Old John:
I don’t know about mid lift units, as hardly anybody up here has them. Tag axles are the unit of choice in the North.
As far as the fifth wheel position is concerned, I try to have mine set so that when you weigh the outfit, loaded, on a dynamic weighbridge, the front and rear axles are bearing almost exactly the same weight. The lorry handles best when set up like this.
Mid lifts with large wheels are usually ok for weight distribution or rather you have more to play with so not likely to have an overloaded axle, far more critical to get the position right when you have small wheels on the mid, without looking at it i think my mid lift max is 5.5 tons which is higher than most makes, but i’ve seen some as low as 4.6 tons which can sail close to the wind on heavy work.
When i got a new MAN with slider i set about getting the slider in the right place for axle weights, what surprised most was the further you moved the slider back the more weight was imposed on the mid lift i’d expected it to be the other way round for some reason, idiot i am, after a few uses of the roll over axle weigher gradually inching the slider forward we’re there at just about the mid way point, no surprise that the vehicle handles noticeably better when the axle weights are where they should be.
Don’t see that many tag axles on tanker work, at least down south, i know of several in use but with some tank designs its the position of the landing legs that causes the issue due to the obvious extra long tractor chassis behind the fifth wheel of a tag unit.
I suspect the higher likelihood of bad weather tends to the preference for tag axles the further north the operator is based?
Simon mentioned a twin steer tractor in his post, i had a new TS Sed Ack in 1984 and a new TS Scania in 2006 and rated the handling of both as excellent with wet cornering far more stable and predictable than with a fixed mid axle, what happened to twin steers as you seldom see one now?