Interesting new caliper from Wilwood: The XRZ4R https://www.wilwood.com/Calipers/Cal...-16464&appid=0

4-piston caliper, 355mm maximum rotor size. Design looks a lot more modern than their older calipers. Pads are somewhat small, but at least they're thick.

Assuming they didn't screw up the design and it's actually stiff, this looks like a good lightweight brake option for autocross and stock power track duty.

I was about to order some Aero6 calipers, but now I'm wondering if I should experiment with this new caliper. With a 14-inch rotor the front brake bias comes out around 93% of stock, which is close to what the Essex CP8350 kit has.

 

Pad area is too far off the mark. The 4.04 in2 is the right size, ~3.5 will push bias rearward too much and ABS wont be happy about it.

 

Pad area is smaller than 6-piston, but that’s expected of a 4-piston.

Pad volume is not bad, though.

The piston area is smaller than ideal, but the effective braking torque with a 14-inch rotor is actually slightly more than what I calculate with the Essex/AP 325mm kit that everyone seems to enjoy. Are you hearing about ABS issues with those kits or other kits with slightly lower front brake bias?

EDIT: This new caliper on a 356mm rotor would have about the same brake torque as a an Aero6 on stock 320mm rotors. Wasn’t someone here doing an Aero6 with stock rotor combo for AutoX?

 

"If" you got to a 355mm (14in) rotor (EvoX size) thats about 11% bigger lever arm. But going from 4.5 in2 to 3.5 in2 (piston count is irrelevant) is a 30% change. Thats way to much.

 

I’m looking at effective brake torque of the system, not piston area. And I’m only considering the 356mm rotor.

This caliper has two 38.1mm pistons and would be used on a 356mm rotor.

The CP8350 caliper has one 38.1mm and one 41.3mm piston, but is used on a much smaller 325mm rotor.

You have to look at the whole system, not just the piston area. The Essex CP8350 system ends up with even less front brake torque, but as far as I’m aware people weren’t having ABS problems with it.

If the 325mm Essex CP8350 kit is working for people, I don’t see why this setup wouldn’t.

 

I just gave you the whole system. Rotor dia and piston area is pretty much the system you’re altering and all the matters. The Essex kit is 3.9 in2 so waaay closer to the 4.04 of most other kits.

You’re obviously free to try what you want, but 3.5 in2 is too far from OEM and will push bias rearward too much even at the max 14” rotor size.

 

Wait it doesn't use staggered piston sizes? That's goofy.

 

the AP racing stuff is great. My track car has it front and rear. It’s amazing to run a 20min tt session and not give a second thought to the brakes - they just work.

 

Stock brakes have 2919 mm^2 of piston area on a 320mm rotor.

The Essex/AP CP8350 has 2480 mm^2 of piston area (85% of OEM) on a 325mm rotor (104% of OEM, assuming D50 pads) for an overall relative brake torque of 88% relative to OEM.

This caliper has 2280 mm^2 of piston area (78.1% of OEM) on a 356mm rotor (119% of OEM, assuming D50 pads) for an overall relative brake torque of 93% relative to OEM.

The setup I'm talking about would be closer to OEM torque than the Essex 325mm system. You have to look at the overall brake system torque (piston area, rotor diameter, and pad depth) to calculate the relative torque.

If the Essex package isn't causing ABS issues with 88-89% of the OEM system's brake torque, then the relative brake torque of this system should be fine at 93%. Or are you saying that the Essex system is also causing problems, and therefore this one will too? I have a hard time believing any system within +/- 10% torque of the OEM system is going to cause problems, given that we routinely mismatch our front/rear brake pads with Mu values that have larger difference than that.

The system still isn't ideal due to the small pad area. It tries to make up for it with thicker pads, but I'd worry about high contact temperatures.

 

They stagger pistons on the version with 5.18 in^2 of piston area, but not on the smaller piston version.

It's too bad they didn't make a version with a 1.38in and on of their 1.50in pistons. I wouldn't be surprised if they're expanding the range of options gradually over time. They only announced this caliper a few months ago.

Would be great if they'd build out a 6-piston caliper in the same weight-optimized style. Aero6 isn't bad, but it feels like a dated design next to all of these modern calipers with FEA-guided structural design.

 

You divided instead of multiplying or something,

CP8350 Relative bias shift = 1 - (2480/2919 * 325/320) = 13.7% (rearward shift)

XRZ4R = 13.1%. So sure, I guess thats less than the essex kit. On paper, that should be more than the stock ABS will be happy with, though maybe its happier with rear bias. Its still a bit of a mystery what it will freak out with.

I use the Aero6 kit with 320mm rotors having an 11% rear shift for AutoX with the idea that I'll swap to X rotors for track for a net zero shift.

 

So totally random thought, those do happen to have the same offset and mounting pattern as the SL6R calipers and I have a set of 1-off radial mount bodies for them speced for a 12.19" rotor. Could run a 44mm spacer and the would mount right up.

 

Wilwood XRZ4R.
Pretty expensive for what they are, you'd be better to use an AP Racing 4POT or 6POT.
Also - pad surface area doesn't effect braking torque.

Things that effect braking torque:
Brake pressure
Piston Area
Co-efficient of friction of pads
Effective radius
(less effect) disc slotting.

 

^^ Hes a brake guru in a different car market if ya'll don't know

 

trying to make a magnetic thingimajig