Why does cars with wide tires tramline? - The Unofficial BMW M5 Messageboard (m5board.com)

atomic80 · 19th January 2002, 17:01

A random thought just came into my head on this fine morning and I thought to myself, "hey, I could ask my buddies on bmwm5.com!"

One would think that cars with narrow tires would be more likely to tramline or even follow the grooves left on the roadway but nope it only seems to happen mainly to cars with wider tires. Thinking about it, the wider tires don't quite fit in the grooves so it sort of is pushed in and out from side to side from what I'd imagine. Is that the scenario that's happening here? If that'd be the case then why don't cars with narrower tires just stay right in the grooves like that of a railroad on tracks? Just wondering.

Aussie2U · 19th January 2002, 17:59

I have thought about this too. I know installing staggered front/rear tires helps alot with this and according to magazines, was the reason BMW changed the "same size all around" for the M3 several years ago.

Of course I have 275/55-17 Pirelli Scorpion's all around on my ML430 and don't feel it tramlining like in my E420 with wide tires.

Does a particular rack & pinion steering setup vs. recirc ball amplify the effect maybe? Just a thought. (I'm thinking of the baja truck steering dampner shocks here...)

Bart Carter · 19th January 2002, 20:52

Never thought about it much, but more tread = more contact = more imput from road variations.

Also, tread design has a lot to do with it.

MEnthusiast · 19th January 2002, 21:07

Im no engineer- but the way I see it is as follows:

weight of the vehicle is spilt on to 4 points.

those points are mostly flat (the part of the tire that hits the road).

because of all the weight on them- they are going to want to conform to their basic shape (flat).

when they encounter road irregularities- like when the road gets worn down in places- they will basically "seek" the place where they can regain their flat stance.

with wider tires theres more surface area to keep flat- so your chances of observing this increase.

Ive discovered that tread design can affect this a little,
suspension design affects it more but the biggest affect is via width of the tire.

interestingly- the tires on the M5 and Z8 are very close to the same width and the Z8 will really seek the road grooves, while the M5 is a lot more benign.....

thats my theory.

Scotty · 19th January 2002, 22:23

Quote:

Originally posted by Bart Carter
Never thought about it much, but more tread = more contact = more imput from road variations.

Also, tread design has a lot to do with it.

Also, increasing sidewall stiffness ( lower aspect ratio), changing steering geometry (offset), and possibly (?) changing the shape of contact patch.

rss

Bart Carter · 20th January 2002, 02:09

Scott, I was going to mention offset. It does indeed cause the steering to react differently. That's why it is important to maintain stock offset if going with aftermarket rims.

rvacha · 20th January 2002, 03:06

As I understand it, tramlining is a two-fold problem caused by front suspension geometry (the two front wheels never point in exactly the same direction - usually by design) and *difference* in right and left tire adhesion. Basically, the car goes more in the direction of the tire with the most grip. High performance tires tend to exhibit more tramlining for a couple of reasons (not an all inclusive list):

1. The tires are capable of more grip - and are therefore likely to exhibit more difference between left and right tire grip over undulating road terrain
2. The tires' contact patches are bigger, again creating the likelihood of more difference in side-to-side grip over undulating terrain
3. The tires' lower sidewalls means less compliance (i.e. more mechanical coupling) between the tread and the wheel, which exacerbates the phenomenon

It doesn't help that in general there is a correlation between high performance tires and the cars they are mounted to - the tires are usually installed on cars with more aggressive steering/suspension geometries. In general some straight-line performance (and tramlining immunity) is given up to increase performance in the twisties.

If your car has a lot of tramlining there are some things that can be done to improve it (undoubtedly not all-inclusive):

1. Change tires
2. Change tire pressure
3. Change the geometry of the car, specially toe in and camber
4. Install stiffer bushings to minimize the amount of geometry change that occurs when the tires fight each other
5. Change wheel offsets (but I think this is mostly just changing camber indirectly)

I'd start with tire pressure first. My 540's Dunlops go from no tramlining at 35 PSI to agitating amounts of tramlining at 36 PSI