DO WE FOLLOW EUROPE?
	Lightweight air suspensions do give a tare mass advantage but this is not the main purpose of their fitment.

Following the editor's challenge in the last edition for technical boffins to get involved in the issues of this industry, Paul van Heerden, GM, trailer products for Meritor Automotive has taken us up on the challenge by joining the debate of air versus mechanical suspensions. What is welcoming is that while he delves into the technical characteristics of the debate, his bottom line of concern revolves around the issue of road damage. 'Should South Africa introduce European-style Road Friendly Suspension Legislation to help save our roads? That is what he is asking?

As far back as the 1950s, the American Association of State Highway Officials (AASHO) performed extensive testing on many different configurations of vehicles. The tests incorporated the use of different suspensions fitted to a range of medium and heavy vehicles which were run on different road surfaces to accumulate a total of over 17 million miles of road tests.

The results indicated that the damage caused to road surfaces was related to the 4th power of the static axle load. In simple terms, a car produces an axle load of approximately 500kg per axle and a heavy vehicle produces a load of 9 000kg per axle when legally laden. This means the heavy vehicle will produce 105 000 times more road damage than the car.

It does, of course, get more complicated than that. There are, for example, theories of spatial repeatability that compound road damage. In other words, when it comes to the number of axles on the road or the space between the axles in the case of a tandem or tridem unit, the time these combinations allow for the road to recover when the tyre rolls over it is also taken into account.

What also came out of these tests is that road damage is further exaggerated with the use of mechanical suspensions due to the resonance created by the mechanical spring and the deflection over the length of the trailer. This causes ripples in the road as several trailers of the same configuration will resonate at the same frequency when travelling on the same piece of road. Air suspension eliminates this.

Road Friendly Legislation
The Europeans obviously took note of all these results and their legislation was written as 'Road Friendly Suspension Legislation'. However, the only way to cost effectively achieve the parameters of this legislation was via the use of air suspension. This legislation only applied to drive axles and trailer axles - not the steer axle of the truck tractor.

In this configuration, the vehicle is allowed to run on a cushion of air and thus has no inherent damping characters. Dampers are therefore fitted to stop the axles bouncing which would cause further road damage. When a vehicle hits bumps, no damping is required on the upward stroke of the axle. However, the axle must be prevented from bouncing back. This means that dampers - not shock absorbers - are fitted with approximately 10 times more damping in the return stroke as in the compression stroke, thus not damping the cushioning effect of the air suspension.

Sufficient evidence exists to prove that air suspension does reduce road damage.

In the Road Friendly Suspension Legislation, it stipulates that not more that 50% of damping can be achieved using coulomb friction. This means that using the friction of the spring against the hanger bracket must do less than 50% of the damping.

This can only be done by combining air suspension and damping. The vehicle is stabilized by the use of what we call a flexible link where the axle acts as a torsion bar or stabilizer. When the axle hits a bump on one side only, some of the torsion induced in the axle must be absorbed or the axle will break. There are various ways of doing this. To name a few, axle seat rubber bushes, tri-functional bushes or parabolic trailing arms.

Further stability in extremes is done by fully compressing the tri functional bush and the rigid trailing arm acts as the stabilizer. In the case of the parabolic trailing arm, the spring resistance increases thus stabilizing the trailer. Stability is further enhanced by having limited cross flow in the leveling valve between the two air springs, which controls the ride height of the air suspension.

Round or square?
There are various arguments for and against a round axle and a square axle under torsion as the axle is a torsion bar or stabilizer. However, it is a proven fact that round axles of the same size and wall thickness perform better under torsion than square axles. However, the connection of the spring seat on a round axle to connect it to the trailing arm is critical (weld).

Air suspension is the answer for off road conditions but this must be stipulated up-front in order for the correct air suspension to be specified. Centers of gravity, track lengths and trailer frame centers are critical when the specification is set up. There is a solution for every application.

Certainly air suspension does induce more stress on the axle and connecting parts and therefore regular maintenance must be performed. However, if the correct suspension is specified for the application, then maintenance will be done at normal intervals.

In South Africa, the cost of an air suspension is probably two times the cost of a mechanical suspension. The fact is that most of the air suspensions are fully imported in South Africa whereas mechanical suspensions are locally made. The heaviest and most costly piece of the air suspension is the trailing arm and there are no spring manufacturers in South Africa with the equipment to manufacture parabolic springs.

In South Africa, the cost of an air suspension is probably two times the cost of a mechanical suspension.

European Legislation of Road Friendly Suspensions has forced the transporters in Europe to use air suspension - and there is a payload advantage both through reduced tare of around 70kg (depending on application) and via legislation which allows an extra ton per axle. Should South Africa be doing the same for the sake of reducing road damage? Should we not have 8tons per axle on mechanical and 9tons on air suspended axles?

The real issue
The question is not whether there is a definite place for air suspension in South Africa but what can we do to reduce or eliminate road damage. Europe did not introduce Road Friendly legislation just to sell more air suspensions. It was done to save money in maintaining their roads. This is the real issue here.

There are, of course, other factors that come into the picture - overloading and type of tyres being two of these factors. Do you know that a super single tyre produces 26% more road damage than twin wheel tyres due to more force being exerted on the road. And here's an interesting fact: The road maintenance costs for the UK were more than the cost of new roads before this legislation was introduced.

Air suspension does come at a premium - and although there are systems on the market which one can retrofit onto a vehicle, this does take surgery. However, the bottom line question that should be asked when debating this issue is: What is the state of our roads going to be in the near future?

On show at last year's Auto Africa was the Henred Fruehauf/Hendrickson self aligning air suspension with the Henred Fruehauf disc brake axle. According to general manager Moses Naidoo, the demand for air suspension has increased in some applications - mainly bottling and soft goods. However, the price premium still plays an important role. "It's still early days for customers to see the benefits," he says.

If the use of air suspension protects our roads for the future, then let us move in that direction and let the government introduce an axle loading incentive such as the Europeans truckers enjoy to encourage the move forward.

Reference: 
Les Price: Road Friendly Suspensions, October 1993.