Disc brake rotor sizing. - Vintage Mustang Forums
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post #1 of 16 (permalink) Old 07-10-2019, 11:12 AM Thread Starter
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Disc brake rotor sizing.

At some point soon I plan on upgrading to 4 wheel disc brakes....there is an unending supply of options to go with with rotors in all sizes. This discussion I want to limit to rotor size....there is a crazy obsession in the car modification world that bigger=better when it comes to brakes. I see new cars being made with 20+ inch wheels so they can fit "bigger brakes" but it seems to me its a case of a dog chasing its tail here....those cars need bigger brakes because they are so much heavier. We will take the example of a base 2015 v6 of 3500lbs compared to a base 1966 I6 of 2500lbs as an example...thats a 30-40% increase in weight. If we assume the base 2015 v6 has a rotor size of 12.6" both front and rear then theoretically for the same braking performance on a 1966 a 9" rotor would suffice(not that you could find a rotor that small). I know that of course only the surface area under the pads actually counts and not my quick and dirty math using the full rotor diameter and that the larger the rotor, the more surface area is under the pad. What I am really trying to determine here is the minimum rotor size I need to get comparable braking capability to a modern mustang without going bigger than I need. Of course calipers are going to factor into the equation...but I want to limit discussion to rotor size first, maybe find a formula that will help me make an intelligent choice.
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post #2 of 16 (permalink) Old 07-10-2019, 11:19 AM
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Originally Posted by wicked93gs View Post
At some point soon I plan on upgrading to 4 wheel disc brakes....there is an unending supply of options to go with with rotors in all sizes. This discussion I want to limit to rotor size....there is a crazy obsession in the car modification world that bigger=better when it comes to brakes. I see new cars being made with 20+ inch wheels so they can fit "bigger brakes" but it seems to me its a case of a dog chasing its tail here....those cars need bigger brakes because they are so much heavier. We will take the example of a base 2015 v6 of 3500lbs compared to a base 1966 I6 of 2500lbs as an example...thats a 30-40% increase in weight. If we assume the base 2015 v6 has a rotor size of 12.6" both front and rear then theoretically for the same braking performance on a 1966 a 9" rotor would suffice(not that you could find a rotor that small). I know that of course only the surface area under the pads actually counts and not my quick and dirty math using the full rotor diameter and that the larger the rotor, the more surface area is under the pad. What I am really trying to determine here is the minimum rotor size I need to get comparable braking capability to a modern mustang without going bigger than I need. Of course calipers are going to factor into the equation...but I want to limit discussion to rotor size first, maybe find a formula that will help me make an intelligent choice.
Swept area (area under the pad) doesn't mean squat except for discussion of how long the pad lasts.
You can carve the pad down to 1" x 1" contact patch and the car will still stop just fine..... as long as you don't exceed the brake material's
operating temperature. How MANY times you can stop with that size pad is the real question.

Braking performance is ultimately a question caliper grip and pad material. Yes, there are other parameters, but those two are a big
deal when designing the system at any given corner of the car.

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post #3 of 16 (permalink) Old 07-10-2019, 11:30 AM Thread Starter
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Swept area (area under the pad) doesn't mean squat except for discussion of how long the pad lasts.
You can carve the pad down to 1" x 1" contact patch and the car will still stop just fine..... as long as you don't exceed the brake material's
operating temperature. How MANY times you can stop with that size pad is the real question.

Braking performance is ultimately a question caliper grip and pad material. Yes, there are other parameters, but those two are a big
deal when designing the system at any given corner of the car.

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So what minimum rotor size(in relation the swept area) would you recommend for a 3000lb car built with the purpose of spirited street driving with an occasional track day(meaning a reasonable amount of stops before brake fade sets in)?
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post #4 of 16 (permalink) Old 07-10-2019, 11:45 AM
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You'd probably be ok with stock front disc and upgraded pads..... this works fine most of the time for what you're
thinking about doing with the car. I consider the front 65/66 rotors to be marginal for track use because they do
not afford much of a "heat sink" when converting kinetic energy into heat energy and dissipating that energy.

If you do a lot of track, you'll want something with a big bigger rotor and caliper. I have the old B302 Trans Am
setup on both of my cars. They are 11.5" and intended to stop a 4800# Galaxie. I ran lots of open track days,
without even needing a race pad. They fit inside a lot of 15" rims. And you are less likely to need rear disc.

Street or Track has a modern day version too.


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post #5 of 16 (permalink) Old 07-10-2019, 12:18 PM Thread Starter
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I don't mind upgrading to rear disc...mainly because I hate fooling with drum brakes, its so much easier to work on discs...I will take the weight penalty for the convenience.
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post #6 of 16 (permalink) Old 07-10-2019, 12:18 PM
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It's not just contact area, but where that force is applied. The larger the rotor, the larger the virtual lever arm is created as the brake pad is moved farther from the hub center.

Chris (Sussex, WI)

California car born an I6, now with warmed up 289 w/T-5z, 3.55T-loc, Granada Discs and 6 bolt Moto-Lita Steering Wheel. Owned since 1983.
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post #7 of 16 (permalink) Old 07-10-2019, 12:29 PM
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I don't mind upgrading to rear disc...mainly because I hate fooling with drum brakes, its so much easier to work on discs...I will take the weight penalty for the convenience.
Yeah. They are a PITA..... that's one of three reasons to upgrade to rear disc.
Another one is shrugging off speed from 140 to 40 to make a sharp bend in a course...... lap after lap after lap.

And lastly, because rear disc brakes look cool.

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post #8 of 16 (permalink) Old 07-10-2019, 02:40 PM
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Quote:
Originally Posted by GT289 View Post
Swept area (area under the pad) doesn't mean squat except for discussion of how long the pad lasts.
You can carve the pad down to 1" x 1" contact patch and the car will still stop just fine..... as long as you don't exceed the brake material's
operating temperature. How MANY times you can stop with that size pad is the real question.

Braking performance is ultimately a question caliper grip and pad material. Yes, there are other parameters, but those two are a big
deal when designing the system at any given corner of the car.

ex-Global West GM
1991-1995
One thing that I think needs clarifying is the definition of "Swept Area" and how that plays into braking. My understanding is that it is the total area the pad contacts the rotor as the rotor makes a full rotation. Considering that brake systems function by turning kinetic energy into heat and that heat is generated as a pad of a given size travels along a path it stands to reason that a 10" rotor would have to rotate ~20% farther to equal the same swept area of a 12" rotor. Assuming a the tire circumference is the same that means the car will have a minimum braking distance ~20% longer than a 12" rotor with the same pads, caliper, master cylinder and etc. Secondly, if constructed of similar materials the larger rotor should be more massive, thus providing a larger heat sink allowing more energy to be absorbed before exceeding the maximum operating temperature of the pads.

Now it is true that the same pad would sweep 20% farther to stop a car in a given distance (a "given" distance being key) on a car with 12" vs 10" rotors. Whether it would wear the pads faster is hard to say as the car would 12" rotors would require less force. It would vary depending on the pad compound.

Is there a simple formula to determine the proper size rotor for a given car? I don't think so. Not only do other system components (pads, calipers, master cylinder) play a role as to how much force can be applied, but the intensity and duration of braking can play a role as to what is needed and what is overkill.

Now braking distance can only be reduced by braking as long as the tires maintain traction. If your rotors are big enough to lock up the brakes you will not reduce stopping distance by increasing the size.

Chris (Sussex, WI)

California car born an I6, now with warmed up 289 w/T-5z, 3.55T-loc, Granada Discs and 6 bolt Moto-Lita Steering Wheel. Owned since 1983.

Last edited by 66coupe289; 07-10-2019 at 02:57 PM.
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post #9 of 16 (permalink) Old 07-11-2019, 09:14 AM
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The simple reason for big rotors is heat. Yeah a small rotor and pad will stop you fine the first time but it won't stop you the 3rd time hence bigger rotors for more mass and more heat dissipation.

No matter the vehicle the best size rotors and pads are the biggest you can fit on it.
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post #10 of 16 (permalink) Old 07-11-2019, 10:37 AM
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Originally Posted by elcam84 View Post
The simple reason for big rotors is heat. Yeah a small rotor and pad will stop you fine the first time but it won't stop you the 3rd time hence bigger rotors for more mass and more heat dissipation.

No matter the vehicle the best size rotors and pads are the biggest you can fit on it.
Nope. Complete hyperbole.
Lotta reasons NOT to put the biggest rotor possible on a spindle. Think about it.
Kind of similar to if 225x50x16 tires are fast 315's will be even faster. Absolutely NOT TRUE.

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post #11 of 16 (permalink) Old 07-11-2019, 02:19 PM
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Many of the "big" brakes use the same calipers and pads as "standard" brakes. The major difference is the caliper bracket. The bracket just locates the caliper in a higher position. Also, bigger rotors look better. 11" rotors look pretty wimpy under a 20" wheel.
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post #12 of 16 (permalink) Old Today, 10:24 AM
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Here’s some food for thought for those that think bigger diameter is always better... an F1 car has a limit on brake rotor size which is 1.10” (28mm) thick by 10.94” (278mm) diameter.

The key for effective and repeated stopping performance is the ability for the brake materials to withstand high temperatures without breaking down and then dissipate that heat away from the rotor/caliper/spindle/bearings/wheel so you can do it all over again on the next corner! A larger rotor will allow more stops before over heating (assuming it has a higher mass) but if the heat can’t be removed it’s gonna eventually fade just like a smaller rotor. Changing the pad material to something that keeps working at higher temps will buy you some more time, but the same removal of heat needs to happen otherwise fade will eventually occur.

I think a large reason modern cars have gone to such large diameter brakes is because tire/suspension technology has gotten so much better that you could actually use more clamping force and not have the tires slip. The old bias ply tires our cars came with way back when wouldn’t benefit from a “big” brake kit, but the sticky tires that we now use and all the suspension mods that we have done encourages us to upgrade the brake system.
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post #13 of 16 (permalink) Old Today, 11:19 AM
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vintage mustang braking limitations are down force.
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post #14 of 16 (permalink) Old Today, 12:23 PM
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I don't mind upgrading to rear disc...mainly because I hate fooling with drum brakes, its so much easier to work on discs...I will take the weight penalty for the convenience.
My nieces 30 year old boyfriend who is a Audi mechanic says the same thing about drum brakes. I think drum brakes are as simple as it gets and they seem to work fine auto crossing and can act as an emergency brake if necessary. I have never been impressed with the e-brake stopping ability on even new cars I've owned with the ratcheting wind back piston type rear brakes while the mini drum within the rotor hub "Crown Vic, F-150" seem to work OKish. Forgive me for not knowing the correct names of the different E-brake systems.

The rear disk do look much better than drums!

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post #15 of 16 (permalink) Old Today, 12:32 PM Thread Starter
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My nieces 30 year old boyfriend who is a Audi mechanic says the same thing about drum brakes. I think drum brakes are as simple as it gets and they seem to work fine auto crossing and can act as an emergency brake if necessary. I have never been impressed with the e-brake stopping ability on even new cars I've owned with the ratcheting wind back piston type rear brakes while the mini drum within the rotor hub "Crown Vic, F-150" seem to work OKish. Forgive me for not knowing the correct names of the different E-brake systems.

The rear disk do look much better than drums!
Funny you mention drums in an E-brake capacity. I modified Dodge Neons for a few years(turbos, engine swaps, anything and everything...surprisingly good little cars, the 1st gen ones) and I always wondered why the Sport models with rear discs had a hybrid Disc/Drum setup...the disc for normal braking, but built into the caliper was a drum/shoe setup that was only used for the E-brake...actually a clever backup I always assumed was for the event of a caliper failure.
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