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How to improve the way our road’s grip is measured

FEMA’s Dolf Willigers explains why the current way of measuring the road’s grip is not good enough for motorcycles.

Some years ago, as part of a North-European tour I made that year, I rode quietly with my wife riding pillion on secondary roads from Gdansk to the small city of Inowroclaw to visit the Polish Vision Zero event where I was to give a presentation.

Suddenly, in a bend of the road my back wheel slipped. No harm done, but I had to promise my wife to avoid secondary roads in Poland as much as possible. Every motorcyclist must have experienced this at least once: totally unexpected your bike loses grip. Usually it is just for a split second and no harm is done, but this unexpected sliding can lead to loss of balance and a crash. The cause of this can be badly executed repairs, joints that are filled with slippery materials like bitumen, wear of the road or pollution by oil or gravel. The road authorities are responsible for the maintenance of the road and for a minimum level of skid resistance (grip, friction) to ensure road safety. To guarantee a minimum skid resistance road surfaces are periodically measured on this. And this is where – in our opinion – things go wrong.

In Europe, the way how the skid resistance is measured differs in every country, but the principle is more or less the same: a partially braked wheel is attached to a trailer, water is sprayed in front of the wheel on the road and by measuring the rotation and the resistance of the wheel the braking skid resistance can be calculated in a very precise way on distances of 5 centimetres. In practice all these results of tiny stretches are averaged on distances of 20 to 100 metres. Another way that is more and more replacing the way that I just described, is to fit a partially braked wheel under a trailer at an angle of 20 degrees to the driving direction. With this method not only the braking grip is measured, but to some extent also the lateral grip. Here too the reports are on stretches of 20 to 100 metres. Both methods have in common that in countries with right hand traffic the measuring is done on the ultimate right track of the road, although sometimes also the left track of the right lane is measured. Of course, in countries where the traffic drives on the left, it is the other way around.

Grip measurement: a new standard needs to be developed and adopted.

To understand the importance of grip it is good to know some basic facts. The first thing is that friction is determined by the structure of the road surface, the pressure on the wheel (vertical force, mainly determined by the weight of the vehicle) and the horizontal forces, caused by changes of speed and/or direction. The structure of the road surface can be distinguished into macro texture and micro texture. The horizontal forces are determined by accelerating, braking and lateral changes, for example by cornering. As long as you don’t brake or corner, there is theoretically no friction: the tire just rolls over the pavement. In practice there is always some resistance and therefore a very small friction. It is important to know that two different horizontal forces at the same time need the same amount of grip as one horizontal force to prevent the vehicle from skidding. To put it in other words: when you corner you use a certain amount of the available grip to do this manoeuvre. When you also brake during that manoeuvre you make use of the remaining part of that grip. When you brake hard you may need more than the available (remaining) skid resistance and the tire(s) start to skid. One of the reasons while it is never a good idea to brake or accelerate too hard in corners and why modern motorcycles have ABS and/or anti-slip systems fitted.

It is obvious that a road surface with a good skid resistance is crucial for road safety, although it is a much forgotten element in reports about accident causes. The famous MAIDS report from 2005 on motorcycle accident causes, mentions that 29.7% of one-sided motorcycle accidents happen as a result of sliding, but other reports don’t mention sliding as an accident cause at all. There the cause is e.g. too high speed. In a survey of FEMA that was part of the RIDERSCAN project, 79.6% of the respondents replied that the road surface (including skid resistance) was one of the main infrastructure problems. It also goes without saying, that a sufficient skid resistance is more important for motorcycles than for cars. Firstly, because motorcycles (normally) have just two wheels: lose grip with one and you are already in trouble. Secondly, because a motorcycle is a balance vehicle: losing grip not only means sliding but also possible loss of balance and a crash. This is why it is important that the skid resistance measurements of the road authorities meet our needs too, which isn’t the case now.

Motorcycles make use of the whole width of the lane: to make themselves visible to other road users, to create a safe distance to other vehicles, to follow the ideal line in corners or just because they have learned to take position on a certain part of the lane. Where they usually do not ride is exactly the part that is measured most: the track that is closest to the road side. Any slippery spots that are outside that track are not measured and thus not reported to the road authorities. To measure the whole width using mechanical means like a partially braked wheel is impossible.

Side-way force measurement with a partly braked wheel attached to a truck on an angle of 20 degrees.

However, there are ways to get at least an indication of the skid resistance of the whole surface. Some of the trucks that are used nowadays are already fitted with laser equipment and camera’s. With the laser and camera equipment it is possible to measure the macro texture of the road and to detect potholes, rutting and other unevenness.

Measuring with laser, which also detects potholes, cracks, rutting, et cetera.

Next to the way the measurements are done, the average reported distance of 20 to 100 metres is too long for motorcycles and need to be reduced: a few metres of slippery road surface can be enough to lose grip. Sometimes stretches of 5 metres with a skid resistance that are below the standard (of the road authority!) are already reported, but usually no action is taken. To ensure that improvements are applied by all road authorities a new standard needs to be developed and adopted, at least for the roads which fall within the scope of the (to be revised) Road Infrastructure Safety Management Directive.

Written by Dolf Willigers

Click here to download Dolf’s full report ‘Skid resistance measurements and motorcycles’.