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Old 04-29-2002, 06:06 PM   #1
davidm_sh
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Default Way too increase rear brake bias/pressure?

I was wondering if anyone knew of a way too increase the amount of psi/pressure/ratio that is transmitted to the rear brakes in a WRX.

I have the SPt springs and struts = stiffer suspension = not as much nose dive in braking = I want to get the rear brakes involved more to get a "squatting brake effect" under hard braking.

I have the Wilwood 6pot front and 4pot rear and the car is back too diving like it did with the stock suspension. So I looked into proportioning valves but it seems that all they do is reduce/restrict baseline pressures (~30% rear and 70% front on WRX). So I could decrease either the front or rear (given two valves) BUT it does not seem that I can shift the percentages to say 40% rear and 60% front without resizing the bores on the master cyclinder? At least this is the way that Wilwood explained it too me .

MY IDEA: (albeit maybe not the best one [heh])

Just wondering if anyone had some info. or help for myself. I was thinking about it for a second and it seems like there should be two lines coming out of the master cylinder (with smaller line going to rear = 30% of pressure and a larger diameter tube going to front = 70% bias).

I was thinking couldn't you combine those two lines into one to regain 100% pressure and then from there you could branch/tee off from with different sized/bores/diameter tubing to front and rear?

Just an idea from someone who really doesn't understand how all the braking compenents work exacally [heh].
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Old 04-29-2002, 06:42 PM   #2
TyrannoSullyRex
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Default

The ghetto way to do it, or at least how you do it in stock class autocross was you have stock-like compound on the front and a more aggressive compound on the rears (in other words a more bite compound on the rear) at least that's what I did on the Miata.
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Old 04-29-2002, 07:28 PM   #3
romanom
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Default Re: Way too increase rear brake bias/pressure?

Quote:
Originally posted by davidm_sh
I was wondering if anyone knew of a way too increase the amount of psi/pressure/ratio that is transmitted to the rear brakes in a WRX.

I have the SPt springs and struts = stiffer suspension = not as much nose dive in braking = I want to get the rear brakes involved more to get a "squatting brake effect" under hard braking.

I have the Wilwood 6pot front and 4pot rear and the car is back too diving like it did with the stock suspension. So I looked into proportioning valves but it seems that all they do is reduce/restrict baseline pressures (~30% rear and 70% front on WRX). So I could decrease either the front or rear (given two valves) BUT it does not seem that I can shift the percentages to say 40% rear and 60% front without resizing the bores on the master cyclinder? At least this is the way that Wilwood explained it too me .

MY IDEA: (albeit maybe not the best one [heh])

Just wondering if anyone had some info. or help for myself. I was thinking about it for a second and it seems like there should be two lines coming out of the master cylinder (with smaller line going to rear = 30% of pressure and a larger diameter tube going to front = 70% bias).

I was thinking couldn't you combine those two lines into one to regain 100% pressure and then from there you could branch/tee off from with different sized/bores/diameter tubing to front and rear?

Just an idea from someone who really doesn't understand how all the braking compenents work exacally [heh].

1. Yes you can get a proportioning valve that shifts the bias without decreasing overall flow. That's what Electronic Brake Distribution (EBD) does. It's the driver adjustable prop valves that act the way described above.

2. The master cylinder has absolutely nothing to do with bias...NOTHING....the TMC (tandem master cylinder) is there just to provide brake fluid volume. 99% of all passenger cars are diagonal systems. This means that both chambers of the TMC are equal. And each sends fluid to one front tire and the opposite rear tire. Change the volume of the chambers just decreases or increases flow overall, but the bias stays the same.

3. Changing brake tubes is impossible unless you plan on completely taking the car apart. Brake tubes are about the 4th thing mounted on the car at the factory. And wouldn't accomplish anything except increase response time.


4. If you have one line off the TMC you no longer have a 2 circuit brake system....and this is very bad.


5. The prop valve is the only thing that will change front-rear brake pressure bias...THE ONLY THING.


6. That pad trick mentioned by TyrannoSullyRex is proven to work and done often. On rare occasion cars have come that way from the factory. With the rear pads having a different friction coefficient (mu) than the fronts.


8. Yes you can buy prop valves with different biases. Many shops that sell racing parts will have these.



I can't believe Wildwood told you that...that's not even close to accurate. Surprised. Master Cylinders are sized based only on the size of the calipers and like I said each chamber provides fluid to one front and one rear tire so changing the volumes does nothing except possibly making your brake system fail.


Some more brake stuff:

http://www.i-club.com/forums/showthr...hreadid=168098
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Old 04-29-2002, 07:39 PM   #4
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By the way you only need to change bias if:

The fronts lock up before the rears. Otherwise I won't touch it.

You can test this by pulling the ABS fuse. Going to a empty and wide open area (hopefully a closed track) and doing hard stops to induce lock up.
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Old 04-29-2002, 09:33 PM   #5
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Quote:
Originally posted by romanom
By the way you only need to change bias if:

The fronts lock up before the rears. Otherwise I won't touch it.

You can test this by pulling the ABS fuse. Going to a empty and wide open area (hopefully a closed track) and doing hard stops to induce lock up.
flat spot hell!!

-justin
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Old 04-29-2002, 10:15 PM   #6
romanom
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Quote:
Originally posted by Blindeye_03


flat spot hell!!

-justin
Well I'm assuming he's going to test to IMPENDING lock up..not actually go sliding all over the place!
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Old 04-30-2002, 12:10 AM   #7
davidm_sh
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Well after talking to some more people it sounds like the easiest way to solve my problem will be too get some more aggressive brake pads on the rear than compared to the front.

Oh well... thanks for your input guys/gals
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Old 04-30-2002, 10:46 AM   #8
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I'm pretty sure nose dive is more suspension geometry-related than than brake bias. If you had just rear brakes for instance, there's no way the rear would squat because you're energy is moving forward. I have a friend that put the ALK on his car, which changed the geometry (convergence?) up front and nose dive was diminished somewhat. BMW has doen a lot in this area, watch one under braking, it squats considerably.

Mark
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Old 06-30-2003, 06:28 PM   #9
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Quote:
Originally posted by Tangmere
I'm pretty sure nose dive is more suspension geometry-related than than brake bias. If you had just rear brakes for instance, there's no way the rear would squat because you're energy is moving forward. I have a friend that put the ALK on his car, which changed the geometry (convergence?) up front and nose dive was diminished somewhat. BMW has doen a lot in this area, watch one under braking, it squats considerably.
Mark
This is a great thread. What are the most aggressive pads you can get for the rear? I have the Cobb pads on all 4 corners now and I'm quite sure I'm not stopping nearly as fast as my tires would ultimately allow. I'm not sure if its due to bias or because of ABS though. Its an area I want to focus on now that I have the other areas sorted out.

If I'm not mistaken, if you pull your ebrake while you are coasting the rear end of the car will squat. I haven't gone through the physics but try it and you will see. Then you can match the physics to what just happened. Its counter intuitive but it happens so I won't bother to do the math.
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Old 06-30-2003, 09:35 PM   #10
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Quote:
Originally posted by TyrannoSullyRex
The ghetto way to do it, or at least how you do it in stock class autocross was you have stock-like compound on the front and a more aggressive compound on the rears (in other words a more bite compound on the rear) at least that's what I did on the Miata.
Just don't try this on the track -- you will melt your front pads, or at a minimum, fade them to non-existence. :-) Less brake dive but increased $$$ or annoyance.

I'm in the same boat, and that didn't work for me, at least on the track...

Autox will prolly work fine though...

Scott
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Old 07-01-2003, 08:44 AM   #11
Legacy777
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For you WRX guys, I would suggest you look at some of the various proportioning valves from other subarus. This will be the only way you will be able to adjust rear brake pressure.

The only other option is to add more rear brake torque, aka H6 upgrade or brake kit for the rear.
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Old 07-13-2003, 09:07 PM   #12
X-VWGLX
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Default nose dive

Good thread.
I have really begun to think about this issue a lot since putting Carbotec Bobcats on the front. those pads bite pretty well, and i notice more nosediving, which is probably causing the rears to do even LESS work. aside from new suspension components or bigger rear brakes, a higher coefficient rear pad seems to be the best solution.
i dont know nuthin about proportioning valves, and i really dont want to do anything to jeoprodize the funtionality of my brakes - i really like when they are working.
the suggestion of using really aggressive pads is not a good solution if you will be at the track. here is why: i saw a friend's WRX at VIR using a track-grade pad on the fronts(they were carbotechs). after one day the stock rotors were fractured and warped, and the rubber boots around the caliper pistons were MELTED OFF. GONE. too hot.
Secondly, the "H6 upgrade" uses a larger rear rotor, but keeps the WRX pads. i am not sure if wider pads are available, but i assume they are.

so, I am going to the track, and need to solve this problem. anyone who can turn a nosedive into a squat for less than $100 please respond.

Last edited by X-VWGLX; 07-25-2003 at 08:29 PM.
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Old 07-13-2003, 11:52 PM   #13
jaxscuby
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Default Re: nose dive

Quote:
Originally posted by X-VWGLX
anyone who can turn a nosedive into a squat for less than $100 please respond.
LEGACY REAR ROTOR UPGRADE!!!

my 2 cents..

Ben
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Old 07-14-2003, 09:28 AM   #14
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Default Re: nose dive

Quote:
Originally posted by X-VWGLX

Secondly, the "H6 upgrade" uses a larger rear rotor, but keeps the WRX pads. that wont change your brake power, just heat dissapation. i am not sure if wider pads are available, but i assume they are.
Incorrect.

By moving the caliper farther out from the center of the hub you do indeed increase rear braking power. In the case of the "H6" upgrade, it is an increase of about 10% vs the stock WRX rear setup. Effective rotor radius and caliper piston area are the two key ingredients to inherent braking power at the wheel. The pad's coefficient of friction is tangentially important if it is used to vary bite of the front vs the rear, but by itself has little effect on braking power.

Brake torque = line pressure X caliper piston area X effective rotor radius(measured at the center of the piston) X pad coefficient of friction.

The "H6" rear rotor upgrade significantly increases torque and marginally increases heat capacity.

The "H6" COULD be the solution, but I think the poster said he had Wilwood 6/4 pots, so without their baseline torque, I dont know if the "H6" would help or hurt the diving problem.

ss

Last edited by subysouth; 07-14-2003 at 09:36 AM.
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Old 07-14-2003, 09:32 AM   #15
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Default Re: Re: nose dive

Quote:
Originally posted by jaxscuby


LEGACY REAR ROTOR UPGRADE!!!

my 2 cents..

Ben
Jax Mod Squad
Ben, I think you are thinking "H6," but remember there is another Legacy rear rotor upgrade, the BC/F Legacy Turbo vented rear rotor upgrade. The Legacy Turbo vented rear upgrade doesnt increase the rotor diameter, so it is a heat capacity only upgrade.

ss
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