This tandem drive thing it best Euro/U.K. verbs, a tandem drive is called a "Double Drive" this is fitted near always to 3 and 4 axle tippers (dump trucks) and cement mixer spec trucks, on tractor trailer outfits it is standard to see (common) 5 and 6 axle outfits, this is 3 axles under the trailer, with 2 or 3 axles under the tractor, the usual set up is twin steer axles with the rear most axle driving, the mid axle will also lift so when empty its not wasting rubber and fuel, its also 2 spare wheels if you need them, the rear tag axle is not as common as it was 15 years ago, on low loader work double drive is the prefered set up depending on the work style of the company, it is common to see a 6 axle outfit when empty to have 2 axles lifted leaving 4 axles only to wear out the tyres, most tractor units fit big super single style wheels to the front steer axle, as they run cooler and have a good effect to the steering, there is 16.5 ton (16500 kgs) axle loading allowed in the U.K. and Europe under set conditions, near all bridges in Europe are built only giving 4 metre hight clearance, anything over 4.080mm will be a bridge strike, so you need to know your hight??, so the reason that double drive is not on every 3 axle truck is, if its not needed why have it, plus why drag around loads of weight for nothing, near all International running trucks carry snow chains by law.
