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Author: Subject: Quicker hack for calculating master cylinder size
rcx718

posted on 7/1/16 at 11:08 AM Reply With Quote
Quicker hack for calculating master cylinder size

I know there's ways of calculating what size master cylinder to fit into a pedal box. But to save me reading through all that technomumph I'm thinking of a quicker method.

If I know what size the original master cylinder was that came with my calipers from factory. And I have the original brake pedal assembly to measure the pedal to MC ratio. And I'm using the original discs that came with the donor calipers.

Could I just proportion the master cylinder size to account for the altered pedal ratio and for the lack of braking assistance?

I know basic hydraulic equations so I know how to proportion. But I don't know how you factor in for lack of power assistance.

Also some people are saying to start with a 0.7" master cylinder and then move up/down sizes. Seems a costly way of doing it though?

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sdh2903

posted on 7/1/16 at 11:34 AM Reply With Quote
If your running a pedal box with a balance bar then its usually 0.625 front and 0.7 rear as a good start. My last car had this and only needed a slight tweak to get more fwd bias.

For a tandem cylinder I've been reading up and the consensus is somewhere around 19mm bore is good for a non servo system. The standard servo assisted mastercylinders tend to be up at 22 or 23mm and give a dead/hard pedal with little feel.

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rodgling

posted on 7/1/16 at 01:17 PM Reply With Quote
Do the calcs, get it right first time. If you hit up google you'll find a good spreadsheet for working it out where you just enter the numbers (car weight, disc size, calliper piston size, etc) and it will work it out for you.
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rcx718

posted on 7/1/16 at 01:25 PM Reply With Quote
quote:
Originally posted by rodgling
Do the calcs, get it right first time. If you hit up google you'll find a good spreadsheet for working it out where you just enter the numbers (car weight, disc size, calliper piston size, etc) and it will work it out for you.


Ooh, do you remember any keywords to make it quicker to find?

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rodgling

posted on 7/1/16 at 02:14 PM Reply With Quote
quote:
Originally posted by rcx718
quote:
Originally posted by rodgling
Do the calcs, get it right first time. If you hit up google you'll find a good spreadsheet for working it out where you just enter the numbers (car weight, disc size, calliper piston size, etc) and it will work it out for you.


Ooh, do you remember any keywords to make it quicker to find?


This is the one I used (which worked well for me):

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwju2v2l8ZfKAh XDtA4KHWLKAE4QFggfMAA&url=http%3A%2F%2Fvitessesteve.co.uk%2FResources%2FGTBrakingmodel.xls&usg=AFQjCNGwMX-D0FX4UHV0v0OmaagSuba8xQ&sig2=8za luwiwxFypVlRCoixgZA&bvm=bv.110151844,d.ZWU

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rcx718

posted on 7/1/16 at 05:12 PM Reply With Quote
Thanks. I'll be having some fun with that.
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rcx718

posted on 21/1/16 at 09:11 AM Reply With Quote
I've had a play with the braking force calculator. I didn't think there would be so much to it! I see now why lots of people say to just experiment with master cylinder sizes.

The Excel calculator at the end gives the brake forces at each axle based on what numbers you've put into the calculator. But how do I know what brake force is required?

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rodgling

posted on 21/1/16 at 10:06 AM Reply With Quote
For your leg or for the car?

For your leg, mock up your seating position with some scales against your feet and figure out what sort of force you want to be applying under maximum braking.

For the car, if you're planning to use track tyres like R888s, I suggest aiming for a maximum braking force of around 1.5g. Maybe a bit more for slicks, bit less (1.25g?) for road tyres.

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rcx718

posted on 22/1/16 at 08:53 AM Reply With Quote
quote:
Originally posted by rodgling
For your leg or for the car?

For your leg, mock up your seating position with some scales against your feet and figure out what sort of force you want to be applying under maximum braking.

For the car, if you're planning to use track tyres like R888s, I suggest aiming for a maximum braking force of around 1.5g. Maybe a bit more for slicks, bit less (1.25g?) for road tyres.


Ah yes, the "g" calculations. I can do that.

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