Here's the email I had if we all wanted to jump in on building one ourselves:The CP9451 rear caliper could be mounted to either a 340mm or 365mm rear disc and it would retain nearly the same front/rear balance as OE. This is based off our data from the Evo 9. In terms of rear caliper brackets, my engineer said:



“I went through some calculations for the bracket geometry. With the 340 mm disc the radial distance between the caliper mounting surface and the mounting hole pattern on the upright is right about 1”. This is made more complicated by the caliper studs at 152 mm being very close to the 146 mm mounting pattern spacing on the upright. Going to a 365 mm disc bumps the radial spacing up to about 1.5” and that would be easier to work with.”



These are the parts mentioned above:

https://www.essexparts.com/cp9451-2s...zed-254mm286mm



https://www.essexparts.com/ap-racing...-rings-34028lh



https://www.essexparts.com/ap-racing...gs-36530d42-lh



Either of these discs would use a 10-bolt hardware kit:

https://www.essexparts.com/10bolt-disc-hardware-kit



You’d also need 10 bobbins per disc, which are about $2 each I believe.



Hopefully the above is helpful. Thanks!



Sincerely,

Jeff Ritter | Mgr.- High Performance Division


The caliper mentioned above assumes a 9660 or 9668 up front already.

Thats a really big rear disk, Id be worried about not getting brakes up to functional temp.

Radical 5000 in the rear is unnecessary, just extra money and weight for no gain.

Looks like the rear kit was listed as CP5108-1003. It's still listed on their website but based on some pics I found it looks like it uses a caliper they no longer produce.

With that said, I'd be hesitant to run a CP8350 in the rear, the bore sizes are much larger than the factory rear caliper. There is a few 2 pot calipers they sell which may work but the pistons are smaller or larger. After doing some digging I'd be inclined to go 2 ways:

1. Using a 2 piston caliper with either smaller or larger pistons depending on how we want to alter bias.

2. Using some cup car rear calipers. After digging through dimensions and piston sizes there are a few options for cup car calipers that would suit the factory disc nicely without gaining much weight and wouldn't alter piston area much. The only hang up is the only reasonable way to get them is second hand from cup car part wholesalers which makes supply spotty at best.


FWIW I have ZERO desire to move away from a factory rear disc. Going larger is just adding unsprung weight and as it stands our rear discs are already larger in diameter and thickness than most front discs on normal road cars. Moving to a bespoke rotor just makes replacements a pain and more expensive.

Well, depends on use right. The simple fact is that the AP J-hook rotor is 1000% better in almost every way then the 3 options we have for "oem" today - Giro (warps) racing brake (cracks) and stoptech/blanks which don't cool well. For a "track day" or TT those all may work but I've gone through enough of them to swear not to ever use them again over an AP or Alcon rotor on a race-purpose build. Even the P cars at our shop are all switching to j-hooks or PerformanceFriction disc's over giro's.

That also being said the smaller Radical in back would actually weigh LESS then any of the oem 9 or X calipers if memory serves. If you made the rotor to eliminate the parking brake thats even less weight. So, if it weighs less, stiffer, cools better, how are those not adventurous over a grocery getter setup.

DBA makes a 4000 series stock replacement rotor. There's not much point in spending a ton of money on a 2 piece rotor if it's not floating.

PFC V3 rotors are super trick and hands down the best 2 piece rotor available IMO, just not easy to get in the US (and they are only made for the front)

It mainly comes down to ease of maintenance. There's probably 5 guys in the US that could legitimately use a larger rear rotor. I can design any brake kit using any caliper and rotor, but if only 3 people are going to buy it there's no appeal for me to do it unless they're willing to cover the expenses of development. I could do a custom center hat that uses a 300mm AP ring to keep bias changes to a minimum, but that requires people to ditch the ebrake which some people do not want to do.

As far as weight goes I expect it to be a wash at best. The lightest cup car calipers I found were around 2.5lbs which sounds about right for the stock caliper.

If we wanted a straight up caliper swap to work with stock diameter rotor at either stock thickness or thicker, the CP5317 is 2 pot and damn near the same piston diameter as stock. Weight is just over 3lbs on those and they're probably not too expensive though I'd need to get ahold of my AP guy to see what they'd be.

EDIT: Just weighed a stock caliper. With pads and hardware the rear caliper is a tick under 5lbs, I'd guess the caliper itself is around 3.5lbs of that. Should be easy to keep weight in that same area with whatever caliper is used.

I haven’t had issues with the rear.

running hard at the track with the blue car (2700lbs 630whp) the front evo X setup left a bit to be desired.

Have a brand matching Motorsports set from and rear would be sweet.

i was hitting up Jeff at Essex as well
it didn’t get the depth of response as you. Did he ever give a number of people needed to get some rears going? Or is the best bet to make one of the existing setups work?

It's the same problem Austin indicated. In Essex's case, he'd likely need to sell ~25 kits to make his numbers. Austin likely at least ~7-10. And there's .....what 3-4 of us that would do it lol. Be he's right - if we could find a caliper small enough that was "motorsports grade" to use the OEM setup then it's just the bracket that needs to be engineered.

I personally love the larger 343mm rears on mine. I would sorta shy away from 365, that just seems excessive.

you dont need to upgrade the rear. The only thing you could consider is a 2 piece disc. We use the DBA's Ayoustin mentioned and dont have any issues.

Put 6 pots in the front as they do all the work and the biggest rotors you can get away with. Ive got 362 but you can go 370 if your wheels will fit. If your obsessed with upgrading the rear you could look into an Evo X rear setup. You can generally get enough balance by putting an aggressive pad in the rear.

Put the money you saved on not buying rear calipers etc into a pedal box with dual master cylinders and you wont look back. safer also

Like @Balrok said earlier, it comes down to use. He's running 30+ minute races in a W2W setting. He spends more time on track, with no cool down laps and in a dirty air stream every time he's behind someone. So his brake needs are different than a car running time attack or hill climb or just doing track days.

The easy button solution (and cheaper option) I could do is design a bracket to use the CP5317 caliper and the factory rotor. This would reduce a tad bit of weight, keep bias fairly unchanged, keep part cost fairly low and give an option for a matching rear brake setup for anyone running AP front brakes and would retain the option of keeping the ebrake. Note, the CP5317 also uses cotter pins to retain the pad hardware, which is a much more robust solution than the plastic compression clips that are known to fail on the factory calipers.

The more involved (and expensive) solution is for me to develop a kit using a cup car caliper (pistons need to be somewhere around 1.0625" + 1.125" on a 4 pot caliper and from what I've seen these sizes are only available on cup stuff) and a 300mm (or larger) AP ring on a custom center hat. This would shave a decent amount of unsprung weight and would eliminate the ebrake.

I would need at least 5 people to commit to either option to get the ball rolling.

Im doing 20 minute wheel to wheel also with 700+ HP, We had heaps of issues with rear locking and too much bite in the rear on standard discs and calipers with aggressive pads. If you get good ducting to them they are fine.

Just saying its an unnecessary upgrade and money could be spent elsewhere more effectively in the braking system.

Agreed with both. But you have to admit bee that the rear oem caliper is sh*t. It shreds the boot on the first session and flexes to hell and back with pad taper. Granted it's better then a non-brembo one, but still.

I'm with you Austin on the AP caliper bracket as the better option for both of your reasons. Keep cost down and use a better part. IF you can find the caliper. You'd need to do the balance math on the existing front setups (CP9660/8, CP8350, Evo X, Evo 9, Stoptech, etc etc. I believe on both CP cases it moves balance to the rear by 0.8% so you'd just need that "number" in a round about way. I can see about getting parts with my account(s).

Yea so the 8350 kit moves bias to the rear by about 13% (not a small change). The 9660 and 9668 kits move bias to the front by about 1%.

The CP5317 caliper in the rear moves bias to the rear by about 6%. So it would work fine with the 9660 or 9668 kits or X front brakes but I wouldn't run it with an 8350 kit unless you're using the 8350 on a larger rotor than the 325mm it comes with.

I guess we need to figure out which way we really want bias going. On my current setup I'm biased about 10% to the rear from stock and I preferred a bit of rear bias.

you mean adding 8350's to the rear, right?

No, the front 8350 kit that Essex sells shifts bias to the rear of the car. The front brake force decreases with their 8350 kit which means the rears are doing more work and thus the bias is shifted to the rear.

wut isn't that a bad thing?

and how so? the rotors are 5mm bigger though the pistons are sightly smaller