Contact patch lateral migration is a metric to measure the amount of lateral movement of the tyre contact patch for a vertical movement of the suspension. It is measured in millimetres of lateral movement per metre of vertical suspension travel, mm/m.
Contact patch lateral migration is determined by the position and movement of the camber pole of a suspension relative to the contact patch. The diagram below shows the projected contact patch trajectory in front view for a vertical motion of the suspension. The trajectory sweeps an arc with the camber pole at its centre (this is a simplified representation of the trajectory, in reality the camber pole moves with suspension travel). If the camber pole is above the ground line as is often the case, the contact patch will initially move outboard in bump. As contact patch lateral migration depends on the position of the camber pole, it is therefore closely linked with bump camber and roll centre height.
In RACE software, contact patch lateral migration is measured as half track change per millimetre of suspension travel. Half track is the distance from the vehicle centre line to the intersection of the wheel centre line and the ground. The graph below from the RACE pdf report shows the contact patch lateral migration plot for a double wishbone suspension. The on-centre (around zero wheel travel) gradient is 50.8 mm/m but the behaviour of the curve off-centre is also interesting. Note how the wheel moves outboard initially but then inboard at 35mm bump travel.
In general, contact patch lateral migration should be minimised. The higher the number, the more the tyre needs to scrub across the ground with suspension travel. For production cars, contact patch lateral migration would typically be less than 100-150 mm/m. Due to its link to roll centre height and bump camber, it cannot be tuned in isolation and as a result values greater than this can sometimes be seen on production cars.