Big rotors?

[Legacy777]

wow....this thread got going....

Ok...first off, changing rotor size most definitely does increase brake torque. Simple physics, If a given force is applied at the end of a moment arm, it applies a torque to that moment arm. The larger/longer the moment arm, the larger the torque will be. By increasing rotor size you are providing a larger moment arm, and there by giving the brakes more advantage to stop the car with the same given input force/pressure.

That's the key! Keeping everything else constant, and only changing rotor size, you will get an increased brake torque with larger rotors.


Brake bias controllers/proportioning valves. You will not find any manual brake bias controllers that will fit on any subaru without replumbing the brake lines. This is because subaru uses a diagonal braking or redundant braking system in the event of a failure. The only option you have with the proportioning valve is to get one from a wagon (or other subaru) if you have a sedan because the split point (point where the valve starts cutting pressure to the rear) is greater. However before the split point (for light/medium braking and for every day driving), changing the proportioning valve does nothing to change bias.


As for the larger rotors vs. venting. Rotors are basically heat sinks. They turn kinetic energy into heat. That's all they do, hold heat. The larger the rotor, or more mass it has, the more heat it will be able to hold. That and maybe cooling is the only advantage a vented rotor has over a solid rotor of the same diameter. Like I mentioned, most people will never come to the point where they are going to generate more heat in the rear rotors then the rear rotors can absorb and dissapate. This again comes down to bias. The stock brake system is considerably biased towards the front. Probably along the lines of 75-80% front and the rest rear.

If you upgrade to the H6 rear rotors, you are shifting bias rearward, and increasing the heat capacity, because it's a larger diameter rotor, ie more mass. So you're getting a 2-for-1 there. Does it have the same amount of mass as the vented rotor....probably not, but like I mentioned it's that big a deal.


The following is a simple formula you can use to determine the % increase or decrease in brake torque from changing certain components in the braking system. It is not an absolute number, but a relative number that is compared against your original setup.

Brake Torque Increase% = [(Caliper piston area new/old)*(effective radius new/old)*(brake pad friction coefficient new/old)]
** For Sliding calipers multiply areas by 2 **

Just a note, I usually throw the brake pad friction coefficient term out, simply because most of the time you don't know it.


BTW Paisan, the subaru 4-pots throw bias rearward because they have smaller pistons. This holds true even when upgrading from the stock turbo legacy brakes.

Here's an excel spreadsheet with quite a bit of info on brake bias and different subaru brake setups.

http://www.main.experiencetherave.com:8 ... kemath.xls

BTW....I have basically redesigned the entire braking system in my car. The only that is stock on it is the hard lines. I say redesign, because I know what changes in brake torque will occur by the mods I've done.

[Legacy777]

i don't know if I answered everyone's questions.....I may have missed some stuff from all the replies. If so, just say so.

[Paisan]

Wow interesting writeup, when I wake up I'll re-read it

-mike

[Paisan]

Since you mentioned it, I'm actually looking at getting the Racing Brake rotors. Mainly because they aren't too expensive (209 from RCE for both). That's about on par with stock price.

Are you looking at the Street-slotted ones? We've been testing these since Feb and are very happy with both the Street and the Race models in our on-track and on-street use of them.

-mike

[BAC5.2]

Thank's Josh! I was waiting for someone to back me up.

Mike - http://racecompengineering.com/brakecomponents.html

First rotors.

[Paisan]

Thank's Josh! I was waiting for someone to back me up.

Mike - http://racecompengineering.com/brakecomponents.html

First rotors.

Yup same ones we have listed for $200.

http://azpinstalls.com/

Shoot me a PM and maybe I can put together a special package for you.

-mike