Tramlining

Tyre pressures, brakes binding, knackered tyres other than that you should find it at the garage. Is the steering wheel centred?? 1st sign of a previous nasty bang.

I have a 328i (96) and it tramlines quite bad aswell. Wheels are standard BMW alloys with 225x50(?) 16.had all tracking and suspension checked. All good,as you would hope for on a 33k car 

Thats the best bet I reckon (watch out for those pesky Gatso's!!) Cheers paul

Omi, what pressures do you run? With those tyres you should have ~37psi up front and a little less in the back.

Tramlining feels like the car is being pulled sideways by any ridge or in-line road anomolly.

Big tyres are a common cause as are incorrect toe angles.

The optimum pressure depends on a lot of factors and need to be fine tuned based on thread wear (measure with avernier, etc.). Running 37f and 32r will cause a little more oversteer as the rears will squirm under load more. Setting tyre pressures is about making sure that the sidewalls and the thread take equal roles in supporting the weight of the car. More pressure means the thread takes more of the % load than the sidewalls and v.v.

You are incorrect about the surface area though - as it remains more or less constant if the tyre pressures are kept within a narrow enough range (see above). Don't forget that the force on the ground is always the same as it is equivalant to the cars mass! It is an urban myth that bigger tyres = more rubber on the road! What actually changes is shape of the contact patch, much less its area.

I have no evidance that it makes the car more understeery - the general consensus is that at 15", the E30 is the most balanced.

The lower sidewall of the 15" will IMPROVE turn in over the 14".

Having a wider tyre can allow more channels for water drainage whilst maintaining the same amount of rubber on the road, which, depending on tyre, can improve the wet weather handling.

Dunno on this one as drving the car with the 14" steel rims and narrow tyres it feels alot sharper but if you push it hard through a bend the rear end will break away very easily (dry conditions) but with the wider 15" rims and tyres the car will take alot more to lose that rear end.  The only reason i could have seen for this was the more rubber in contact with the road theory.

The force on the ground /square inch will change with wider wheels, and the car will be more likely to aquaplane on water with wider wheels also, as thinner tyres will be more effective in 'cutting' through the water to reach the tarmac below, the wider tyres will trap water easier in the centre of the tyre.

Wider tyres can be better in the wet as you say but it does depend alot on the tyre make and the design of the tread as you also say but i have always found that a car with narrower wheels will perform better in the wet.

Paul,

Can you explain what you mean by "The force on the ground /square inch will change with wider wheels".

Narrower tyres are less prone to aquaplaning in principle but a wider tyre can accomodate much better thread patterns to make up for it. Nowadays, there is not so much of a difference - and in damp conditions, the wider tyre will be often better.

Kitty,

Nice resolution.

No, the weight of the car is the same so the force in total is the same. If the tyre pressure is the same, from P=F x A (pressure = force x area), the area will be the same too. What happens is that the shape of the contact patch changes. A narrow tyre has a contact patch more along the length of the car and a wider tyre has a contact patch more across the car. The area will be almost identical.

This is not 100% correct because the sidewalls will play a different supporting role (and tyre pressures are usually higher on wider tyres because of this), but the point is that it is an urban myth that bigger tyres give a bigger contact patch.

If you put on truck tyres, they would hardly deflect at all - you would almost have point contact!

Regarding the wet, my exprience is different. I don't have enough information about aquaplaning, but in damp conditions I have always found bigger tyres to be better. Usually they are better quality too! Try and find some braking tests within a model range. You will find that the car with the bigger tyres brake better - even thought they have the same ABS systems.

Your point about wide tyres squaring to the road needs to be tempered by the fact that you will get less roll before they skid because they will slide at a lower cornering force. Your point also suggests that it is NOT advantageous to have the tyres squared to the road too - or have I picked that one up wrong!?

tramlining is caused by wider tyres. the wider you go the worse it will get . its combated a little by the slightly less wider wheels /tyres at the front like bmw do on the sport / m3 models etc .
if its a problem have skinny tyres on your car. myself i prefer the fat tyres and handling .

the more rubber on the road the better grip you have , the only difference wide tyres make in any weather is the snow, in countrys with a lot of snow they use a second set of wheels with snow tyres ,and these are skinny .
i drive very fast most of the time in all weather and i have used several different cars/tyres , i prefer wide tyres as the grip is increased in rain and dry over skinny tyres, its the compound as well thats involved not just the size .
p.s i have not crashed a car yet . fingers crossed
a racing driver once told me to drive through my tyres . this was the best advise ever.
no matter how good a car is if you put crap tyres on it ,it will handle crap and visa versa.

I will disagree at all times don't worry , thank you for enlightening me on the topic.