The 4x2 tractor can't get the rig moving on a slippery upgrade, so the trailer's powered rear axle provides a boost. Photo: Tom Berg from SAF-Holland video Powered trailer axles are unusual, even in ...

Have you ever wondered why some semi trucks have one set of wheels and others have two sets crammed together? That's because some semis use a single axle while others have tandem axles, but why is it ...