^ Same thing happens to me on mild turns when I have brand new rears in and old fronts...vdc off or new front tires fixes it, it has nothing to do with size...I promise.

 

Based on our experience, (and apparently yours), size is what caused it... When you have new rears and old fronts, you have 11/32 treads on rear and something less than that on front. In my case, I had 7/32 on the fronts.

The VDC was programmed with the fronts being 2.7% smaller than the rears.

If you run 245s on the front, then your fronts are already 1.0% smaller than stock, making your stagger 3.7% Now if you throw on new tires on the back, and keep your old fronts on, your stagger will be off even more... THIS is why I think the VDC is flipping out. Putting new fronts on, makes the stagger smaller and closer to stock.

The VDC has no idea what tires you have on. The VDC system works by looking at steering angle, throttle angle, yaw sensors, and speed of each wheel as reported by the ABS sensor. This is how tire size plays a role. If you have new rears on, and worn out fronts, (which are already smaller than stock), then the VDC may think that your fronts are spinning too fast compared to the rears, so it may think that the front is slipping, and apply brakes to the corner, to try to pull the back end back in.... But since the back end really sn't stepping out, it'll just cause the back end to step out when it brakes the front corner... Which is what happened in my case.

 

Turn it off.

 

Point taken, then why is it when I have 4 brand new tires the vdc is fine, but when I have old fronts and new rears it goes crazy? I run 245/35/19 F and and 275/35/19 R as well. I still think it has to do with more so with grip and not size.

 

great insight everyone, I guess I'll go w/o VDC for the time being. But damn, it is freaky as hell when it happens. The first time it happened, I almost ended up going into the curb on the exit :X

 

When you have 4 brand new tires, you have 11/32 tread on back, and 11/32 tread on front. So you have a 3.7% stagger front to back, assuming 245 on front and 275 on back, like me....

When you have new rears and old fronts, you have 11/32 on back, and something like 6/32 on the front.

Using my shady math.....

The old fronts have a smaller radius by 5/32", or 0.15625"
Circumference = 2*pi*r
The circumference of the "worn" tire is thus smaller than a "new" tire by about 1".

Assuming an 19" tire, this means the worn tire is about 1.7% smaller than the new tire.
That means your stagger front/back with new rears and old fronts is now 5.4%

It could be that this is outside the normal spec for the VDC...

I doubt it's grip related, because the VDC does not know "grip". It only knows how fast the wheels are turning, and how fast they "should" be turning compared to the other wheels, based on what the throttle/steering/yaw sensors say...

If you are taking a mild curve at 30mph, you wheels should not be slipping, thus there is nothing for the VDC to compensate for..... But if the stagger is off by 5.4% when the VDC thinks it's only 1.7%, then it may think one of the wheels is slipping when it really isn't.

 

I get what you are saying. The way I always looked at it was new tires in the rear meant the rear was out driving the front which caused the VDC to kick in and compensate. Good info though. Much appreciated.

 

Just to add to the mystery....I put 275's on the back 19's because of a nail in the side wall...the tires weren't old. The tread depth of the fronts are deep....pretty much the same as the rears. But, I'm getting the VDC to kick in now too.

I don't think it is 'grip'. I tend to get it when the turn causes the front wheels to be turned more sharply.

I think avs007 is on the right track with his thoughts around steering angle. Plus I wonder if the newer tires have less sidewall flex than the fronts and this causes the differential diameter of the tires from front to back to be magnified. This would add more credibility to avs007's point on diameter differences causing the problem

 

It is definitely tire size but not quite what you think. I've had this problem even with the VDC turned off! The coupes use the same VDC as the sedans which have four tires all the same size. This makes the tire sizing on the coupes that much more critical. The dealer could not figure out what was going on until they drove the car with my tires and then swapped them out with wheels and tires from a new one. They even got the factory reps involved - nothing helped. It went away on its own in time - confirming the learning curve in the computer theory. After having swapped tires multiple times since then it has never recurred. Go figure.............

 

I just drive wih VDC turned off.

Way more fun that way, IMO.

 

+1 the VDC is a pain in the @ss. The first thing I do when I get in my car is turn it off.

 

Did you check the tread-depth of the shoulders? Cuz originally I thought my fronts were still like new, but then I found that the inside shoulder was worn to about 6/32", while the center of the tire was closer to 8/32" (This type of wear, I was told is normal for the coupe, as those tires had 20,000 miles on them)

Good point about sidewall flex, I didn't even think about that.

 

I've got to get both the front & rear tires swapped from left side to right side in order to balance out the wear....I'll have the shop measure the tread when I do that. You probably are right on the tread difference....it sounds like the computer is very sensitive to even small differences.