73 duster brake question
First, let me clear up a misconception. Just adding power brakes does
nothing to increase your stopping power. Nothing. Adding a power booster reduces the amount of pedal effort needed, it does not increase the stopping power. Changing the master cylinder bore size can change the line pressure which does change the clamping force, but that's true without the power booster.
Someone is going to throw out the answer to go to 11.75 disk brakes vs 10.95 brakes. That's only a 7.3% increase for all the extra work plus probably 15 inch rims.
Um, yes, 11.75 is a 7.3% increase in the diameter of the rotor from 10.95, but again that has
nothing to do with the change in stopping power. You need to use areas and lever arm calculations, not the percentage increase of the diameter. That number doesn't mean anything.
There are several things going on when you swap to 11.75" rotors. First, the diameter of the rotor is larger, so, the lever arm acting between the caliper and the center of the hub is longer, and that directly increases braking force. On the stock 10.95" disks, the middle of the brake pad falls at ~4.59" from the spindle axis. On the 11.75" disks, it's 4.975". That might not sound like a huge difference, but that represents an
8.4% increase in braking force because you multiply the force applied at the pad by the length of the lever arm.
You're also increasing the braking surface area. The pad area is the same, but because the pads are further out they sweep out a larger area as the rotor turns. Applying the same amount of force over a larger swept area means the brakes don't heat up as fast, and there's more area to dissipate heat as well. If you figure the outside of the rotor is 11.75" and the width of the braking surface is 1.8" ( I measured it), then the total braking area is π (11.75/2)^2 - π(9.95/2)^2 = 30.67 square inches for the 11.75" rotors. For the 10.98 rotors the braking area is π(10.98/2)^2 - π(9.18/2)^2 = 28.5 square inches . That's an 7.6% increase in surface area on the rotor.
And both of those assume that you keep the A-body caliper. The 73+ A-body calipers use 2.6" pistons. The B/E/F/M/J/R body calipers had 2.75" diameter pistons. Don't be fooled and think this is a 5.7% increase, you must use the area, not the diamter. The 2.75" piston has a surface area of 5.94 square inches. The 2.6" piston has an area of 5.31 square inches. That's an 11.9% increase in area for the piston, and if you keep the line pressure the same that's an 11.9% increase in braking force because you're multiplying the line pressure PSI by the surface area of the piston to get that braking force.
All of those things combine to increase the stopping power. The direct increase is the 8.4% if you keep the same calipers and upgrade from the 10.95" rotors to the 11.75" rotors, which you can do. Under heavy braking conditions, the larger surface area will also reduce brake fade by slowing down heating and speeding up cooling. That doesn't change braking force, but it will improve braking performance under difficult conditions, and that's a big deal. And you can change the calipers to the later larger diameter piston too. If you assume the same brake line pressure, that's an 11.9% increase in force on the pads. If you increase both the diameter of the rotor and the diameter of the caliper pistons, your 11.9% at the caliper multiplied through by the 8.4% increase in lever arm turns into
21.3% increases in braking force at the spindle which is what's acting on the wheel. That's a
big difference.
But remember that just because you increased the braking power by 21.3% doesn't mean you've decreased your stopping distance by that much. And really, that's what counts. Stopping distance is tied directly into the tires, the better the compound you have there the more friction you have and the more you can benefit from the larger brakes. I've done the 11.75" rotor upgrade a couple of times now. On my Challenger all I did upgrade the rotors and caliper brackets. The calipers for the E-body were already 2.75", so, no increase there. Even just that upgrade was noticeable out on the streets, the brakes felt much better, and can lock up the 275/40/17's I have on the front of my Challenger just fine. On my Duster I went to 11.75" rotors and changed the calipers to the later ones as well. Again, a very noticeable improvement even with just the 225/60/15's I had on the car at the time. Now, I doubt my stopping distance made a big jump because it would be tire limited with those small tires. But braking felt a lot better, more confident. And I'm sure it reduced the stopping distance a little even under normal conditions. Under repeated heavy braking it would have held up better too.
You do need 15" rims to use the 11.75" rotors.
