I just caught the very end of a show last night on speed channel and apparently they had shown (using a chassis Dyno?) that going to a brake upgrade - bigger rotors - had caused the car to loose 12 HP. Even though the rotors were lighter than stock - the larger diameter and mass concentrated on the outside had done the trick.

Did anyone else see this? What did I miss? can you give me the readers digest version?

Carl Johansson

 

Rotational Inertia.

The farther away from the center the higher the interia because the speed is higher.

So even something lighter can have more inertia if it's a larger diameter.


I (rotational for a disk): 1/2 * Mass * the radius squared

as you can see with a "radius squared" little changes can make a big difference.

Inertia eats up energy..or horsepower, since it takes power to get that disk spinning.

 

It all has to do with rotational inertia. It takes a significant amount of power to spin the rotors/hubs/rims/tires upto speed. This is why people pick up quite a bit of time in the 1/4 by switching to skinnies and small rotors.
The other issue is how far the mass is away form the center of rotation. It takes more power to spin up to speed a rotor of the same weight with more mass further from the center. Think of a bicycle rim with 1lb of mass right around the hub compared to the same mass added equally around the bead area. If you were to try to spin each one, the mass on the outer edge of the rim would have to move "more" than the mass just in the center.
It is hard foe me to explain better, maybe someone will be able to do a better job...
BTW: I think it was that Mugen Acura RSX on Dream Car Garge you are talking about, right...

 

Interesting. Guess its worth it if you really need brakes that large.
Never would have guessed anything, let alone 12hp

 

interesting facts!! good work guys!!

 

Larger brakes in performance terms are only needed if heat is a big problem.

And heat is an issue if the car is heavy, your braking from high speeds or using the brakes a lot.

Oddly the street vehicle that needs the biggest is a heavy SUV, towing a big boat up and down the hills of the Carolinas.


If the brakes are way undersized in a very heavy vehicle you can get thermal shock, that's where the brakes heat up so quickly that all pressure and/or friction goes away in an instant.


This said, the truth is most OE brakes are undersized for aggressive use, but just fine for everyday street driving.

 

There is a real good article on brakes in this months (Jan.) Sports Car magazine, one of the benefits of SCCA. membership, you get it free. An example: Jim Konte, "Brembo" .. "Peope often confuse the car not stopping with the brakes getting to hot when in fact they're running to cold," explains Brembo's Kontje. "I would rather run a brake system too hot than too cold. A lot of times people think it's brake fade - in actuality their not running the temperature on the disk that's optimum for the friction material to work correctly".

"If you run the disk to cold, it has a tendency to crack; if you keep it in a range that's just on the edge of to cold it tends to thermal shock the disk, he says." They show a picture of a cracked rotor due to thermal shock. Whats also interesting is, they say that, Bedding or pre-bedding is absolutely critical for both longevity and performance. He say's "Babying them is not the answer" .. That means at race speed, hard on the brakes for 2 secs. on and off 3-4 times. Let it cool down and do it again. Then really hammer on the brakes for a lap. That process will not only bed in the disk, but will also heat cycle and thermally prep it as well". Up until now I alway's figured it was to much heat that would cause the rotor to crack, go figure.

 

I would accept this for racing (or track) pads that have a low friction coefficient when cold. Because of that the rotor also doesn't heat up.

For OE street pads, IMO this doesn't hold. Compounds used in street pads are designed to work best at low and intermediate temperatures.

And trust me, the number of opinions on brakes is straggering. That's because there really isn't a complete understanding of what happens in all conditions during braking.

So, that's why I NEVER say it's wrong outright, just if I agree or not.

 

I am in agreement with you. This article needs to be seen in proper perspective, as it is viewed from a "racing" perspective. At the very beginning: "Original equipment systems are not designed for the high temperatures that racing demands put on calipers, disks and mounting hardware, says John Slagle, director of motorsports for Hawk Performance".

I also have tried various pad compounds & rotors. I have used Bendix pads that transmitted the heat into the system and boiled the fluid out of the master cylinder, and consequent brake failier. I have not had that problem since switching to drilled rotors, SS brake lines & Hawk pads. My car is not a daily driver, even so I do drive on the street on occasion. One thing nice about the Hawk pads is they work great right away, when cold. Some other pads didn't work so good until they warmed up a bit. My car is setup for AutoCross, so I really need to have the brakes work right away, on the first series of corners. The Bendix pads were a little scary at times because they wouldn't really start working til the end of a lap.

I also was assuming that this thread was started in direct relationship to higher performance braking applications. As some members here are building bigger brake systems, that if only used on street driven cars may not reach proper or optimum operating temperature and may consequently cause other issues.

 

This is a very good point to bring up!


Brakes do have to be sized and materials choosen for the circumstances and the vehicle.

Sometimes this is difficult, for example that boat towing SUV may have a difference of a couple thousand pounds from loaded down to empty with just the driver.

And then of course there's the issue with parastic power losses which effect MPG which effects the CAFE rating.