Some vehicles have one or two axles at the rear which steer passively, i.e. they are not turned by the steering wheel. For example, when a trailer is pulled to the right, the rear-steer axle pivots the wheels to the left to help the trailer turn, and to reduce sideways scrubbing on the tyres. This means reduced tyre wear (estimated at 4 tyres per 100,000km) and lower fuel consumption (estimated at up to 1000 litres per 100,000km, or 1%). It also provides for a tighter turning radius, assisting with manoeuvring.
The maximum steering angle is usually set between 8 and 27 degrees, depending on the type of vehicle. Modern systems have a steering lock which activates when driving in reverse.
The axle itself functions like a regular axle in terms of its specification – it will have its own brakes and be capable of supporting a standard axle loading. It can have single tyres or duals.
The axle may be able to be locked using a switch on the side of the vehicle.
The types of vehicles that tend to have rear steering axles are:
- Quad trailers (i.e. a trailer with four axles) – the rear one or two axles
- Tri-axle trailers (i.e. a trailer with three axles)
- 3-axle buses and coaches
- 8×4 and 6×2 rigids (uncommon)
- 5-axle prime movers (uncommon)
The rear-steer axle might also be a lift axle which can be lifted up when not required.
Rear-steer axles might be actively steered on some vehicles such as mobile cranes where a 9 axle crane could have up to 5 front-steer and 2 rear-steer axles.
Hydraulic modular trailers have every axle able to steer, but these are not passive.
In the late ’80s several cars, including the Honda Prelude, featured four-wheel steer which steered the rear axle. This was abandoned by car manufacturers fairly quickly because of its unnecessary cost and complexity.
Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists
