with a conventional ‘big mc to front, small mc to back’ set up. I ended up using a Mini stepped master cylinder but plumbed opposite to the way it was in the mini - small to front and large to back. With the axle weights it meant the fronts still locked first, but there was at least some braking effort on the back.
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AdamR20

posted on 12/9/18 at 07:33 AM Reply With Quote
Depends on a lot of things. Head to www.brakepower.com to see what I mean!

I ended up with 0.75 front and 0.625 rear on my previous Westfield, the current one will use 1" front and rear. Most people run the small one on the front (and have crap brakes )

[Edited on 12/9/18 by AdamR20]

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Ivan

posted on 12/9/18 at 07:57 AM Reply With Quote
quote:
Originally posted by tegwin
Mmm.

Smaller diameter will exert less pressure than a larger one surely for the same stroke.

Id have said the larger one would be fronts so there is higher pressure and thus the fronts lock first.

The above assumes the slave cylinders are all the same size.

[Edited on 11/9/18 by tegwin]


I don't agree - smaller pistons will always exert more pressure for the same stroke than larger piston when pushing against the same size caliper piston. Which MC piston is connected to which end of the car will depend on the relative piston sizes of the front and rear brakes. The decision on which way to go will depend on your brake balance. If your car is way over braked in the front (the front locks up when you brake lightly then look at using the bigger MC to feed the front. If way over braked at the back use the larger MC on that side.

Normally the choice is Smaller MC in the front and larger to the rear and minor imbalances tuned out with the balance bar. In 7's using donor parts and with their very different CG location to the cars that supplied the brakes the normal MC size ranges need another look, as the rear will often be under-braked and need smaller MC to give the right balance. In my Cobra using Jag XJ6 brakes front and rear it is difficult to get the rear brakes hot during track days so they are significantly under contributing to the braking effort whilst the front is easy to lock up and over heat.

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tilly819

posted on 12/9/18 at 09:46 AM Reply With Quote
Pressure = Force / Area (MPa = N / mm^2)

For the largest braking force you want the smallest master cylinder connected to the largest slave cylinder.
Therefore, small master cylinders for the front and larger master cylinders for the rear if everything else is equal.

Hope this helps.
Tilly

[Edited on 12/9/18 by tilly819]





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