rossba

Hi All,
as above have a Sport Tourismo in winter I use 20 inch wheels on Winter Friction Tires in Summer I have 21 inch wheels on Pilot Sports.
on the 20 inch wheels the car handles amazingly. On the 21 inch wheels however when trying to drive in a straight line the car is all over the place dragging we left to right on small inclines. It’s amazing going round corners just a handful in a straight line. Is this normal? I put up with it last year but this year I have had enough. Tire pressures are set as per cars instructions. Wheel size changed on the computer. The tires have about 5 to 6 mm tread left on them. I am thinking maybe the alignment and cambers have been set on 20 inch wheels and putting 21 inch on is messing up the geometry? I am guessing now but the tires wear evenly. Finland does have grooves in the road from stud tire users but this is getting super annoying. Any ideas anyone before I get porsche to check it?

rossba

Forgot to mention proper wheel sizes….winter tire sizes friction tires

Front 275 40 R20

Back 315 35 R20



Summer tire sizes Michelin Pilot Sport 4S

Front. 275/35R21

Rear. 315/30R21

PandaSPUR

Thats only a 0.3% difference in diameter in the front and 1% difference in the rear when comparing winter to summer. Shouldn't be enough to cause issues.
Any chance one of the 21" wheels are damaged or not properly balanced?

rossba

Hey thanks for the reply.
wheels are like brand new no dents, scratches or inner cracks.
don’t get any shaking from the wheels so not sure it’s balancing.

PandaSPUR

Yea if you're not getting any vibrations/shaking then its probably not a balance issue.

Only other DIY thing I can think of would be to check tire pressures with a gauge rather than rely on PCM, and being sure to check when the tires are cold.

rossba

Yeh I just put the summer wheels on yesterday and done the tire pressures before the car moved.
38 psi front 42 on the rear using digital gauge.
it’s weird since like you say the tires are the same width as the winter tires.
the winter tires in the groves made by all the other cars on winter stud tires on the road you would think I would have the same problem when they are on but I don’t.
my winter tires are non stud friction tires.
I am thinking it might just be the age of the summer Michelins tires. I inherited the tires and wheels from the previous owner so could be the original ones from 2018 are just old when the car was sold for the 1st time.
just worries if I purchase new tires and it’s the same issue.

justabout

Try running comfort tire pressures and maybe a couple pounds lower (for testing only). I’ve found that high performance tires and higher pressures cause instabilities on some surfaces. Get the alignment verified as well. Could be off a small amount.

rossba

Yeh I tried lower pressures last year worked in sone scenarios like you say.

yeh I’m taking it to Porsche tomorrow get the alignment etc checked and have a chat with them.
If all else fails thinking to maybe slap some new 20 inch tires on the winter wheels and be done with the 21’s.

not sure yet though.

healthnut76

My guess is that you have an undiagnosed alignment issue. Keep us posted.

skiracer

Try to find a set of 21" wheels w/ new tires and put them on to see if the issue goes away. This tramlining effect is more pronounced w/ low profile tires. Have you checked the production date codes on the tires to confirm their age? Best of luck sorting this out!

E39Nutz

try dropping the fronts down to 36 psi. Pressure too high on the fronts will give you the sense of tremoring when going in straight line. I run 21” in the summer and no issue at all. I do 36 fronts and 42 backs.

Marcus13190

Have you put yours wheels in the good position (left right) ?

E39Nutz

OP has michelin PS4S, those are non-directional tires.

rossba

yeh non directional.

rossba

This is a good idea thanks.
Will give this a try.