Tight manoeuvre 08 December 2010

Truck and trailer steering are seeing significant improvements. Dave Young travels to Eindhoven to test drive Tridec's latest hydraulic innovations

Rear steer trucks and trailers are far from new: operators have specified them and trailer builders constructed them for years. But most have been self-steered, mechanical designs, driven from the kingpin by simple levers and built to lock in straight at high speed and in reverse.

While these have worked well enough, there has always been the bulk and weight – and hence also payload – penalty. And there has been that irksome maintenance problem: never easy when we all know that semi-trailers are among the most neglected vehicles on haulage fleets.

That is why Tridec's (the world's largest independent maker of truck steering systems) latest products are so interesting. Starting with its HF-S, which has been designed for lightweight distribution trailers, it scores over previous designs, because it is hydraulic. That means lower maintenance, less weight and less space – in fact it occupies a very small area on the swan neck above the fifth wheel. Further, it can be bolted on, rather than welded in, meaning that, if there is a component failure, it's much easier to whip out.

The HF-S also works without electronic control and, according to Tridec, gives "high directional stability and precise steering". Using single master and slave cylinders and hydraulic stub axle steering, rather than a rim bearing, not only means negligible payload penalty, but also the potential for fitment where positively steered axles would be impossible – for example, on sliding floor trailers or tankers.

What's more, it can be used as a steered single axle or third axle in a triple-axle suspension, or mounted on a tridem set-up, with steered first and third axles and a reversed steering angle. This latter gives an axle spread of four metres, which is handy since it reduces the possibility of individual axle overload due to multiple deliveries from the rear of the trailer.

As for the mechanism itself, it comprises a revolving base with integrated kingpin, camming wedge and built-in master cylinder, a valve block, the hydraulic steering cylinder on the axis and a pressure accumulator. Effectively, it is a displacer steering system, in which the dual-effect master cylinder operates the axle steering in a closed system. Be advised, however, HF-S can only be used with single tyre equipment.

This steering equipment is going to be attractive in all sorts of delivery applications, from forestry to supermarkets, where the yards are often not quite big enough. It's not just about manoeuvrability, but also reducing tyre scrub, saving damage to the yards, and reducing strain on the half shafts and kingpin.

Moving on, though, Tridec's WST electronically-controlled, hydraulically-activated steering could soon feature in the UK, since it is designed for rigid trucks with tandem axles. Yes, there are already plenty of lift and steering axle systems, designed to improve manoeuvrability on three-axle 24 tonners that are increasingly replacing two-axle 18-ton rigids, for example, but these, too, are all mechanical. So this is a useful development – and it could also work well on 6x4 and 8x4 chassis employed by utility contractors in tight urban settings.

That's not what WST was designed for. Dutch C&U regulations allow a four tonne payload increase if the two drive axles are more than 1.81 metres apart (hence the three axle grab loader tipper pictured, at 32 tonnes gvw). However, the long wheelbase reduces manoeuvrability and increases turning circle and tyre scrub. So, by designing a steering system for the last drive axle, driveability is significantly improved, even off-road.

In operation, a sensor transmits the steering angle of the front axle to the system's computer, which calculates the steering angle for the third axle and controls its hydraulic cylinder via a valve block. Another sensor on the steered drive axle then provides feedback. And it works: when we attempted to take a rigid tipper through a skidpan slalom course, it predictably understeered off the track. With WST engaged, it easily and safely completed the slippery course.

Tridec is currently fulfilling a Scania commission to equip three-axle vehicles with the WST electronic-hydraulic system, for the Dutch fire service.

That said, also of interest to UK operators, if the proposed EU regulations restricting trailer height to 4.0 metres is enforced, will be Tridec's HF-O steerable single wheel suspension, aimed at double-deck trailers. Because of the absence of axle beams, a floor level of around 320 millimetres becomes possible, which allows two 1.80 metre-high loading decks, each offering space for 1.75 metre euro-pallets. An option also allows the loading floor to be lowered flat onto the road surface using the air suspension.

Tridec claims that the HF-O system is easy to fit. All parts – controller, pneumatic springs and shock absorbers – are supplied as modules, with the geometry pre-set in the factory. Also, the wheel suspension is low maintenance, having no spring clamps or male spring connectors – meaning retightening bolts and exchanging male spring connectors is unnecessary, and there are no spring leaves to wear and break.

Author
Dave Young

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