orys:
Carryfast:
Exactly you can’t load a train’s drive wheels to the point of creating sufficient traction to pull it’s overall gross weight without breaking the tracks therefore you use lots of drive wheels to reduce the individual axle weights and the amount of tractive effort needed to be applied by each drive wheel.I am glad I managed to get through for you.
Which is exactly the same logic as the Americans are ( rightly ) using in the case of double,or even tri,drive tractor units.IE they don’t want the high axle weights because of road damage over thousands of miles of roads.In addition to which,in deep snow,just like on soft slippery ground,putting the type of weight needed,to overcome the loss of traction,caused by slippery snow and ice,just digs the drive axle in deeper and means that you’ve added the issue of having to climb out of a deep hole in the snow to the one of loss of traction on the flat.
So they don’t do it because double drive is better than single axle. They do it to reduce axleweights.
As for the offroad 4X2 Tatra yeah right see how far it would get loaded to 18 t gross let alone as a 44 tonne gross artic.Which is why any landfill operator with any sense used a double drive Foden for the job wether 6/8 wheeler rigid or artic.While those like me who had a 4x2 Clydesdale usually needed the help of one to pull the thing out of the zb.The same applies in the case of gritters/snowploughs.
Of course heavy tippers are build on more axles, for the same reasons as americans or constructor of train engines are doing it. But did you said before, that your 4X4 snowplougs were struggling in winter? So please stop laughing at Tatra:
http://www.youtube.com/watch?v=lLxMS4tCIHk
I don’t think it was a case of you getting through to me I think it was more a case of you arguing with me about something that you actually now finally agree with me about now that I’ve successfully used the train locomotive example. 
But no the Americans don’t ‘just’ use double drive units to reduce axle weights,although that’s one of the reasons,they also use them because,even if they were using just one heavier loaded drive axle,as I’ve said previously,having two reasonably loaded drive axles is better than having just one heavier loaded one,after factoring in the halving of the tractive effort required to be applied at each drive wheel to overcome the inertia of the overall weight,compared to having just one drive axle.Which is why,as I said,a double drive Gritter/Snowplough doesn’t struggle as much as a ‘4x2’.I never actually mentioned a 4x4 gritter/plough at all which really is just an expensive,complicated,flawed ( considering that a steer axle can’t be loaded to the same weight as each drive axle can ) way of doing the same job that a 6x4 one could do.While the fact is the idea of 6x2 and 4x2 tractor units is just a compromise that’s all about saving a few quid in purchase,fuel,payload and maintenance costs. 
