PSM kicking in after tyre change - Front/Rear tyre mismatch?

Following discussion as to why Porsche changed from 265/35/18 to 285/30/18 between gen 1 and 2 on this thread , I decided to replace my standard fit 265's for some 285's this time as an experiment to see what difference it may make for myself. It seems this may have been a mistake, the PSM is now kicking in constantly on under normal driving (when it gets twisty) to the point I switched it off as it was a dodgy driving with it on. Would the slight difference in the tyre size is too much for the PSM to handle? Or could it be the grip of the old tyres up front not matching the grip of the new rubber out back? It's a '99 C4.

Maybe you should try the back ones on the front.

As you have a C4, I'd imagine the PSM is not happy with the difference in rolling radius. Looking at a tyre calc. the new tyre is 2.4% smaller in diameter. It would seem to the PSM that the tyres are always slipping (over rotating) and hence the PSM kicks in. To maintain the RR in a 30 Section tyre you would need a 305/30/R18 which is huge. Only other option is to match the front RR to the new rear.

I don't think it should matter that it's a C4 as the PSM is an independent system to the AWD. You are right though, the new tyres are slightly smaller in diameter. I assumed as Porsche used the same size front tyre (225/40/18 ) on both the gen 1 and gen 2 cars despite the change in rear tyre size then this wouldn't present me with a problem. One thing I didn't account for was the gen 1 cars having a 7.5" front wheel, the gen 2s used an 8" front wheel. Would the extra 0.5" width have such a dramatic effect on the profile of the front tyre that it would balance the two alternative rear tyre sizes? The car definitely doesn't like something about the new set up...

Looking at the tyre comparison again there's not much in it when compared to the relative size of the front tyre, although as above none of these figures account for the difference in the actual wheel width. One thing I did just notice is that relevant to the front tyre, the 265's are larger, the 285's are fractionally smaller. Could this be the issue? I guess the same size front stretch over a wider gen 2 front rim could drop the diameter just enough to bring it back inline to the wider gen 2 specced tyre?

Comparison;

I have had this issue just mixing different brands with new rears and old fronts problem solved when I went back to all same brand just a few mm difference causes the problem as the car thinks different wheels are travelling at different speeds .I.e. spinning or sliding so the PSM kicks in My car is a c4s but I'm not sure it makes a difference I posted a thread about it previously here http://911uk.com/viewtopic.php?t=131161 Winny

Thanks Winny, reading your thread it's interesting the comment made by toohey regarding the experience they had had with the differing wheel sizes. Another thought I had and similar to your experience, my front tyres are a bit long in the tooth (still plenty of tread) so I guess it could it be that they've gone a bit hard and are getting pushed through the corners causing the under rotation? I'll be replacing the front tyres imminently which will confirm that either way, but if anyone can contribute any experience in the meantime it would be very much appreciated!

Let me know how you get on with the new tyres I'm betting that's the case and new fronts all being the correct size will fix the issue

Might i suggest you get the steering angle sensor calibrated using a tester then see how it goes .

You may For the (genuine) purpose of enlightenment, can I ask what that involves and how it being out of kilter may be causing the affects I've been experiencing following the change of rear tyres please?

Let's say the steering sensor is out of calibration .. say more to the left when driving in a straight line .. car thinks its cornering a bit to the left .. no big deal .. When you come to a sharp corner the car starts to think your getting out of control .. PSM kicks in .. i took the comment .. when it gets twisty to mean cornering . Just a hunch but different tyres fitted might make this active when the original ones and it was still inside limits so it still drives ok . Steering angle sensors should be calibrated after a geo .. it doesn't always happen and in which case it will be out . It's just a thought anyways and all it needs is a capable tester and about 2 mins of pressing buttons to calibrate it... even at a garage it's going to be a cheap option just to eliminate this .

I have had this loads on BMWs with mismatched tyres front and rear on BMWs. Really don't like it, traction light constantly flashing and feels skitty at the back. I first realised how much it can be an issue with the CSL. With 3k miles on it it was a twitchy mess, nearly lost it slowly overtaking something on the M11 in the wet. BMW agreed it felt weird, they changed everything, control arms, dampers, springs and all bushes, still terrible. Then got a call one day to say they had someone working there from another dealership who raced M3s and would have a look, so I popped down, we went out and at the roundabout less than a mile down the road he said "You have different tyres front and rear." I was adamant they were Pilot Sports all round, but when we pulled over sure enough they had put Pilot Sports on the rear instead of PS2s. We went back, put some PS2s on the rear so all matched and....BINGO! The car was sorted, once again absolutely planted as you would expect. Ever since then I have been able to tell, the more powerful the car the more obvious it is, and there have been times when I have bought a car with different tyres front and rear have been fine, but if anyone says about traction kicking in or the back end feeling a bit lose it is the first thing I will ask.

Following a week of fairly unhappy driving (it's a daily) I had the front tyres replaced today for the same Pirelli model & size and I'm delighted to say it's a huge difference. They were still well within the legal limits of tread but they were getting a bit old so presumably past their best. The issue of the PSM over-zealously kicking in seems to have completely gone away so I assume the new rear tyres were simply 'out-gripping' the old (harder) fronts which was most notable when in tight twisty corners, although even on a straight road above 50mph the front end felt like it was wandering which was quite disconcerting. Well, all that has gone away and the car feels balanced again. I'll likely get the geo checked on the front as per deMort's suggestion anyway as it seems like a sensible routine thing to do and it's been 2 years+ since it was last checked. Would the swap from 265 to 285 tyres on the rear have any influence on the required front geo setup?

Glad it's fixed sounds like exactly the same problem and solution I had Winny