As riders we are perhaps more aware of the changes of road surface, than any other group of road users. The importance of finding the safest riding line and one that gives the maximum road holding and braking capabilities is vitally important to our safety.
But have you ever wondered what is actually running beneath the tyres and just how much grip is available?
This page provides an introduction to what is involved and things that you should be aware of when out riding.
On roads with a good clean driving surface there will be a high skid resistance although this can change with the weather and other factors during the life span of the road surface itself. Roads are built up through many layers until we finally reach the top dressing, the one that we ride over. This is generally Bitumen based into which is rolled an aggregate (chippings). The aggregate provides the sliding resistance between the road and the tyre.
The aggregate undergoes testing to determine the level of skid resistance. There are agreed minimum standards that determine which aggregates are used and where.
As new surfaces are laid down the size and quantity of the aggregate are measured and tested to ensure that the gap between the chippings is to a minimum standard as is the height that they protrude above the bitumen surface.
The surface of the chippings is called the microtexture. It is the main contributor to skid resistance and the dominant factor in determining skid resistance in wet conditions.
The macrotexture is essentially the gap between the chippings and serves to drain water away form the road surface.
Skidding and skid resistance
What is a skid?
A skid occurs where one or more of the tyres loses normal grip on the road. This happens when the grip of the tyres on the road becomes less than the force or forces acting on the vehicle.
The skidding resistance on a wet road is reduced by the film of water acting as a lubricant on the surface.
The relationship between the aggregate, its surface and its contact with the tyre are important in dispersing water and become increasingly important as the speeds get higher. The surface of the aggregate (Microtexture) can only penetrate the remaining water film if the chippings are sharp enough to build up a “dry” contact with the tyre.
As roads are travelled there is a polishing effect on the chippings and over time the Microtexture wears down so reducing its effectiveness to skid resistance.
Sideways forces acting between the road and the tyre allow the bike to travel through the curve. The increased amount of skidding resistance is determined by the speed of the vehicle and the severity of the curve. So at locations where there is a high level of acceleration, braking and cornering, the polishing effect on the aggregate is at its highest and so skid resistance reduces.
New road surfaces
You might think that when a new road surface is laid down that the skid resistance is at its most efficient. However, like a new bike tyre, the surface has to bed in and wear away a little to reveal the true surface.
As road surfaces are laid down the aggregate is immersed in bitumen and rolled out so that the effective surface of the chippings is not revealed and needs to be worn away though the action of traffic passing over its surface. This allows the bitumen over the chippings to oxidise and break down to reveal the rough area that effectively provides the grip.
“Slippery Road” signs will often appear at these sites and will remain for some months until the road has matured before being taken away. Do not assume that after a couple of weeks that all is well, caution still needs to be applied.
So those riders that like to ride fast and exert a lot of force through the tyres when cornering should consider that over new surfaces they could be facing a reduced level of skid resistance and the need to slow down becomes a real MUST.
This equally applies to well worn surfaces where the polishing effect has also reduced the skid resistance of the road surface.
There are many factors, including the weather, traffic flow and spillages to contend with, that can change the characteristics of the road surface.
Winter v Summer
Winter Roads remain wetter for longer during the winter months between October and March. The grit and dirt washed onto the road works in with the road surface making it harsh and so improves the grip available.
Summer, the roads are a lot drier and the debris is very dusty and assists in the polishing of the road surface and so effectively reduces the level of skid resistance available.
Therefore during the very months that most riders are out enjoying the bike, the roads offer a lesser degree of skid resistance than in the winter months.
So next time you’re out there look for the signs and understand what they mean and the information that they provide to you as a rider. Don’t take the road surface for granted.
If you know of any road problems that you would like bringing to our attention then please e-mail us.