is a measure of the hardness of cured rubber. Hardness has an effect on
how the rubber grips, how it resists flexing back and how quickly a
tire built from it will wear. A tire’s hardness is checked by using a
durometer gauge, which has a needle point that penetrates the rubber and
reads its hardness on a dial.
The durometer gauge is named after instrument maker Albert F. Shore.
There are several different scales of durometer used for materials with
different properties. The two most common scales are the Type A scale
for softer compounds and the Type D scale for harder plastics. Motocross
tires are measured using the Shore A durometer and given a number
ranking that represents their position on a scale from soft to hard. The
higher the durometer number, the harder the rubber compound.
Hard compound rubber registers 70 to 80 durometers. Soft compound rubber reads in the 60 to 70 range. Anything less than a 60 durometer (a very soft rubber compound) would be too flexible to use in motocross knobs. As a rule of thumb, softer rubber compounds are heavier because they contain more oil in their formula. Soft compound tires can weigh as much as one pound more per tire (for an equal amount of rubber).
But don’t be fooled by durometer readings. Just because a tire’s rubber is 60 durometer on paper doesn't mean it will consistently measure at 60 durometers. The knobs could read between 58 and 62, which is considered acceptable quality control. For the best overall tire performance, the durometer readings between a cold tire and a hot tire should remain as close to the original durometer number as possible. While it is true that all rubber gets soft when it's warm, what you want is a minimal drop in the durometer reading. Rubber that remains stable gives the most consistent traction during the course of a moto.
Many factory race tires use dual-compound treads. These tires use a higher durometer rubber in the center of the tire’s contact patch for increased durability in the part of the tire that wears out first. The sides of the tire are molded with a softer durometer rubber for better cornering grip. There are also dual-compound tires that use two different durometers of rubber from the top to bottom of the knob's height. The goal with this design is to use the hard compound at the base of the knobs to keep them from flexing and the soft rubber on the top of the knobs for increased traction. Production versions of dual-compound tires have yet to make it big, but they do exist. The Bridgestone M401/402 is an example of a dual-compound tire with a stripe of harder center knobs, while the Bridgestone M59 has dual-compound knobs.
Although you wouldn’t think it, intermediate tires use the same rubber hardness as hard terrain tires (a durometer reading in the 60 to 70 range). The tread pattern is what delineates a hardpack tire from an intermediate tire.
A sand tire uses the firmest 70 to 80 durometer rubber compound. Although sand tires are the easiest tires to design (because they work in an arena of plentiful traction), there are design considerations. (1) On cement starting pads, a softer and grippier rubber works best. Since sand tires, by their very nature, are comprised of hard rubber, tall knobs and wide tread spacing, they don’t like concrete starts. (2) Choosing the correct hardness for sand tire knobs is a delicate dance. If the rubber is too hard, the knobs can break off on hard patches of dirt. If the knobs are too soft, the knobs will flex too much and lose traction. Smart tire designers use as soft a durometer as possible and increase the size of the knob base for more support for the tall knobs.
Rubber compounds can affect tire pressure as the heat builds up in a tire. The classic example of this is with soft compound tires. Because soft rubber has more adhesion with the dirt, it generates more heat. Heat causes the tire’s air pressure to increase. Hardpack tires can heat up as much as four pounds in a long moto. The pressure starts to build up on lap two. In this situation, it's common to drop the starting tire pressure by one to two pounds (in an effort to reach a compromise for the majority of the race).