Tandem axles are a set of two axles positioned close together on a truck or trailer. By spreading the vehicle weight over more tires, tandems allow the truck to carry heavier loads without damaging road surfaces. On trailers, these axles are often mounted on a sliding subframe that can be adjusted forward or backward to balance the weight distribution.
Adjusting the tandem position is a key skill for truck drivers. Sliding the axles backward increases stability on the highway but makes turns wider. Sliding them forward makes maneuvering in tight yards easier but shifts more weight to the rear. Federal laws restrict the maximum weight allowed on tandem axles, making regular weigh station stops necessary.