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6x4 equals Foden

10th March 1984, Page 60
10th March 1984
Page 60
Page 61
Page 60, 10th March 1984 — 6x4 equals Foden
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Which of the following most accurately describes the problem?

Alone among the vehicle manufacturers, Foden favours 6X4 tractive units. Bill Brock explains the technicalities

FOLLOWING the implementation of the higher maximum weight almost a year ago, most operators' initial instincts were to make the easy choice to use the 4x2 tractive units and threeaxle trailer configuration. Through the course of experience many are now looking closer at articulated combinations, using a tri-axle tractive unit and tandem-axle trailer, to solve their weight distribution problems.

Axle drive and type of suspension used vary from one manufacturer to another, but alone among them Foden Trucks advocates the operation of the 6x4 tractive unit axle configuration in preference to the more conventionally accepted twin-steer 6x2 unit.

I recently visited Sandbach Engineering at its modern assembly facilities in Cheshire to meet two of the company's American Paccar executives, Ross Canterbury, marketing manager and Mrs Laurie Baker, who as Foden's chief engineer is responsible for final policy decisions on points such as this.

Using the latest flowline techniques, Foden Trucks builds only to order, and would need to enlarge its order book many times over to work the plant at full capacity.

Foden's thinking is clearly influenced by the American experience, which differs from the European situation. While the

British commercial vehicle industry is now served by our extensive national network of motorways, the large articulated vehicle still has to contend with narrow, twisting roads tight bends and even mini roundabouts. European commercial vehicles designers believe that the steering of the 6x4 is less suited to this environment, which also induces higher stresses on its chassis frame when compared to the twin

steer machine.

To date more than 80 UK operators have decided to operate Foden 6 x4 tractive units at 38 tonnes gvw. Ross outlined some of the reasons that in his opinion have persuaded them to take this decisive move.

Unladen weight concerns all hauliers. The better the payload offered by a vehicle, the more chance there is of making a reasonable return on a considerable investment. Differences in specification from one make to another infl uence the vehicle's kerb weight. Typically, the Foden S106 is equipped with a RollsRoyce 290L engine, Lipe Rollway clutch, Fuller transmission, Spicer drive-line, Foden FF20 rubber suspension, Rockwell axles, Eaton brakes, steel spigot wheels, Jost fifth-wheel, cornposite sleeper cab and 80 gallon aluminium fuel tank.

A bolted chassis construction incorporates high-tensile steel channel side-members with extruded aluminium crossmembers, forged aluminium gussets and aluminium bumper.

The S106, lighter than some 4x2 tractive units, is spec for spec, the lightest of all existing three-axled units available in the UK. At the same time, it offers the most load advantage through the kingpin.

Loading tolerance is indicative of the flexibility of the fifthwheel location in accommodating different types of loads and trailers. Apart from offering the highest bogie-plated weight at the full 22-36 tonnes, no other three-axled tractive unit can match it on load tolerance except for the Scammell 20.32, which is plated at 21.73 tonnes.

The 6x4 Foden can be tailored to meet trailer coupling require ments for any trailer with a 760mm (30in) kingpin position or more, simply by adjusting the wheelbase and therefore the propshaft and sider-rail specification to suit.

In altering the wheelbase di mensions of a twin-steer chassis, the manufacturer has the added complication of having to compensate drag-link lengths and recalculate the Ackerman steering angles.

Relocation of the fifth-wheel

installation to accommodate either landing leg clearances or

front swing clearance, reduces the loading tolerance. A rearward movement of just 200mm (7.9in) from the ideal position reduces the maximum imposed load potential by 12 per cent.

Foden's rear swing clearance advantage can be demonstrated by comparing the centre bogie position to the rear end of frame dimensions. The true picture exaggerates the difference, as swing clearance is determined by a diagonal measurement from end of frame to fifth-wheel pivot point.

Foden argues that the S106 offers greater wheelbase and fifth-wheel flexibility and is therefore better matched to a wider range of trailers than most if not all twin-steer models.

Ross claimed that the Foden S106 offers tractive effort that is up to 100 per cent better than twin steers, and to back up his statement he quoted from a SAE paper 811264 "Heavy truck tugtandem versus drive tandem axle considerations." This was based on tests carried out by the Western Highway Institute on a 12m (40ft) trailer coupled first to a 6 x 2 tractive unit and then a 6 x 4 tractive unit to identify maximum gradeability at 11.34 tonnes unladen and 34.94 tonnes laden.

On wet asphalt the 6x4 was able to climb a 17.1 per cent gradient loaded and 21.5 per cent unloaded, while the 6x2 managed only an 8.4 per cent gradient loaded and 11.7 per cent unloaded.

The same differential was monitored on packed snow using chains. To improve traction, most 6x2 units are equipped with devices which permit the weight on the second steered axle to be transferred to the drive axle.

While in theory this should diminish the difference between the two designs, invariably the driver does not activate the device until after traction is lost and even then duration of the transfer is usually limited.

With so many variables, maintenance costs are difficult to quantify, but by taking a 4x2 tractive unit as the standard vehicle, it is then possible to compare the difference between a 6x4 and 6 x 2.

The 6 x 4's extra drive-axle will be worked only half as hard while the 6 x 2 has a load transfer device and steering linkage to maintain. Plumbing, linkages and particularly alignment will be the more common cause of attention.

It might be assumed that a 6X4 would be harder on tyre wear, but in Foden's experience, I was told this is wrong. Tyre wear on the Foden S106 is evenly distributed from front to rear, and 120,000km (75,000 miles) life is not unusual.

With a greater number of miles travelled on motorways tyre wear is, to a large extent, a function of traction and the torque transmitted through the axle.

A greater torque transmitted through the single drive-axle will cause those tyres to wear at a faster rate, but to some extent this is offset by the wear rate of the free rolling steered-axle.

Against this, the tyres will need to be rotated between axles if an even wear rate for all tyres is to be maintained.

A restricted traction capability puts limits on the vehicle's operational roll, but it a 6x4 is involved in a lot of slow, shunting work, then tyre scrub must make its toll.

Data supplied by Rockwell, reinforced by SAE 811264, suggest that there is no measurable difference between the driveline efficiencies of a 6x2 and a 6 x 4 tractive unit. Both are approximately 96 per cent while the efficiency of a 4x2 drive line ranges from 96 to 99 per cent.

On braking, Foden was one of the first companies to introduce asbestos-free linings. The S106 employs one brake chamber size on both drive axles, activated by a single valve. Some twin-steers have a more complicated arrangement where small brakes are specified on the third axle.

The Foden has a comparatively large brake lining area of 8,787sqcm (1,362sqin). Manoeuvrability is an area in which the 6 x 4 might have been thought not to fare so well as the 6x2 twin steer, but can the figures tell a lie?

Kerb to kerb turning cicle measurements clearly show the Foden S106 at 14m (46ft) bettering the Seddon 401 at 16.38m (53.7ft), Daf 2500 at 14.2m, Mercedes at 16.8m (55.1ft) and lveco at 15.2m (49.9ft).

Handling tends to be a subjective assessment and therefore difficult to quantify. But here again the Foden engineers referred me to another SAE report, No 821260, "Yaw roll stability of heavy trucks" based on a computer simulation study, concludes that the use of additional non-steered axles at the rear of a vehicle improved stability.

In practical tests made in conjunction with a major fleet operator, Foden claimed to be able to substantiate this point and also to show that the Foden S106 at 38 tonnes was more stable than some 4 x2s at 32 tons.

One might say that with a design weight of 55 tonnes the Foden S106 is over specified. Foden prefers to look towards future developments and says that its vehicle will be suited better for 40-ton or more operation if or when the UK enjoys the full benefits of European weight harmonisation.


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