fouraxle

Fouraxle, or four-axle, is a designation used in transportation to indicate a vehicle that has four axles in total. It is applied to both rail rolling stock and heavy road vehicles. The configuration affects axle load distribution, ride quality, traction, and allowable payload.

In railroading, a typical four-axle vehicle uses two bogies, each carrying two axles. The common wheel-rail notations are B-B in AAR terminology or Bo-Bo in UIC classification, reflecting the two-axle bogies on both ends. Four-axle locomotives and passenger cars are common in many fleets. The four-axle setup provides a balance between traction and weight, enabling greater tractive effort than lighter two-axle units and often a smoother ride on standard-gauge tracks, but at the cost of higher mass and maintenance compared with lighter designs. Track wear and axle-load considerations influence wheel profiles, suspension, and wheel-rail contact geometry.

In road transport, four-axle configurations are used on heavy trucks and tractor-trailer combinations, often with two axles on the tractor and two on the trailer, or with a tandem drive on a single semi-trailer. This arrangement increases payload capacity and helps distribute weight. However, it also imposes higher vehicle weight and stricter axle-load, length, and permit requirements in many jurisdictions. Some systems employ lift or steerable axles to improve maneuverability and weight distribution.