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Could Damping on Trucks Reduce Road Damage ?

If more attention was paid by the traffic authorities to damping on trucks, South African roads would last longer. This opinion was expressed by the marketing manager of Gabriel SA, Rudi Bombal, in a statement sent to FleetWatch. Given this, he advocates that traffic authorities implement a system of checking whether trucks are adequately damped. Do you agree with him? Let's first give him the podium.

Bombal says wear-and-tear caused by truck wheels is a major factor in road maintenance costs and that road-induced bounce can be reduced by efficient damping. He adds that this becomes more pertinent when one considers that many truck operators favour using the 'super single' wheels which, he reckons, are less friendly to the road than the double wheel system.

"A truck axle which is carrying a five-tonne load but is oscillating in an uncontrolled manner may be imposing a load on the road which varies between about two and ten tonnes. Ideally, the load should move forward on a constant straight line and the wheels should move up or down to cope with the irregularities of the road surface. The closest approach to this ideal is achieved by a correctly damped air suspension. The smoother the load is carried, the less impact force is imparted to the road and the wear-and-tear is minimised on both the road and the truck."

He reckons the reason that air suspension achieves this smoothness is that it is able to match the spring rate closely to that which is needed for the load condition. This enables the wheels/axles to move freely and the low friction inherent in air suspension enhances this freedom of movement. The free axle movement necessitates good damping and means that the shock absorber has to do more work than with a leaf spring suspension system.

"Even conventional truck springing can be improved in terms of truck wear-and-tear and causing road damage if it is efficiently damped," Bombal says. "An oscillating axle or load does more damage than a controlled one and that damage is costing the trucker and road maintenance authority money."

Another view

We put Bombal's views to FleetWatch's correspondent on legislation, Jack Webster, who replies as follows:

Rudy Bombal's comments deserve much thought as there is a lot of truth to what he says but it is not the whole story! It is not denied that when a road deteriorates, the blame is usually attached to the big overloaded trucks and it is conceded that high axle massloads do reduce the life of a road before needing major rehabilitation. But there are other causes of premature break-up of roads.

During the mid 1960's, the Railways were busy electrifying the rail-line between Witbank and Johannesburg during which time the railways service was depleted and a heavy burden fell upon road transport to bring coal from Witbank to Johannesburg and Pretoria.

The vehicles used were four-axle artics with two-axle trailers and an exemption was obtained from the TPA to permit a payload of 30 tons - 20 tons on the semi-trailer and 10 tons on the trailer. After a few months, the Witbank road "broke up" and inevitably the coal trucks were blamed.

BUT – much to the consternation of the roads engineers, it was not the Witbank to Johannesburg road which suffered damage; it was the Johannesburg to Witbank road and the trucks were certainly not loaded going to Witbank. It was eventually conceded that over-stabilising of the gravel sub-base with cement plus the pounding of the road by the impact caused by the bouncing of the unladen axles was the cause.

Another example of the damage caused by undamped axles is the corrugations which appear on unpaved roads. These corrugations are caused by lightly laden, badly damped axles travelling at high speeds "bouncing" along the road. Heavily laden - not overloaded - axles travelling at reasonable road speed tend to have a compacting effect on the road. This is explained in cases when the roads are under construction and the sub-base is compacted by a heavy roller (compactor) travelling slowly over the road. Also, when the road is paved with asphalt, a heavy roller is used to level out the road.

Finally, Rudy Bombal refers to air suspension which is usually efficiently damped for stability and smoothness of ride. Technical analysis has shown that an axle or axle-unit fitted with air suspension has at least a 10% advantage against steel suspension in respect of reduced road damage.

South Africa badly needs to protect its road network which is the life-line of the country. It is time we stopped thinking we know what causes premature break-up of our roads and indulge in more through research and take appropriate action.

What do you think?

What do you operators think? What do you traffic authorities think? What do you road engineers think? Is Rudi Bombal's call for the traffic authorities to implement a system of checking whether trucks are adequately damped a sensible or practical one? Add your voice to the debate. Write to the editor at P.O. Box 3097, Honeydew, 2040 or e-mail us on: fleetwatch@pixie.co.za.

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