Self-tracking axles can give supermarket trolleys a
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life of their own, so putting them on semi-trailers L.
could seem decidedly risky. However, some fleet operators are going for them in a big way and reaping the cost benefits, as Move danne reports.
nybody who has ever spent a happy hour or two trundling a supermarket trolley around Asda or Tesco might well query the value of '0 self-tracking axles on semi-trail ▪ ers. Neither locked in place nor • steered positively, the little wheels on these trolleys head deter?, minedly in every direction except • the one you want. Surely there's a danger that a selftracker—usually fitted at the rear of the trailer bogie—will do the same, but with rather more dramatic consequences? And doesn't that negate the supposed benefits of greatly improved tyre wear and fuel consumption, and reduced stress on the trailer?
The apparent waywardness of self-tracking axles—sometimes referred to as self-steering only becomes an issue when the driver has to back up, explains Andy Richardsbn, engineering director at Don-Bur: ''Then the self-steer has to be locked into position, because if you don't do so, the wheels will swing to one side."
It can be locked manually or automatically. BPW's self-steer, which was introduced in 1991, is among those which can be installed with automatic locking—a single-diaphragm, midaxle mounted air-operated brake chamber.
A solenoid valve is fitted in the air line and connected to the reversing lights circuit. When reverse is engaged, the valve exhausts the air from the chamber, and the self-steer locks.
When reverse is disengaged, the locking mechanism is released. The system BPW uses means drivers don't have to pull forward 5 or 6 metres to bring the wheels into line before the lock is engaged.
The automatic approach also ensures they don't leave the selfsteer locked in place all the time because they can't be bothered to climb out of the cab and release it.
Paul Billson, an engineer at BPW's UK headquarters in Leicester, says some makes of axle have to be locked straightahead at some forward speeds otherwise the truck may become unstable.
Andy Richardson argues that investing in self-steers leads to lower tyre, fuel and trailer maintenance bills, with far less risk that high lateral loads on the chassis frame and suspension will cause premature wear and tear.
That's because with self-tracking, one set of trailer wheels will always be heading in the right direction rather than slipping sideways when the driver has to swing forwards into a tight depot entrance, for example. They'll suffer from far less scrub than their stablemates, and won't use more fuel to overcome their resistance.
Cornering
BPW contends that not only does the driver enjoy better manoeuvrability with its LL self-steer when heading forwards, but cornering safety is improved too.
"That's because the combination exhibits favourable understeer characteristics, similar to passenger car design," says a BPW spokesman. "And up to lo% less space is required by the vehicle when it negotiates bends."
Hauliers operating at high gross weights have a further reason for favouring self-steers, says Andy Richardson. They cart move the trailer bogie further back and impose more weight on the kingpin, thus improving weight distribution without breaching the turning circle regulations, Changes to the regulations on r January this year, mean that fitting a front lift axle to a maximum length semi-trailer can create too wide a turning circle, warns Paul Billson. Having a self-steer installed moves the steering centre to between the front and middle axle of a triaxle bogie, and should ensure the vehicle complies with the rules. Have one installed in conjunction with a front lift axle, and with the axle lifted the steering centre shifts to the bogie's centre axle, Billson adds.
A self-steer will add around 20 okg to the weight of your new trailer, and L1,800 to i'z,000 to the cost.
So why fit just one self-steer? Why not fit two, or even make all three axles on a triaxle bogie selftracking? "It's possible to make two axles on a triaxle bogie self-steering, although at present we haven't installed any," Richardson replies. "Make all three self-tracking, and you'd lose some straight-line stability."
"Fit two self-steers and you'd need two rigid axles to maintain stability," BPW's Paul Ralson says.
Newark, Nottinghamshirebased temperature-controlled transport specialist Phil Hanley is convinced of the virtues of selftrackers. The firm has just put eight Schmitz Cargobull 13.6m refrigerated triaxle trailers fitted with them into service, and runs 35 trailers with self-trackers. The ones it favours are made by BPW.
Dramatically reduced tyre wear rather than enhanced fuel economy is the key argument in their favour, says fleet engineer David Furnival. "With a normal triaxle trailer we'd get too,000km out of the rearmost tyres, i6o,000km out of the fronts, and 350,000km out of the centres ." he explains.
"With rear steers you're looking at 350,000km out of the rears, and 300,000km out of the rest."
BPW self-steers need little extra maintenance, says Paul Billson. The kingpins require lubricating every six weeks unless an automatic chassis lubrication system is fitted. Bear in mind that they require Class 2 or Class 3 grease," he points out.
"You get some wear between the steering wedges, but they'll cover 1,5m km before they need replacing."
The weight penalty is of little consequence for Phil Hanley. "We've had them on every trailer we've ordered over the past three years, and our entire trailer fleet should be on them in three years' time," says Fumival.
"And with BPW's system, the self-tracker's wheels flick to the centre the minute reverse is engaged."
Another fan of BPW self-steers is Swatragh, Northern Irelandbased tanker operator BP McKeefry Transport. It specified them on two Thompson Carmichael tankers which went into service last September, transporting sewage sludge.
"The greatest saving is on the rear axle tyres, which are well on their way to lasting twice as long, with major savings also being seen on the front axle," says Brian M cKeefry.
A third operator to opt for self steers is groupage specialist StIS Services of Tonbridge, Kent, with SAF self-steers on all but four or five of its 32 trailers.
The 19.5in wheels on one triaxle double-deck used on regular runs to Birmingham carrying palletised cargo are shod with tyres that have clocked up 204,000km, and still have 7mm to 8mm of tread left.
Fleet engineer Peter Golding believes self-steers probably provide some fuel economy benefits, but cannot quantify them. Most hauliers who've used them believe slight benefits can be realised, but add that better tyre life is a more important.
Popular
Don-Bur's Andy Richardson believes self-steers have become noticeably more popular as OE fittings over the past 12 to 18 months, while Mentor calculates that they feature on around 15% of new trailers sold in the UK each year. However, not everybody is persuaded that there is a swing in their favour.
Brendan McIlvanna, sales manager at SDC Trailers, says operators aren't always prepared to pay extra for them when profit margins are so thin: and they may not be convinced that the savings promised will be realised.
BPW, SA F distributor Industrial Machinery Supplies (1MS), and Mentor all agree that there is little or no demand for self-steers to be fitted to trailers already in service. It seems slightly odd," says Sophie Wright, sales and marketing co-ordinator at IIvIS. "After all, the benefits of self-steers far outweigh the cost."