945 Dual front axle steering

Operating large rigid trucks with heavy payloads makes it necessary in addition to utilizing tandem axles at the rear to have two axles in the front of the vehicle which share out and support the load.

Both of the front axles are compelled to be steer axles and therefore need to incorporate steering linkage such as will produce true or near true rolling when the vehicle is driven on a curved track.

The advantages gained by using dual front steering axles as opposed to a single steer axle are as follows:

1 The static payload is reduced per axle so that static and dynamic stresses imposed on each axle assembly are considerably lessened.

2 Road wheel holding is improved with four steered wheels as opposed to two, particularly over rough ground.

3 Road wheel impact shocks and the subsequent vibrations produced will be considerably reduced as the suspension for both sets of wheels share out the vertical movement of each axle.

4 Damage to one axle assembly or a puncture to one of the tyres will not prevent the vehicle being safely steered to a standstill.

5 Tyre wear rate is considerably reduced for dual axle wheels compared to single axle arrangements for similar payloads. Because the second axle wheels have a smaller turning angle relative to the foremost axle wheels, the tyre wear is normally less with the second axle road wheels.

A major disadvantage with dual front axles is that it is unlikely in practice that both instantaneous centres of the first and second stub axle turning circles will actually intersect at one point for all angles of turn. Therefore tyre scrub may be excessive for certain angles of steering wheel rotation.

Dual front axle steering geometry (Fig. 9.29) When a pair of axles are used to support the front half of a vehicle each of these axles must be steered

Twin Steering Axle Geometry

Fig. 9.29 Dual front axle steering Ackermann geometry

Twin Front Axle Steering Mechanisms

Fig. 9.30 Dual front axle steering linkage layout with power assistance

Fig. 9.29 Dual front axle steering Ackermann geometry

Fig. 9.30 Dual front axle steering linkage layout with power assistance if the vehicle is to be able to negotiate a turning circle.

For a dual front axle vehicle to be steered, the Ackermann principle must apply to each of the front axles. This means that each axle has two wheels pivoted at each end of its beam. To enable true rolling of the wheels to take place when the vehicle is travelling along a curved track, lines drawn through each of these four stub axles must intersect at a common centre of rotation, somewhere along the extended line drawn between the tandem rear axles (Fig. 9.29).

Because the wheelbase between the first front axle is longer than the second front axle, relative to the mid-tandem axle position, the turning angles of both first front wheels will be greater than those of the second front axle wheels. The correct angle difference between the inner and outer wheels of each axle is obtained with identical Ackermann linkage settings, whereas the angle differential between the first and second axles is formed by the connecting rod ball joint coupling location on both relay drop arms being at different distances from their respective pivot point.

The dual steering linkage with power assistance ram usually utilizes a pair of swing relay drop arms bolted onto the chassis side member with their free ends attached to each axle drag link (Fig. 9.30).

The input work done to operate the steering is mainly supplied by the power cylinder which is coupled by a ball joint to the steering gearbox drop arm at the front and the power piston rod is anchored through a ball joint and bracket to the chassis at the rear end. To transfer the driver's input effort and power assistance effort to both steer axles, a forward connecting rod links the front end of the power cylinder to the first relay drop arm. A second relay connecting rod then joins both relay arms together.

A greater swivel movement of the first pair of stub axles compared to the second is achieved (Fig. 9.31) by having the swing drop arm effective length of the first relay AB shorter than the second relay arm A'B'. Therefore the second relay arm push or pull movement will be less than the input swing of the first relay arm. As a result, the angular swing of the first relay, 0 = 20°, will be less than for the second relay arm angular displacement, 0' = 14°.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


  • jens
    How work the twin front axle steering system?
    2 years ago

Post a comment