a couple of things weren't mentioned here. in the 40's and 50's some tandems implemented a system of belt, or chain drives linking a forward "drive axle" to a "dead axle" or "tag axle". even today a person can buy the wheel separator that goes between the dual wheels of a semi, and has the slots cut in it for the belt.
companies like the now gone p.i.e., riss, and mc lean trucklines who purchased the "r" series cabover and conventional tractors often used the belt drive system. the belt did help, but not totally solve the problems of getting stuck when the dead axle would cross an object and lift the drive axle off the ground, or on icy roads. the system was only used for a few years, as far as belts went. even today, it's easy to get stuck in some places with a tandem, so, a system called a power divider lock is used to effectivly lock in all 8 tires on the rear to get out a certain situation. it is to be used very short term, as many a driver who forgot to unlock it has discovered when they burned up a power divider by forgetting to unlock it and return to regular driving. these days, tag axles are fewer, and most are air lift, or air adjustable to add or decrease weight distribution. most air actuated axles are seen these days on dump trucks.
