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Brake FAQ - Rotor, caliper, MC sizes

63K views 72 replies 29 participants last post by  Ignatius_Reilly 
phatvw said:
You could try. Why not get a full 93t set and full 91t set side by side and take some measurements for us? Perhaps its as simple as fabbing a small adapter out of billet aluminum or something?
the holes would be too close together to work out. they're even pretty close going from a stock 91t to a 12" rotor. i actually have brackets drawn for a 12" rotor upgrade using the 91t calipers, but i'm going to use porsche 996 calipers/supra (if they fit, or LS400 if they don't) rotors in the front instead.

the stock caliper upgrade would have used acura RSX-S rotors up front, and front rotors from the 96-01 rav4 on the rear. they're almost exactly the same dimensions as the 91t rotors, only larger diameter.

i'd be happy to fork over the details to someone who wanted to actually contemplate building it, so long as it's not for profit...

i've done a LOT of brake research in the past 5 years :D
 
jazzcornet said:
also, for the solefact of superior area, has anyone tried to use the front calipers in the rear? can it be done?? it would mke more sense to me to have more bias in the rear anyway and upgrading the rear brakes would prove to be more logical than upgrading the fronts.
the pistons in the front calipers are too big to ever get the bias forward enough to make the car safe to drive.
 
jazzcornet said:
makes sense, but then why do people buy big brake kits?
for the same reason they buy the tornado fuel saver, electric superchargers, $5 "performance chips" and super fuel mileage pellets- because they don't fully understand how the system they're trying to improve actually works.

most people just assume that bigger is better. truth is, balance is better. bigger only improves fade resistance, but can have detrimental effects on actual stopping distances. if you want a brake change to be an upgrade, it should not only improve fade resistance, but improve braking distance as well. most people assume that going bigger accomplishes both.
 
phatvw said:
  • Front
  • 1993+ Turbo dual piston front caliper [2x36.5mm = 458.673mm?]
  • 1991-1992 Turbo dual piston front caliper [2x36.5mm = 458.673mm?]
  • 1991+ N/A single piston front caliper [1x51mm = 320.442mm?]
  • Rear
  • 1999+ JDM Turbo single piston rear caliper [1x42.9mm = 269.549mm?]
  • 1993-1998 Turbo single piston rear caliper [1x41.3mm = 259.496mm?]
  • 1991-1992 Turbo/91+ NA single piston rear caliper [1x41.3mm = 259.496mm?]
just a side note, but i think you may have been calculating the circumference of the pistons (diameter x pi) rather than the area of the pistons (pi x r^2).

36.5mm piston area is 1046.34mm^2, and we multiply that by 4 (because they're sliding calipers), so total effective area for dual piston front calipers is 4185.3 mm^2.

NA front calipers have 50.8mm pistons, so their area is 2026.82 mm^2. multiply that by 2 (also sliding calipers) for a total effective area of 4053.84 mm^2.

the turbo front calipers have about 3% more piston area than the NA calipers.

also, according to centric (brake caliper remanufacturer), the 93 turbos also got the 43mm pistons in the rear calipers, not just the 98 jspec cars.

your list should look like this:

  • Front
  • 1993+ Turbo dual piston front caliper [2x36.5mm = 4185.3mm?]
  • 1991-1992 Turbo dual piston front caliper [2x36.5mm = 4185.3mm?]
  • 1991+ N/A single piston front caliper [1x51mm = 4053.8mm?]
  • Rear
  • 1993+ Turbo single piston rear caliper [1x42.9mm = 2904.4mm?]
  • 1991-1992 Turbo/91+ NA single piston rear caliper [1x41.3mm = 2679.28mm?]

a mod can delete this post once the info is updated.
 
sliding calipers act as if there are pistons on both sides.

from your first link:
reaction force from a sliding caliper is the same as an opposed piston one.
i suppose the math works out either way. i've always thought it easier to work the opposition forces in right away and just ignore the number of friction surfaces, in order to more easily compare when looking at both fixed and sliding calipers. for me, it's easier to think of a twin 36mm piston sliding caliper as effectively the same area as a 4 piston fixed caliper of the same piston size.

i suppose we can do it either way though, since the math works out the same in the end.
 
the pistons are the same size. the calipers on the earlier models are narrower though, accommodating up to a 25mm thick rotor, while the later gen can accommodate up to a 30mm rotor. the brackets are also made for smaller diameter rotors on the earlier cars as well.
 
FWIW, I'm still amazed at how little of the swept area of the supra rotor the Wilwood calipers use. If you want big, meaty pads on there, find some Brembo or Sumitomo OE calipers. It seems a waste to install that much larger rotors if you're only going to use 50% of the extra area with the Wilwoods.
 
Oteck, you need bigger pistons in the back. That's the only way. The prop valve can't increase pressure to the rears, only decrease it. If you're low on clamping force there to begin with, you're kind of SOL.

- You could have the rear calipers bored out to fit a larger piston (LS430 rears are 43mm).

- You could have the front calipers sleeved to run smaller pistons (the LS430 pistons are 43mm and are massive compared to the stock 36mm pistons).

- You could find another caliper that uses larger pistons, though there aren't many on the market that run large enough rear pistons. Even porsche's rear calipers are slightly undersized. I can't think of any fixed calipers at all that have big enough pistons. GM uses some decent sliding calipers of about the right size though. I use 88 Fiero calipers (48mm piston, but I'm using smaller rav4 rotors in back, 302mm diameter)

- Running a larger rear rotor would help, but piston area is an exponential increase, while the lever arm is not, so you'd need really huge rear rotors to get the right balance.

- You could try significantly higher ? pads in the rear than the front, but you'd need a pretty big differential to get the balance back to stock.

- You could build a dual master cylinder setup with a balance bar. Honestly, this might be the best option, since it would give you better control over the brake bias than any of the others, and knowing your fab abilities, probably wouldn't cost much more than having the calipers custom machined. I have a pic somewhere of a DMC on an SW20 chassis, and it doesn't look very complicated at all.
 
Phat, he's no longer on stock brakes. He's running LS430 calipers front and back, with I *believe* either LS400 or supra front rotors in front with supra rear rotors and a custom drum style E-brake. My comments were in reference to his front to rear piston area ratio, but I know he didn't mention that in his post (I only know because I've followed his brake threads on the other board :D).

I'm not sure how wolfkatz got his hydraulic balance the same with the LS430 and OE rear calipers. I'm assuming he sleeved them for smaller pistons, because in stock form the LS430 calipers are monstrous- 42% more area- compared to the stock calipers. They're the same size as EVO/STi/350Z calipers, which is why I've said in the past that those wouldn't work well for our cars, because it's so hard to source a rear caliper that's big enough to balance them out. An equivalent rear caliper would have a 51mm bore or so- much bigger than the 43mm pistons in the LS430. That's another reason I went with the porsche 996C4 calipers- they're 38mm pistons, which is only 11% more than stock, and much easier to balance.

I wish manufacturers would list calipers by piston area rather than bore diameter. Then I wouldn't have to do as much math..
 
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