Thread: Brakes 101: question

What are the effects of or benefits of brake rotor diameter vs. sweep area width? Does it matter? Is total swept area all that counts or does rotor diameter equate to leverage and applied torque?

Example: Corvette rear brakes are large in diameter but have a very narrow sweep area.

Thanks

I'm no expert but I would think a larger sweep area would be better than a large rotor. Just look at the ceramic brakes on exotic cars, they have huge sweep area.

Certainly has to be a point of diminishing returns as far as leaving a % of surface area directly exposed cooler airflow at all times otherwise you'd see the caliper and pads wrap around the entire rotor. Found this in relation to bikes...

Swept area of a brake is an important measure of it's effectiveness. The swept area of the disc is the total area contacted by all the brake pads in one revolution of the disc. This combined swept area of all brakes can be divided by the weight of the bike to give an indication of how effective the brakes are likely to be.

Originally Posted by badboy00z

I'm no expert but I would think a larger sweep area would be better than a large rotor. Just look at the ceramic brakes on exotic cars, they have huge sweep area.

They (ceramics) seem to have both, large diameter AND large sweep area, they are budget no object too though.

Based on that analogy it raises another question though: Heat. You never see rotors as large as the ceramic one made of iron. Ceramic rotors are not affected by heat. Perhaps there is a local mass component and heat associated with that mass that works into the equation.

I'm just brain storming here. I hope someone with more engineering background than me can chime in here.

Originally Posted by UCrazyKid

They (ceramics) seem to have both, large diameter AND large sweep area, they are budget no object too though.

Based on that analogy it raises another question though: Heat. You never see rotors as large as the ceramic one made of iron. Ceramic rotors are not affected by heat. Perhaps there is a local mass component and heat associated with that mass that works into the equation.

I'm just brain storming here. I hope someone with more engineering background than me can chime in here.

i think steel rotors with a sweep area as large as a ceramic would be far too heavy.. but i might be wrong. but on race cars the sweep area is small and the rotors are really wide.

Originally Posted by UCrazyKid

They (ceramics) seem to have both, large diameter AND large sweep area, they are budget no object too though.

Based on that analogy it raises another question though: Heat. You never see rotors as large as the ceramic one made of iron. Ceramic rotors are not affected by heat. Perhaps there is a local mass component and heat associated with that mass that works into the equation.

I'm just brain storming here. I hope someone with more engineering background than me can chime in here.

Short answer is: You want both - period.

Rotor diameter = brake torque = stopping power.

Swept are = large surface area that takes longer to heat up = no fade repetative performance.

You haven't even covered rotor width (thickness) and radial vane construction yet.

Iron rotors as big as ceramics do exist and are used in racing applications.

The other advantage of CCMs is you can run large swept area, and large diameter, and have less unsprung weight than a comparably sized iron rotor. Most people don't consider it, but the ceramic rotor due to its reduced mass, also retains less rotational inertia which takes less energy to slow down, AND makes it easier to turn the front wheels at speed = less gyro effect. Think about driving your car with a flywheel bolted to each corner. When I worked for Honda in the Champ Car era of Indy cars, we used "qualifying" calipers and rotors - particularly at Laguna, to make it easier to steer the car. For the race we would switch back to a "race" brake package that was a little heavier, but more consistent over the duration of a race.

One last note: the friction characteristics of an iron vs CCM rotor are different, therefore the two rotors of the same size but differing material perform differently. I went through this first hand developing an iron rotor competition brake package for my Stradale customers. You cannot just swap the rotors and pads and be good. The brake bias ends up being "off" due to the different friction coefficients of the different materials. The package ends up using different Brembo calipers with the bias adjusted through the caliper piston size.

Of course he^^ nailed it.

Regarding the rear rotors of some cars having a larger diameter then the fronts.. Those rotors are a lot of time not vented(no radial vanes), they are solid discs. Being larger(and a lot of times cross drilled), help to keep them cool. If those rears were vented, they'd likely have a smaller diameter.

Also like stated, thickness of the rotors plays a large role in the rest of the sizing(rotor diameter, caliper choice, pad choice etc). They all work as a package and there would be pros and cons to each decision of size, weight etc. It's a pretty complicate set of decisions based on a lot of variables based on more variables(what they are trying to achieve, and what they are trying to achieve that in).

In my library of automotive books I have a book that has a very large section on brakes. It covers them in the areas mentioned and also discusses spring weight, contact points, thrust angle, caliper design, drum design, pad design, shoe design, piston design, fluid dynamics, fluid types, property of the fluids and more that I don't recall at the moment.

There are brake designs that work best on competition what are horrible on the street and vis versa.

So I would ask this question. What are you trying to determine by asking the initial question?

Originally Posted by N360LL

So I would ask this question. What are you trying to determine by asking the initial question?

OH, you had to ask that. I was just walking through my parking lot at work on my lunch and looking at the cars there, 996 TT, E96 M3 and my ISF all with very different braking set ups but all very competent in their performance. That's what I get for "day dreaming". I think I missed my calling, this all seems more interesting to me than the medical devices I work with, but hey, grass is always greener I guess.

Thanks for all the great information here. Tony, you never fail to impress. Thanks Boss.

Originally Posted by UCrazyKid

OH, you had to ask that. I was just walking through my parking lot at work on my lunch and looking at the cars there, 996 TT, E96 M3 and my ISF all with very different braking set ups but all very competent in their performance. That's what I get for "day dreaming". I think I missed my calling, this all seems more interesting to me than the medical devices I work with, but hey, grass is always greener I guess.

Thanks for all the great information here. Tony, you never fail to impress. Thanks Boss.

In your examples above, you can see the reason for different brake packages. 996TT = Rear engine, short wheelbase. The E96 and ISF are front engine - longer wheelbase. Weight distribution and has a huge effect on the brake performance/bias. The 996TT may have rear biased "static" weight distribution, but under heavy braking, the dynamic weight distribution actually moves twoards the center of the car, and all four brakes are doing close to the same amount of "work". This is one of the reasons the "911" platform Porsches brake so well in a straight line. The ISF and E96 may be "closer" to 50/50 static weight distribution, but under heavy braking, the dynamic weight distribution moves forward, and loads up the front brakes even more. Ever look at the brakes on an Audi closely? (R8 excluded) crap static weight distribution, and worse than crap dynamic weight distribution. The rear rotors are so small its laughable.

We can go on for ever. There is so much involved in the engineering of the "dynamic aspects" of a high performance car its really amazing. This is why I have no time for people who just compare specs. (not referring to you).

Thanks for the kind words Andy, it was good seeing you for a millisecond at the Palo Alto concourse.