changing tire size and ECU/abs problem
 

Guys,

My 2009 Z is mostly used on track. This summer I had the bad idea to change the tire size on my 370Z tire size, thinking I would increase handling. I bought a set of RE71R. 265-35-19 in the front and 285-35-19 in the rear. That's when all of my problems started. I soon as I hited the track with this setup, the ABS would overly interveen which cause braking distance to be much longer. I month a a half later, I chated with a tire specialist who told me that he had seen similar beaviour on GTR when tire diameter between front/read are outside a certain range (+- 3% according to this guy). I then tested his theory by removing the ABS relay and braking came back as strong and powerfull as it was before I changed tire size. I can drive the car without ABS on dry pavement but I would like to keep ABS on wet tracks. My question is: Does anyone knows/understand what is the algorithm used by the ECU to drive the ABS module and what are all the parameters to take into consideration. As an example, I bought back a set of RE71R 245-40-19 (OEM size) and I kept my 285-35-19 in the back. Doing calculation proved that the front to rear wheel size ratio is with 1%. It improved the ABS behavior but it is still kicking in to much, to early. WHY?????
Can anyone help ?

I was thinking about your problem, have you ever experienced ICE mode while braking, sounds a lot like that, try entering your brake zone a little easier on the brakes and see if that helps, I'd do the normal checks first, pads and fluids

Dude, check your maths ....... 265/35R19 is ~ 668mm in diameter and 285/35R19 is ~ 682 mm in diameter.

Difference is outside tolerance (just) and is enough to upset the ABS calibration.

The ABS brake controller is monitoring the number of rotations front and rear to detect a locked (or about to lock) wheel and because ofn the sizes, it is getting confused as it will not tolerate more than ~3% difference in diameter and rotation speed. This is NOT in the ECU, it is in the ABS controller.

I run 265/35R18 front and back on 10" wide rims for track duty - for DD use I have a set of 245/45R18 and 275/40R18.

Your tyre size choice is the problem and a fix in the ABS controller is probably beyond anyone other that the ABS system OEM.

I have ran those size tires front and rear and didn't have any problems at all. Most folks run a 305/30/19 on the rear with a front 265/35/19.

I run 275/35-19 front and 345/30-19 rear. I'm at 2.2%. The only time I have problems is when I forget to turn off the VDC.

Looks like I wuz wrong ......... humble pie coming

My fronts are 26.6"/675.64mm. The rears are 27.2"/690.88mm. My math tells me a 2.2% difference. I might be on the hairy edge of it.

Think the OP might need to do the yaw sensor bypass.

That I understand ... I originally started with that setup I it became clear to me that the problem was with diameter diference. where I am confused though, is when I brought back my 245-40-19 while keeping my 285-35-19. Diameter difference is now within 1%. and I am still experiencing ABS over reaction. is there some other criterias like rear tires diameter must be lower or equal than front tires !

thanks for you help

The issues you are having is not tire diameter, but VDC. With it off. It still will affect the car. To help out. You have to disable the yaw sensor.

http://www.the370z.com/diy-section-d...er-switch.html

Run square. Problem solved :P

What effect on the car behavior will disabling the yaw sendor do ?

It disables the last of the driving nannies, including ABLS (anti-brake limited slip), which basically applies the rear brakes any time slip is detected even if VDC is already OFF.