Porsche Stability Management w/ New Tires

[ironman1]

Replaced Michelin Pilots on rear of 2005 boxster s due to excessive camber rare at 25,000 miles. The front are 1/2 wore so only replaced the rear with Bridgestone Potenza, same size as original 265/40 /18. Now the stability management kicks in way early. A turn I could take easily at 65 before, now the front brakes will grab and PSM kick in at 50. Any ideas ?

[JFP in PA]

Replaced Michelin Pilots on rear of 2005 boxster s due to excessive camber rare at 25,000 miles. The front are 1/2 wore so only replaced the rear with Bridgestone Potenza, same size as original 265/40 /18. Now the stability management kicks in way early. A turn I could take easily at 65 before, now the front brakes will grab and PSM kick in at 50. Any ideas ?

 

 

Sounds like the tires are mismatched and the system is seeing it as a reason to kick in.  Front to rear mismatch can do this.

[AWDGuy]

experienced the same thing at calabogie motorsports after switching my rears for Michelin pss's.

[ironman1]

Double checked tires are the same size as originals and pressure is spot on.. Is the circumference of the new  tires that much bigger than as being read by PSM than the 1/2 worn fronts to throw it off ? 

Is the only solution to just bite the bullet and buy new Bridgestones for the front ?

Is that normal to have that much inside camber wear on the rears ?  I had a 2000 Boxster S and I don't remember them wearing anywhere like these ? 

[AWDGuy]

I would lean towards the level of grip the new tires are providing over the old ones before laying blame on the wear/circumference of the tire. thats what I chalked it up to anyways as I could feel the fronts loosing traction way before the rears did. I just live with it.

I've read that the tires wear from the inside on the 996's. my tires are wearing out very evenly though and I havent expereinced this kind of wear but have seen it on others 911's.

Edited August 6, 2014 by AWDGuy

[JFP in PA]

Double checked tires are the same size as originals and pressure is spot on.. Is the circumference of the new  tires that much bigger than as being read by PSM than the 1/2 worn fronts to throw it off ? 

Is the only solution to just bite the bullet and buy new Bridgestones for the front ?

Is that normal to have that much inside camber wear on the rears ?  I had a 2000 Boxster S and I don't remember them wearing anywhere like these ? 

 

Problem could be differences in grip front to rear, which can be considerable,  but is often more of one of "rolling diameter".  If you have ever watched an NHRA top fuel car do a burnout and have the tires grow in height by nearly a foot, you have seen a visually extreme example of rolling diameter in action.

 

All rolling tires grow due to the impact of speed induced centrifugal forces and their own weight.  Because your car has asymmetrical sizing (front and rears are different sizes), the rears will tend to grow slightly more due to their heavier weight compared to the fronts.  Add in the wear differences and it compounds the issue.  The clue was your comment about the PSM invoking at a lower speed on the same turn; the difference in front to rear diameters fed the PSM differential wheel speeds outside the limits of its algorithm, and it suddenly thinks you are in trouble when you are not.

 

A lot of Boxster owners experience the high rates of inside wear on the rear tires.  Usual mitigating factors are the car's four wheel alignment specs and general driving style.  Those that push the cars harder seem to see accelerated wear.