picking the right pedal ratio for manual brakes

so i decided to just up and forgo power brakes in favor of a couple wilwood masters and a balance bar and just use the stock pedal. but then the question of where to mount the the balance bar into the pedal ran through my head.

any rules of thumbs to go with on picking where to mount the balance bar. it's a 13" pedal, running 7/8"s on the front and 1" on the rear. and mazda 4 pots on all 4 corners.

any thoughts on what ratio to run?

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"Rally racing makes a heroin addiction look like a vague craving for something salty"

don't have a good answer but I have been playing with the math using some guidance from the stop tech guys. I'm sure its all road related but seems to be good info.

Stoptech Dual Master PDF

try these:
http://www.automedia.com/Better_Brake_Performance/pht20080501gh/1
http://www.stoptech.com/tech_info/PedalSetup-DualMaster-Guide.pdf
http://books.google.com/books?id=vIPlH06yzWYC&pg=PA68&lpg=PA68&dq=brake+pedal+ratio+calculation&source=bl&ots=LISdI6YK4y&sig=DvuQUZqpYWNFFFTUDt-6AYhOKk0&hl=en&ei=fJ9yTKHoK4HGlQfGsu2vDw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CBgQ6AEwAjgK#v=onepage&q&f=false

Nick

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Nick Leone
Former scca member

this might help
brake math spreadsheet
http://www.google.com/url?sa=t&source=web&cd=1&ved=0CBIQFjAA&url=http%3A%2F%2Fsth2.com%2FZ-car%2FBrakemath.xls&rct=j&q=brakemath&ei=vZpyTKLjLcP48AbG07i0Cw&usg=AFQjCNEqYJd1aVZuFeZfs_7IBmSfsVjjQg&cad=rja

you know the more i really think about this the more i think i'm over thinking this. ok lets say i was to buy some fancy wilwood brake pedal set up. most of the normal ones are are 6.25:1 ratio. and for what i have been thinking is, i needed to figure out what the right travel and pressure and blah blah blah to fit this in there right.

but sadly it's math that comes to our rescue here. oh math i hate you.

wilwoods brake pedals are 11.88in. a 6.25:1 ratio puts the pins from the MC at 1.9in from the pivot point. so if i cram my foot down with a 100lbs of force i'm pushing on the MCs with 625lbs of force. and the MCs travel in at .16in for every inch the pedal is pressed.

so lets take my stock 13" pedal. if i place the pins at 2.08in from the pivot point, i get the same 6.25:1 ratio, i get the same 625lbs of force from the 100 i give, and i get the same .16in of MC travel for every inch of pedal travel.

i get the same ratios that a wilwood pedal box would give you, just for $200 less.

stupid math telling me i don't need to over think this.

but really, if i was to get a wilwood pedal box, i'm about 99% i wouldn't think twice about "oh is this the right ratio" i would just run it and live with it. so really, no need to over think something that wilwood figured out already.

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"Rally racing makes a heroin addiction look like a vague craving for something salty"

You should run more than 6.2 ratio. And the willywood pedal box is far from fancy. In fact its at the bottom of the totem pole. But anyways, the reason I say you need a higher ratio is because you have 2 bigass masters. So, you're going to have a very hard pedal and not have enough torque to stop the car at speed.

So yeah, even though its all about math, you have yet to solve all the equations. So that being said, I dont think you'll have any decent amount of brake no matter what ratio you choose, even going with a 9:1 ratio is still going to give you a hard pedal.

Go with different masters.

ok then what size masters should i go with?

they are 4 pots all around and if i remember right 1.2in pistons. but i can double check later

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"Rally racing makes a heroin addiction look like a vague craving for something salty"

Quote
mellow65
ok then what size masters should i go with?

they are 4 pots all around and if i remember right 1.2in pistons. but i can double check later

Check the pistons, 1.2 (30.5mm) sounds way too small, maybe for the rear.

Are the pistons the same size on the front and rear calipers?

Quote
mellow65
ok then what size masters should i go with?

they are 4 pots all around and if i remember right 1.2in pistons. but i can double check later

36mm or 1.41 portions of some mythological kings thumb.

Quote
Cosworth
Check the pistons, 1.2 (30.5mm) sounds way too small, maybe for the rear.

Are the pistons the same size on the front and rear calipers?

i checked they are 1.27" . i think you might have missed that they are a 4 piston caliper also.

and yes, i'm running same brakes front and rear. calipers and rotors. yes it's over kill, but i'll never have to say again, i wish i had more rear brakes.

but also from the simplicity of spare parts, the fact these calipers are like $40 new.

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"Rally racing makes a heroin addiction look like a vague craving for something salty"



Edited 1 time(s). Last edit at 08/24/2010 12:34PM by mellow65.

Holy dogshit.... you're shit out of luch with that setup then.

3 reasons:

1st: The pistons are WAY too small. With the size masters you have you would need at least a 8+ pedal ratio and even then the pedal would still be too hard.

2nd: You'll have to go down on master sizes dramatically just to get any decent torque out of the system.

3rd: Using a 5/8's front would be ideal with a 6.2 pedal but you'd have keep the rear high in the 1" range to have a decent bias. The problem with that is the massive difference in both sizes, so your balance bar is never going to act right because you wont be able to tune it properly. You rear timing will always come on first no matter how much you adjust the m/c rods and it will have so much stagger than it will just make the car feel terrible under braking.

The only sulotion to be able to run the same caliper in the rear is if you run crappy street pads in the back and very aggressive in the front. Even then its still going to have to run a 13/16's master for the back, and about 2 turns on the bias knob to the front.

hmmmmm well shitty yo, thats not what i wanted to hear.

so with the fronts what they are, what should i be looking for in the rear?

im totally still in mock up stage for the calipers so i can go else where if needed, but im stuck with the rotors, i had to pick the rotor when i had my axles machined.

im always up for some input. i dont know this shit, i talked to some car builders and they pointed me this way, clearly they may have been blowing smoke up my ass.

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"Rally racing makes a heroin addiction look like a vague craving for something salty"

Oh gawd all my credibility has gone right down the shitter!!!
OH NOES!!!!

I was wrong on RX7 4 pot piston diameter!!!!!

They're 36 POINT FAWKIN ONE!!!!!

How could I be so sloppy?
I resign....
Gonna get a Hair Shirt, give away everything and become a hermit.


Goodbye cruel world!



Edited 1 time(s). Last edit at 08/24/2010 05:06PM by john vanlandingham.

oh wth john, clearly you know nothing,

wth

wth i say

Quote
john vanlandingham
Oh gawd all my credibility has gone right down the shitter!!!
OH NOES!!!!

I was wrong on RX7 4 pot piston diameter!!!!!

They're 36 POINT FAWKIN ONE!!!!!

How could I be so sloppy?
I resign....
Gonna get a Hair Shirt, give away everything and become a hermit.


Goodbye cruel world!

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"Rally racing makes a heroin addiction look like a vague craving for something salty"

What year are you working with??

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SteveL
This is the point in the killing spree when you really should turn the gun on yourself