Three axle bus

A multi-axle bus is a bus or coach that has more than the conventional two axles (known as a twin-axle bus), usually three (known as a tri-axle bus), or more rarely, four (known as a quad-axle bus). Extra axles are usually added for legal weight restriction reasons, or to accommodate different vehicle designs such as articulation, or rarely, to implement trailer buses.

An early example of a multi-axle bus was a one-off three-axle Crossley Condor, shown here, built for the 1932 Scottish Bus Show.

Usually vehicle licensing authorities of different countries will set legal limits on the amount of weight that can be put on each axle. In the UK, a recent extension to the legal limit on the length of rigid buses and coaches has led to the increased use of three axles to accommodate the heavier chassis and passenger load. Certain countries apply exceptions to vehicle rules for specific operations.

Extra axles may also be provided on shorter buses and coaches to accommodate extra equipment loads, such as passenger lifts, or comforts such as toilets, televisions, climate controls. In addition, in some cases the need is bus cargo transport, when large cargo compartments and heavy weight of cargo needs extra axles.

Adding axles to chassis designs is done for specific reasons such as weight or legalities, as having extra axles means extra costs for the operator in terms of tyre replacement, and to an extent, higher fuel consumption.

In some buses the rearmost axle is connected to the steering, with the rear most set steering in the opposite direction to the front axle. This steering arrangement makes it possible for the longer triple axle buses to negotiate corners with greater ease than would otherwise be the case.

Tri-axle double-decker buses are common in high capacity operating environments where articulated buses are not used, such as Hong Kong and Singapore. Tri-axle buses also see high volume use in Dublin, Ireland. Imported tri-axle buses were used initially by Megabus for high capacity low cost intercity travel in the UK.

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Wikipedia