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Location:
Melbourne
Registered:
January 2003
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FC Caliper downgrade
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Sat, 30 October 2004 00:19
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Found this really good write up by Gabriel Tyler on a small US forum .
I have also seen the reverse of this theory on a S13 sr 180sx which had GTR front calipers fitted . It had no rear brakes , due to the smaller area of the Four piston compaired to the Big single piston used normally on that car . And thats not as easy to fix as more brake power to the rear . Basically don't just bolt on shit thinking its better , it might not be safe .
Well Heres the Write up By GT .
Did some research, some of you may find this interesting...others may not. It was brought up in my thread about our big brake kit, but I figured this particular info was more pertinent to the Tech Talk Forum.
MidTenn Drifter brought up the issue of the FC front caliper upgrade causing the rear calipers to lock up first. So I figured I'd look into it.
I did some research on the FC calipers today and figured out exactly why they make the rears lock up first sometimes.
Suprisingly enough...shock shock horror...the FC calipers are NOT an upgrade for the AE86... I know I know...but they are 4 piston etc....
But lets to the math...Remember, you have to use Pie R Squared and all that to get actual surface area of the piston. 2x1" pistons does NOT equal a single 2" piston for surface area.
FC calipers use 2 pistons pn each side that are 1.375" in diameter Making a TOTAL Piston surface area of 2.97" per side.
Remember that with a Fixed Caliper, you can't add the force together from both sides, since the SAME master cylinder is pushing both sides. All that having 2 pistons per side does is split the force in half that goes to the pad. For example, a 2 piston sliding caliper puts EXACTLY as much force on the pads as a 4 piston fixed caliper. Every time you double the number of pistons, ALL OF THEM MOVE HALF AS MUCH.
Stock AE86 Calipers use 1 piston 2" in diameter Making a Total Piston surface area of 3.14" The stock AE86 caliper is a sliding caliper and puts out an equal amount of force that a 2 piston fixed caliper with two 2" pistons would. Again, all that multiple pistons do is cut the force aplied to each piston in half.
What all this means is that the FC calipers have LESS clamping force than the the stock AE86 calipers!!!!This could easily cause the rears to start locking up before the fronts.
Now the FC kit does still offer the larger rotor, which equals more leverage and more braking, as well as more heat handling capabilities, but all in all, the caliper is not something to get up and shout about.
Anyone that want's to check my math or just mess around with piston calculations etc... there is an excellent calculator here:
http://sth2.com/Z-car/Brakemath.xls
A reply to the post :
neat info. But I'd argue that the bigger rotors would still make this mod worth while. Better braking generally comes from better heat dissapation which the bigger rotors will do. But yes, this mod should be done with a an adjustable brake bias valve to get the front locking up first again. A more suitable master cylinder might help restore the pedal pressure lock up point back to stock too.
Another Reply :
well it may be an upgrade but for the price of the adaptor kit and all the parts needed you could probably get alot more out of your oem brakes with that money
Another Reply :
Gabriel,
I like your approach in trying to analyze this braking problem. However, I think you have misinterpreted a couple of points. My comments are included below, encased in { }.
**********
But lets to the math...Remember, you have to use Pie R Squared and all that to get actual surface area of the piston. 2x1" pistons does NOT equal a single 2" piston for surface area.
FC calipers use 2 pistons pn each side that are 1.375" in diameter Making a TOTAL Piston surface area of 2.97" per side.
Remember that with a Fixed Caliper, you can't add the force together from both sides, since the SAME master cylinder is pushing both sides. All that having 2 pistons per side does is split the force in half that goes to the pad. {Not quite true. The force is equal to the hydraulic pressure divided by the area, so if the area is the same the force is equal.} For example, a 2 piston sliding caliper puts EXACTLY as much force on the pads as a 4 piston fixed caliper. {but only if the area on each side is equal.} Every time you double the number of pistons, ALL OF THEM MOVE HALF AS MUCH. {The distance traveled is a function of how much the piston(s) retract, and perhaps the rotor run-out. In an opposing piston setup, each piston only needs to move half as far. But, this doesnt affect the braking force.}
Stock AE86 Calipers use 1 piston 2" in diameter Making a Total Piston surface area of 3.14" The stock AE86 caliper is a sliding caliper and puts out an equal amount of force that a 2 piston fixed caliper with two 2" pistons would.
{That is correct.}
Again, all that multiple pistons do is cut the force aplied to each piston in half.
{This is not correct. As you stated above, the SAME master cylinder is pushing on both sides, therefore the pressure and force is identical provided the areas are the same.}
What all this means is that the FC calipers have LESS clamping force than the the stock AE86 calipers!!!!This could easily cause the rears to start locking up before the fronts.
{This is correct.}
{Because the brakes have been modified the front/rear braking ratio has changed. Perhaps an easy fix is a brake proportioning valve, which allows the amount of braking to the front and rear to be adjusted.
Hope this helps!
[Updated on: Sat, 30 October 2004 03:47]
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Location:
sydney
Registered:
May 2003
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Re: FC Caliper downgrade
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Sun, 31 October 2004 13:32
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any 1 else got anythin to say bout this??
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: FC Caliper downgrade
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Sun, 31 October 2004 13:50
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when changing brakes, it pays to actually do the math beforehand 
more pistons = more even (or uneven as the case may be) clamping force, but not necessarily more clamping force, thats all to do with the MC to caliper area ratio.
front to rear bias is a function of piston area x disc diameter.
i think it's funny that the 'upgrade' is a 'downgrade'... but if it was "JDM spec", i'm sure most AE86 owners would still be happy 
Cya, Stewart
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Location:
Rosanna, Melb
Registered:
June 2002
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Re: FC Caliper downgrade
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Mon, 01 November 2004 05:24
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Just one minor mistake in your comments improved, piston force is brake line pressure TIMES piston area (not divided by).
While all this maths may be correct, I think the major point is being overlooked. Big brake upgrades aren't primarily useful in increasing the brake force available, they just improve the fade resistance of the brakes meaning you can keep pulling up lap after lap.
I could lock up the fronts on my stock oz-spec 86 from a decent speed if I tried (with 185 tyres), but would I consider keeping them after doing a GZE conversion? No way. I've been unable to notice brake fade using FC calipers on 280mm discs even when really trying to get them to fade around Calder Park.
And if you are concerned about the clamping force of the caliper on the disc, you can always REDUCE the diameter of the master cylinder. For a set pedal force this increases the hydraulic pressure in the system and so increses clamp force.
Hen
PS, forgot to mention the most important fact about this upgrade, 4piston brakes have great bling value.
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Location:
Melbourne
Registered:
January 2003
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Re: FC Caliper downgrade
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Mon, 01 November 2004 05:28
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Gabriel Tyler's maths Henn , looks like you are a better engineer than him . Good on you for pointing ou the line pressure from the master cylinder , i guess there don't think of a master cylinder change in the US
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Location:
Rosanna, Melb
Registered:
June 2002
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Re: FC Caliper downgrade
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Mon, 01 November 2004 06:05
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Sorry about the misunderstanding Improved.
In spite of the point about smaller masters meaning more clamp force, I upped my master cylinder size when I went 4 piston since they need more fluid volume pumped to close.
The bigger master size means you don't have to push the pedal as far, but the trade off is you may have to push it harder. I've never found this to be a problem though. It's amazing how hard you can kick when there's a solid wall approaching at 100+ km/h.
Hen
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Location:
Sydney
Registered:
February 2003
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Re: FC Caliper downgrade
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Tue, 02 November 2004 12:09
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that bit about it being a downgrade is just silly... why would you do such a massive upgrade (single to 4 piston) and not use a brake bias valve???
a mate of mine had 6piston AP's installed on his supra which made the rears lock up first too... until that is, he installed a brake bias adjuster valve.
larger caliper needs more fluid to travel the same as a smaller caliper, so obviously, with an upgrade, you'd adjust the bias more towards the larger caliper till you get it even.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: FC Caliper downgrade
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Tue, 02 November 2004 12:34
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| R-jay wrote on Tue, 02 November 2004 23:09 |
that bit about it being a downgrade is just silly... why would you do such a massive upgrade (single to 4 piston) and not use a brake bias valve???
a mate of mine had 6piston AP's installed on his supra which made the rears lock up first too... until that is, he installed a brake bias adjuster valve.
larger caliper needs more fluid to travel the same as a smaller caliper, so obviously, with an upgrade, you'd adjust the bias more towards the larger caliper till you get it even.
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silly?
your mate "upgraded" to 6 pots, that made the rears lock up first...
so now the rear brakes are more effective than the front and lock up first? what part of downgrade doesn't work here?
the point is that even tho a '4 pot' or '6 pot' caliper is used, that may result in less force being applied to the brake pads by the new calipers, so thus more force needs to be applied to brake pedal to get same braking force (for same disk size), and if the rears lock up first due to the fronts now doing less, you install a brake bias valve to make the rear brakes less effective??
so, we've made front less effective due to caliper "upgrade" and we've 'dumbed down' the rear brakes because the front "upgrade" meant they were too effective..
ie, reduce front braking force, and thus need to reduce rear braking force...
and this is not a down grade how??
maybe i'm confused, but if you decrease the braking force, i don't see how that is a straight "upgrade" (of course pad area, heat capacity of the disk etc is also a factor)
Cya, Stewart
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: FC Caliper downgrade
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Tue, 02 November 2004 12:42
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the point of this being posted int he first place is becuase people ASSUME that more pots = better braking (as you just did)
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why would you do such a massive upgrade (single to 4 piston)
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if the piston area of one side of the 4 pot is less than the area of the single piston... it is NOT an upgrade in terms of force applied to pad for a given pedal force.
not having a go at you, but people need to stop and think abotu it, rather than just assuming that thigns are upgrades because they are JDM or fooly sik...
fwiw, i probably will use RX7 calipers, because for my KE15, they _will_ be an upgrade, as will the disc upgrade from 218mm (or 200mm stock) to 260mm
Cya, Stewart
this sums it up.
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A reply to the post :
neat info. But I'd argue that the bigger rotors would still make this mod worth while. Better braking generally comes from better heat dissapation which the bigger rotors will do. But yes, this mod should be done with a an adjustable brake bias valve to get the front locking up first again. A more suitable master cylinder might help restore the pedal pressure lock up point back to stock too.
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also, here's a nice place to start.
http://www.wilwood.org/pulpfriction.pdf
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Location:
Sydney
Registered:
February 2003
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Re: FC Caliper downgrade
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Tue, 02 November 2004 23:44
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if you're going to do such a large conversion.. from single to 4 or even 6 pistons, why not do it the right way and use a bias valve???
with the bias valve, you dont actually reduce the braking force of the rears... you just allow more fluid to the front than previous (stock).... but the resultant effects show the rears to be just as effective as stock, with the added extra braking from the upgraded fronts.
as for surface area, yeah i understand that if your brake surface area decreases, then it is a downgrade....
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: FC Caliper downgrade
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Wed, 03 November 2004 00:23
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ahhh k... i see..
a proportioning valve, or bias valve, is usually used to reduce the braking force at the rear of the car, so that in a fast brake application (with more weight transfer to front of car) the rears don't lock first. it's not that it allows more fuild to front of car, it reduces fluid to back of car...
anyway is all semantics 
Cya, Stewart
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