With turntable steering the whole axle, including its suspension, is mounted to a turntable; when driving through a bend the whole axle rotates.
The main advantages of this steering configuration are that they are relatively easy to manufacture/install and that axles equipped with single & twin tyres can easily be used.
These are the characteristics of turntable steering for consideration:
- The forces in the mechanism are much higher than in an axial pivot steering system, in some cases up to five times greater.
- More space under the trailer is needed, i.e. the chassis must be manufactured to include the fitment of turntable(s).
- During steering, the tire track and spring centre decreases. This means that the stability of the semi-trailer is greatly reduced by comparison with an axial pivot steering system, especially when fitted to tank trailers.
Axial pivot steering system axles
In this system, the body of the steering axle is fixed, and has rotating points (kingpins) at each end. The main difference an axial pivot axle has as a comparison with a self-steering axle is that the kingpins are positioned on the axle centerline, instead of ‘offset’ as they are on a self-steer axle.
These are the characteristics of an Axial pivot steering system axles for consideration:
- There is very little tire track decrease during steering with the spring centre remaining the same resulting in maintaining stability.
- Because of the small track decrease, the wheels stay outside the chassis, even during full steer, so that the semi-trailer chassis can be lower. Again, bringing better stability.
- Simple to mount.
- Lower axle spacing compared with turntable steering.
- Lower weight.
- Lower influencing forces required to steer a trailer.
- Higher cost at capital purchase, due to the need for the higher engineered steering axle.
A self-steering axle has the points of rotation (the kingpins) located in front of the centerline of the axle. This type of axle must always be used in conjunction with a fixed axle, this is to ensure that a ‘fixed point of pivot’ is there when sideward forces create the self-steer demand. On a semi-trailer with a self-steering axle, the wheelbase will always pass through the centerline of the fixed axles.
Because of friction on the road surface, the wheels of a self-steering axle will always turn until they point in the direction of the turning point of the combination. This type of steering is mainly used on semi-trailers where it is necessary to make the wheelbase so long that, without them, legislative requirements would not be met. Alternatively, they can be used to save tyres.
These are the characteristics of self-steering axles for consideration:
- They give a relatively cost effective form of steered semi-trailer.
- The axle must be locked when the vehicle is reversing.
- They can be susceptible to vibration, which can increase wear on tyres and bearings.