I think because; as cranes have evolved and become bigger people have followed the same path in fitment, and in all fairness where else can you put it? 6x4 tractor unit would weigh 23 tons with that on and have a pin load of 3 ton, and you would have to stretch the tractor unit as well to get it on, so you would be over length and over weight.. 8x4 tractor unit would be the same as in to long and heavy... and even an 8x4 tractor in C&U is only allowed to be 26 ton gross... So really it is an 8x4 draw bar which would work or a trailer... Stability also would not be achieved with a short vehicle ( 6x4 tractor unit). And from the DVSA's point of view you are not allowed to use crane trucks in STGO... Why you ask? Because STGO is a non divisible load that cannot be carried on a normal vehicle ie C&U therefore the load would have to weigh in excess of say 28 tons and thats a mighty crane to lift that load, and even then if you put the load on a normal vehicle you would run in CAT 1 ( 44-50 tons gross train) and with a big crane you would be in CAT 2 ( 50-80 tons gross train) .And if the load can be carried in a lower category without the use of an abnormally heavy vehicle, then that is the way it should be transported......It's a minefield that DVSA are toughening up on..... So legally the only way you can lift and load a 17 ton lump is on my style of trailer.... If you carry lighter volume that you need a lot of reach on then it's draw bar....Hope that is of some help... Cheers Pat

And what is the crane that you have fitted?