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Old 12-26-2009, 11:23 PM   #1
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Default Some brake calcs

Just thought I'd share some calcs I ran on the new V-8 Roadsters BBK. I've been running numbers on every combination I can think of (easily 20 combinations so far), so it's not like I went to a book and looked up a bunch of formulas just to come up with this post. In fact quite the opposite. I plugged the numbers into the spreadsheet basically just for thoroughness, and when I saw the result, felt it appropriate to share. I'm not selling any caliper upgrades for Miatas, and don't work for a company that is or has any plans to. Still, some may take this as pissing on Emilio's campfire, so in order to try not to start a flame war in a sponsor's thread promoting his latest product addition, I started a new thread.

With the stated piston sizes, this BBK has 53% more front caliper piston area (looking only at the pistons on one side of the caliper) than stock and 138% more rear caliper piston area (essentially like two of the '01+ Sport rear calipers on each side of the car). The 1-in. master cylinder has only 31% more piston area than a 7/8-in. stock one.

When you further consider the effective radii of the rotors and pads (neglecting differences in friction characteristics), for a given hydraulic circuit pressure, the front has 88% more brake torque than stock and the rear has 175% more brake torque than stock.

Someone else may round differently or use a different method to calculate effective radius and thus come up with slightly different percentages. I'm not going to debate that, nor am I going to predict or debate the likely implications of these numbers, nor the design elements of the kit as described. I may not even post in this thread after this. I am going to sit back with my popcorn and watch what happens. Kinda curious how it runs.
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Old 12-26-2009, 11:44 PM   #2
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brake torque requires knowledge of pad compounds and friction coefficients.

but adding more rear bias on a car with R-comps will probably lead to death by trailbraking.
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Old 12-27-2009, 12:49 AM   #3
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Originally Posted by y8s View Post
brake torque requires knowledge of pad compounds and friction coefficients.
Getting exact numbers does yes... but if you assume same pads in both cases it doesn't make a difference when comparing systems because Cf stays the same.
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Old 12-27-2009, 12:57 AM   #4
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Did you run the calcs on the optional six pot fronts?
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Old 12-28-2009, 06:43 PM   #5
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I hope by effective radius of the rotor, you mean center of area radius? Which would be somewhat difficult to do since most brake pads are not a simple geometric shape. However this would be the true lever arm of the pads to the rotor. This would be a very simple way to determine the effective torque given the same brake pads, and applied force on the rotor.

In order to do an accurate comparison you will need the center of area of each pad then the surface area of each. Next you could assume the same brake pads and rotor material for each. However, the line pressure must be known for each since they pads are of different areas.

This might be useful to find the theoretical torque appiled on the rotor, but this doesn't really tell you much about how the brakes will perfom.
Heat transfer is a HUGE part of desiging brakes and assesing there performance capabilities.

If you would like to go over your brake calcs more thoroughly, I would be happy to. Pm me or post what your spread sheet and how you derived your numbers, or what exactly your trying to compare? torque on the rotor?

For crediability, I designed our brake rotors and did all the brake calcs for our formula car at Oregon state University. And it would be fun to do some work or comparison on miata stuff
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Old 12-28-2009, 07:09 PM   #6
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post the spreadsheet, id like to take a look.
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Old 12-28-2009, 07:30 PM   #7
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prop valve... ///thread
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