Craig911

I bought my 996tt several years several years ago with HRE 19" wheels together with Michelin PS2 tires: 315/25 in the rear and 235/35 in the front.

Recently I replaced the worn rears with Invo 315/25 tires. The car generally handles the same except occasionally on hard turns, the front inside wheel seems to momentarily brake--it almost feels like he car is tripping over itself. It recovers right away but it is very annoying.

I assume this is probably related to the mismatched tires and the AWD and PSM? Anyone have any info on what could be happening and how it could be corrected? Is it simply because the fronts are more worn or is it because of the mismatched brands?

mcbit

I just checked the manufacturer's specs.

The Michelin 235/35 19 have a rev/mile rating of 817 and the rear Michelin 315/25 19 have a rev/mile rating of 824, this set up is known to be correct for the car with the rears rotating faster than the fronts.

The Nitto 315/25 19 have a rev/mile rating of 798 which means that the rears are rotating slower than the fronts which means that the input of VC is being driven backwards relative to the output and if your VC is working correctly it will produce a braking effect at the front wheels. The net effect of this is that the handling becomes a little quirky with a slight tendency to over steer as you turn in followed by being dragged into understeer if you get any slip as you apply power. I felt this with the rears having only a 2 rev/mile lesser rating than the fronts; the car felt twitchy and it only got worse the faster I went and my issues were with Michelin PS2s front and rear.

The other thing you might notice is reduced acceleration at high speed.

"02996ttx50

psm kicking in during cornering is exactly as stated. annoying. also, as noted the tire sizes are within spec but new vs old is quite often responsible for triggering abs/psm activation, and is only possibly exacerbated by the mismatched brand as the sensitivity of the system is such that even the most minute differences front to rear will be sensed and acted upon by the ecu, in the only way it knows how.

by triggering the abs. i would go back to the same size michi or swap the fronts with the same size you ran before ( presumably issue free? ) with new invo's

but even more optimally, i would switch out the used fronts with either a PSS or a PS2 ( again ) in the same sizes as had worked before, issue free?

once you scrub them and the ecu "adapts" that should eliminate the slight difference in tread depth etc, that the ecu is currently acting upon. you will also have to scrub them, as the issue may again present itself until the ecu ( again ) "adapts".

Craig911

That sounds almost exactly what is happening periodically when I am in high speed turns although I have not noticed any loss in acceleration in high speed straight aways.

How do you find these rev/mile ratings? I went to the Nitto website and could not find any of that kind of data on those tires. It sounds like I need to do the up-front research on this when I go to buy tires since my tire guy who put me in these is clueless.

So I guess the best solution is to buy some new front tires that match the rears better? If I drive with the current set-up is there any risk of damage to anything on the car?

My other sports car is a 2004 GT3 so all this PSM/AWD stuff is a mystery to me.

Road King

I experienced this first hand with my car when I first bought it over the holidays...car had new rear tires, 1/2 worn (and loud) front tires. Tires were same brand, but differrent models. PSM would kick in upon aggressive turn-in.

I threw on new matching front tires, and I haven't felt PSM since...and I'm able to drive the car much harder the more comfortable I get with it!

"02996ttx50

as your "comfort" level increases, your reliance upon or desire for the psm will diminish commensurately. at least it did in my case. i just havent yet removed the pump or installed the bypass switch diy. thats on my never ending "list"!

mcbit

I got the Nitto information from here.

You can check the handling to see if its the same with PSM on or off, it made no difference to mine.

On the acceleration issue in the lower gears you wouldn't notice anything but I drive the same off ramp every day and between two markers i can accelerate from 75mph to 150 with stock tyres whilst with the GT2 set up with slightly larger rears, this dropped to just over 140 mph at the second marker.

...and before any mentions the tyre size difference, these are GPS measured speeds.

Craig911

Thanks for all the info--it's been a lot of help!

I was thinking of getting the matching Nitto 235/35 19 tires for the front but after checking the site, I noticed that they have a rev/mile rating of 814 which is not much different from the Michelin fronts I have now at 817 rev/mile. They may not help the problem.

When I turn the PSM off, it does help--the problem with the quirky steering goes away and the handling becomes much more like it was before. I may stick with this until it is time to buy new rear tires.

PK911

Had the same issue when I replaced my rear tires with PZeros (I wanted the PS2s to match the front but they were not available in my size). After picking up the car from my Indy the car was a complete mess in fast corners with the PSM kicking in pretty much all the time. Called my Indy who told me that the rear tires needed to be broken in. I drove the car "slowly" for a few hundred miles and now everything is fine. BTW, I'm really happy with the PZeros.

"02996ttx50

very common issue even with tires that are within proper "spec" until the highly sensitive ecu adapts tp the differences in tread depths by scrubbing the newer additions, you WILL get abs/psm cels until scrubbed. though i'm still a bit dubious about running mismatched brands/treads with awd intact.

one solution around all of this stuff is to go rwd then you can run whatever tires/brands front/rear you want!