For the Techies – How Hard is Rubber?
Natural rubber is the sap of a rubber tree, converted to a solid by coagulating it with acetic acid. Rubber used in tyres is generally the product of an oil refinery.
When combined with chemicals such as carbon black, antioxidants, and hosts more, sulphur added, the mixture subjected to heat and pressure, it comes out vulcanised. Sulphur makes the process irreversible.
Part of the “black art’ of making up the various mixtures used in the many components of a tyre, is varying the “hardness” of the rubber compound. For example, the rubber around the bead wire is compounded up to be quite ‘hard’ since it doesn’t move when the tyre rolls along. Conversely, the sidewall is ‘soft’, because it moves around a lot- it flexes.
How is it measured?
The tool used in the trade is a Shore A Hardness Durometer. Never heard of it? There is a range of them, designed to measure the hardness of different materials.
It simply is a small hand held tool with a domed plunger that is pressed into the surface of the rubber. The reading obtained on the quadrant scale when the needle is first pressed squarely against the rubber, is the hardness. Some cautions though- rubber hardness varies with temperature, it softens as it gets hot- every race car driver knows that. Also, the rubber ‘creeps’ away from the plunger, and as you hold the plunger against the tyre, the initial reading falls away.
So that is absolutely no help when the salesman tells you that the tread of the tyre that he’s selling is hard and long wearing. You might get a clue by pressing a reasonably blunt pencil into the rubber. If the rubber is “soft” the indentation might stay around a while after you remove the pencil. If it’s “hard”, the indentation might disappear quicker than for a softer rubber.
That’s not much help either, is it?
This is because there are many variations on a theme to make a tread compound. But beware of the salesman who tries to tell you that the tread is long wearing, and hard, and gives good grip, particularly in the wet. That’s nirvana, and hasn’t been achieved yet, to my knowledge.
So treads and sidewalls might have 1-2% sulphur, bead wire compounds 6% sulphur, and the old fashioned black ebonite ruler that granddad had, contained 35% sulphur, and was as hard as the hobs of hell.
I did say that it was a “black art”!