Technology: What do you measure with a Durometer?
You can directly relate a durometer to the tires of your dirt bike. The device, which resembles a dial indicator, does nothing other than measure the hardness of rubber. It is not common to check this on the rubbers of your motocross bike, but it tells you a lot about which tires you are on the road with.
Tires are marketed in hard, medium or soft variants. But this is not a hard and fast rule. You may and may use a tire in various circumstances because they can be used more widely than you would think. The hardness of rubber in a tire is expressed in durometers. A durometer is a device for measuring the hardness of rubber and other soft (plastic) materials.
It goes without saying that hardness affects the grip, resistance to bending and life of the tire. The hardness of a tire is therefore checked with a durometer, which has a needle tip that penetrates the rubber and reads the hardness on a dial. There are different durometer scales used for materials with different properties.
The two most commonly used scales are the Shore Type A scale for softer compounds and the Shore Type D scale for harder plastics. Motocross tires are measured with the Shore Type A durometer and are given a number value that represents their position on a scale from soft to hard. The higher the durometer number, the harder the rubber compound.
Hard rubber indicates 70 to 80 durometers. Soft rubber has a durometer between 60 and 70. Anything with less than 60 durometer is too soft for motocross use. To make it even more difficult: Intermediate tires have the same rubber hardness as hard terrain tires.
So from a purely technical point of view they are the same tires, but the rubber composition may differ. This means that the use of a Durometer in motocross is rarely used. In sports such as F1, where tires play a more crucial role, it is used. However, hard and intermediate tires have different tread profiles, tread spacing and tread heights.
Rubber compounds can affect tire pressure as the heat in the tire increases. The classic example of this is tires with a soft rubber compound. Because soft rubber has more grip on the surface, it also generates more heat. Heat causes tire pressure to increase.
Photos: Danny Hermans, Huub Munsters and archive
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