The longer semi-trailer (LST) trials commenced in 2012 and involved 14.6m and 15.65m trailers, equating respectively to total vehicle lengths of 17.5m and 18.55m. One of the non-negotiable technical requirements of LSTs is when connected to a tractor unit they meet the standard minimum turning circle requirement as set out in UK Construction and Use (C&U) regulations. That means that articulated vehicles with an overall length of 15.5m or less must be able to turn within concentric circles with radii of 12.5m and 5.3m.
Steering axles have become synonymous with LSTs. But as trailer manufacturer Don-Bur’s technical support specialist and marketing manager Richard Owens points out, they were not a DfT minimum requirement for this type of trailer. He explains: “To maintain standard C&U turning circles, a wheelbase of 8,135mm cannot be exceeded, although there can be an increased tail swing of up to 0.74m. A quirk in the C&U regulations allowed a standard single bogie to be split into two: a forward tandem axle bogie with an 18-tonne capacity and an added single axle bogie set further back, rated at 10 tonnes, so a rear-steer axle has been the solution. The steered rear axle is typically needed to firstly ensure the trailer continues to pivot around the front tandem axle bogie and secondly to overcome the huge tyre scrub the rearmost axle would otherwise have to endure.”
Steered axles generally fall into two categories: ‘self-steer’ and ‘positive-steer’, according to Owens. Self-steer refers to a fixed axle with wheels that can be compared to shopping trolley castor wheels. As the angle of direction changes during transit, the wheel simply steers freely to prevent any resistance. As there are relatively few parts, this technology is the cheapest, is available from most of the major axle manufacturers and is the most widely adopted. There is an advisory warning here, however.
When reversing (as the wheel steering is biased to a forward direction), they would normally twist unnaturally and there would be a real risk that tyre scrub might rip the tyre off the rim. To counter this, the manufacturers have generally included a locking system so that, when the trailer is reversing, the angle of steer can be locked in a straight position, adds the technical support specialist.
Positive-steer axles [also referred to as command-steer] are affixed to turntables or are electro-hydraulically controlled, and automatically force the wheels to steer. This is controlled by the relative angle of the tractor unit and trailer. This technology reduces the transit corridor width and improves manoeuvrability both in a forward and reverse direction.
The downside is, says Owens, positive steering solutions tend to be heavier and more expensive, due to the complexity and increased number of parts and require additional maintenance, predominantly greasing for the turntable, which can be time consuming, and there are more moving parts which will wear over time. Hydraulic command steer has additional electro-hydraulic components which require regular maintenance including seal changes, oil changes and filter changes.
Another option is ‘active steer’. The term was used by the Cambridge Vehicle Dynamics Consortium in its variation of the positive-steer theme but controlled by smart electronics rather than a linkage to the fifth wheel. And an additional variant was designed by Silvertip Design working with Don-Bur in the late 1990s. Owens adds: “Active steer – whether the Cambridge Vehicle Dynamic Consortium solution or the Silvertip Design – is still very much on the table as a viable option. The regulations state that LSTs have to comply with turning circles and other C&U requirements. The method, provided it can be proved safe, is irrelevant.”
The recently introduced legislation governing the use of LSTs requires operators to plan and risk-assess any route that they intend to use the trailer on. It is here that operators need to think carefully about which type of steered axle they will specify for their trailer. While all LSTs must conform to the C&U turning circle regulations, the way the trailer tracks the tractor unit on the road, as well as reversing and low-speed manoeuvring characteristics, will differ depending whether a self-steer or a positive-steer axle is fitted.
Richard Owens explains: “Self-steer solutions only react relative to the angle of the trailer versus the angle of the road. This means that the turning effect is delayed, so the travel corridor width is at maximum because the trailer unit is just a dead dragged object. A positive- (or command-) steer on the other hand reacts as soon as the tractor unit turns. The result is a much slimmer travel corridor width when turning. There is also some less scrub on the tyres on a positive-steer because the tyres are actively steering in the direction they are going, rather than being dragged.”
Axle manufacturer BPW provides further insight: “From the amount of self-steer equipped LSTs used in the trial, we would assume these trailers are generally used as anticipated: primarily on motorways and modern trunk routes, with minimal road width, curvature and turning restriction causing manoeuvrability fears. Command-steer systems can be designed to provide steering angles that exceed self-steer axles. This is only necessary where the route is narrower or demands very tight manoeuvres. Command steering is more driver-friendly, as it operates with greater ease and control whether driving forward or in reverse. Within a tridem bogie, unlike self-steer, two axles may steer, giving even more flexibility with greater steering angles for increased manoeuvrability. Power override is also possible.”
BPW’s main offering to LST trailer manufacturers is its LL (load dependent steering stabilisation) self-steering axle. The company claims it is worthy of consideration for any duty cycle.
This is because the axle beam and axle stub are connected to corrugated thrust bearings mounted under the kingpin assemblies. When driving straight (zero position), the corrugations in the thrust washers keep the wheels on track, while the vehicle’s weight is used to keep the washers in contact, providing load-sensitive damping and ensuring optimal performance, with forces acting on the tyres when cornering is distributed more evenly.
BPW says its LL steering axle has been designed with a self-locking facility when reversing. For more challenging situations, the axle can be fitted with BPW’s Active Reverse Control (ARC). ARC is an electro-hydraulic auxiliary system which automatically controls the steering axle when reversing. The ARC is an optional extra and consists of control and hydraulic units and a steering cylinder with an integrated steering angle and rotational speed sensor. When a driver engages the reverse gear, the steering system is automatically activated via the reversing lights. Additionally, for precise movement, steering can be manually operated with a remote control, accurately manoeuvring a trailer into a defined area.
BPW steering axles (whether LL or Ackerman type) used with active steer systems require lubrication of the top and bottom pivot bearings (four points) every six weeks. Auto lube systems are not allowed with BPW LL 23-to-27-degree steer. The BPW LL axle comprises kingpin pivots and steering thrust washers. These require greasing at six-week intervals.
The company says: “Mechanical command-steer systems have more moving parts, comprising turntables and connection rods incorporating pivot pins and spherical bearings. These all require a similar greasing regime. It stands to reason that command-steer systems therefore require more time to carry out maintenance, thus increasing costs.”
It is evident that there are few restrictions on how you steer an LST bogie, as long as it meets C&U requirements. But while complexity improves performance, it also hikes cost (initial and maintenance) and lowers payload productivity. Where LSTs win is when used on trunking work where an advanced self-steer axle, such as BPW’s LL axle, will suffice. If the operation’s duty cycle demands it, there are positive- and command-steer options. As already highlighted, they have drawbacks, but will in the long term cost the operator less in fuel and tyres, so it’s worth doing the maths.
BOX: JOST EXPANDS COMMAND-STEER AXLE RANGE
Following the market launch of a command-steer axle in a 9t capacity version, a 10t axle has been added to the portfolio in 2023. Like the previous model, the CSA 10T (previously named ZGA) can also be used in all conventional steering systems whether mechanical, hydraulic or electronic. Its low roll radius is said to reduce the required actuation force. This removes the need for additional batteries and also provides greater flexibility when designing steering systems. The Jost CSA can integrate with other TRIDEC-brand steering systems. Up to three Jost command-steer axles can be used. The steering angle can be adapted to the available space for installation and the required maneuverability, according to the supplier.
BOX: AXLE WEIGHTS
Use of LSTs will be restricted through Special Types General Order (STGO), which will include LSTs as a new category. This means all new or existing LSTs must be fitted with onboard weighing to operate legally over 38 tonnes gcw, according to equipment supplier Axtec. A six-month transition period for those operators who are already running LSTs under the trial to ensure that they have an onboard weighing device fitted finishes at the end of the month.
In response to the Department for Transport’s (DfT) mandate to fit axle weighing equipment to longer semi-trailers, Axtec has launched a new version of its Axtec OnBoard Load Indicator.
This new, weather- and vibration-proof version of Axtec OnBoard is now available specifically for LST combinations. The standalone trailer system also communicates wirelessly via Axtec’s new Connected Load Indicator Protocol (CLIP) system. CLIP automatically connects to any tractor unit fitted with Axtec OnBoard. The whole combination, with its axle weights, will then be displayed on both screens, in the cab and on the trailer chassis.
To adequately guard against overloads, the tractor unit must also be fitted with Axtec OnBoard, although the system on the trailer will still operate independently when parked, or when used with a tractor unit that is not CLIP-enabled. This complies with the new law, and the system can be installed on new trailers, or retrofitted.