Unstable at speed - Please help

Hi, Can anyone offer any suggestions as to what may be the problem with my car. It is a 996 C2 Cabriolet which I have owned for around 2 years now. I've always thought it felt a little unstable when travelling at motorway speeds but always put it down to the front end being light because the engine is in the back. It never worried me too much until recently when I lost control and spun the car at 65mph on the motorway in the wet whilst out with my girlfriend. There are no noises coming from the suspension and the tyres have recently been replaced (Conti's) as I thought this may of been the cause. Can anyone shed some light on what may be the cause of the problem? I love the car but unless I can sort this out I will probably decide to sell it. Thanks, John

I've always found the faster I go the more stable the car feels so there must be something wrong with your car. Maybe tyre pressure or geometry - someone will be along to point you in the right direction But having said that, if there was enough water on the road, any car with wide wheels could aquaplane

Unstable as in excessive tramlining - having to hold the wheel to prevent it tugging either way? Check it isn't running too much negative camber on the front (and even the back actually). My first intelligent answer

Unstable at Speed - Please Help Hi, Thanks for the replies. It is as if it's tramlining, it feels as if it's wobbling and I have to keep adjusting the steering to compensate. It makes the car quite difficult to drive when on the motorway because you just can't relax. When the tyres were changed the nearside front was quite badly worn on the inside edge. Would this suggest the camber needs adjusting? I spoke to the tyre fitter and he said he changes loads of tyres on the front of porsches and theyre always worn like this. Something to do with cornering speeds? I appreciate any suggestions? Thanks John

The tyre pressures are very important. Check them and put them here for people to see, also list your tyre sizes.

225/40zr18 Fronts & 285/30zr18 Rears all replaced less than 1000 miles ago. I was running 36psi front & 42psi rear but was told the car would drive better if I lowered these. Not sure how true this is? Thanks, John

For those tyre sizes it's common for people to run them 32 or 33 at the front and 38 or 40 at the rear. I'd try changing them to those and see if it feels better, if it's better but still not right then it's likely that a 4 wheel geometry set-up will be needed. All of the 996's I've driven feel more planted and heavier the faster you go.

Questions: * how hard was it raining and was there lots of surface water? * were you travelling in a straight line? * were you accelerating or cruising?

Re: Unstable at Speed - Please Help IMO that does not sound right and suspect it needs the geometry checked

It wasn't raining too heavy but there was quite a bit of standing water. I was cruising in the middle lane when it happened. The front seamed to drift slightly then felt like it had got grip again before it spun out. My car certainly doesn't feel more planted as I go faster. I'll get the alignment checked/reset and post my findings. Can anyone recommend somewhere for getting this done and how much should it cost? I'm in South Manchester. Where can I find the recommended settings? Thanks, JohnL

Sounds like you have 2 issues - (1) geo setup and (2) aquaplaning which 'could' have been made worse by the first

I had my geometry done a few months ago at this place. http://www.abpmotorsport.co.uk/abp-homepage They are not far from JCN16 on the M6. Highly recommended, cost about £100 or £120. They test drive and then get it on the ramps etc etc then test drive again. Took about 3hrs, I waited while they did it. Avoid Demon Tweeks near Wrexham like the plague. Perhaps Ninemeister or Unit 11 near Warrington do 4 wheel alignment, worth checking their sites. It is worth altering the pressures beforehand though to see what difference it makes.

I would echo the comments about geometry and tyre pressures, etc. Do you have a body kit of any sort? Larger rear wing perhaps? If so, does it have a suitable front splitter to balance it? If you just have the standard rear spoiler, are you sure it is extending at speed?

:agree: very good point!

The car is standard apart from a GT3 mk2 front bumper. It has the standard rear spoiler and it does come up at 75ish mph. Is there any way to manually raise the spoiler? I had the bumper fitted at Christmas hoping it may help cure the problem but it didn't. Thanks, John