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Thread: Brake/clutch pedal set specifications with brembos.

  1. #1
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    Brake/clutch pedal set specifications with brembos.

    Hey guys.
    I have a supra jza80 and am wanting to delete brake booster and ABS, i have some jeep srt8 2012 front and rear calipers on it (6 pot and 4 pot).

    Now, im wanting to do a pedal set, the one im interested has a ration of 5:1 and adjustable to like 5.45:1 , comes with 3 cylinder (twin for brakes and one for clutch), balance bar etc.
    Options for cylinder size are .625/ .7 / .75 / .875 / 1".

    Is anyone able to help me work out what size bore should i go with for the brakes (front and rear size)?
    The supra uses 1 1/16" master cylinder.
    The jeep srt8 im told uses 1.062" master cylinder.

    I prefer a firmer pedal feel if that matters.
    And from reading, the front and rear bore sizes should be different.
    Even if you were to guess, which would you say is a most likely combo that will work?

    Any help appreciated and many thanks.
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  2. #2
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    What are the piston diameters for the calipers? This is needed to work out master cylinder sizes.

    Rear master cylinder is usually smaller than the front due to smaller rear caliper piston area, relative to the front caliper piston area.

  3. #3
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    Im reading 44mm for the fronts(obviously 6 of them), 32mm and 28mm for rear as they seem to have different size.

    Everywhere i read, on a twin cylinder pedal set you want rear to be bigger, less pressure when cylinder is bigger for the same ampunt of force.

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    Quote Originally Posted by MAZMAN View Post
    Everywhere i read, on a twin cylinder pedal set you want rear to be bigger, less pressure when cylinder is bigger for the same ampunt of force.
    This would be how you compensate for the absence of a balance bar. If you are going to the trouble of installing a pedal box, you want it to have a balance bar.

  5. #5
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    Quote Originally Posted by MAZMAN View Post
    Im reading 44mm for the fronts(obviously 6 of them), 32mm and 28mm for rear as they seem to have different size.
    .
    I would have thought a 6 pot caliper would have differential piston sizes, do you want to double check that before I bust out the calculator?

  6. #6
    Non Compos Mentos Gammaboy's Avatar
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    Quote Originally Posted by Sprinkles View Post
    This would be how you compensate for the absence of a balance bar. If you are going to the trouble of installing a pedal box, you want it to have a balance bar.
    No, it's how you centre the balance bar. Relying on the balance bar alone is a shit idea.
    "Where can we get hold of a Vincent Black Shadow?" "Whats that?" "A fantastic bike," I said. "The new model is something like two thousand cubic inches, developing two hundred brake-horsepower at four thousand revolutions per minute on a magnesium frame with two styrofoam seats and a total curb weight of exactly two hundred pounds."

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    Quote Originally Posted by Gammaboy View Post
    No, it's how you centre the balance bar. Relying on the balance bar alone is a shit idea.
    It's worked fine for me in a race car previously.

    Anyway, at the end of the day it's going to ultimately depend on the dimensions of his caliper pistons.

  8. #8
    Non Compos Mentos Gammaboy's Avatar
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    If it's racked all the way over to one side and you lose a circuit, it may be possible to bottom the bar out in the tube and not have any brakes...
    "Where can we get hold of a Vincent Black Shadow?" "Whats that?" "A fantastic bike," I said. "The new model is something like two thousand cubic inches, developing two hundred brake-horsepower at four thousand revolutions per minute on a magnesium frame with two styrofoam seats and a total curb weight of exactly two hundred pounds."

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    Quote Originally Posted by Gammaboy View Post
    If it's racked all the way over to one side and you lose a circuit, it may be possible to bottom the bar out in the tube and not have any brakes...
    Correct. Mine wasn't. It boils down to the piston sizes and required brake bias.

    Currently working on the spreadsheet for OP to figure out his master cylinder sizing.

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    Mazman, you are going to want the larger sized master cylinders, either the .875 or the 1". It's going to boil down to your idea of a "firm" pedal, so it's a bit subjective.

    If you get both master cylinders the same size, you will have an initial brake bias distribution of 76% front/24% rear with the balance bar centered, which should be a pretty good starting point for track use. This is assuming your 6 pots do have all the same piston diameters, if they are all the same, your pads are going to wear unevenly.

    Anyway, PM me your email and I'll send you this spreadsheet with a bit more explanation.

  11. #11
    GTFO of my ED doctor ed's Avatar
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    Quote Originally Posted by Gammaboy View Post
    If it's racked all the way over to one side and you lose a circuit, it may be possible to bottom the bar out in the tube and not have any brakes...
    Sounds like youíve been driving my Gokart

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    Rotor diameter will play a part too. What have you got installed?

    Sent from my SM-G950F using Tapatalk

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    360mm front rotors from isf.
    345mm rear rotors from isf.

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    So I finally sat down and had a look at this.

    Assumptions used for calcs:

    General:
    Vehicle Mass (including driver): 1670
    Centre of Gravity Height: 550 mm
    Wheelbase: 2550 mm
    Front-to-rear weight distribution: 53% F, 47% R
    Tyre Outside Diameter: 610 mm
    Brake pad coefficient of Friction: 0.325
    Pedal force supplied by driver: 500 N
    Pedal Ratio: 5:1
    Bias bar centred between two master cylinder (ie: not adjusted)


    Front:
    Number of Rotors/Calipers: 2
    Number of Pistons per caliper: 6
    Piston Diameter: 44 mm
    Total piston area: 9123 mm^2
    Rotor Diameter: 360 mm
    Effective Rotor Diameter (Rotor Diameter minus piston diameter): 316 mm

    Rear:
    Number of Rotors/Calipers: 2
    Number of Pistons per caliper: 4
    Piston Diameter: 30.07mm (average piston diameter for 28mm/32mm combo pistons)
    Total piston area: 2840 mm^2
    Rotor Diameter: 345mm
    Effective Rotor Diameter (Rotor Diameter minus piston diameter): 315mm

    Calculated values:
    Assuming some sort of weight transfer at 1g decceleration, a 75% brake bias would lock up fronts/rears at the same time. Requried braking torque is ~2980 Nm per front wheel and 1015 Nm per rear wheel. Using 7/8" master cylinders for both front and rear would give you pretty close to these values, with the bias bar allowing some fine adjustment.

    If you can brake harder than 1 g, then your will want your bias to go further towards the front.

    Spreadsheet is attached for you to have a play if you like. Edit the assumptions I have used as required, then look at "JZA80 brakes" worksheet. Select a braking g-force at the top which will give you the required brake torques front and rear.

    Disclaimer: It's your car, not mine. Follow the above at your own risk.
    Attached Files Attached Files

  15. #15
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    Thanks guys, much appreciated.
    I'll order the .875" cylinders as recommended and start with that as my base, then go from there.

    Thanks again.

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    Surely it doesn't have 6 x 44mm pistons per caliper? I would have assumed that they would be differential bore sizes.
    That's significantly more piston area than an R35 gtr which runs 30,34 and 36mm pistons.

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    The only thing a quick 5 second Google search reveals is that Jeep people are weird.

    I just went off what the OP said. He can confirm with what's on his car pretty easily before spending the cash.

    Sent from my SM-G950F using Tapatalk

  18. #18
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    Its all on the car atm, I'll try and confirm about the front piston sizes when i get a chance ,but all searches say same pistons for the fronts.
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    I started installing everything.
    One thing i cant find is.... are brake/clutch fluid reservoirs allowed inside cabin? And if so, are there any rules in regards to size, location, sealed etc!!!

    Ive seen plenty cars with them inside, same as cylinders being located inside cabin, but want to make sure before i start cutting/drilling up things
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    What category are you running in?

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    time attack, supersprints, hillclimbs.
    no door to door stuff at all.
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  22. #22
    Registered User Stix Zadinia's Avatar
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    probably safer to have it in the cabin than anywhere near the exhaust! lol

  23. #23
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    shouldn't be any restrictions on reservoir location - lots of IPRA cars have them inside the cabin. But i would prefer to keep them in the engine bay to avoid inconvenient spillage.

  24. #24
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    Quote Originally Posted by MAZMAN View Post
    time attack, supersprints, hillclimbs.
    no door to door stuff at all.
    So CAMS events then. If you enter it as either 3J (IPRA) or 3D (Sports Sedans) you are OK to put the reservoirs in the cabin. In 3J the brake master cylinder is free, in 3D the entire brake system is free, with some exceptions unrelated to the reservoir location.

  25. #25
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    Bit of an update on this and some of the issues of had along... bit of a long story...

    Initial setup came with 0.625(front) and 0.725(rear) master cylinders, i thought why not, i'll try it.
    Didn't know how to setup the balance bar properly so did some reading and finally figured it out, with the front master cylinder rod being longer lengh so more fluid volume would be pushed.
    Tested it and didnt like it, had to double pump the brake pedal to get a good pedal and pressure ( when i did double pump it tho, the brakes felt good and braked awesome).

    I then installed 0.725 front and 0.875 rear masters... pedal pressure improved where i didn't have to double pump it but travel was too much for my liking, close to the floor.

    Installed 0.875 front and 1" rear masters...pedal travel now was much better at about half the pedal travel, i was going to leave it at that, but i had a spare 1" master so thought i would replace the fronts with a 1" master as well to see what happens, so now i have both front and rear masters at 1" bore.
    Pedal travel now was tops! nice and firm with not much travel, just the way i like it but a heavyish foot was required... i thought i'll give this a try.

    Went out testing at smsp, here is when things got interesting!
    1st session out went to test it and all felt great, minimum pedal travel but car braked good but needed to be pressed hard, once i was confident and everything felt good i did a hot lap and managed 1.40 flat.

    Went out 2nd session...here i was going to try to go quicker as everything felt good 1st session!
    Warm up lap done, and i headed off for a hot lap!( this is lap1 after a warm up lap) as i come to the finish line i do a 1:38.9 yay!!! mad i thought, head to do another quickish lap but this time on turn2 i go to press the brakes and pedal goes to floor! pump once, pump twice, pump 3 times!!! nothing! no pressure at all , as if i've removed the brake lines... i quickly down changed the gears and slowed down enough but still went off the track and into the grass/dirt section, luckily avoided the walls.
    Waited for traffic to clear and say about 1minute, go to drive it back on the track to go back into garage to see whats going on, as i slowly moved onto the track again at slow speeds i went to press and test the brakes again and now the pedal is back to normal and firm again, as if nothing happened! all the way back to the garage.
    Back in the garage, can't see anything wrong or out of the ordinary, checked brake lines, brake fluids, the rods on the master and balance bar etc... nothing i could see to fault. I didnt end up going back on the track as i was afraid it would happen again, so i need to solve this issue...

    Anyone with any idea's?
    If not, can anyone recommend a motorsport shop that can look over it, adjust the setup properly if needed in NSW? ( im near blacktown).

    Brembo 6 and 4 pots.
    Brembo pads.
    Hard lines all the way to strut towers then braided lines to calipers.
    OBP motorsport pedal set.
    93 tt supra,black 6 speed
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  26. #26
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    Quote Originally Posted by MAZMAN View Post
    Bit of an update on this and some of the issues of had along... bit of a long story...

    Initial setup came with 0.625(front) and 0.725(rear) master cylinders, i thought why not, i'll try it.
    Didn't know how to setup the balance bar properly so did some reading and finally figured it out, with the front master cylinder rod being longer lengh so more fluid volume would be pushed.
    Tested it and didnt like it, had to double pump the brake pedal to get a good pedal and pressure ( when i did double pump it tho, the brakes felt good and braked awesome).

    I then installed 0.725 front and 0.875 rear masters... pedal pressure improved where i didn't have to double pump it but travel was too much for my liking, close to the floor.

    Installed 0.875 front and 1" rear masters...pedal travel now was much better at about half the pedal travel, i was going to leave it at that, but i had a spare 1" master so thought i would replace the fronts with a 1" master as well to see what happens, so now i have both front and rear masters at 1" bore.
    Pedal travel now was tops! nice and firm with not much travel, just the way i like it but a heavyish foot was required... i thought i'll give this a try.

    Went out testing at smsp, here is when things got interesting!
    1st session out went to test it and all felt great, minimum pedal travel but car braked good but needed to be pressed hard, once i was confident and everything felt good i did a hot lap and managed 1.40 flat.

    Went out 2nd session...here i was going to try to go quicker as everything felt good 1st session!
    Warm up lap done, and i headed off for a hot lap!( this is lap1 after a warm up lap) as i come to the finish line i do a 1:38.9 yay!!! mad i thought, head to do another quickish lap but this time on turn2 i go to press the brakes and pedal goes to floor! pump once, pump twice, pump 3 times!!! nothing! no pressure at all , as if i've removed the brake lines... i quickly down changed the gears and slowed down enough but still went off the track and into the grass/dirt section, luckily avoided the walls.
    Waited for traffic to clear and say about 1minute, go to drive it back on the track to go back into garage to see whats going on, as i slowly moved onto the track again at slow speeds i went to press and test the brakes again and now the pedal is back to normal and firm again, as if nothing happened! all the way back to the garage.
    Back in the garage, can't see anything wrong or out of the ordinary, checked brake lines, brake fluids, the rods on the master and balance bar etc... nothing i could see to fault. I didnt end up going back on the track as i was afraid it would happen again, so i need to solve this issue...

    Anyone with any idea's?
    If not, can anyone recommend a motorsport shop that can look over it, adjust the setup properly if needed in NSW? ( im near blacktown).

    Brembo 6 and 4 pots.
    Brembo pads.
    Hard lines all the way to strut towers then braided lines to calipers.
    OBP motorsport pedal set.
    My first thought is the fluid boiled. What fluid are you running, and how old is it?

  27. #27
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    Rbf600.
    Very much doubt fluid boiled due to a few reasons.
    1. Old setup that was 4 and 2 pot brakes with smaller rotors would never boil the brakes, even tho it got much hotter and a lot of laps at once, and master cylinder in engine bay heat.
    2. These brakes are massive compared to old setup, measured almost 200deg less temp on rotors to previous setup, pads are twice as big surface wise and fluid is inside the cabin.

    Fluid was brand new as i had to bleed the system, this track day was its forst with these size masters and all new fluid.

    3. Was only 1 hot lap! No way it should overheat in one hot lap!?
    4. Session 1 i did 5 laps, 2 of wich were 141 and one that was 140 and brakes were awesome...

    But i dont know, anything is possible, just ive been racing for so long ive had all kinds of things over the years while perfecting the car, from shit pads that overheated, faded, using shit brake fluid even at the beggining when i started etc never had pedal go to floor on first press!
    On smsp straight, its a long straight and not much braking happening before t2 so to have pedal go to floor on first application is a bit weird to me.

    Is there a way i can test the brake fluid now thats in the reservoir to test and see the condition?
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  28. #28
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    Based on what you say above, boiling seems unlikely, despite the symptoms matching your situation. New fluid is unlikely to be contaminated with water. No idea how you could test for it, if in doubt chuck it out.

    While not solving your problem, itís a good habit to give your brake pedal a ďconfidence tapĒ with your left foot on the straight to make sure itís still got pressure.

    Iím sssuming the brakes have been properly bled? My next thought is to check your master cylinder pistons are fully retracting.

  29. #29
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    One more thought.... are you running fixed rotors? If so could be pad knock off.

  30. #30
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    Yeah i do a little nab at the end of the straight to wash off some speed (i was doing 252.8kmh on the straight that day) and i felt the brakes were good and after taking the t1 corner at a average 180kmh coming to turn2 where i usually brake at the 100m mark, there were no brakes!.

    I would also assume they were bleed properly but who knows if there was air trapped that i couldnt get rid of? But again the pedal felt great...

    Yes 1 piece isf rotors made by dba4000.

    The master retracting is an interesting one, but hard to diagnose now when its fully operational at the moment.

    Knock off i would assume pumping up would bring the pressure up tho? As the pistons fill with fluid again?
    Thanks for the ideas.
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