Abstract: A convertible traction device for permitting a road vehicle to be used in an off-road configuration is described. The device is provided as a trailer having a pair of retractable road wheels with a pair of traction belt assemblies mounted thereon. The traction belt assemblies each include an endless belt, a plurality of support wheels about which the belt is entrained and a supporting framework, each assembly being pivotally attached to the trailer chassis. During towing the road wheels support the trailer on the ground with the traction belt assemblies being held in a retracted position above the trailer platform. For off-road use the traction belt assemblies are pivoted downwardly until the belts engage the ground, the assemblies are locked in position and the road wheels are retracted. The vehicle is driven onto the platform so that the drive wheels thereof engage drive rollers mounted in the trailer chassis, which rollers, as the vehicle's drive wheels rotate, impart driving movement to the traction belt.