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Brake Master Cylinder Sizing - Stroke Considerations
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Author:  planet6racing [ Fri Sep 04, 2020 5:00 am ]
Post subject:  Brake Master Cylinder Sizing - Stroke Considerations

All:

I am working to improve the existing braking system in my car and am moving to a dual circuit race setup with Tilton pedals and master cylinders. As such, I am trying to perform the calculations for master cylinder size to know how much force input will be required to meet the previous performance.

I am also looking at how much stroke will be required to displace the brake caliper pistons a specified amount. This is where I am getting confused. My calculations show that, for an 80 lbf input and a 5:1 pedal ratio, I can generate 400 lbf of force going into the Master cylinders (assuming the bias bar is set to the middle 1:1 position). Using a 0.875" master cylinder size for the front and a 2" caliper piston size (single piston caliper), I get a pedal stroke of 0.522" to move the piston 0.010" (arbitrarily picked number). Using a 0.625" master cylinder for the rear and a 1.25" caliper piston size (again, single piston caliper), I get a pedal stroke of 0.400" for the same 0.010" of piston movement.

My confusion is with this difference. Do I want to balance these front to rear? My initial thought was that, yes, these should be balanced such that, as I push on the pedal, I do not have the caliper pistons in the rear generating torque before the fronts. That said, as I thought while typing this, wouldn't the system balance for that on the return pedal stroke and "suck" the rear piston back farther (closed hydraulic system) such that pressure builds at the same time?

Thank you in advance for any help and advice.

Author:  Bob Clark [ Fri Sep 04, 2020 5:28 am ]
Post subject:  Re: Brake Master Cylinder Sizing - Stroke Considerations

Call me at work...

Bob Clark

Author:  Al Seim [ Fri Sep 04, 2020 5:29 am ]
Post subject:  Re: Brake Master Cylinder Sizing - Stroke Considerations

A. Jay Lutz (aka Brake Engineer) is the best person on this forum for all things brakes.

B. But I can help some from a mechanical engineering perspective:

A typical dual master cylinder / bias bar setup, assuming that it is bled etc, is inherently going to develop pressure in both circuits / cylinders simultaneously. Think of the balance bar as "resting" on BOTH master cylinder pistons, with the pedal pressure in the middle of the bar. The bar is free to "tilt" and thus won't put any substantial amount of force on one master piston until it is meeting resistance from the other. Think of two paint cans two feet apart with a 2x4 laid across the top and you standing in the middle. Doesn't matter if one can is an inch taller than the other, both will see half your weight if you are half way in between.

Also - the master really isn't "sucking" the caliper piston on force release, the caliper piston (aided by the pad, pad knockback and hydraulic seal and groove profile, are going to push fluid back into the master. Any "sucking" will result in fluid entering the master from the reservoir.

Hope that helps!

Al Seim

Author:  Bob Hess [ Fri Sep 04, 2020 10:41 am ]
Post subject:  Re: Brake Master Cylinder Sizing - Stroke Considerations

Not sure if comparing Spridget brakes with Saturns is possible but we use the Tilton 3 master cylinder setup on all our cars, some with drum rears, some with Willwood caliper rears. Have run both combinations with stock front calipers and the slightly larger MGB calipers. We set the rears to put the brake pressure just a little less than the fronts and use test days to fine tune the settings. If you are a trail breaker like I am the settings have to be almost perfect and set for how you drive.

One thing we found that affected the less than huge brakes on our cars was the master cylinder size. Tried both the 5/8s and 3/4s and found the 5/8 gave better control and "feel" over the 3/4. All that is relative to the size calipers you have as the GT1 cars do the same thing you are trying to do but with much larger hardware, but ironically not that much bigger master cylinders. Hooking up brake pressure gauges to your master cylinders and hitting he track is a great way to set up the system.

Bob

Author:  chois [ Fri Sep 04, 2020 11:15 am ]
Post subject:  Re: Brake Master Cylinder Sizing - Stroke Considerations

Tilton will also do the analysis and make a recommendation if you contact them.

Author:  racingfool [ Tue Sep 08, 2020 8:51 am ]
Post subject:  Re: Brake Master Cylinder Sizing - Stroke Considerations

I recently ran across this. Learned quite a bit.

https://www.joesracing.com/master-cylinder-math/

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