Why are the 11.75 Brake Better?

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72DMag

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I recently purchased the bigger 11.75 caliper brackets. After buying them I found out they take the same brake caliper and brake pad as the smaller 11 inch rotor.

If the clamping force by the brake caliper and the surface area of the brake pad remain the same, then how does only a bigger rotor improve braking? Heat disapation I can see but I was shocked to find out the 12 inch rotors take the same caliper and brake pad as 11 inch.
 
More surface area for heat dissipation would be the main benefit, which can be huge if you are doing repeated high speed/intensity braking.
 
But the more leverage in this case is .75 of an inch. That doesnt seem like alot to make a significant diff.

Just asking cause I'm kinda reconsidering spending the money for these larger brakes. They are also heavier than the stock 11in a body rotors.
 
The way I read your post, you have the big rotors already. So, run em! If I had the 11s on my car, I doubt I'd bother moving up to the 11.75.
 
What it amounts to is the "swept area" of the rotor- the amount of area covered by the pad in 1 revolution of the rotor. The amount of area covered by the pad in one revolution of a 10.85 rotor is significantly less than the area covered by the same size pad on a 11.75 rotor. Pardon me for not doing the exact math on it, but you get the idea.
More swept area = better braking. If it takes X amount of force to stop a car of a given weight in a given distance, that amount of force can be generated easier and quicker with a larger diameter rotor, with all other factors being equal. And since braking is a conversion of dynamic energy into heat (through friction), that heat is also more readily dissipated over a larger area when using a larger rotor.
 
The closer the brake pad is to the outer circumference of the tyre, the greater the leverage the pad has on braking effort. Everything else being equal, for the same pedal pressure, the car will stop quicker.
 
Let me give you a quick story here if i may.
I factory ordered a 71 Cuda with disc brakes, and kept that car for 45 years, before i sold it.
In the later 70's when it came time for a front brake job, i had already collected front brakes off Cordoba's, St Regis, and Dodge Magnum cars that had the 11 3/4 inch front disc brakes on them.
Back in the later 70's, and then the 80's, it wasn't hard finding these cars at the Pick-N-Pull wrecking yards, out here in Commiefornia.
I got the brainy idea, to convert the Cuda to the larger 11 3/4 inch brakes as i had everything off the donor car, the spindles, rotors, calipers, brackets, bearings, nuts and bolts, everything, as that car came into the wrecking yard, with new brakes on it, before it got hit, and wrecked.
I put all that on the Cuda, and when i got done, i NEVER noticed any difference at all between the 10.85 inch rotors, and the bigger 11 3/4 inch rotors.
Guess that's because the car was already a disc brake car, and i was driving it for years, before i did the conversion to the larger diameter rotors.
Guess whoever owns that car now in 2023 is gonna hafta figure out the brakes, whenever parts might need to be purchased for that car, as it's changed hands a few time since i sold it off.
Jim V.
hemi71x

Jim 1971.jpg
 
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It doesn't take a large change in the lever arm to make a significant improvement in braking force. As pointed out, the larger rotor will give the caliper a larger lever arm, which multiplies the braking force. It also adds a larger swept area on the rotor, which will dissipate heat faster and spread the energy of braking over a larger area to begin with (heats up slower).

There are also two different calipers, '73-'75 A-bodies used a single piston caliper with a 2.6" piston, 73+ B/E's used 2.75" calipers, so did '76 A bodies. The larger piston diameter in the caliper also increases braking force.

Screenshot 2023-03-05 at 10.46.16 AM.png


If you do the math on the braking force, just going from a 10.98" rotor to an 11.75" rotor with the same exact caliper results in an ~8% increase in overall braking force. The actual numbers are less important than the percentage, the math assumes 100% of the force generated at the caliper is transferred to the rotor which is not the case. But since we're comparing the same calipers, the percentage change should be the same. And on that note I used the same pedal force, master cylinder diameter, and wheel diameter for all the calculations, since all of those things change the total force number.

If you currently have 2.6" piston calipers, and you go from 10.98" rotors to 11.75" rotors with 2.75" piston calipers, the difference in braking force is ~21%. That's massive!!! And this is just the change in braking force, it doesn't even consider the heat dissipation and temperature differences.

And really, even a small percentage increase in the braking force can make a big difference. If your stopping distance changes by a few feet, what's that worth? Well, if it means stopping a few feet short or hitting something, it could be worth a lot.

I've run the 10.98" and 11.75 rotors on my Duster and my Challenger. The 11.75's make a noticeable difference, especially as you start to push the car a little harder. Now, I will say tires also make a big difference, and running 275's on the front of both of those cars I wouldn't use anything less than the 11.75's, that width of tire works the 11.75's pretty hard. If you're going to run 195's up front it will not make as much of a difference when you start translating braking force to stopping distance. Obviously the better your tires are the more important the larger brakes can be.
 
Let me give you a quick story here if i may.
I factory ordered a 71 Cuda with disc brakes, and kept that car for 45 years, before i sold it.
In the later 70's when it came time for a front brake job, i had already collected front brakes off Cordoba's, St Regis, and Dodge Magnum cars that had the 11 3/4 inch front disc brakes on them.
Back in the later 70's, and then the 80's, it wasn't hard finding these cars at the Pick-N-Pull wrecking yards, out here in Commiefornia.
I got the brainy idea, to convert the Cuda to the larger 11 3/4 inch brakes as i had everything off the donor car, the spindles, rotors, calipers, brackets, bearings, nuts and bolts, everything, as that car came into the wrecking yard, with new brakes on it, before it got hit, and wrecked.
I put all that on the Cuda, and when i got done, i NEVER noticed any difference at all between the 10.85 inch rotors, and the bigger 11 3/4 inch rotors.
Guess that's because the car was already a disc brake car, and i was driving it for years, before i did the conversion to the larger diameter rotors.
Guess whoever owns that car now in 2023 is gonna hafta figure out the brakes, whenever parts might need to be purchased for that car, as it's changed hands a few time since i sold it off.
Jim V.
hemi71x

View attachment 1716059304
take your +1 for the stylin' and profilin' pic

slot mags, plaid pants and green with black vinyl will never go out of style

(btw, are those sta-prest w/ a mr california?)
 
It doesn't take a large change in the lever arm to make a significant improvement in braking force. As pointed out, the larger rotor will give the caliper a larger lever arm, which multiplies the braking force. It also adds a larger swept area on the rotor, which will dissipate heat faster and spread the energy of braking over a larger area to begin with (heats up slower).

There are also two different calipers, '73-'75 A-bodies used a single piston caliper with a 2.6" piston, 73+ B/E's used 2.75" calipers, so did '76 A bodies. The larger piston diameter in the caliper also increases braking force.

View attachment 1716059334

If you do the math on the braking force, just going from a 10.98" rotor to an 11.75" rotor with the same exact caliper results in an ~8% increase in overall braking force. The actual numbers are less important than the percentage, the math assumes 100% of the force generated at the caliper is transferred to the rotor which is not the case. But since we're comparing the same calipers, the percentage change should be the same. And on that note I used the same pedal force, master cylinder diameter, and wheel diameter for all the calculations, since all of those things change the total force number.

If you currently have 2.6" piston calipers, and you go from 10.98" rotors to 11.75" rotors with 2.75" piston calipers, the difference in braking force is ~21%. That's massive!!! And this is just the change in braking force, it doesn't even consider the heat dissipation and temperature differences.

And really, even a small percentage increase in the braking force can make a big difference. If your stopping distance changes by a few feet, what's that worth? Well, if it means stopping a few feet short or hitting something, it could be worth a lot.

I've run the 10.98" and 11.75 rotors on my Duster and my Challenger. The 11.75's make a noticeable difference, especially as you start to push the car a little harder. Now, I will say tires also make a big difference, and running 275's on the front of both of those cars I wouldn't use anything less than the 11.75's, that width of tire works the 11.75's pretty hard. If you're going to run 195's up front it will not make as much of a difference when you start translating braking force to stopping distance. Obviously the better your tires are the more important the larger brakes can be.
you mention some very meaningful things here that are subtle nuances most gloss over.

small upgrades can reap huge benefits if the recipe is just right. things like when you have the correct master cylinder bore, with the correct caliper bore you can increase your braking power without increasing the force needed at the pedal. the same for adding stainless flex lines in place of rubber can give better pedal feed back.

i upgraded a car for a client from 10.98's to the cross drilled/slotted 11.75's and some decent pads and it was a noticeable difference. then when combined with a 17" rim and larger rubber they could really be put to use and made for an even better system.
 
But the more leverage in this case is .75 of an inch. That doesnt seem like alot to make a significant diff.

Just asking cause I'm kinda reconsidering spending the money for these larger brakes. They are also heavier than the stock 11in a body rotors.
Figure percentage. That is a HUGE change based on the old rotor surface. Also, are the caliber bores larger than the old ones?
 
you mention some very meaningful things here that are subtle nuances most gloss over.

small upgrades can reap huge benefits if the recipe is just right. things like when you have the correct master cylinder bore, with the correct caliper bore you can increase your braking power without increasing the force needed at the pedal. the same for adding stainless flex lines in place of rubber can give better pedal feed back.

i upgraded a car for a client from 10.98's to the cross drilled/slotted 11.75's and some decent pads and it was a noticeable difference. then when combined with a 17" rim and larger rubber they could really be put to use and made for an even better system.

It all makes a difference, small changes add up.

But eventually you have to put it all on the road. Like @hemi71x ’s experience shows- he already had 2.75” calipers because his car was an E-body, and rocking old bias ply tires, or even all season white letter radials, is going to limit the improvement you see in the real world. Yeah, the brakes work 8% better, which is a big improvement. But if the tires are already capped out, you might not see much of a difference on the road. And unless you actually measure stopping distance it will be hard to tell, your “butt dyno” might not notice a foot or two of stopping distance, even if that might make a difference in an emergency situation
 
@72bluNblu I got the calipers for a 78 cordoba since that's what the brackets were originally on.

Thanks everyone for all the info!! This turned out to be a very good thread with alot of details. Learned good bit that I bet most didn't know. Thanks again!!
 
@72bluNblu I got the calipers for a 78 cordoba since that's what the brackets were originally on.

Thanks everyone for all the info!! This turned out to be a very good thread with alot of details. Learned good bit that I bet most didn't know. Thanks again!!
Perfect! Those should have the larger 2.75” pistons.

That should be a nice set of brakes for you. Obviously there are some pretty fancy aftermarket brakes out there, but the 11.75’s are (in my opinion anyway) the best “bang for the buck” available. And you have the added bonus of all the parts being factory. Maybe not the fanciest, but very consistent and reliable with widely available replacement parts.

I run DoctorDiff’s 13” cobra brake kit on my Duster, and while I do think it was an improvement over the 11.75’s the difference between them and the 13” cobras wasn’t as much as from the 10.95’s with 2.6” pistons and the 11.75’s with 2.75” pistons. And of course the 11.75’s will work with 15” rims, the 13’s will not.
 
The differences will show when you use the brakes hard with real aggressive driving or down a long grade with traffic.

When they get hot and fade.

The 13” brakes in my 5.7 2015 Challenger with track pack suspension will fade going down a long mountain grade with traffic and I’m 80 mph in the fast lane and come up to cars braking at bottom of hill. 6am on way to work always pack of cars doing 75-80. If I’m late and aggressive they will even pulsate until they get a chance to cool off.
 
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Perfect! Those should have the larger 2.75” pistons.

That should be a nice set of brakes for you. Obviously there are some pretty fancy aftermarket brakes out there, but the 11.75’s are (in my opinion anyway) the best “bang for the buck” available. And you have the added bonus of all the parts being factory. Maybe not the fanciest, but very consistent and reliable with widely available replacement parts.

I run DoctorDiff’s 13” cobra brake kit on my Duster, and while I do think it was an improvement over the 11.75’s the difference between them and the 13” cobras wasn’t as much as from the 10.95’s with 2.6” pistons and the 11.75’s with 2.75” pistons. And of course the 11.75’s will work with 15” rims, the 13’s will not.

Not to bring back a (somewhat) old thread but I'm glad I saw this post. My '70 Duster has 10.95" '73-up discs and today my passenger-side caliper seized up on me; drove 8 minutes to the grocery store to pick up some things and when I got there I could smell burning brakes and smoke was coming from that wheel. I tried to hit the caliper with random tools in my trunk in an attempt to free it up but once I got back to my neighborhood it was locked up so bad it was pulling the steering wheel to the right and there was so much smoke billowing out I had to leave it parked in the driveway to let it cool off.

I've been debating whether to go with Dr. Diff 11.75" brakes or the 13" Cobras but now it's time to make a decision, these brakes are done and that side specifically has always given me problems with dragging and such. Replaced the piston seal once which kinda helped but it was always still not quite right. I know I need more brakes from doing track days and I am on a somewhat tight budget so it seems to me from reading your post it's not worth the extra 30%+ in price to go with the Cobras for what I need. I think I have the 2.6" diam calipers currently too so going to fresh calipers with bigger pistons and bigger new rotors (likely slotted and drilled, not a ton of street miles I do anymore and these stock rotors have lasted 30k+ miles) with the braided stainless lines will be a nice improvement. I just wish there were more options for pads.
 
Not to bring back a (somewhat) old thread but I'm glad I saw this post. My '70 Duster has 10.95" '73-up discs and today my passenger-side caliper seized up on me; drove 8 minutes to the grocery store to pick up some things and when I got there I could smell burning brakes and smoke was coming from that wheel. I tried to hit the caliper with random tools in my trunk in an attempt to free it up but once I got back to my neighborhood it was locked up so bad it was pulling the steering wheel to the right and there was so much smoke billowing out I had to leave it parked in the driveway to let it cool off.

I've been debating whether to go with Dr. Diff 11.75" brakes or the 13" Cobras but now it's time to make a decision, these brakes are done and that side specifically has always given me problems with dragging and such. Replaced the piston seal once which kinda helped but it was always still not quite right. I know I need more brakes from doing track days and I am on a somewhat tight budget so it seems to me from reading your post it's not worth the extra 30%+ in price to go with the Cobras for what I need. I think I have the 2.6" diam calipers currently too so going to fresh calipers with bigger pistons and bigger new rotors (likely slotted and drilled, not a ton of street miles I do anymore and these stock rotors have lasted 30k+ miles) with the braided stainless lines will be a nice improvement. I just wish there were more options for pads.
There are options. You can (or used to) get Hawk racing pads.
 
There are options. You can (or used to) get Hawk racing pads.
I searched Hawk's website and can't find anything for classic Mopars. I know EBC sells pads for them though. Dr. Diff includes pads in their kit I might send an email and ask what brand/type they are.

Another note, the piston area of the 2-piston Cobra calipers is almost half of the 2.75" single-piston Mopar calipers. Doesn't that mean the clamping force is also much lower? I guess the bigger 13" rotors make up that difference? Oh yeah... and a ton lighter too lol I forgot about that part.

Looking on AmericanMuscle.com there are bolt-on brake upgrade kits for SN95 Mustangs from Baer, Brembo, Wilwood etc. so if you have the caliper brackets and rotor adapter hubs for our classic Mopars there's room to grow if you want to have a real race-ready track machine.
 
I searched Hawk's website and can't find anything for classic Mopars. I know EBC sells pads for them though. Dr. Diff includes pads in their kit I might send an email and ask what brand/type they are.

Another note, the piston area of the 2-piston Cobra calipers is almost half of the 2.75" single-piston Mopar calipers. Doesn't that mean the clamping force is also much lower? I guess the bigger 13" rotors make up that difference? Oh yeah... and a ton lighter too lol I forgot about that part.

Looking on AmericanMuscle.com there are bolt-on brake upgrade kits for SN95 Mustangs from Baer, Brembo, Wilwood etc. so if you have the caliper brackets and rotor adapter hubs for our classic Mopars there's room to grow if you want to have a real race-ready track machine.

Bergman AutoCraft and FirmFeel also sell high performance brake pads for the Mopar calipers.

As far as the clamp force goes, if you just work off piston area you are correct. And actually even when you account for the rotor diameter a calculator will tell you that a 11.75” rotor with the 2.75” calipers will have more brake force than the 13” Cobra kit with the PBR calipers.

The problem is you’re comparing a single piston caliper to a dual piston caliper, and a slider to a pin style (but both floating). There’s flex and efficiency losses there that don’t get accounted for when you do that calculation. It gets weirder if you start comparing floating calipers and fixed calipers, there’s losses there that the calculators just don’t address.

That said, the 11.75’s with the Mopar 2.75” piston calipers is a good set up. I think the 13” cobra kit in practice is a little better, but like I said before it’s not a night and day kind of difference.
 
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