Subaru Master cylinder Sizing

Did you double area for sliding calipers? I did forget that in calcs.
Paulinho posted somewhere about ideal balance for gravel but don't recall what he said. I think it was 80%ish front?

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Grant Hughes

Yes. I believe I did it correctly, should have been 4pir^2 for the 2 piston and 2x for the single piston correct?

Just stick something in there and go drive. Follow conventional wisdom for these things, no need to agonize over decisions for weeks. Keep in mind that you've got a fancy pedal set, so you can adjust the balance later to fine tune it for different surfaces.

Dave

0.7 front, 0.75 rear will give you 74% front bias at even split. That'd be a good starting point and will leave you the 0.625 for the handbrake, which will allow you to give easy yanks with lots of line pressure.
Ironically, that's probably what John would have recommended based on the books.
If you need more front, you could go 0.625f and 0.75r for 78.5%.
You can also fine tune the bias with you balance bar and with different pads if you want more front or rear.

What are you using for gearbox and diffs?

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Grant Hughes

Gearbox for now i have the wrx ratios with a 4.11 final drive. I have a Modena front diff.

Grant sent me this link on FB, I've been away for medical reasons and haven't had time to check anything. But here it goes.

Is this for an AWD car or FWD? If its an AWD car then about 75% split will be fine. The master sizes being talked here are good for torque and split but with the 6:1 pedal ratio and those kinds of calipers the pedal is going to feel very mushy. Running a .625 (5/8) in the front is asking for trouble because it will make a long pedal and if the fluid starts to overheat then you'll start to run out of pedal real quick.

7/10s in the front and 13/16s in the rear is the best compromise for a good bias and firm pedal. Turn the bias to the rear 2 turns, elongate the front mc rod 5mm longer than the rear and it should be all set. Run the same friction pads front and rear.

Thanks! It is awd. I do want to upgrade calipers to at least the subaru 4/2pot but I don't know how soon that all be. Are there more options for subaru besides the subaru calipers and AP?

Will the .625 hand brake be all right ? I forgot to measure its ratio.

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Thanks! It is awd. I do want to upgrade calipers to at least the subaru 4/2pot but I don't know how soon that all be. Are there more options for subaru besides the subaru calipers and AP?

Will the .625 hand brake be all right ? I forgot to measure its ratio.

it'll be fine

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John Vanlandingham
Sleezattle, WA, USA

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Finished this up finally. Well, still need to permanently mount the resivoirs. Still a little worried about doing that in the cab but I see that it's not uncommon.





For masters I used 7/10" clutch and front brake, 13/16" rear, and 5/8" for the hand brake as suggested above. They all feel great. The brake pedal travels a little more than an inch, and is firm but has that little squish right at the end just how I love my bike brakes. Hand brake is a tad less firm but still feels good and doesn't have much of a throw either. Hope to try them out this weekend but I find new problems just as I finish other ones. Now the car looks like a cross eyed kid because of the wider arms and frozen tie rod ends. Some day soon...

Finished mounting reservoirs and a temp cover for the masters. Been playing a bit with the bias and it was pretty apparent tonight in the snow and ice that the rear locks up much sooner than the front, unless bias bar was moved all the way to the front. I thought I noticed it in the gravel on Sunday but only confirmed it tonight.

Quick refresher, I have a 0.70" cylinder in front, a 13/16" cylinder for the rear and same pads front and rear.

Trying to decide what course of action to take first. Different pads in the front? Smaller front master cylinder? Looking for different calipers which will be the most expensive since my collection of wheels won't fit over anything larger, I don't believe

Mike Hurst is going to have kittens when he sees those brake fluid reservoirs mounted inside the car. They could burst into flames at any moment! I don't have a problem with it myself.

I doubt I will make an RA event this year anyway. Looking at NASA events and CHCA hill climbs and Dave Kerns BMW passed tech for the climbs with them. But I totally forgot about Hurst. Damn. And my stilo helmet will burst into flames this year too anyway... Or is that next December? Either way it was the cheap one and its currently at ~$120/event

Bigger rear master so you don't loose pedal feel? It is nice having the same compound pad front and rear so they warm up the same ect.

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Finished mounting reservoirs and a temp cover for the masters. Been playing a bit with the bias and it was pretty apparent tonight in the snow and ice that the rear locks up much sooner than the front, unless bias bar was moved all the way to the front.

You should also be able to adjust how quickly one side sees pressure as well with how much you thread it into the bias bar, no? Then you can adjust line pressure by fiddling with the bias bar.

Dave



Edited 1 time(s). Last edit at 12/27/2012 11:33AM by DaveK.

What the goal? Lock up at the same time?

A quick little box around the master cylinders with grommets where they exit to the master will suffice when the time comes to deal with Rally America scrutineering.

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Grant Hughes