Front wheel setback is a measurement taken through the spindles perpendicular to the centerline of the vehicle. Rear wheel setback is when one of the rear wheels sits back further than the other. Front wheel setback causes a vehicle to drift to the side with the most setback. Rear wheel setback causes the vehicle to drift to the opposite side. Rear wheel setback is often caused by a damaged leaf spring or a broken center bolt. Start looking for a bent component when it exceeds around ¼ in. or so.
Setback is typically caused by collision damage or a bent part, but it’s still a good idea to check the vehicle manufacturer’s specifications for built in setback. It may be built into the vehicle to compensate for road design. In particular road crowning.
A crowned road is designed for water drainage, the center or dome of the road is higher than the shoulder. A vehicle with even setback will drift towards the shoulder of the road. This drift can be compensated for by setting the wheel closer to the crown of the road back slightly further than the other.