×ばつ4

A ×ばつ4 or six-by-four is a vehicle with three axles, with a drivetrain delivering power to wheels at the ends of two of them.^[1] It is a form of four-wheel drive ^[2] but not one of all-wheel drive.

It is the most common form of drivetrain of semi-tractors ^[2] and heavy haul fixed-chassis cargo trucks in larger countries such as the United States and Australia; in Europe, ×ばつ2 and ×ばつ2 variants are more commonplace.

The First World War saw a great adoption of petrol-engined lorries, beginning to replace horse transport. These were almost all simple ×ばつ2 chassis, often based on commercial lorries or buses. Only a handful of ×ばつ4">×ばつ4 vehicles existed and these were slow and lumbering, with complicated servicing needed to their steering axle.

Between the wars there was a great interest in the development of vehicles with better off-road performance, both for the military and for the growing oil exploration industry. More driven axles were needed, for better grip, but also a heavier load capacity which itself required an additional axle. This led to designs favouring the ×ばつ4 layout with only the non-steering rear axles driven. ×ばつ4 was largely ignored at first, at least from the major truck makers.^[3] Military ×ばつ4 chassis were also easier to produce for civilian manufacturers, as they were more similar to chassis for the larger civilian market, often just with uprated components. A few aberrations of ×ばつ6">×ばつ6, ×ばつ8">×ばつ8 and even half-tracks were produced,^[4] but these were mostly as gun tractors or recovery vehicles and were too complex and expensive for General Service trucks. The standard design became a ×ばつ4 truck of 3 ton capacity,^[5] along with larger trucks of 10 tons.^[6]

With six wheels, the off-road performance often relied on articulation between the axles, so as to keep all the wheels in contact with uneven ground. A standard lorry chassis with live axles suspended by semi-elliptic leaf springs was inadequate for military use.

A solution to this was a bogie suspension. This coupled two rear wheels to a rigid frame or bogie but allowed this to tilt. The bogie pivoted freely for much of its range, allowing a greater movement than was possible with springs.^{[i] [7]} One of the best-known heavy tractor units of World War II, the Scammell Pioneer, used this bogie system.^{[8] [9]}

To encourage development, the War Office offered free use of a suspension design developed by Lt Col H. Niblett (1882–1969), then the professor of mechanical engineering at the RASC training college, Aldershot.^{[10] [11]} This gave increased and independent articulation to the two rear axles. A pair of semi-elliptic leaf springs on each side were mounted lengthwise on the chassis, cambered downwards. The axles were attached to the ends of both of the paired springs, with each suspended by what was effectively two quarter-elliptic springs, in what was a relatively conventional design. This design also gave rise to the ×ばつ6 AEC 850.

Nearly all ×ばつ4 have an unpowered leading steering axle, with two driven rear axles. However some mass-produced ×ばつ4">×ばつ4 ^[ii] vehicles have had their load-carrying capability increased by adding an additional, unpowered, rear axle.

The Carmichael Commando chassis, a stretched version of the Range Rover, was used for such extended ×ばつ4 conversions in the 1970s and '80s. Some German conversions provided full ×ばつ6 performance, but most were like the TACR2 and just ×ばつ4. This gave a vehicle suitable for airfield emergency services; with fast performance on tarmac, useful off-road performance for crossing obstacles, increased load capacity and a much cheaper price than dedicated ×ばつ6 vehicles.

• Four-wheel drive
• Six-wheel drive

• Off-road vehicles
• Car layouts
• Drivetrain
• Six-wheeled vehicles
• ×ばつ4 vehicles">×ばつ4 vehicles
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