How We Test Tire Hardness
The hardness (or softness) of a tire has a tremendous amount to do to with the way it handles the terrain it is placed on. Generally, a harder tire will see less tread damage, longer tread life, less rolling resistance, and will perform better in hotter temperatures. Conversely, tires with softer compounds will keep traction in colder temperatures better, they will wear faster, they will conform to the terrain better, and the softer tread lugs can be damaged (or worn out) more easily.
Rubber compound hardness and its consistency during temperature changes is a big concern to manufacturers (and consumers, though they may not realize it). A tire that becomes too hard in cold climates can essentially become a large hockey puck on a wheel, and will exhibit such manners when trying to brake or turn. A tire that gets too soft in hot temperatures will see accelerated wear and potential tread separation if given the right conditions.
Many vehicles sold today come with all-season tires that have minimal change in rubber compound hardness, though some purpose-built cars (like sports cars) may have single-season tires that cannot handle such a wide temperature range.
It's important to note the temperature of the tread surface when recording tread hardness, as the hardness of a tire's tread can change at a different temperature.
Things That Can Change Tire Rubber Hardness
Temperature: Tires get softer as it gets hotter, and harder as it gets colder
Age: Tires typically harden with age
Tread Depth: Sometimes there is more than one type of rubber compound in a tire, and the contact surface hardness can change as a tire wears
Chemicals: Many everyday chemicals can change rubber hardness if put in contact with the tread
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