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09/04/2008
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Tom Shelley reports on a purely mechanical solution to steering the rear axles of heavy goods vehicles
Following the publication in February of our article, ‘Active steering corners with precision’, Yorkshire farmer George Mitchell got in touch to tell us about a purely mechanical trailer axle steering system which he has built himself and tested full size.
It uses links and bell cranks, and includes a simple method of improving handling in reverse. While a patent was granted in 1996 and the system received favourable test reviews at the time, it has yet to find a commercial backer. Meanwhile, Mitchell is continuing to improve and refine the system.
In its current embodiment, three semi-trailer axles are mounted on a turntable and, as the vehicle makes a turn, the tractor pulls the trailer round – causing the turntable to rotate relative to the trailer in the opposite sense. This acts on two link rods attached to bell cranks, which manoeuvre the front and rear sets of wheels into the turn. This eliminates tyre scrub and helps the trailer to follow the tractor, thus reducing any cutting in.
A particular feature of the system is that, in order to aid reversing, the point of connection of the link rods to the turntable can be slid across to the other side. That way, when the semi-trailer starts to move to one side, the steering wheels on it will turn in the opposite sense, tending to return it to a path behind the tractor.
A very simple system is how Mitchell sees it and one that evolved from chaining a loose axle to a farm trailer and watching the wheels follow those of the tractor in front. ‘Commercial Motor’ had many positive things to say about in a review more than a decade ago. However, of the 20 trailer manufacturers that Mitchell has contacted, only one has replied and that was to express no interest.
Mitchell believes cost may be a factor here – at 1996 prices, he estimated that it would have added £5,000 to the cost of a conventional semi trailer.
“I have now redesigned it to do away with the changeover mechanism to assist reversing. I still have the trailer, but it needs modifying. Working on Ministry turning circles, my system easily keeps within the 25m outer circle/10.6m inner circle and turns down to 6m circle, without tyre scrub or body twist. I feel there is potential for a 13.6m trailer with four axles, all steering, to allow increasing gross weight, without increasing axle weights. ”
The semi trailer prototype measures 9.5m from pin to centre axle, while the steering turntable bogie reduces the effective wheelbase by about 2m. The original article in Commercial Motor concluded: “The system works smoothly and has considerable potential.”
Pointers
* System mounts the three axles of a semi-trailer on a turntable.
* Turning the vehicle causes the turntable to move, so that links manoeuvre the front and rear wheels into the turn, minimising tyre scrub, while reducing the turning circle and cutting in
* The patented design includes a means of changing the position of the attachment point for the links, so as to assist reversing; in its latest form, this is eliminated to save cost.
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Author
Tom Shelley
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