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Old 10-08-2002, 09:40 PM   #1
Legacy777
Scooby Guru
 
Member#: 4800
Join Date: Mar 2001
Chapter/Region: TXIC
Location: Houston, Tx
Vehicle:
1990 Legacy & OB XT
AWD 6MT EJ22T AWIC Swap

Default Master Cylinder sizing, brake pedal dead area, brake math

second edit: have added info for the sti 4-pot & 2-pot calipers

Here is the explanation for my assumption for the Sti calipers. On my original calcs I assumed the pistons would move .125" When doing the 4-pots I assumed each piston would only move half that amount because you have opposing pistons which will hopefully oppose each other's forces and reduce the amount of piston movement.

In the real world it will probably be a little more then half, but not quite the whole amount


Ok......here's a little info/background on my problem and current setup.

There are two distinct and different issues I have gotten, due to the front WRX brake swap & rear turbo legacy brake swap.

I also have SS lines & Mintex 1155 pads.

The first is that the pedal stroke is longer. It takes more travel of the brake pedal before the brakes start to engage. It does not give a smooth transition from light braking to heavy braking. Most of the modulation has to come from my foot.

I just got a set of FSM's for 92 & 93 legacy MY's. I did find out something new. My legacy, stock came with a Master cylinder with 1" effective piston diameter, front calipers with a 2.252" effective piston diameter, and rear calipers with a 1.374" effective piston diameter.

In swapping to the current setup of WRX fronts & turbo legacy rears I now have same master cylinder, front calipers with 1.685" * 2 effective piston diameter and rear calipers with a 1.5" effective piston diameter. So I have increased the volume of fluid that must be displaced front and rear. I had thought I only increased the fronts, but the turbo legacy's were larger.

The turbo legacy has a master cylinder with a 1.059" effective diameter. So it's a tad bigger then my stock one. The WRX has a master cylinder with a 1.0625" effective diameter, which again is slightly bigger still.

I basically have a WRX brake setup. The piston areas are exactly the same as the WRX.

What I am thinking is either swapping over to a turbo legacy master cylinder or WRX master cylinder. I don't know if the WRX one will be too much of an overkill because I don't have ABS. With a larger bore on the master cylinder the more mushy the pedal feels.

I have also heard you can adjust the brake pedal and it's travel, but I have not really checked it out on the car yet.

A possible ideal setup might be turbo legacy master cylinder with single stage booster, as it won't provide as much braking assistance so the braking force will be more linear......I'm not sure yet. I have to do some calculations and such.

The next issue I have gotten is the braking bias is weighted too much front, due to the increase in piston area in the front and such. I have to redo the calculations to see how much it increased over the stock setup now that I know the rears also increased.

I have looked. There are no good aftermarket solutions to adjusting brake bias on a subaru, without re-plumbing the entire car. So what I'm possibly going to look into is getting a proportioning valve from a wagon. The split point for the sedan is 533 psi. The split point for the wagon is 640 psi. It's about 100 pounds greater on the wagon. So it's possible in putting the wagon prop. valve on, I will gain more rear brake bias. BUT.....again, I need to do some calcs and see that it won't be too much rear bias.

I've got some whiteline springs to install, which will prob help nose diving and such a dramatic weight transfer when compared to the stock setup, so that too may help.

So....those are my ideas. All the numbers and info I gave you, that refers to specific diameters and such came directly from the factory service manuals, so I know they are correct.

Here's some general calculations I have done in reference to the master cylinder sizing issue and pedal travel for the stock setup, current setup, and possible solutions.

**List of Assumptions for my calcs.**

1. Assume 0.125" of caliper piston travel on all sliding type calipers, front/rear & left/right.

2. Assume 0.0625" of caliper piston travel on all opposing piston type calipers, front/rear & left/right.

2. Pedal travel due to unmeasureable issues, or small amounts have been omitted. i.e. some of the stuff romanom mentioned here

Calculations:

Legacy MC diameter 1"
Turbo Legacy MC diameter 1.0625"
WRX MC diameter 1.0625"

Legacy MC radius 0.5"
Turbo Legacy MC radius 0.53125"
WRX MC radius 0.53125"


A=Pi*r^2
Legacy MC area 0.785398163 in^2
Turbo Legacy MC area 0.886640895 in^2
WRX MC area 0.886640895 in^2


Legacy Front caliper diameter 2.252"
Turbo Legacy Front caliper diameter x 2 1.685"
WRX Front caliper diameter x 2 1.685"
Sti 4-pot Front caliper diameter x 4 1.60"

Legacy rear caliper diameter 1.374"
Turbo Legacy rear caliper diameter 1.5"
WRX rear caliper diameter 1.5"
Sti 2-pot rear caliper diameter x 2 1.5"

Legacy Front caliper radius 1.126"
Turbo Legacy Front caliper radius x2 0.8425"
WRX Front caliper radius x 2 0.8425"
Sti 4-pot Front caliper radius x 4 0.8"

Legacy rear caliper radius 0.687"
Turbo Legacy rear caliper radius 0.75"
WRX rear caliper radius 0.75"
Sti 2-pot rear caliper radius x 2 0.75"


A=Pi*r^2
Legacy Front caliper area 3.983149927 in^2
Turbo Legacy Front caliper area 4.459844201 in^2
WRX Front caliper area 4.459844201 in^2
Sti 4-pot Front caliper area 8.042477193 in^2 (Note: area was calculated for one piston and then multiplied by the number of pistons)

Legacy rear caliper area 1.482734343 in^2
Turbo Legacy rear caliper area 1.767145868 in^2
WRX rear caliper area 1.767145868 in^2
Sti 2-pot rear caliper area 3.534291735 in^2

Assume 0.125" of caliper travel for sliding calipers
Assume 0.0625" of caliper travel for oposing piston calipers

(Calcs for both left & right)
V=(A*0.125")*2 or V=(A*0.0625")*2
Volume for Legacy Front Calipers 0.995787482 in^3
Volume for Turbo Legacy Front Calipers 1.11496105 in^3
Volume for WRX Front Calipers 1.11496105 in^3
Volume for Sti 4-pot Front Calipers 1.005309649 in^3

Volume for Legacy rear Calipers 0.370683586 in^3
Volume for Turbo Legacy rear Calipers 0.441786467 in^3
Volume for WRX rear Calipers 0.441786467 in^3
Volume for Sti 2-pot rear Calipers 0.441786467 in^3

Combined f & r volume for Legacy 1.366471068 in^3
Combined f & r volume for Turbo Legacy 1.556747517 in^3
Combined f & r volume for WRX 1.556747517 in^3
Combined f & r volume for Sti 1.447096116 in^3

Travel=V/A
Pedal travel for Legacy 1.739845"
Pedal travel for Turbo Legacy 1.755781315"
Pedal travel for WRX 1.755781315"

Current Pedal travel 1.9821125"



So.....out of all that math.....

The take home numbers are.

I have close to 2" of pedal travel with my current brake setup due to the small 1" bore on the MC.


If I were to put a WRX MC on, I would have around 1.76" of pedal travel.

Again......these numbers are purely generalistic type numbers, and are not necessarily correct numbers by any means. There are just to judge and compare the different caliper/MC setups against each other.

Like I mentioned before. It looks like the turbo legacy one should provide enough volume of fluid, but be a tad smaller to help against mushy pedal.

With the addition of the single stage booster I would assume pedal feel would improve and brake force would be more linear. Am I correct in thinking that?

Comments/suggestions?

Josh

Last edited by legacy777; 05-08-2003 at 11:35 PM.
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