try for a C con:versi When general-purpose 44-tonners hit the
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street they'll be rolling on six axles. This will doubtless boost demand for 6x2 tractors. Which begs the question, should you buy new trucks or add an axle to your 4x2s? Chassis manufacturers and conversion specialists are busily preparing the tools for the job; Sharon Clancy reviews the options.
Chassis conversion specialists have always had a pioneering spirit, working with operators to convert standard chassis to boost payload and productivity. Mid-lift axles, for example, were developed not by truck manufacturers, but by independent specialists such as Granning and Phoenix in response to operator demands.
The arrival of 38-tonners on five axles in 1983 proved a turning point for the axle conversion companies as operators had to convert either two-axle tractors or tandemaxle trailers to take advantage of the higher limit. Converters have never looked back, but it's doubtful whether they can expect a similar bonanza if gross weights are increased to 44 tonnes at the end of the year.
44-tonne limit Some operators will doubtless want to take advantage of the 44-tonne limit by converting their 4x2 tractors to 6x2s, but many 4x2s simply lack the power to make the extra axle worthwhile. And conversion specialists will face stiff competition from the chassis manufacturers themselves.
ERF and Mercedes-Benz are among those with 4x2 tractors specifically designed to take a bolt-on third axle. ERF's 4x2 convertible EC tractive unit is essentially a 4x2 built on the 3.48m wheelbase of the 6x2 chassis. This built-in "convertability" can even be extended to engine technology. ERF points out that the Cummins Celect electronically controlled Mll engine can now be programmed at one power rating for lower gross weights, then reprogrammed to deliver up to 405hp for running at a higher weight.
Granning Lynx and Phoenix are among the best-known suppliers of axles and suspensions for heavy truck conversions. Some axle manufacturers, such as Gigant of Germany, will supply independent converters with axles; others, such as Hendrickson, prefer to work with the original chassis manufacturers.
Dealers are another source of innovative ideas: North EastTruck and Van, for example, has its own specialist engineering department.
At the IRTE show last month Granning unveiled a range of drum-braked mid and tag axles that are said to be 20kg lighter than their predecessors. They're built using new production methods that were developed for the disc-braked axles that are expected later this year. The latest axles have heat-treated seamless axle tubes, friction-welded spindle ends and cold-forged, induction-hardened, drop-centre beams. Capacities range from 7,500-13,500kg.
The 400-Series drop-centre axle is popular for car transporter conversions and municipal chassis, says Granning.
Axle converters have been doing brisk business with the car transporter companies as cars have been getting heavier. Tripledeck drawbar outfits designed to carry nine or 11 cars could no longer do so without risking axle overloading. Adding a third axle was not practical because of the Construction & Use regulation limiting body overhang to 60% of wheelbase.
But now rigid vehicles can comply by meeting the swing-clearance turning circle requirements that is applied to artics...the trick is to squeeze the axle and suspension into a limited space. Granning's 400-Series axle meets this challenge with a single offset wheel and hub, giving it a compact snub nose that provides maximum track width.
Gigant had to modify the air-bag mountings on the axles it sold to East Cumbria Motors to uprate 40 Mercedes-Benz 1834L chassis with Lohr bodywork. The standard mounts occupy the same space at the bottom of the air-bag used on the Lohr bodies to raise the deck. Gigant's four-tonne axle is plated at three tonnes on 17.5in wheels shod with 215/75 tyres: it lifts gross weight on the prime mover to 21 tonnes.
Phoenix says its SK drop-beam axle solves the problem of lifting axles fouling the underside of chassis. Instead of the pronounced Ushape found on other drop-centre axles, the SK has a gentle curve that provides more space to lift the axle without striking the chassis, even when air is dumped from the suspension or the axle hits a speed bump or ferry ramp.
Phoenix has been working with Iveco Ford on an increasingly popular configuration: rear axles that are positively steered using hydraulic actuators on the axle. The original rear-steer configuration was developed for drinks company Carlsberg Tetley. Installed on a 15-tonne Cargo chassis, it boosted payload potential to an impressive 12.5 tonnes.
A rear-steer 6x2 retains much of the manoeuvrability of a 4x2 because it has a smaller turning circle than a fixed-axle 6x2. The drive axle is moved forward to accommodate the steer axle so the wheelbase is very short: typically 4.50 or 4.88m compared with about six metres for a fixed-axle 6x2. Tyre scrub is reduced because the axle steers when reversing as well as going forward.
When cornering, the long body overhang results in a quicker and bigger tail swing-out than most drivers are used to, but Ken Brewis, sales engineering manager at Iveco Ford, says it is no worse than on a 12m bus. Nonetheless, this is one reason why other manufacturers predict the main application for rear-steer axle conversions will be on 4x2 bulk tankers used on milk collection or domestic heating oil deliveries.
ERF, for example, has developed a rearsteer option that uses ZF's Servocom hydraulic steering system, while Leyland Daf has just introduced a right-hand-drive version of its rear-steer FAN chassis.
Leyland Daf product engineering manager John Beveridge says that in many cases front axle capacity and tyre sizes will reduce a theoretical 26-tonne GVW to a practical 25.5 tonnes. Foden is adapting its rear-steer conversion on the 3000 for the new Alpha.
Electronically controlled Phoenix and Iveco have come up with an electronically controlled rear-steer axle. Tests have been completed and Iveco hopes it will be available later this year. One benefit is that positive steering will be possible at speeds up to 40km/h: few hydraulically actuated rear-steer axles operate above 15km/h.
Some operations, such as drawbar trailers, benefit from some positive steering at speeds up to 40km/h, but this requires electronic control. Another possibility is that the tag axle can be made to steer crab-like, helping manoeuvre into tight spaces.
For the past year two Volvo FL7 chassis with electronically controlled rear-steer axles have been in service on milk-collection work with Fane Valley Farmers in Northern Ireland. Gigant supplied the axle from its French SAE subsidiary and developed the steering with Mobil Elektronik, a German electronics company. The Vehicle Application Centre at Leyland handled the conversion work.
An electronic control unit behind the front axle controls two angle transducers and a hydraulic valve that steers the rear axle. The wheelbase steer-angle limits are programmed into the control unit, but can be changed using a lap-top computer. Should any part of the system fail the axle simply functions as a conventional self-tracker.
Higher payloads are not the only reason for converting 4x2 chassis into 6x2s. Pickfords has taken delivery of a Volvo FL6 17-tonner pantechnicon converted into a 6x2. By running with three axles on 19.5in wheels (instead of two axles on 22.5in wheels) the floor height is lowered by 150mm. The extra axle does lift the design gross weight to 20.4 tonnes, but Pickfords has dowplated it to 17 tonnes to reduce its Vehicle Excise Duty bill. Volvo extended the wheelbase to 5.8m on-line; TVAC added the axle later.
Maximum payload is not the be-all and end-all, however, as subcontractors working for Pioneer Concrete have discovered.• Drinkwater Chassis Engineering at Leyland has converted seven Foden 3000 6x4 26tonne chassis to four-axle 29-tonners by adding a lifting Granning 7.1-tonne axle ahead of the drive bogie.
The wheelbase has been extended, but it's still 750mm shorter than a typical eightlegger. Pioneer points out that they are cheaper to run than eight-wheelers, and the axle spacing at 29 tonnes means the VED is lower than on a 26-tonne 6x4, let alone an 8x4.
The Fodens can take 7m3 of concrete with the axle lowered; a cubic metre more than a 6x4 and one less than on an 8x4 (see Fleet Focus, pages 46-7, CM18-24 June). CONTACTS • Chassis Developments: 01922 410880.
• Drinkwater Chassis Engineering: 01772 456888.
• Gigant: 01858 434813.
• Granning Lynx: 01925 810400.
• North East Truck and Van: 01642 290290.
• Phoenix: 01279 501176.
• The Vehicle Application Centre: 01772 457116.
• Wheelbase Engineering: 01254 53278.