'75 Duster B body front brake upgrade impressions wanted

[340fourspeedDuster]

I have a 1975 340 four speed Duster. I am currently running the factory 10.95" disc brakes. The master cylinder is the 15/16" manual system with metallic pads and ss front lines. Anyone with the B body 11.75" brakes on their A body car, please chime in. Any stopping distances recorded before/after the upgrade? Looking to shortening the stopping distance at a discount price on parts vs the Wilwood systems. Leaning towards the Dr. Diff stage 2 system. Hesitant that the discs are "drop Shipped" from a supplier.
I am not racing this car. It is a fun weekend driver that I would like to have shorter stopping distances. The car has been lightened somewhat. It had the heavy front and rear bumpers (5 mph) removed and replaced with glass front, and a '73 bumper rear. That saved nearly 50-60 pounds on the ends of the car. The car has the factory anti-sway bar up front, and weld-in frame connectors. The 1.03" torsion bars have been installed replacing the 45 y.o. .867" bars. Wheels are 17" with BFG G-Force tires. Suggestions please.

 

[72bluNblu]

The 11.75’s are a nice upgrade from the original 10.95’s. Just the rotor swap by itself results in about a 9% increase in braking force because of the longer lever arm.

Your ‘75 may already have them, but it would also be a good time to check if your calipers have the A-body 2.6” piston bore or the 2.75” piston bore used by the B/E and later A bodies. I think ‘75 was the swap over year.

I ran 11.75’s on my Challenger for 70k miles of daily driving, they work well. A bunch of that was with 275/40/17’s on the front, and with those tires the braking was pretty darn good. I also ran them on my Duster for awhile, they work great with 11x2.5” drums out back to keep things balanced. The Duster was upgraded to DoctorDiff stage III brakes, I run the 13” cobra style kit now. But the 11.75’s are a great less expensive option. They’re very reliable, and if you do the math on the caliper clamp force they’re capable of more clamp force than the standard Wilwoods too.

And don’t worry about the drop shipping. The 13” rotors were the same way. They took a couple days longer than the rest of the parts that came straight from Cass, but they’re quality parts.

 

[cudamark]

I'd start by seeing which brakes will lock up first.....front or rear. Then get an adjustable proportioning valve and get it to where both ends lock up a the same time. That should shorten your braking distance about as much as you're going to get IMO. Going to bigger brakes will just keep them from wearing out as fast and keeping pedal fade from happening as easily. Going to a tire with a better traction rating can help on your stopping distance too.

 

[AndyF]

The 11.75 setup up front is a good idea. Mopar never put the big brakes on the A body car but no reason you can't. Don't forget the rear brakes. If you a big bolt pattern rear axle then you should be able to swap over to the biggest B body drum brake. Just have to search around to find the correct backing plates. Not sure if anyone is reproducing them or not.

 

[72bluNblu]

I'd start by seeing which brakes will lock up first.....front or rear. Then get an adjustable proportioning valve and get it to where both ends lock up a the same time. That should shorten your braking distance about as much as you're going to get IMO. Going to bigger brakes will just keep them from wearing out as fast and keeping pedal fade from happening as easily. Going to a tire with a better traction rating can help on your stopping distance too.

Big brakes don’t just increase fade resistance and wear. Yes, the swept area is larger which improves both cooling and wear. But the caliper being further away from the hub also increases the “lever arm” used to calculate the braking force. It’s a direct increase in braking force. About 9% in this case, which doesn’t sound like much but it makes a big difference. I’ve run these cars with both the 10.95 and 11.75” rotors, the 11.75’s make a noticeable improvement.

The OP is already running more modern tires, so he will benefit from the larger brakes.

As for the prop valve, all that does is reduce the pressure to the rear. It does not change pressure in the front. And since the front brake do more than 70% of the braking, it’s really hard to go wrong with larger front brakes. Even if currently his rear brakes do lock up first, adding the larger rotors up front could very well solve that balance issue.

The 11.75 setup up front is a good idea. Mopar never put the big brakes on the A body car but no reason you can't. Don't forget the rear brakes. If you a big bolt pattern rear axle then you should be able to swap over to the biggest B body drum brake. Just have to search around to find the correct backing plates. Not sure if anyone is reproducing them or not.

Totally agree. I found the stock combination valve worked fine on both my Challenger and my Duster using 11.75” disks up front and 11x2.5” drums out back. There are also two different wheel cylinders available for the rear drums, so you can dial it in even closer. I’m sure an adjustable valve can get you closer than my cars were, but they were better balanced than what the factory sent out.

I didn’t have much trouble finding 11x2.5” rear drum set ups. You just have to be careful if you’re only buying backing plates because the 11x2” and 11x3” backing plates have different axle flange offsets, so you actually need backing plates for an 11x2.5” drum. Buying them or finding them all together makes that a bit more certain, some sellers may not know that there are different plates for different drum/shoe widths.

 

[cudamark]

That's kind of my point. Once the brakes are locked up, they're not braking......the tires are. Getting a good balance between front and rear is the best you can do in a non-ABS vehicle. Changing the modulation, pedal effort, brake fade, etc. can help in overall performance, but, IMO won't change your stopping distance if you can lock up the brakes, both front and rear.

 

[72bluNblu]

That's kind of my point. Once the brakes are locked up, they're not braking......the tires are. Getting a good balance between front and rear is the best you can do in a non-ABS vehicle. Changing the modulation, pedal effort, brake fade, etc. can help in overall performance, but, IMO won't change your stopping distance if you can lock up the brakes, both front and rear.

Just because a particular set of brakes can be locked up doesn't mean that they're already the biggest that will benefit you. Sliding and rolling friction are different values, the time of the application (impulse) matters. If you stomp the brakes as hard as you can and they lock up it doesn't necessarily mean they're too big, a larger brake applied smoothly can still get you more braking to the ground. That's the physics, not an opinion. Guys can lock the fronts with 10x2.25" drums up front, but the stopping distances will clearly tell you 10.95" disks get the car stopped faster.

And the 11.75's will get him stopped faster, I know because I have DONE that swap, and the next one past that. The 13" rotors on the front of my Duster still get it stopped faster than the 11.75's did, with 275/35/18's up front I was working those 11.75's.

And yes, they do have to be balanced front and rear for the shortest stopping distance, that goes for 10.95's with 10x2.5" drums, it goes for 11.75's with 10x2.5" or even 11x2.5" drums, and it goes for the 13" rotors I run in the front with 11.7" disks out back. But every one of those steps gives better braking performance, I've tried them all.

 

[340fourspeedDuster]

I am looking at doing this one step at a time. 11.75" first. My rear brakes on the 8 3/4" are 10" x 1.75". I do have a backing plate to drum set of 10" x 2" in the garage. So it sounds like the Dr. Diff stage 2 setup will be what I am looking for. The tires are the BFG G Force Comp 2 A/S 215/45ZR17 up front. These clear the fender. Putting large drum brakes on the rear (more coefficient of friction) means that there needs to be more rear pressure bleed-off, yes?

 

[72bluNblu]

I am looking at doing this one step at a time. 11.75" first. My rear brakes on the 8 3/4" are 10" x 1.75". I do have a backing plate to drum set of 10" x 2" in the garage. So it sounds like the Dr. Diff stage 2 setup will be what I am looking for. The tires are the BFG G Force Comp 2 A/S 215/45ZR17 up front. These clear the fender. Putting large drum brakes on the rear (more coefficient of friction) means that there needs to be more rear pressure bleed-off, yes?

If you upgraded the rear brakes without upgrading the front, yes, you'd need to use an adjustable valve to lower the pressure to the rears to keep them from locking up first. But, if you upgrade the front brakes first it may not be necessary. The bias needed front to rear depends on a lot of things, the size of the brakes is one. The weight balance of the car, size of the caliper pistons and rear wheel cylinders, tire sizes front and rear, even the suspension set up changes what the bias needs to be.

I know on my cars, when I was running 11.75's up front with 11x2.5" drums out back they were pretty well balanced for brake bias with the factory combination valve.

10x1.75" rear drums? So you're still running SBP axles out back?

 

[340fourspeedDuster]

Yes, SBP axles that I had drilled for the 4.5" pattern. Work well so far. I had a station wagon rear 8 3/4 axle under the Duster for a while. Wide stance in the rear.

 

[72bluNblu]

Oh ok, was wondering what you did about the bolt pattern. With those rear brakes you won't need to do anything with the pressure to the rear. And when you get the chance to go BBP axles and brakes you'll notice even better stopping.

I run a '68-70 B body rear axle in my Duster, I just take care of the additional width with a 7" backspace on my 18x10" rims.

 

[340fourspeedDuster]

72bluNblu, have you tried any aftermarket brake compounds? I relined a set of the Porterfield R4-S shoe linings on my '63 Dodge M37. These are 14" x 2" shoes on all four corners of the truck. Stops much better than the woven asbestos linings that came with the truck.https://www.porterfield-brakes.com/images/portstrt2008_11_06_02_45_30.pdf

 

[DionR]

If anyone cares, here is the formula I found years ago in a Hot Rod magazine (Jan '82) to calculate brake torque:

Tb = (Ph x Ap x u x 2) x Er

Where:

Tb = Torque (brake)
Ph = Hydraulic system pressure
Ap = Area of piston(s) (one side of the caliper on fixed calipers)
u = co-efficient of friction of the brake pad
Er = Rotor leverage length

Basically it takes the average lever arm of the caliper/pad, hydraulic force and pad friction and calculates a torque.

Based on the above, the following brake torque numbers are generated assuming 1000 psi out of the MC and .4 for the coefficient of friction:

10.87 w/ 2.6" piston - 19524
11.75 w/ 2.6" piston - 21393
10.87 w/ 2.75" piston - 21842
11.75 w/ 2.75" piston - 23933
13" w/ Cobra calipers - 15431
13" w/ C5/6 calipers - 17436

In all the above cases, I used the same pressure and coefficient of friction just to compare them. I used .4 for the pad friction as that is (as I understand) a middle of the road number for an FF pad. And all the to above numbers are in ft/inches.

Based on the above, it would seem that an 11.75" rotor with a 2.75" caliper would be the go to solution. It almost equals the brake torque for the big 6 piston Brembo brakes on the Hellcats which calcs to 24239 ft/inches. But there are two problems; 1. the 15.4" rotors on the HC will hold far more heat than the "big" 11.75" rotors and 2. available pads.

On the street, I doubt the big heat sink advantage of the 15.4" HC rotor will ever show itself, so the 11.75" rotor is probably more than adequate. On the other hand, if you want to more brake performance in the 11.75" setup, it's not as easy as just swapping in better pads since they don't make any. Which is why people have to swap to aftermarket kits at some point (besides just wanting bigger rotors to fill in their wheels).

Being that the single piston calipers take a brake pad that (AFAIK) don't have any aftermarket or performance pads available, it's hard to find something with a good coefficient of friction. As an example, if we drop the 11.75" rotor with a 2.75" piston down to a low end EE pad of about .3, the brake torque changes to 17950 ft/inches. At the same time, if the Cobra caliper on 13" rotors gets a GG pad which averages .5, it's torque numbers change to 19289 ft/inches. So pad selection can have a huge impact on the brake torque.

In the end? I think the 11.75" setup is a great way to go if you don't want to jump up to a 13" setup. But if you ever want more, just know that it will probably require a different brake kit all together.

Note that based on the picture on RockAuto, Bendix does sell an FF pad (SBM84) that would work for the 11.75" setup. That should at least give you a coefficient of friction of .35 and put you close the Cobra setup from Doctor Diff.

Lastly, it should be noted that these numbers ore only theoretical and ignore any design flaws that could negatively impact the actual function of the brakes. Sticking pads, caliper flex, stiction in the caliper? Any one of those and probably others will reduce the actual output of the brakes. Which is partly why I think 72bluNblu saw an improvement when he went to the 13" setup; better pads, better pad support design, more pistons to spread the force consistently and create less caliper flex, etc.

BTW, here a link to the brake coefficient of frictions I have been using - DOT Pad Codes

 

[72bluNblu]

If anyone cares, here is the formula I found years ago in a Hot Rod magazine (Jan '82) to calculate brake torque:

Tb = (Ph x Ap x u x 2) x Er

Where:

Tb = Torque (brake)
Ph = Hydraulic system pressure
Ap = Area of piston(s) (one side of the caliper on fixed calipers)
u = co-efficient of friction of the brake pad
Er = Rotor leverage length

Basically it takes the average lever arm of the caliper/pad, hydraulic force and pad friction and calculates a torque.

Based on the above, the following brake torque numbers are generated assuming 1000 psi out of the MC and .4 for the coefficient of friction:

10.87 w/ 2.6" piston - 19524
11.75 w/ 2.6" piston - 21393
10.87 w/ 2.75" piston - 21842
11.75 w/ 2.75" piston - 23933
13" w/ Cobra calipers - 15431
13" w/ C5/6 calipers - 17436

In all the above cases, I used the same pressure and coefficient of friction just to compare them. I used .4 for the pad friction as that is (as I understand) a middle of the road number for an FF pad. And all the to above numbers are in ft/inches.

Based on the above, it would seem that an 11.75" rotor with a 2.75" caliper would be the go to solution. It almost equals the brake torque for the big 6 piston Brembo brakes on the Hellcats which calcs to 24239 ft/inches. But there are two problems; 1. the 15.4" rotors on the HC will hold far more heat than the "big" 11.75" rotors and 2. available pads.

On the street, I doubt the big heat sink advantage of the 15.4" HC rotor will ever show itself, so the 11.75" rotor is probably more than adequate. On the other hand, if you want to more brake performance in the 11.75" setup, it's not as easy as just swapping in better pads since they don't make any. Which is why people have to swap to aftermarket kits at some point (besides just wanting bigger rotors to fill in their wheels).

Being that the single piston calipers take a brake pad that (AFAIK) don't have any aftermarket or performance pads available, it's hard to find something with a good coefficient of friction. As an example, if we drop the 11.75" rotor with a 2.75" piston down to a low end EE pad of about .3, the brake torque changes to 17950 ft/inches. At the same time, if the Cobra caliper on 13" rotors gets a GG pad which averages .5, it's torque numbers change to 19289 ft/inches. So pad selection can have a huge impact on the brake torque.

In the end? I think the 11.75" setup is a great way to go if you don't want to jump up to a 13" setup. But if you ever want more, just know that it will probably require a different brake kit all together.

Note that based on the picture on RockAuto, Bendix does sell an FF pad (SBM84) that would work for the 11.75" setup. That should at least give you a coefficient of friction of .35 and put you close the Cobra setup from Doctor Diff.

Lastly, it should be noted that these numbers ore only theoretical and ignore any design flaws that could negatively impact the actual function of the brakes. Sticking pads, caliper flex, stiction in the caliper? Any one of those and probably others will reduce the actual output of the brakes. Which is partly why I think 72bluNblu saw an improvement when he went to the 13" setup; better pads, better pad support design, more pistons to spread the force consistently and create less caliper flex, etc.

BTW, here a link to the brake coefficient of frictions I have been using - DOT Pad Codes

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.

***Edit- these effective diameters are incorrect. Sorry! See below***

 

[DionR]

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

I actually checked my spreadsheet before posted just to make sure I wasn’t doing something wrong. But will see if I can compare mine to yours and see if I see something.

Ft/inches really isn’t much different that ft/lbs, just 12 times bigger.

I will admit it wasn’t my choice for units, but it works out and is what Marian did in the HR article.

But I will compare later when I get back home.

Will be back.

 

[DionR]

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.

 

[72bluNblu]

I actually checked my spreadsheet before posted just to make sure I wasn’t doing something wrong. But will see if I can compare mine to yours and see if I see something.

Ft/inches really isn’t much different that ft/lbs, just 12 times bigger.

I will admit it wasn’t my choice for units, but it works out and is what Marian did in the HR article.

But I will compare later when I get back home.

Will be back.

Uh, ft/inches is length/length. I don't care who used it, it's not a unit of force. Probably a typo, but whatever. Just because it's in a magazine doesn't mean much. Inch-Lbs is what you're talking about.

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.

Yeah my diameters are wrong. I subtracted from the diameter like it was a radius. I measured a set of pads I have

So the effective diameters for the 10.87's should be like 9.095" and the 11.75's should be 9.975" based on that. I'm sure there's some pad variation out there anyway so that makes them basically the same as your numbers. My numbers in my previous post above are wrong. Should be more like this...

As for the mustang calipers, these are listed at 45mm.
2002 FORD MUSTANG Power Stop L4723 Power Stop OE Replacement Calipers | Summit Racing

These are listed at 40.3mm
More Information for CARDONE 184885

I didn't measure mine, I suppose I could ask Cass. I assumed they were 40mm because the later PBR calipers were (2000 up I think?). Of course, that's not the only thing that goes into braking performance. The 13" cobra style kit uses aluminum hubs, and even with the 13" rotors that assembly is quite a bit lighter than the 11.75". Which is all rotating mass the caliper also has to stop. And the efficiency thing about multi-piston fixed vs. single piston sliders we talked about before. Honestly, and this is just my subjective opinion, I think the difference between the 10.87's and 11.75's was probably bigger than the 11.75's and the 13" cobra style kit. The 13's were an improvement IMO, but it was not as dramatic. And it could just be that they "feel" better, because again that's just my subjective experience. That could be pad compound, it could be the change in the size of the piston area changing the pedal feel, it could be the reduction in flex with the fixed calipers, who knows.

 

[DionR]

Uh, ft/inches is length/length. I don't care who used it, it's not a unit of force. Probably a typo, but whatever. Just because it's in a magazine doesn't mean much. Inch-Lbs is what you're talking about.

You are completely right. Not sure how I overlooked that repeatedly.

Supposed to be lb/inches or lb/feet. Pretty sure the article had it right, I just completely biffed it somehow.

As for the mustang calipers, these are listed at 45mm.
2002 FORD MUSTANG Power Stop L4723 Power Stop OE Replacement Calipers | Summit Racing

These are listed at 40.3mm
More Information for CARDONE 184885

I didn't measure mine, I suppose I could ask Cass. I assumed they were 40mm because the later PBR calipers were (2000 up I think?).

Interesting. I was always looking at the '98's or so and not the later ones.

Some of what I based my info on was the old Baer kit that used the same calipers. Don't remember the name (Sport?) other than they called the calipers PGC "Pad Guided Calipers" (I think) because the pad actually clips into the caliper. The next step up was their GT kit with PDC "Pin Driven Calipers" and was basically the C5/6 caliper. Either way, pretty sure they listed the Cobra calipers as 38mm and I didn't see others so I thought they all were. Looks like the '99+ used a bigger piston in the Cobra calipers.

Good stuff. Thanks.

 

[72bluNblu]

You are completely right. Not sure how I overlooked that repeatedly.

Supposed to be lb/inches or lb/feet. Pretty sure the article had it right, I just completely biffed it somehow.

Interesting. I was always looking at the '98's or so and not the later ones.

Some of what I based my info on was the old Baer kit that used the same calipers. Don't remember the name (Sport?) other than they called the calipers PGC "Pad Guided Calipers" (I think) because the pad actually clips into the caliper. The next step up was their GT kit with PDC "Pin Driven Calipers" and was basically the C5/6 caliper. Either way, pretty sure they listed the Cobra calipers as 38mm and I didn't see others so I thought they all were. Looks like the '99+ used a bigger piston in the Cobra calipers.

Good stuff. Thanks.

Eh crap happens. I subtracted half the pad width from the diameter instead of the radius.

Yeah the calipers that DoctorDiff is selling with his kit look to be RightStuff signature calipers. I need to measure mine, but they're either 40.3 or 45mm. The calculator is nice, but obviously there's more to it. For example, '03+ Viper calipers are 4 piston, with "only" 40 and 44mm pistons (2 of each). But comparing the calipers to the PBR's, you can see there's far more material and the caliper is much larger. The calculator wouldn't be much different, but the efficiency and heat resistance would be decidedly different because of the additional stiffness and material.