Originally Posted by
JohnL
The following may or may not have anything to do with your problem, but it's good to know...
Some tyres pull ('lead') slightly to the left because of a phenomenon called 'bias steer', and some tyres 'lead' to the right (for the same but oppositely manifested reason). This is to do with how the steel belts are laid up in the mould when the tyre is made (i.e. which of the two belts is angled diagonally left and which angled to the right).
Most tyres have some degree of lead (even if only very slight, but some more than others), and the tyre makers use it to counteract the predominant tendency for the car to pull toward the side of the road because of road camber / crown.
Since we drive on the left side of the road, tyres intended for use in this country (and others where the left side is the 'right' side) should be made with an inherant tendency to lead to the right, thus counteracting (in some degree) the tendency for the car to pull left due to left leaning road camber (which exists more often than not).
A problem can arise when a tyre intended for sale in a right side of the road market ends up being sold in a left side of the road market (e.g. a tyre made for sale in the USA being sold here, etc), because the tyre will have a tendency to lead slightly to the left (to counter road camber found on the right hand side of the road), which in this country would get added to any tendency to pull left due to road camber, i.e. rather than counteracting the problem the tyre lead adds to it.
This probably happens quite frequently because tyres can go through quite convoluted pathways before ending up with the end user, they don't all follow a simple linear path from manufacturer to end user. It's probably less likely to occur with bigger brands bought from official distributors, but perhaps more often from independant tyre stores or with lesser known brands?
Having said that, I recently got rid of an irritating left pull by moving the front tyres to the rear. These were a well respected brand tyre that were wearing very evenly and the same on each tyre, but which had been pulling increasingly left as they wore down (the problem usually seems worse as tread gets thinner)...