About 6-7 months ago I installed my rear Stop Tech kit, 13" ST22. I added it to complement my Stop Tech 13" ST40 kit in the front. Since then I've had a few issues, I had my ABS controlled module fail and I still have the red dash BRAKE light on, but my ABS is now working again. However, since I've installed the rear kit I've had a lot of rear bias. I hoped it was due to the ABS module but it would appear not. I realize there are different front kits for the pre and post 06 models ,but from what I've heard the difference is pretty subtle. The amount of bias I have in the rear is quite substantial. I have tried but street and track pads but the bias is persistent.

I'm basically looking for ideas as to what would be causing this or if I will need to go to a 14" front kit or a Brembo rear kit. What kind of bias do other people using the 13" front and rear kits have?

 

my guess is piston size, possible wrong st-22 kit. There are different forms of the st-40 kit with different sized pistons.

When I was shopping for my front ST-40 kit there was a different model for use with the stock single piston rear caliper, and the rear brembo/2 piston caliper (I had done a brembo swap on my enthusiast).

So.. the answer is - check with StopTech. And be prepared to share specifics about the kits you have.

 

Do you have the part numbers for the kits you purchased?

 

I've sorted through all the stop tech part numbers for the Z33. From what I've heard, I haven't looked into it myself, the differences between the kits if you match up the worst case to get the most rear bias the change is about 5%. To me that doesn't sound like a huge amount. And I'm pretty sure I can change the bias more than that with pads.

Erik, I'll take the calipers off again tonight and see if I can find a part number. I'll try and get a micrometer home and see if I can actually measure the piston diameters too.

Is it possibly to adjust the piston diameters without major work by going to thicker seals for smaller rear pistons?

 

The visible part of the piston is not the actual size, you would need to remove the dust boots to get the actual size. Your best bet is to provide us with the lot number which will be stamped on the caliper around the piston area.

There is no option to resize a caliper after its been machined.

 

The only numbers on the front caliper are 03 and 04. I can't find any numbers on the rear.

Erik, can you reference the piston diameters for both pairs of ST40 and ST22 calipers for me?

 

Okay,
My front pistons are 36 and 42mm and the rears are 40mm. Sound correct?

 

You have the front and rear kits for the 03-05 models so the problem is more to do with under-sized front calipers than over-sized rears, but either way you have too much rear bias. Because of the big change in OEM rotor size and caliper torque outputs from 03-05 to 06+, there is also likely some changes to the brake proportioning which can exacerbate the issue even though your BBK calipers have the built in bias for 03-05 models. At this point you have 14% less front output and 10% less rear output than is designed into the kit for 06+ non-Brembo cars, but I'm sure your pedal feel is amazing.

 

Well I just tried out putting 14" rotors on the front with the 13" ST40 calipers to increase my front torque. It helped but I'm still locking up the rears first. And now my ABS light is back on. Grrrr. Back to the drawing board.

 

Well for what it's worth. I've given the front rotors more time to bed in and I re-bled the brakes. My ABS light has gone away and I've cleared all the codes. None have appear to return. My front bias is actually pretty decent now running Carbotech XP12 pads in the front and Stoptech Street pads in the rear, at least by feel. I'm not sure how well the stop tech pads will hold up at the track but I hopefully will find out soon enough.

I also had a chance to datalog some of the ABS parameters last night. I plotted the wheel speed at each corner and the ABS signal. It looks like when the ABS fires the largest differential in wheel speed is at the RR and the FR. However, the initial lock up is being shared but all tires to some degree, which indicates my bias is much more balanced now.

 

The StopTech pads will be fine in the rear on track, they are rated to 1300F, similar to that of Ferodo DS2500 which are commonly used at track days.