Rolling Diameters And Attessa System

[CeJay]

Probably a stupid thing on my part, but I need help.

Ok Just had my new rims put on and discovered an issue on the way home.

Fronts are 8 inch 235/45 tires with 35 offset

Rears are 9 inch 255/40 with 42 offset

cars a r32 btw.

While driving home the torque split guage was basically playing silly buggers and putting power to the fronts on light throttle in 3rd and 4th. Noticed this happening a few k's after leaving the tire store (normally only activates on heavy throttle). Pulled over and pulled the fuse, drove the car home in rwd mode. Now I asked the tire store if the different profile tires would cause an issue with the attessa system, they said no, since they were the same rolling diameter.

Well the rolling diameter of the fronts seem to be around 645 mm and the rears seem to be about 638mm, What I'm wondering is this enough to stuff up the attessa system or is the problem possibly something else?

I still have 4 reasonable 235/45 profile tyres on my old rims, was thinking of putting a set of them on my rear rims to see if this was the issue.

[adam-__-]

You should have the wheel/tyre with the larger rolling diameter at the rear as far as I know. If you've got larger ones at the front, the attessa system will think you're losing traction at the rear and will be 'confused'.

Either drive without the new wheels or with the fuse ripped out.

[neil_se]

I had 245/45/17 tyres all round but different brands front to rear and that was enough to upset the system. I could feel it sort of lurching or surging when driving on the highway.

[heller44]

Simillar here, 235/45/17's new on front, old on back. Surging torque gauge and seat of pants at >80kph, swapped and all good.

[GTS4WD]

Definately change..

It will wear ur clutches between the gearbox and transfer box out.. its an an expensive re-build..

[CeJay]

Yeah don't worry I'll definately change, suppose it just goes to prove don't believe everything a shop tells you.

[funkymonkey]

providing your tyres aren't stretched, you're looking at a total rolling diameter of 25.3 inches in the front and 25 inches in the back. the widths don't matter and the profiles don't matter, as long as you get the rolling diameters even or slightly larger in the rear.

When you have a larger diameter wheel in front it'll think that there's traction loss in the rear and send power to the front.

If you have larger diameter wheels in the back, then it'll detect that there's traction loss at the front and won't send as much power to the front as it would if it were even.

My advice to you would be to go UP to a 265/40/17 in the rear. It'll make the overall rolling diameter of the front and back even.

[CeJay]

So I can either put a 235/45 with some decent tread on the back, or 265/40's to even out the rolling diameter? whatever I choose will get it sorted on the weekend hopefully.

[ferni]

Easiest way is to keep the tyres and wheels exactly the same size front and back. Since you have 4wd you don't need a bigger rear tyre like the gtst guys do for grip.

[CeJay]

Getting two of my old rear tyres put back on on Saturday, only reason I had these tyres left on my new rims was they were in good nick, and like I said the tyre shop said it wouldn't be an issue, which it obviously is.