318i Touring Very Light & Twitchy Steering at 70mph

[Flybynight]

OK, so I took my new Bimmer out for a drive along the motorway today and it was pretty scary! I was doing around 50 mph and all was OK as soon as I hit 65mph it started behaving very strangely. It would start to veer off slightly but then when I corrected for it the steering was real twitchy and very sensitive.

I spent the whole journey (about 35 mins) clutching on to the steering wheel and concentrating hard in order just to stay in my lane. Pretty tiring stuff and not at all what I expected.

I have a '00 318i SE Touring with 18'' BMW alloys. When I looked at the tyres I noticed that the fronts were two different makes and sizes (a 225/40/R18 and a 225/35/R18). Is it likely that this could be causing the problem? Could it be over inflation of the tyres? What tyre pressures should I be running in this size tyres? I jacked up the front wheels and checked for play in the joints and rod ends etc but they were OK except for the rearmost bushes on the suspension arms which had a small amount of play in them - could that be enough to this kind of problem?

I didn't have much load in the luggage compartment and there were no rear seat passengers.

I have never owned a car with tyres of such a low profile before. Is this how low profile tyres feel and do I just have to get used to the feel of them?

All opinions welcomed as I am really disillusioned with my 318 at the moment.

Paul.

[dkerley03]

Have your front end checked and get MATCHING tires!!!! My 1988 325IS felt like that in the rain, but fairly normal on dry days, it turned out to be my Rack and Pinion and inner tie rod ends had play in them! I had then replaced and got a four wheel alignment, all is Great Now!

[Flybynight]

Hey, thanks for that info. I have ordered a couple of new tyres for the front end and will have to get it all checked out. I tried playing around with the tyre pressures and it did make a big improvement but it's still not great.

Thanks again,
Paul.

[darkhorse737]

change the tires first and see... you're on the right track as i see it.

the car will always pull to the side where you have the 35 series tire. because it is actually smaller in diameter than the series 40 tire. The effect is... 1 wheel will be travelling slower than the other tire, even if both are 18 rim

[Flybynight]

That sounds like good advice, thanks. I can't wait to get new tyres on the front end and see how differently it behaves. It figures that if one tyre diameter is smaller than the other the speed of revolution will be higher and therefor will experience different amounts of gyroscopic precession when the wheels change direction.

Thanks again,
Paul.