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January 24, 2010 - After a fair bit of calculation, I'm starting to figure out my brake options.
From my measurements, it looks like the MG brake pedal has a ratio of 4.48. That means that 100 lbs of pressure on the pedal is transformed into 448 lbs on the master cylinder. It also means that it takes 4.48" of pedal movement to move the master cylinder pushrod by 1". Basically, it's the same as a gear ratio. The Miata pedal has a ratio of 4.1 according to the manual.
I know the Miata setup pretty well, and the Targa Miata uses the hydraulics from a later Miata setup. The standard Miata master cylinder is a 7/8" unit, and the Sport master is 15/16".
As you can see from the picture, the stock MG master cylinder is 13/16".
So, based on pedal ratios and master cylinder sizes, I compared how much force on the pedal is needed to get 250 psi of line pressure. Why 500 psi? No reason, it's just a constant so I can get an idea of how the different setups would compare in feel for the same amount of braking. I also figured out how far the pedal would have to move in order to push 1 cubic inch of brake fluid, as the leverage ratios and hydraulic multiplication always trade off travel for effort. Here's what popped out of my spreadsheet:

  • MG pedal, MG master: 57.87 lbs, 4.32"

  • Miata standard setup: 73.3 lbs, 3.41"

  • Miata Sport setup: 84.2 lbs, 2.97"

  • MG pedal, 1" Wilwood master: 87.7 lbs, 2.85"

So no surprise here, the further you move the pedal the less you have to push on it. Simple leverage. And if you're doing these calculations at home, remember that it's a dual circuit system so you'll get half the pressure and twice the fluid flow as you would with a single circuit of the same size.
According to my calculations, a 15/16" master on the MG pedal (77 lbs, 3.25")would be pretty much right between the two Miata types. But the dual circuit remote reservoir master cylinder available from Wilwood is a pretty unusual piece, and only available in a 1" bore. I need the remote reservoirs to clear the hood, I believe. I do have a few other single circuit master cylinders I can use for test fitting for clearance.
Still, since Janel and I both prefer a nice firm pedal, I'll probably go with the Wilwood 1" setup.

But of course, there's a catch. The booster. All the calculations above give the numbers for an unassisted brake setup. The booster will multiply the force on the pedal to give more force at the master. If the booster has a 2:1 boost ratio, that means it would only take a bit less than 44 lbs of force on the pedal to generate my 500 psi of line pressure, instead of 87.7. And here's the problem. I don't know what the boost ratio of the MG booster is. The Miata one should be around 4.6 according to the manual.
So the end result is going to come down to trial and error, with a bias towards a too-firm pedal instead of a too-soft one.

This stuff is fun. I get to learn all sorts of things.
tags: brakes