So it was time to get new tires for the rear (due to the inner sides wearing down to the belts) I had to go with the Pzero's (295/30/18) due to the everlasting backorder of the Mich. pilot sports. I didn't change the front tires (Mich. pilot sport OEM size (225/30/18) because they have plenty of tread left to get me though the winter season.
After the new Pzero's were put on, I hopped onto the 110fwy (nice twisty portion) and the car felt very different. Almost as if the car is pulling slightly left and right when I'm making small turns. At one point I was going over 75mph and the car was pulsing and pulling to the left in a pulsing matter with one of the dash lights pulsing on at the same rate ( couldn't tell if it was a PSM or an ABS light. I slowed down and kept it under 75mph for the remainder of my journey (about 15 miles) and didn't have any other problems.

Yes the front tires are old comparably speaking to the new rears. But I kept the same OEM sizes for the rear due to the decent OEM size front tires. And to keep the -1% rotational mass between the F/R
Am I missing something??
I really want to upgrade to some newer wheels in the GT2 size including tires. So I bought the Pzero's to just get me through the winter before buying the whole new wheel/tire setup.

 

ya gotta give em a bit of time to scrub in and they should settle down. i think it'd be far more proscribed and unfavorably evident with psm triggering if you were on angeles crest or pushing it up some canyons. also make sure your tp is in spec. they often overfill using the door sticker guide which would be "overinflated" unless you have the car packed with stuff/people etc.

try 36/40 and scrub em and see if it settles. i'm betting it will, provided alignment and tp are all proper.

 

Considering Angeles Crest is my backyard and a pretty frequent Sunday drive for me, I'm scared ****less now about taking it up the crest after encountering what I did today.
When you say "scrub" what do you mean exactly? 80 grit sandpaper?

 

I didn't even think about the TP. I just looked it over to make sure they didnt put the rear wheels on the wrong sides, and took off for work. I'll have to check that tomorrow.

 

break them in. angeles crest near the top where the rough stuff begins is a perfect place. take it up tomorrow am before the traffic and put about 75 miles on em. again, start at 36/40 and adjust to your liking. see how that changes ( or not ) things.

you probably will experience the abs/psm. dont freak out, just wear some of the new rubber down. it doesn't take much. GL!

 

I fit new rear Sumitomos with 50% worn Mich PS2 fronts and experienced near the same thing. Car actually slid in a turn on the freeway, then caught, PSM flashed. Car felt nervous and the faster I went the worse it got. I gave it 2 weeks and it didn't get any better. I fit new Sumi fronts and the car felt perfect. I checked beforehand, even measured tire circumferences thinking I was close enough to new front to new rear specs and everything would work out. It didn't. I'm now RWD so it no longer an issue.

It is normal for new tires to feel a bit unpredictable if pushed a bit. I give it a couple to around 500 miles or 2-3 runs up the canyons pushing it a bit more with every turn.

 

Thanks for the input. I will have to take it 50% of my normal canyon driving. Hopefully they will settle down

 

I bet if you put new matching Pirellis on the front all your problems will go away. Just keep the Michelins until you can get some rears and then use them up. Different wear levels and different tread patterns can lead to all kinds of funky behavior.

 

We had the same on my friends 997, mismatched brand front and rear the car felt weird and very disconnected to drive. Replaced with a matching set, back to normal.

 

^all the foregoing about mismatched brands fvr is true, but doesn't affect those that have seen the wisdom and light that emanates from running rwd. you can run anything that fits

but truly, i neglected to consider the mismatch of tires AND brands. fageddaboudit. it's unstable squirrel. but all that said, the scrubbing is still the major issue.

 

Gee I don't know much about tires but think maybe the mismatched tires may be the problem. Just an opinion.

 

Quick update for others who may have the same issue at some point. I have yet to change the front tires. So they are still a mismatched pair front to rear. After a few spirited canyon drives, and the occasional 110 hwy trips, I only get the "Abs" light and pulse/jerking when making left hand swoops at 75+mph. No issues what so ever on the right hand swoops.
If I put the car into "no PSM" the car doesn't seem to have the jerking/pulsing as often at those speeds. But when it does happen, the "abs" light doesn't flash on.
I am going to order up a set of front tires, I was hoping that after 1k miles and some canyon runs it would settle down, but it has not.
In reality it didn't think that having two different brand tires on front and rear would make or create such problems. Even by keeping the same tire sizes. Now I know.

 

similar behavior for me with brand new tires at mid-Ohio track. Turned off PSM, and all was fine.

djcooper@rennkit.com

 

my favorite admonition regarding PSM directly from Porsche: to wit:

"While confident in the system’s ability as a dynamic handling aid, Porsche cautions drivers that PSM cannot counteract the laws of physics."

LOL

 

While new tires need some "break in" to eliminate the greasy feeling, they should be past this by now. Given the wear pattern of the old rear tires the alignment is quite suspect. My advice would be to get the car aligned -- before the rear tires wear in much from the bad alignment.

If you go the alignment route, fill the gas tank, empty the car of "junk", and be sure the spare tire/tool kit/etc are properly stowed.