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F , lexibility is the key to successful container haulage and
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is the reason the majority of UK skeletal trailer sales are those where the chassis can be ortened or lengthened—called tendible, rolling or sliding ,gie skeletals.
The complication is necessary cause operators need kit that n carry containers ranging )rn. ISO 20ft, 30ft, 4oft and 45ft .im, 9.1m, 12.2m, 13.7m) sizes 13.6m (44ft 7in) swap body forIts. Some skeletals .ave two posi tions—closed and extended— while others have an intermediate position to help operators get the weight distribution correct when carrying heavily laden centrally. mounted short containers.
To accommodate all common container sizes, sliding bogie skeletals need no fewer than 14 twistlocks. This sounds a lot, but is necessary to ensure containers of varying length can be secured in the correct position to comply with legally permitted overhangs and tractor-trailer swing clearances. The front kingpin-to-twistlock :.-entre-line measurement varies 'rom 7oo-85omm and many rafters also have pivoting or slidng rear bolsters to ensure the -earmost twistlocks are in the cor-ect position for longer bodies. It's not just the various sizes hat complicate container iaulage—container weights can ilso vary widely. Although the veight uplift to 44 tonnes gross las helped remove payload contraints (a typical skeletal unladen veight now varies from 4,700j,90 okg), operators must still nake sure trailers meet the .ccompanying rules. The usual iroblem at 44 tonnes of ensuring nough weight is put through the ingpin to comply with the 24anne maximum weight limit on he trailer bogie is made more cute with sliding bogies because he critical kingpin-to-centre-of-ailer-bogie measurement can be ltered to suit the container being arricd.
rypical scenario
Ihn Deeming, technical sales ianager at S DC Trailers, escribes a typical scenario: When the skeletal is carrying a oft or 3oft centrally-positioned 3ntainer, the centre line of the Dntainer is behind the centre ne of the trailer bogie. A heavily [den container [and they weigh p to 29 tonnes] can impose so nich weight on the trailer ogles, the landing legs actually me off the ground. Even when le trailer is coupled, the tractor nit is not carrying enough eight, so there's lack of traction a the tractor unit."
One solution adopted by SDC id other UK skeletal manufactrers is to have either a load ansfer system, where air is
dumped out of the front trailer axle, or a lift-axle. Load transfer is designed primarily to improve traction when manoeuvring. Usually, the driver simply presses a button which exhausts the air on the front trailer axle, effectively shifting the centre of gravity forward. The air bags automatically re-inflate once the vehicle reaches a certain speed, between TO and i5kph.
On S DC skeletals, the load transfer system is standard, with the ELDAS (Extendible Lift Axle System) optional. When the skeletal is completely closed with a fully laden 20ft container on it, better weight distribution is achieved if the front axle is raised. Apart from increased traction, tyre wear is also minimised, points out Deeming.
"The operation of ELDAS is not determined by speed, but by wheelbase length. The trailer will always comply with both the turning circle and swing-clearance parameters. because once the trailer is extended beyond a given wheelbase, a valve is activated which automatically lowers the axle."
Dennison claims to be the first manufacturer to have introduced twin front twistlock positions on a slider in anticipation of legislation changes regarding rear overhang and swing clearance, and also to be the UK market leader in sliding bogie skeletal sales. Twin twistlocks are now available on Dennison fixed skeletals. Fitted with double bolsters the trailers can carry a 45ft container using the rearmost pin positions on the front and back double twist locks. With the pins in the front locations the trailer can handle a 13.6m or a 4oft container within legal dimensions and axle loadings. The current standard model slider skeletal has 14 retractable twistlocks and somm deeper chassis beams that allow an allbolted mudwing to be fitted, reducing the need for welding. There is also a lighter fabricated steel pivoting rear bolster.
Flexible skeletal
Dennison also produces a platform skeletal (PSK) which is basically a conventional platform trailer fitted with twistlocks. It remains popular with operators that need the flexibility to carry other types of loads.
GT Fruehauf's new Multiliner skeletal is a result of a user survey which identified ease of use and protection from operational abuse as key requirements. For example, the rollers on which the chassis slide are now wider, covering the whole width of the flange, and have phosphor-bronze bearings with an external grease feed. The top flange is i8mrn thick for smoothness and has a stainless steel surface to ensure a flat surface for the rollers to run on and to eliminate in-service problems of paint chipping and rust.
A lighter fabricated steel pivoting rear bolster weighs a third less than the previous design, says product manager Trevor Chapman, while the pin-andchain locking system used on earlier (and some rival) designs has been replaced with what GT Fruehauf calls a gravity lock. This
is a hook which automaticall engages as the bolster is pivote, into position. "True, the rear bo. ster is only used when driver need to carry longer container5 but they often have to flip it ove in the dark or in bad weather, points out Chapman. "Making lighter helps and there's n searching for the pinto secure it.
All the electrical and brak lines are neatly housed in a flex ble caterpillar-style conduit calk an Energy Track, and the twis locks are galvanised wit chromium-plated tops to enabl them to resist knocks better.
Double twistlock
Schmitz Cargobull announced i intention of competing in the U skeletal market at the CV Show i March, showing its 44-tonr design adapted for the UK ma ket. The trailer is unusual in ha ing a colour-coded double twit lock position at the front, and single lock in the chassis with sliding rear twistlock. The fir front twistlock is 850mm forwa: of the kingpin and is used for z ISO containers, while the secor lock is 7oomm ahead of the kin pin and is used for 13.6-met swap bodies. "If you have tl double twistlock at the front, 37( also need a method of lining the rear twistlock to suit the co tainers. We think our system easier for drivers to operate," sa Schmitz Cargobull's UK sal manager, Colin Ess, "and there less potential for mistakes."
Montracon has also modifi its sliding skeletal trailer, whi now incorporates a load trans] system. The chassis main r centres are now roomm wider increase stability and because t actual chassis is wider, the twi lock overhang is reduced, affol ing them better protection. T rollers are bigger and the positi has been changed so there more overlap between the fr( and rear chassis sections, maid opening and closing the trai easier. The rear twistlock ope tion is improved—the locld mechanism is now a sprii loaded slam lock, so there is need to align locking pins.