Originally Posted By: CapriRacer
In tires, pull is caused by a tire property called "conicity" - root word "cone" - and like a cone, the tire pulls slightly to one side or the other. Conicity is the result of the centeredness of the belts. Conicity has both a direction and magnitude, and the dimension is pounds force.
Decreasing the deflection - either by increasing the inflation pressure or using a lighter load - results in less conicity. Needless to say, on a vehicle, lightening the load isn't practical, so changing the inflation pressure is the only thing that can be varied.
But pull is caused by the vector difference between the 2 tires - AND the sensitivity of the vehicle.
Perhaps an example would help clarify this:
Let's assume you have a vehicle that is sensitive to anything larger than 12 and you have a pair of tires - +7 and -6. The vector difference is 13 - and that would cause a slight pull.
Swap the tires left to right and the magnitude of the pull is the same, but the direction is towards the other side.
Replace either tire with something smaller - say a 4 (either plus or minus) - and the vector difference is also smaller - and the result is no pull.
Take both the +7 and -6 and put that on a vehicle that is sensitive to anything larger than a 15 and this also results in no pull.
So it's not that one particular tire is bad, it's that the pair are not matched AND exceeds the sensitivity of the vehicle.
Going back to the original +7 and -6 pair of tires: If you take the -6 tire and mount it inside out (If this was a white sidewall with the white side normally mounted out, that would be WSW in), the conicity for that particular tire is the same except that it is pointed the other direction - and no pull.
This phenomenon seems to be strictly confined to the front axle. I've never heard of this occuring on the rear axle, so moving the front tires to the rear may eliminate the pull - with the unfortunate result that a rotation would result in a pull.
Vehicle manufacturers will usually specify a maximum value for conicity and commonly request that a shipment of tires all have the same direction - with the theory that the first (and last) set of tires from a shipment won't get mixed with the previous (or next) shipment.
In cases where they know their vehicle is more sensitive than normal, they may request "quad sorting" within a shipment - that is a given shipment is not only all positive (or all negative), but also all high (or all low).
But Mercedes specifies that ALL their tires have the same direction, so the tires are either purposefully built with the belts slightly off center - OR - the tires are identical on both sides and the side to be mounted out is indicated by an almost invisible dot.
Unfortunately, it is possible for a misalignment condition to cause a wear pattern that changes the conicity of a tire - and a tire that had none before could develop high values of conicity.
You might think that a tire with a large value of conicity would wear towards one side or the other, but I have never been able to detect any abnormal wear patterns that stem from conicity. I suspect that is because toe has a much larger effect on wear, coupled with the fact that conicity values are pretty small compared to the forces generated by even a small amount of toe in.
AAAHHH, AAHHH, MY HEAD HURTS!!!
