Quick Dips

Road Wave

"Bow waves" in roads result in significant economic and environmental costs, according to an ongoing roading study.

Heavy trucks create a "bow wave" in a road, which moves ahead of each wheel and causes a flatter dish under and behind the wheel. This uses more fuel because truck wheels are always trying to climb out of a "groove". The strength of the road and the load being carried by the wheel determines the depth of this groove.

This "bow-wave" effect is a preliminary finding of a six-year study, by Opus International Consultants, Central Laboratories and the Department of Mechanical Engineering at Canterbury University, into how roads interact with tyres.

Graph showing the actual pavement distortion under a tyre for a weak, medium and strong pavement. Note the axes of the graph are different scales to exaggerate the effect, with the x-axis in metres and the y-axis in millimetres

Most of New Zealand's sealed roads are low-strength, "flexible" pavements covered with thin, bituminous chipseal surfaces. These roads are comparatively cheap to build, but require careful management and timely maintenance.

"This approach contrasts with typical practice in most developed countries where the emphasis has been on high cost, high strength "rigid" roads such as concrete or thick asphalt," says researcher Peter Cenek of Opus.

The research is, therefore, looking at how the flexible nature and high texture levels of New Zealand's mainly chipseal roads affect the cost of running cars and trucks. Cenek says a "bow-wave" of just one millimetre pushes up the fuel use of heavy trucks by 13%.

The research has shown fuel use of trucks to be sensitive to the size of the bow wave, with the roughness of the road having only a secondary effect. But cars' fuel use is directly related to the texture of the road surface. The coarser the texture, the more fuel is used.

A reduction of four percent in fuel use was measured for about a one millimetre decrease in road surface texture depth.

The research is now turning to finding out how chipseal surfaces affect tyre noise, tyre tread wear, skid resistance, splash and spray and ride quality, and how important these are to motorists.

This will allow chipseal roads to be better designed and built to reduce vehicle running costs, while retaining their inherent safety characteristics.

A view of the truck used in the tests.