Artic Tractor Axle configurations

In the US all artic tractor have 2 drive axles. Here our truck are heavier and have only on drive axle, why is this?

Also there are loads of axle configurations. The merchant that hauls our grain has some with the centre axle driving and a full sized rear axle, some with the rear axle driven but a full sized centre axle and some a driven rear and the centre axle with very small wheels. What's the story?

In the UK there are different applications of a three axle tractor unit, 6x4 (six wheels sort of, four driven), is a double drive as mentioned which are predominently used for off road or heavy haul. There are then 6x2 configurations (six wheels sort of, two driven)which come in different forms. The most common in a driven axle at the rear with a mid lift axle just in front, some manufacturers also offer this axle as a steering axle known as a "twin steer". Then there is the recent trend towards the mid lift axle being smaller, it's designed to provide the benefit of running at full 44t weight whilst minimising tare weight of the vehicle-usually used on bulk carrying applications such as tankers and tippers. Finally there is a tag axle configuration, this is where the drive axle is ahead of a rear lift axle. The idea with these (I think)is to offer a better turning circle but they don't really sit right when lifted- the fifth wheel is often behind the centre point between the axles giving an odd drive when empty. I've driven a few but never really thought the benefits were that great. Hope that makes sense, it's the best I can do in a hungover state and happy to be corrected.

To confirm, rear drive mini axle saves about half a tonne on a scania, I was told by the boss of a company we work with who has them.
We have all mid lift scanias, we used to have a mid lift Volvo. The normal sized mid lift scanias have twin steer, volvo's and Renaults, don't, I think. As far as I know, dafs and ivecos have a steering midlift, not sure about mans or mercs. Any of the tiny wheel mid lifts don't steer as far as I know.

We also have one tag axle scania. The idea is that you get better traction as the weight of the trailer is pressing down on the drive axle mainly (especially if you drop the air out of the rear axle), rather than on a normal unit where plenty of weight is pressing down on the mid axle and the front axle too - owing to fifth wheel being positioned forward of the drive axle. Plus, as mentioned earlier, a shorter turning circle. I don't like driving it empty, but I think it drives better than anything else we have when the axle is down. I always try make it wheelie by holding the lift axle up when getting it loaded and accelerating off. It does have a habit of under steering like crazy when accelerating if it's loaded, particularly if accelerating around a roundabout, with the axle on the ground. Though probably not much worse than our fixed steer midlift Volvo.

However, I don't think our tag axle scania has any better traction than our mid lift Volvo. The tag axle scania does weigh a good chunk less than the mid lift ones we have, maybe down to it losing all the lift axle steering equipment.

I'd love to have a go in one of the quarry spec or logging spec double drive artics, especially one of the long wheel base ones (which I assume understeer less than the normal wheelbase midlifts and tag axles). I know the tag axle bullnose scania t cab my grandad drives doesn't understeer and rides well, despite being a tag axle, and I'd put it down to its longer wheelbase.

And we (Sweden in my case) run heavier loads than the UK and the U.S aswell I think. 60 ton maximum weight in Sweden, I used to run steel for a living, sometimes with a 4 axle, 38 ton load fully laiden, 17 meter long trailer attached to a 6x2 Volvo, with tag lift, mid lift are not that common in Swedish sold trucks here tbh. In my very limited driving experience driving mid lift I thought it understeered alot more than a tag lift, that being said the mid lift was a short wheelbase Scania, and all the other ones were long wheel base Volvos, to handle all that weight.

I used to load bales onto straw merchants' trucks as a summer job. The rear lifts were favoured by those running artics because lifting the rear axle transfers more weight to the driven axle than lifting a mid axle would. Handy when trying to get through a sticky gateway when you've trapped the only nearby towing vehicle in the field behind you.

Wow, are those solid sided trailers the 17m ones? They look considerably longer than our new 15.65m trailers. Is that a 4x2 Volvo on the top photo? Yard shunter?

The longer wheelbase tag axle volvos you have look to be quite a bit longer than our long tags, even our double drive heavy haulage stuff, but it might be the angle the photos were taken from.

True, mid lifts are useless in winter conditions, espacially if you run loaded, takes more weight off the driven axle making it understeer and spin alot.

In the frozen North we run a max of 46500kgs with a tri axle or 64000 in B train config , max of 5500kgs on the steers . Very antiquated technology this side of the pond , discs have only recently appeared on trucks . Fuel economy is a relatively recent concern , super singles , 6x2's , and lift axles are becoming more popular at last . Current ride 2013 W900 6x4 ,

This^^.

I could really push on the return runs home while empty, even on the twisty bits, especially with the really long wheel base on mine.

Haha, so true, one of the Nowergian guys, spoken to him on a few occasions does that thing, cowboy hat, boots, american flag in the cab, the works