If I understand you right: yes. In the ‘80s, at least, you could have your 38-tonner on a tri-ax tractor and tandem-ax (or tri-ax if you want) trailer; or you could have a tandem-ax unit with a tri-ax trailer: 3+2 or 2+3. So either your eight-tyred rear cluster or your four-tyred rear cluster digging into the road.
I’m talking about ‘80s but I suspect CF is talking about the present so things might have changed. A modern-day twin-steer is so much more sophisticated than 40 years ago that such a set-up may indeed be a better bet than 6x4. But, like I say, my whole piece in the above posts is about the past.
The point is single drive and lazy (6 or 8x2) is mutually exclusive with equally sharing the load across the bogie without severe loss of traction.It still means a heavy drive axle ironically possibly even heavier if air dump is used or needed to get enough traction for the extra gross weight in the wrong contitions..In our case the lazy axle mid lift or rear tag is usually just a relatively light weight helper auxiliary to provide relatively similar drive axle weight at 44t gross, than 4x2 running at 40t gross.Obviously air suspended and controlled to control the amount of axle load on the ‘helper’ so as not to compromise traction on the single drive.Which really confused my steel suspension mindset for a while getting my head around the retrograde alien thinking.
I never understood the flak I got for daring to point out the mathmatical advantages of double drive or why.
Welcome to the 1970’s!
Air suspension is generally lighter, and is much more adaptable. Generally it gives roads a longer life, I would think cheaper on truck maintenance too?
There are advantages to double drive, anddisadvantages.
Generally they are more expensive, and heavier, and consume more fuel.
If not needed they are a waste of money.
They are nice to keep in your back pocket, but for UK on road use, not often needed.
I didn’t know about AEC, but air suspension, in various guises has been around for over a hundred years!
I chose the 1970’s because that is when I remember seeing air on trucks on UK roads, rather than as rare oddities. I wasn’t driving but I seem to recall seeing Volvo 6 wheelers with the 3rd axle in the air. Amazing sight at the time!
That is not very precise admittedly, but I hope my point was made.
I remember seeing one, and assuming it was some sort of suspension failure. Seeing others later did show the error of my guesses. I would put that about 1975-ish. Is that when they were first sold in the UK?
