Longer semi-trailers are here to stay after successful trials showed them to be as safe as conventional designs in operation, and as well effective in reducing truck miles, and therefore pollution, cost and road risk.
Instrumental in their success has been the self-steering axle, which allows them to conform to UK turning circle requirements. This technology is also found on other specialist trailers, including low-loaders and urban artics.
Many of these trailers feature BPW axles. German-owned BPW produces product for the British market in Leicestershire and supplies most of the leading trailer manufacturers in the UK. The most cost-effective solution for most applications is the LL axle (pictured above), which requires no external hydraulic or electrical inputs.
HOW THEY WORK
The principles behind the product are not actually a great deal more sophisticated than those of the supermarket trolley castor: trailing wheels are free to pivot on a vertical axis, although the wheels of each axle are connected by drag-link to maintain the Ackermann angle and follow a radius around the same central point.
This allows the trailer wheels to conform closely to the track of the tractor unit when moving forward: benefits include not only a tight turning circle, but also reduced tyre wear and fuel consumption.
But that flexibility brings with it two problems: stability moving forwards, and the ability to straight-line and turn predictably in reverse.
The latter of these is simply resolved by locking the steering up when reverse is engaged via a pin in the drag-link actuated from the trailer’s reversing lamp circuit.
The first issue is rather more problematic, but is solved by using technology borrowed from the humble rising-butt door hinge to return the wheel to the straight-ahead position. Thrust washers incorporating cams are built into the pivots that connect the axle beam with the wheel hubs. The cams keep the wheels in line when the tractor-trailer combination is travelling in a straight line. When a cornering force is applied as the tractor unit is steered into a bend, the trailer wheels castor-steer as the force overcomes the resistance of the cams on the thrust washers. When the steering force diminishes as the combination comes out of the corner, the weight of the trailer and load serve to force the wheel pivots back down the cam into a straight-ahead position. Usefully, the heavier the load is on the axle, the more stable it becomes.
So, what can possibly go wrong? David Etty is a technical engineer working with BPW supporting British trailer-makers who fit the product. He says that the pivots themselves are reliable, but are subject to obvious wear and tear, given the high loads imposed on them and the dirty environment in which they operate. Regular greasing is a necessity, but just applying a grease line to the nipples installed top and bottom will be insufficient.
“It’s vital to get grease into the spaces between the thrust washers, and not just on to the shell bearings on which the wheel pivots,” he warns. “These washers have wearing surfaces: if they wear down, then the self-centring effect will diminish.
“The axle must be jacked up so the road wheels are clear of the ground. Then each wheel must be turned on its steering pivot while the grease is applied so the cams ride over each other. The application should continue until fresh grease appears between the washers. Ideally, you’d have the trailer over a pit.”
He recommends that the application be made at six-weekly intervals, regardless of mileage.
“Trailers that cover low mileages are often those that do the most arduous work: particularly in urban and off-highway applications.”
The type of grease used is also important. “We developed our own synthetic formula at BPW; it’s a very sticky grease designed to maintain a strong film on the cams. When it comes to lubricants, cheap really isn’t cheap.”