Driving the Yankee 41-tonner
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From January 1999 UK artic operators will be able to run at 41 tonnes on six axles. But while the industry holds its breath on what the Treasury will charge on VED, CM has been to the US to drive the only truck in the world already purpose-built for this configuration.
by Danny Coughlan • A standard American artic grosses 80,000lb (36.32 tonnes) on five axles. And the 3+2 combination makes few demands on suspension type. A proposal now before the US Senate calls for 90,000lb (40.86 tonnes) on six axles with air suspension. The suggestion, from market leader Freightliner, the Daimler-Benz subsidiary, also calls for an increase in the permitted trailer length from 53ft (16.2m) to 58ft (17.7m). This is coupled with the addition for the first time of an overall length limit of 69ft 6in
(21.2m), around 600mm shorter than existing US bonneted combinations.
The proposal aims to improve efficiencies, reducing the number of trucks on the road and tackling an eight-year driver shortage. There are also road wear and safety gains, says Freightliner.
As with UK proposals, air suspension is required on all but the front steer axle. In this case the extra axle appears at the rear of the trailer as a steered unit, designed to crab sideways in tight turns. Other features fitted by Freightliner in an attempt to sway regulators include low-profile tyres, electronically controlled brakes and a radar-based collision warning system—all of which are commercially available in the US. And, conveniently for Freightliner's marketing department, the proposal requires a forward-control tractor to stay within the new overall length limit. Optional safety features include a lane departure warning device, and disc brakes.
Trailer Proposals All the proposals have been brought together in Freightliner's Safety Concept Vehicle: this uses the latest Argosy tractive unit, which has a riveted aluminium-built cab that meets European ECE R29 safety standards, and includes a driver'sside airbag. The trailer is a Wabash National van with a load volume of around 136m3. Like the tractor, it runs on 19.5in wheels. Our test truck was loaded to 5.44 tonnes on the front axle, 15.42 tonnes on the tandem drive bogie, and 19.96 tonnes on the triaxle trailer, for a GCW of 40.82 tonnes.
A drive-axle weight limit has kept US operators on doubledrive bogies. The benefit here is to keep the individual drive axles to just 7.72 tonnes. The 15.44-tonne (34,0001b) bogie weight is seen by Freightliner as essential for good traction, even with ASR anti-spin control.
John Prescott's plan to 'maintain a single 10.5-tonne drive axle looks doubtful given Freightliner's test results and the experiences of some UK operators with lightly laden 6x2 tractors.
Dimensions aside, the 41tonne combination feels little different to an average 38-tonner. In its favour there's an extra set of brakes at work, and the three extra tonnes have no measurable effect on engine braking where a Jake or similar compression-type engine brake is used— we tried another truck along the same route at close to 38 tonnes and found no noticeable difference in this respect.
However, there's a real change powerwise. Three tonnes may not sound much, but when you're dragging one extra axle, acceleration is noticeably slower. Those going for 41 tonnes will need to think about power ratings if they are going to maintain performance. A 330hp 38-tonner has about the same power-to-weight ratio as 360hp at 41 tonnes—the test tractor was fitted with a Cummins M11, but can take any proprietary engine up to 600hp.
Traction
Ride and handling also change in the move to 41 tonnes. The six-axle ride is smoother, but thought must be given to the control of lifting axles for extra traction, and whether or not twinsteers will offer the best directional stability when fully laden. A lifting pusher axle ahead of the drive axle may still prove the simplest and lightest solution in the UK.
If traction proves a problem we might see the re-emergence of the light 6x4 as a 41-tonne solution. Such tractors were previously offered by Foden and Renault mainly for stability— the 4000 and Magnum ranges have been offered with super single tyres to tanker and fridge fleets in particular.
Tractor Freightliner Argosy 6x4 with llOin (2.79m) bbc (bumper to back of cab) raised-roof sleeper ca h. Wheelbase: 4.65m. Front axle rating: 5.448kg (12,0001h). Drive bogie rating 15,436kg (34,00 01b). Fifth wheel height: 990mm. Unladen weight 7,270kg. Engine: Cummins M11.