Subaru Master cylinder Sizing

The amusing thing with the data in the link is it does tell the poor bastid looking at it what he should do with dual masters.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
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Damn it! John why am I not seeing it?

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phlat65
Just remember, with a slider calper you double the piston area. That may explain the lower torque of the fixed caliper.

I don't think that spreadsheet accounts for that.
I feel like the doubling would not be 100% effective either as there will be some deflection, bridge flex, etc. So the four pots would have a better pedal feel than a similar sized 2 pot on slider. Not to mention the slider is an additional failure point. I've had a few times with a car where the slider got dirty/corroded/neglected for years and basically wasn't fully releasing leading to brake drag and fast pad wear.

How about effective radius? Given the same diameter of rotor a smaller 4 piston caliper should have a greater effective radius than a larger 2 pot slider correct?

This thread needs more Paul F.

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Grant Hughes

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FunctionAuto
Damn it! John why am I not seeing it?

I think John is saying that it tells you to toss them and use stock stuff.

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Grant Hughes

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phlat65
Just remember, with a slider calper you double the piston area. That may explain the lower torque of the fixed caliper.

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I don't think that spreadsheet accounts for that.
I feel like the doubling would not be 100% effective either as there will be some deflection, bridge flex, etc. So the four pots would have a better pedal feel than a similar sized 2 pot on slider.

Everything flexes. And thinking is useless here. A cheesoid POS flexy slider may flex at stomp on it levels, but if the OEM asked the caliper MFG to make it stronger than shit and they did then maybe it doesn't flex appreciably or even more important, noticably..

Faster and cleanest 2wd guy I've ever seen in North American rally kicked ass bad, I mean BAD in a car with boring ol' stock 740 Volvo 2 pot slidey thangs.

And those worked better, gave better feel than the slidey thangs on his beatiful Misterbitchy Evo.

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Not to mention the slider is an additional failure point. I've had a few times with a car where the slider got dirty/corroded/neglected for years and basically wasn't fully releasing leading to brake drag and fast pad wear.

That's not a design failure thats a maintenance failure.

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How about effective radius? Given the same diameter of rotor a smaller 4 piston caliper should have a greater effective radius than a larger 2 pot slider correct?

¿Que?

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

I guess it's more a function of the caliper size, not count?
Simplified thought process:
If overall radius is the same, let's say 150 mm from center line to outside of pad.
Now say you have a 40 mm piston. That means effective radius is 150-20 = 130 mm.
Now say you have a 30 mm piston. Effective radius is 150-15 = 135 mm.
Making the assumption that both calipers have top edge at top of pad which may be a faulty assumption if they are centered at same location...

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Grant Hughes

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Carl S

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FunctionAuto
the 2pot rear wear through very quickly

Wheel scrapers will help emmensly with that.

You HAVE to have wheel scrapers on the rear or you will destroy the 2pots in 1-2 events. Sandblast (one event) w/o scrapers for me nearly destroyed them. (I had scrapers, but was unable to get them modified enough to fit the night before the event.)

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NoCoast
I guess it's more a function of the caliper size, not count?
Simplified thought process:
If overall radius is the same, let's say 150 mm from center line to outside of pad.
Now say you have a 40 mm piston. That means effective radius is 150-20 = 130 mm.
Now say you have a 30 mm piston. Effective radius is 150-15 = 135 mm.
Making the assumption that both calipers have top edge at top of pad which may be a faulty assumption if they are centered at same location...

Back up, stop thinking.
Hydraulics is hydraulics.
Leverage is leverage.
Pad volume is all about wear---ie how long the pad wears

Hydraulic ratios sets up HOW the brakes will feel. Like rock hard no travel, or way long pedal with OK function but generally mush-u-lar-ized feel.
Leverage is how hard you have to push for X redardation.

Gotta get them all right...there's thick steel backing plates that distribute the piston force pretty good

And yeah just about every OEM had the pad nearly at the edge.
AP says ideal is to overhang the pad by a half mm so that you don't have a hatchet shaped lip when the pads eat the disc---they say it makes pulling pads out a pain.
They're right.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

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EricW

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Carl S

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FunctionAuto
the 2pot rear wear through very quickly

Wheel scrapers will help emmensly with that.

You HAVE to have wheel scrapers on the rear or you will destroy the 2pots in 1-2 events. Sandblast (one event) w/o scrapers for me nearly destroyed them. (I had scrapers, but was unable to get them modified enough to fit the night before the event.)

I guess I can't think of it really. But how do wheel scrapers help them not get destroyed?

Dirt/mud accumulate on the inside of the wheel and it acts like a grinder on the caliper

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BillyElliot

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EricW

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Carl S

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FunctionAuto
the 2pot rear wear through very quickly

Wheel scrapers will help emmensly with that.

You HAVE to have wheel scrapers on the rear or you will destroy the 2pots in 1-2 events. Sandblast (one event) w/o scrapers for me nearly destroyed them. (I had scrapers, but was unable to get them modified enough to fit the night before the event.)

I guess I can't think of it really. But how do wheel scrapers help them not get destroyed?

The scrapers set out (radially) from the hub a bit more than the caliper, with a piece of urethane bolted to them. The urethane ideally conforms to the shape of the wheel itself. Without this, the rear wheels would get packed with crap, and the caliper itself would be the only thing that would (by default) be preventing more crap from going into the wheel. So in effect, the caliper would be the scraper.

eg:

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FunctionAuto
Dirt/mud accumulate on the inside of the wheel and it acts like a grinder on the caliper

The worst thing is the rocks embedded in the dirt/mud. The dirt/muds gets shoveled by the bucket load into the wheel which since its rotating gets carried up and if its just dirt, slowly grinds the back side of the caliper, if you're unlucky, one nasty rock can gouge a groove right thru to the fluid passages.

Iron is a good deal tougher than cast aluminum and that's one reason my default rear caliper for mods is Volvo 240 cast iron which had a good sized pad and is the very nice 1.5" or 38mm x2 pistons---the same size more or less as the Subie aluminum things... It's pad is same as Portch 911 frontfrom the 60s to around '75 so piles of good pad choices..

Scrapers are good period, cause even cast iron can die with one unluckily placed rock....

(this is all assuming the disc diameter is such that the caliper is close....most stock shit has miles between the wheel and back side of the caliper.)

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

That's what I was thinking. How Yeah I had an issue like that in my STi. Was on the rears but I had a whole layer of paint removed off the rear caliper. And I didn't even do any crazy mud bogging type stuff to get enough shit in the rear calipers with 17" wheels too. Most of it would have been snow from fucking around in the winters.

How do the scrapers attach to the hub/knuckle anyway? I had thought of getting some on the Civic. Pretty much any muddy stage I'll have a nice thick layer of mud on the insides of my wheels that I would rather not have there.

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BillyElliot
That's what I was thinking. How Yeah I had an issue like that in my STi. Was on the rears but I had a whole layer of paint removed off the rear caliper. And I didn't even do any crazy mud bogging type stuff to get enough shit in the rear calipers with 17" wheels too. Most of it would have been snow from fucking around in the winters.

How do the scrapers attach to the hub/knuckle anyway? I had thought of getting some on the Civic. Pretty much any muddy stage I'll have a nice thick layer of mud on the insides of my wheels that I would rather not have there.

Every application is different since it all depends on the configuration of the hubs and stuff. Maybe somebody has some good clear photos..

By the way, I scraped out the mud out of my 5.5 x 15 wheels swept it up then let it dry once, and when dry I had over 9lbs of dirt for one wheel.
Mud is heavier than dirt... Yikes!

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Ok I made up a quick spread sheet and granted I did the calculations that Grant gave above correctly, these were the values I came up for for brake torque front and rear.

My pedal ratio is 6:1 and I assumed 100lb leg force and a mU of 0.4.

• Master Cylinder____Brake Torque Front______Brake Torque Rear
• 0.62-----------------------5306.19---------------------2121.22
• 0.70-----------------------4162.65---------------------1664.08
• 0.75-----------------------3626.13---------------------1449.6

Do those values look correct? And what do I look for from those numbers? I haven't weighed the car since I had the cage installed and I don't have the new struts yet so I don't know what the weight.