The other side
of the suspension
COIN

To get the other side of the story, we forwarded Goodyear's claims to Jim Campbell, chairman of the technical committee of the Institute of Road Transport Engineers (IRTE). While he stresses that his response is based on his personal views, he is confident they reflect the situation in the road freight industry arising from practical experiences in the use of air suspensions over the years. What his comments show is what FleetWatch has often found - that supplier claims are not always fully in sync with the hands-on realities faced by operators. Here is Cambell's response:

While the comments and claims made by Goodyear are fair reflection of the advantages of air suspensions, the successful use of the equipment is greatly influenced by conditions under which the air suspended vehicle is to operate - and there a number of points relating to the use of air suspensions that the potential user must be made of.

1. There may be a tare mass penalty when comparing air suspensions with newer generation light-weight steel suspensions.

2. Air suspensions cannot be readily retro fitted to existing vehicles or trailers as major and expensive surgery to the chassis is usually required to accept the air suspension components. It is therefore preferable that air suspension is specified at the time of the initial build of the new trailer.

3. Air suspension equipment available in South Africa is usually more expensive than conventional steel suspensions. However, I understand this may not be the case in some of the source countries such as Europe or the USA.

4. The claimed ride benefits of air suspension over steel suspension are more apparent when the difference between laden and unladen mass on an axle increases. For example, note the number of new generation trucks using air suspension on the driving axles while retaining steel suspension on the steering axle.

Jim Campbell '...as always, it is a matter of choosing the correct specification and equipment for the operation and job to be done.'

5. As air suspension does not have inherent damping characteristics as does a multi-blade leaf spring, separate dampers (shock absorbers) are required for all axles fitted to air suspensions. Further, the design of the damper is critical to ensure optimised damping characteristics for both upwards and rebound movement of the axle. Too soft a damping effect does not allow sufficient control in the laden condition and too stiff a damping action can cause the suspension movement to lock up under certain road conditions. Experience has shown that with an incorrectly spec'd damper fitted to an air suspension unit travelling on rough road surfaces - such as a gravel road with corrugations - the heat build up in the dampers can lead to damper seal failure and subsequent loss of the oil in the dampers. This, in turn, results in loss of control over the vertical movement of the suspension and can lead to instability of the vehicle or trailer as a whole.

6. Further, tests undertaken in the past by the Road Research Laboratory in the UK have indicated that air suspended vehicles with worn shock absorbers can actually cause more damage under certain conditions that steel suspended vehicles.

7. A typical air suspension/beam axle unit has an inherent anti roll characteristic, with the trailing arms, generally quarter elliptic single or multi-blade springs, together with the axle beam forming an extra heavy duty roll bar. This means that an upward movement of one side of the axle, when travelling over a hump in the road, will tend to lift the other side of the axle as a result of the anti-roll characteristic. The can have obvious beneficial effects but can lead to problems under certain road conditions as indicated in the point below.

8. There has been poor experience in this country in the use of air suspensions in off-road conditions such as timber or sugar cane haulage for reasons other than stated above. The inherent anti roll characteristics of a typical air suspension/beam axle unit can have severe negative influences if the tyres on either side are not carrying a reasonably equal load. For example, if the vehicle is crossing over a raised hump in the road, or through a donga, set at an angle to the direction of travel, each side wheel will attempt to rise or fall at a different time. This action of alternating vertical movement sets up tremendous forces in the interface between the trailing arm and axle beam, and the connection point of the trailing arms to the chassis brackets. This is not normally a problem with a typical multi-leaf steel suspension as there is usually sufficient flexibility in the springs to absorb the relative movement of the axle. On four axle drawbar trailers using air suspension, the above forces have also led to excessive forces to act on the turntable ring leading to greatly accelerated wear on the ball races and even failure of the turntable mounting bolts. The same accelerated wear characteristics have also been experienced on drawbar trailers to a lesser degree. While there have been a number of proposed solutions to this problem, it has never been fully resolved and this has prompted a number of transporters to revert to steel suspension.

9. While air suspended trailers used on-road require a similar amount of maintenance as steel suspension trailers, it is certainly my experience that off road trailing equipment requires a higher level of maintenance to the extent that one suspension manufacture called for all suspension related bolts and 'U' bolts to be re-torqued every 10 000 kms - which was on an almost weekly basis!

Notwithstanding the above comments, air suspensions have a definite place in the South African road transport scene as the positive aspects detailed in the original article definitely provide user benefits and, as always, it is a matter of choosing the correct specification and equipment for the operation and job to be done.

A further interesting point is that as air suspension ensures better equalisation of load between coupled axles, some European countries allow increased axle loads when "road friendly" suspensions are used as there is a reduced factor of road damage when using these types of suspension.

Local tests carried out at Gerotek some years ago showed that there is a reduction in vertical "G" forces of up to 21/2 times when comparing air suspension to steel suspensions. This results in obvious benefits to the chassis superstructure and the load carried - and of course, the road over which the vehicle is travelling.

Unfortunately, however, as yet we have not been able to persuade the local legislative authorities to consider differential axle load allowances dependant on the type of suspensions used.