The exact same thing as the first drive axle.
A tandem truck usually refers to the amount of axles on the trailer or tractor. A single drive axle on a tractor would be referred to as a single, or the same for one axle on the trailer. I have seen it referred to the amount of tires on the end of an axle. When there are two tires on the end of the axle, that would be referred to as a dual, not a tandem.
The "tandem" part means it has two drive axles, although it is possible for a tandem truck to be a 6x2 vehicle, and have only one of those be a live axle.
Tandem by definition is two. So tandem axle is two axles. Example would be a tractor trailer having two axles next to each other is a tandem axle.
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Three - the two drive axles, and the steer axle.
Tandem axles are connected by spring Multi axle trailers Shipyard Transporters
The length from the middle of the steer wheels to the center point between the drive tandems (for a tandem truck) or center of the single drive axle (on a single axle truck).
There's no such thing as a one axle truck. There has to be a minimum of two axles. "Single axle" if a term referring to a truck with only a single drive axle, and doesn't count the steer axle. Same with "tandem axle", "tri axle", "quad axle", and "quint axle".
Both of them.
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Five, all told. The steer axle isn't included in the count, so a single axle with have two - a steer and a single drive axle, a tandem will have a steer axle and two drive axles, a tri-axle will be a tandem with an additional lift axle, a quad axle will have two additional lift axles, a quint will have three additional lift axles, and a centipede will have four additional lift axles. A superdump is usually a quint with an additional Strong Arm mounts to the top of the dump body which extends out to maximize the vehicle's wheelbase.