Big rotors?

scottzg · Post by **scottzg** » Thu Sep 02, 2004 8:52 am

Citing this:
http://www.bbs.legacycentral.org/viewto ... inflatable

Can we do this: http://www.ravensblade-impreza.com/modi ... brake.html

To pique your interest, the impreza folk get a different caliper bracket and install the wrx rotors.

vrg3 · Post by **vrg3** » Thu Sep 02, 2004 1:10 pm

Unfortunately, no. WRXes and RSes happen to have the same calipers. Our calipers are different. There aren't any brackets that move our calipers out to the right position for WRX brake rotors.

But that's not a big deal, is it? I mean, complete WRX brake setups including calipers, brackets, and rotors are pretty easy to find on NASIOC for good prices.

Kostamojen · Post by **Kostamojen** » Sun Sep 05, 2004 9:29 am

Yup, you need the caliper too. Same goes for the rears if you want to do an H6 rotor swap... Need the calipers.

I wonder about the new 05 brakes...

BAC5.2 · Post by **BAC5.2** » Mon Sep 06, 2004 1:11 am

When I get my wheels/tires, and go to swap my front brakes for WRX brakes, I'll be going to my friends' house to do the work.

When we do it, I'll check his 05GT brakes, and we can work from there.

If the 05 brakes WOULD work, then that would be SWEET, as the 05 GT brakes are HUGE (12.3" front, 11.3" vented rear. Same as the JDM B4). They fill his new 17x7.5 5Zigen rims quite impressively.

They will NOT fit under 16" wheels though, so you HAVE to have 17's (lucky for me

).

Paisan · Post by **Paisan** » Mon Sep 06, 2004 5:22 pm

I'm running subaru 4-pot front calipers with RacingBrake front racing rotors that I got through http://AZPInstalls.com

The HP+ pads will put you through the windshield on these cars!

Best upgrade I've done on my car thus far.

-mike

Paisan · Post by **Paisan** » Mon Sep 06, 2004 5:28 pm

There's the pic of the Racingbrake rotors w/4-pot Calipers on my '94. More pics at http://azpinstalls.com/miscpics/azp-05-15-04/index.html

-mike

Paisan · Post by **Paisan** » Mon Sep 06, 2004 5:30 pm

Yup, you need the caliper too. Same goes for the rears if you want to do an H6 rotor swap... Need the calipers.

Why would you want the H6 rears? They aren't even vented! The Legacy Turbo ones are much better IMHO.

-mike

Post by **Legacy777** » Tue Sep 07, 2004 2:56 am

Paisan wrote:
Yup, you need the caliper too. Same goes for the rears if you want to do an H6 rotor swap... Need the calipers.
Why would you want the H6 rears? They aren't even vented! The Legacy Turbo ones are much better IMHO.

-mike

Brake bias.

You'll get an increase in rear brake bias which will improve the overall chassis braking since the factory ususally puts the bias a little bit too far forward.

Most people really aren't going to utilize the added heat capacity of the vented rotors.

Paisan · Post by **Paisan** » Tue Sep 07, 2004 3:03 am

Legacy777 wrote:
Paisan wrote:
Yup, you need the caliper too. Same goes for the rears if you want to do an H6 rotor swap... Need the calipers.
Why would you want the H6 rears? They aren't even vented! The Legacy Turbo ones are much better IMHO.

-mike
Brake bias.

You'll get an increase in rear brake bias which will improve the overall chassis braking since the factory ususally puts the bias a little bit too far forward.

Most people really aren't going to utilize the added heat capacity of the vented rotors.

So you are saying you would suggest along with the larger rear rotors a manual brake bias controller?

If you don't need the venting, then why would larger ones help? I'm a bit confused, if you are saying you need larger rotors due to "extreme" driving, yet you don't need them to be vented?

If you are going to go through the trouble to upgrade the brakes I would hope you get out on the track and at least put them to use! I've had my car at Pocono, Limerock, and Summit Point. The stock rears with upgraded rotors (Mountain Rotors) and 4-pot subie fronts with the Racingbrake rotors and HP+ pads worked very well even after 4- 35 min sessions on the road course. And after 40-35 min sessions and 10K street miles I don't even have a lip formed

-mike

vrg3 · Post by **vrg3** » Tue Sep 07, 2004 3:10 am

I think the difference, put simply, is: venting brakes improves heat capacity, while increasing their diameter improves brake torque. You only notice vented brakes when you're working your brakes hard. You always notice larger brakes.

Paisan · Post by **Paisan** » Tue Sep 07, 2004 3:22 am

yeah more rolling resistance, for the 10%-15% braking the rear does.... Not worth the effort and that's coming from someone who actually races their cars on the track.... Stick to upgrading the front brakes with WRX rotors/pads/calipers/brackets, significantly easier and most bang for the buck.

-mike

Kostamojen · Post by **Kostamojen** » Tue Sep 07, 2004 4:00 am

Its worth it when your rear brakes go bad and you spend the extra cash at that time to upgrade.

The vented rear brakes are cool and all, but too small. I hope the 05 brakes do turn out to be a financially reasonable upgrade...

BAC5.2 · Post by **BAC5.2** » Tue Sep 07, 2004 4:07 am

Our cars have tremendous front bias though. The increased rear torque would help keep the car level. I'm ALMOST considering doing the H6 rear upgrade if the 2.5GT rears won't bolt up.

Paisan · Post by **Paisan** » Tue Sep 07, 2004 4:15 am

maybe you guys need suspension work, cause I don't have a problem stopping with either my '96 Impreza L nor my '94 Legacy Turbo both running WRX size front rotors and 4-pot calipers with Hawk blues on the impreza and HP+ on the Legacy. And the impreza has the rear drums....

-mike

BAC5.2 · Post by **BAC5.2** » Tue Sep 07, 2004 4:39 am

It's not to do with stopping, it's to do with feel and bias. My car stops just fine in a straight line. It's when I trail the brakes into a turn that the car doesn't feel as level as it should. Suspension tuning would help, but that doesn't change the bias, and that's what's important.

It's like stiff springs or huge sway bars. It's a bandaid for the real problems, but it doesn't actually solve them. Bigger rear brakes would help the whole braking system work better.

Have you ever auto-x'd or tracked an STi? Trail the brakes through a turn in the STi and then do the same in your Legacy. You'll notice the difference, I know I could.

Paisan · Post by **Paisan** » Tue Sep 07, 2004 4:56 am

My point is you cannot change the bias by putting larger rotors. The bias will still be the same either way. I've tracked RSs, Legacy Turbos, my Impreza. I haven't noticed that big a difference between the brakes on the STi on the track v. the others with upgrades. Guess I'm not trying hard enough

-mike

BAC5.2 · Post by **BAC5.2** » Tue Sep 07, 2004 5:08 am

Since when won't bigger rear brakes change rearward bias?

The fluid and pressure split front/rear will be the same, but the increased rear rotor diamater increases rear brake torque, which effectively increases rear bias.

Follow?

Paisan · Post by **Paisan** » Tue Sep 07, 2004 5:41 am

The valve changes the bias, not the rotor size IIRC. I'm not 100% sure but as far as I've learned from all the car prep is that it won't change the bias by slapping on larger rear rotors. You may very well be correct but this is the first I've heard of it.

The other logic behind not doing the H6 upgrade in the rear is availability of the brackets/rotors is much less than finding front WRX brackets/calipers/rotors, which are plentiful on NASIOC.

-mike

BAC5.2 · Post by **BAC5.2** » Tue Sep 07, 2004 5:50 am

Bigger rotors = More torque. While actual line pressure doesn't change, more torque in the rear increases rearward bias. I'm fairly certain of this.

Edit: (in response to your edit)

The front WRX swap increases front bias, the rear H6 upgrade balances this out a little. It's not a one-or-the-other mod. The H6 rear would be in addition to the WRX front. You COULD do it with the stock front brakes, but then you'd have larger rear rotors than front rotors, and that'd look funny, lol. If anything it would be TO rear biased at that point.

Paisan · Post by **Paisan** » Tue Sep 07, 2004 6:04 am

Gotcha, I'm not rich enough to do that on my impreza and the vented rears work fine on my legacy so I'm sticking with them for now. Good discussion though!

-mike

scottzg · Post by **scottzg** » Tue Sep 07, 2004 6:10 am

Physics exam question #4:

Paisan says that since the same amount of pedal pressure is being distributed as before, the biasing has not changed.

BAC says that since the piston has been moved further from the hub, it has more mechanical advantage over the rotor, and thus increases the bias.

Who is right?

I side with paisan on this one, although the caliper has more mechanical advantage, the rotor is moving faster per revolution, and it cancels out. A larger pad is going to apply less pressure per cm^3, and so it cancels out.

One way you could figure it out would be to find a performance model somethingorother that has larger front brakes and stock rears and see if the biasing is the same.

BAC5.2 · Post by **BAC5.2** » Tue Sep 07, 2004 6:15 am

You should sell the 4-pots from your Impreza and buy the 2-pots for the rear of your Legacy.

Trade the stock front setup from a Turbo Legacy + some cash for your front 4-pots from the Impreza.

You can get the 2-pots from www.RaceCompEngineering.com For $994. Everything you need.

Paisan · Post by **Paisan** » Tue Sep 07, 2004 6:21 am

Yup, on the impreza race car once I put in the rear disc brakes I will also put in a manual brake bias controller. Makes it nice for controlling tha bias, especially on a rainy track

-mike

Paisan · Post by **Paisan** » Tue Sep 07, 2004 6:23 am

BAC5.2 wrote:You should sell the 4-pots from your Impreza and buy the 2-pots for the rear of your Legacy.

Trade the stock front setup from a Turbo Legacy + some cash for your front 4-pots from the Impreza.

You can get the 2-pots from www.RaceCompEngineering.com For $994. Everything you need.

Nah the legacy is fine as it stands for the street and backup track duty. The impreza however is the one that would need the rears if anything. Racingbrake one of our sponsors will likely have out a rear 2-pot kit shortly which will go on the impreza

-mike

BAC5.2 · Post by **BAC5.2** » Tue Sep 07, 2004 7:05 am

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.

Scott - Isn't pressure directly related to piston size, NOT pad size? If a piston applies, say 200lbs of force, it's going to apply that force reguardless of the pad size. The pad is going to distribute that force evenly, but not reduce it. I'm pretty sure it doesn't work that way.

Look at a scenario.

Given exact caliper and pad specs, and the only variable being rotor size, larger rear rotors would produce more torque. No? More torque changes effective brake bias. No?

If this is NOT the case, then why are we upgrading brakes in the first place? Why do you want larger fronts? They stop better? Why? Increased brake torque, right? Same goes for the rear. Increase the rear diameter, increase the torque.

Now, lets play this out.

Stock brake system. It's set up 60% front and 40% rear line bias (hypothetical). That's just line bias though.

Say your front brakes exert 200lbs of force, and your rears exert 150lbs of force. If you increase the rear brake torque, to say 185lbs of force, what happens?

Line bias remains the same, but torque bias is changed.

Follow? I wish it wasn't 2am, and I could work out some concrete numbers. Josh (Legacy777) knows more about brakes than me, so maybe he could chime in and help clear it up.

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Big rotors?

Big rotors?