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Commercial Motor has taken delivery of a new triaxle trailer
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Page 33
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and we are replacing our tandem-axle semi-trailer for the start of our 1989 test programme.
i• Our new trailers replace units supplied and maintained by Britain's foremost trailer manufacturer, Crane Fruehauf. They were put into service in 1983 so that we could test tractive units at the newlyintroduced 38-tonne maximum weight. Both semi-trailers have proved very reliable and still have many years of service left in them.
In the intervening years, specification trends have undergone a fundamental change. In 1983 Crane Fruehauf had bare' ly introduced its purpose-designed 'T' air suspension as an option. Today the overall take-up is over 50% — higher on tri-axle semi-trailers, at about 70% — and says CF, almost 100% on its tipping semi-trailers.
CF initially imported the design from its former parent company in the US, but soon introduced an im proved version to match our higher weights and much narrower, twisty roads. Continued development has led to a reduction in weight of about 100kg.
The T' suspension uses a tapered forged steel spring arm to support an aluminium piston, mounted off-centre on a reinforced base. The wide spring base contributes to the semi-trailer's overall roll stability.
VERY RELIABLE
Dunlop's rolling-lobe air bag, produced to CF's specification, has proved very reliable, even in the toughest off-road conditions. While most air suspension systems incorporate at least two control valves, CF holds the patent for a single-valve system controlling the air pressure within all four or six suspension bags.
In the event of a failure in the system, the single valve design offers a distinct advantage allowing an even deflation of the suspension on to the bump stops.
In two-valve systems, a valve usually controls the pressure on one side, so if a bag fails only one side deflates, resulting in an uneven load across the axle.
The forged-steel trailing arms are bolted to the axle tube acting in torsion to provide the necessary roll stiffness. Air pressure, hence ride height, is set automatically to compensate for the load condition, but the inclusion of a manual control allows adjustment through 212mm travel to match dock heights.
Steel multileaf suspensions remain less expensive and are lighter but can't match the performance of air, CF's air suspension adds about 180kg to the unladen weight of a tri-axle trailer and due to heavier axle tubes, about 250kg to a tandem axle trailer, but it pays for its extra weight with much better load equalisation between axles. This is claimed in turn to offer improved tyre life — so much so that the extra premium can be recovered in as little as 18 months: maybe sooner where lifting axles are incorporated.
Air suspension is also much kinder to both the body and the load, and gives a quieter ride.
The compatible design of the chassis, built at CF's North Walsham, Norfolk plant, incorporates all the necessary stiffening to withstand the high roll and axle scrub forces imposed by the air suspension. Some weights quoted for proprietary systems don't take account of the extra work that may be needed on the trailer.
While allowing for an evenly-distributed load, the bogie position is determined by the king pin depth: in this instance using the British standard 900mm position with a second position at 1.350mm to allow coupling to a three-axle tractive unit for five or six-axle operations.
Both of our trailers will measure 12.36m overall, incorporating a platform length of 12.2m. This is less than the legal maximum as we already have difficulty in coupling to some Continental tractive units within the permitted 15.5m limit. Recent proposals make use of king-pin-torear-of-trailer and swing-clearance dimensions to determine trailer length. This might lead to longer trailers and consequently longer overall lengths. At 1,600mm, the king pin position is quite common on the Continent, and the French have already gone to 16.5m for artics.
The CF semi-trailer chassis incorporates medium-high-tensile steel fabricated I-beam members with a 210mm ISO neck profile increasing to 485mm depth over the main section.
Wide-flange crossmembers at 600mm centres will suit the later addition of curtain-sided bodywork. They pass through the main chassis members to meet the channel-section side raves which fit flush with the 29mm half-lapped hardwood floor. These are interspaced with the main frame box section members.
DOUBLE-ACTION
Double-action Jost landing legs incorporate the CF patented rocking feet which compensate for forward movement of the trailer if the air suspension deflates when left standing.
Skidchek antilock brake control — standard equipment on all CF semi-trailers — operates on large (420x18mm) S-cam brakes. The park brake operates on two of the axles. At the rear, a polyethylene panel houses the 24V light clusters and long-vehicle" signs.
After completion, a CF. semi-trailer is delivered either to one of the company's 12 branches or to a specialist bodybuilder of the customer's choice for the body kit
to be added, Both CM trailers are specified with curtain-sided bodies in which the front bulkhead is panelled internally with 2.65nun steel sheet. Externally the top half is protected with a GRP wind deflector while the lower portion is skinned with a 1.6mm aluminium sheet.
The rear frame is constructed to provide maximum access through a pair of 19.5mm plymetal doors faced on the inside with galvanised mild steel. Four hinges support each door and heavy-duty locking guards against racking.
CLEAR ACCESS
Steel-channel roof members at 650mm centres have hollow section front and rear headers which are gussetted to cant rails at each corner.
They are overlayed by a single sheet of lmm-thick aluminium which is sealed and capped. The roof is supported on each side by a stanchion which can be released from the side rave by a lever action and moved along the vehicle to provide clear access for loading and unloading. Curtains, running in nylon glides, are tensioned by a ratchet housed in the vertical corner members.
A cargo strap pole, installed within the roof space, provides for internal cargo restraint while the PVC-coated 900gan polyester curtains contain 50mm-wide fixed polyester webbing at 600mm centres, terminating at positive-lock buckles for tensioning.
The red and white livery is suitably inscribed with CM and CF logos so that we will be easily identified with every new tractive unit on test on our route, from the Midlands to Scotland and back.
111 by Bill Brock