'75 Duster B body front brake upgrade impressions wanted

72bluNblu said
So, a couple things. First, I think your math is incorrect. Ft/inches isn't a useful measurement, so, somewhere your units may have gone wrong. Inch pounds, foot pounds, pounds per square inch, something like that. I would have assumed it's a typo, but you put it down multiple times. The numbers also don't match up with anything I'm familiar with, you may also have an order of operations issue with your equation (I tried to correct it for inches, but that didn't match up either). Brake force should typically be in the several thousands of pounds range, not tens of thousands.

There are EBC pads available for the mopar calipers. You can get EBC yellowstuff and redstuff pads from Summit. The EBC redstuff pads supposedly have a .50 friction coefficient. I'm sure there are others out there.
1975 DODGE DART EBC Brakes DP3678C EBC Redstuff 3000 Series Ceramic Brake Pads | Summit Racing

In general, the large single piston calipers mopar used do generate more clamp force, at least by the math, than newer multi-piston set up do. However, if you've changed the pads on the mopar calipers a bunch you know they frequently don't wear evenly, which means they're not putting all that theoretical force to the rotor. The multi-piston systems tend to be more efficient than single piston calipers. What that efficiency rating actually works out to I don't know.

As far as the 13" cobra style brakes I run, also remember that there are several sizes for those caliper pistons- 38mm and 40mm were available stock depending on the year, and some aftermarket calipers are up to 45mm pistons.

For example, 2.6" pistons with 10.87" rotors vs 2.75" w/10.87 and 2.75" w/11.75" rotors. Normally I run this calculator with 70 pounds for the pedal force, that's a bit more realistic. But I ran it with 106.145 to match your 1000 psi line pressure so you can compare your math and see where you went wrong. This is with a 15/16" master cylinder and the standard 6.5 pedal ratio. The result shows the ~9% increase in total force between the 10.87 and 11.75" rotors (with the same calipers) I was talking about earlier, although you can get that just from the change in the lever arm (rotor effective diameter), because the caliper force doesn't change unless you also change the piston diameter.

View attachment 1715655258

Ok, I can match your numbers (for the most part) by taking my effective rotor diameter and converting it to feet, and thus returning lb/ft (or ft/lbs). In the end, the calculation is really only useful if the units are the same when comparing. So, don't see anything wrong in my calculations, other than not doing the conversion to feet on the effective rotor diameter.

If I convert to feet, I get the following numbers:

10.87 w/ 2.6" piston - 1627
11.75 w/ 2.6" piston - 1783
10.87 w/ 2.75" piston - 1820
11.75 w/ 2.75" piston - 1745
13" w/ Cobra calipers - 1286
13" w/ C5/6 calipers - 1453

One place that can significantly upset any comparisons between your numbers and mine is the way the swept area is dealt with. In my spreadsheet, it was simpler to use the total rotor diameter and the width of the pad at the centerline to calculate the effective rotor diameter. I did this because I didn't have one to measure from and it was easier to find the pad dimensions and assume it was at the edge of the rotor. I know they are probably 1/16" or more down, but I figured it was good enough to compare the numbers between my calculations. Looks like yours requires you to input the effective radius, so you must be coming up with a number somehow, and we ended up not matching there.

So in my calculations, my effective rotor diameter ended up being 9.1935" for the 10.87 rotor and 10.0735" for the 11.75" rotor. This is a pretty good departure from your numbers. I got to my numbers based on a pad width of 1.6765". Now, it's been 9 years since I started this spreadsheet, so I can't say for sure where I got it other than probably RockAuto since I think I got a fair amount of info off that site. I think I probably took a picture from one of the pad manufactures that had dimensions on it and scaled the width.

An image like the fourth in this link - More Information for CENTRIC 10200840

In regards to the 1000psi at the piston, it was a general number I read about some time ago when I was looking at building my own custom brake setup and trying to get a handle on a direction. As I recall, the point was that the pressure out of the MC should be 1000 psi and the whole MC bore size and pedal ratio was a different calculation and I didn't really care since most of that was pretty well fixed by then. But I am reaching way back to remember what all was going on. My only point is, that the 1000psi number is similar to the coefficient of friction in that I standardized on a value so that I could look at the specifics of the rotor size and piston diameter(s).

Lastly, I have looked for bigger piston calipers and have come up pretty blank. All of the Cobra calipers I have found online are 1.5" (38.1mm) pistons. I used the C5 calipers on my swap because they us 40mm pistons, and just recently discovered that the later Challenger calipers look to be a direct swap for my C5 calipers and have 42mm pistons. I did find 51mm piston calipers but they are on GM pickups and appear to be significantly larger over all and thus don't swap. I would love to step up to a 45mm piston caliper that bolts to my existing abutment. Not argue that they aren't out there, only that I can't find them.