Originally posted by Novarossi
Here is a question you might be able to answer : Why are a set of tyres that come out of a box brand new never the same shore as rated on the box? Is it because over time the tyres will have changed readings (as you mentioned above) or is it because of the different techniques/durometers used when measuring the tyres?
If you do have two identical 40 shore tyres both 60mm and both brand new. You true one down to 55mm and the other down to say 58mm - will the shore ratings now change for each tyre? -In other words, the 55mm tyre will actually have a higher rating (harder) then the 58mm tyre?
I guess the rating on the box applies to the foam material before they were turned into tires. When a manufacturer rate the material, they would follow the guidelines per ASTM standards. Meaning they would obtain a specimen from the batch with specific dimensions (usually a square/rectangular block or disc with a certain thickness) and tests the specimen under a specific condition/method per the standard. This specimen, I am sure, does not have the same shape and sizes as the foam tires one took out from the box.
A thin (about 7mm), curved piece of foam, reinforced by a semi-rigid plastic (the rim) would have a different reading from the specimen that was tested by the manufacturer even if it is the same material. The reading can also be affected greatly by how the material is able to deform under stress. Notice the readings between the center of the tire and the edges of the tire are different? At the center, the foam could and would deform in all directions, at the edges, the deformation is limited to one side.
I would expect the 55mm tier to have a higher reading when tested at the same spot because it has less material to deform and is closer to the reinforcing rim.
Shore rating gives an idea of relative resilience of various grades of rubber/foam but it does not give any information on properties like wear or abrasion and they could be different between manufacturer even if the Shore rating is the same. I wouldn’t worry too much about this Shore rating thing and I don’t understand why the manufacturer even bother to put it on the box since it is really meaningless to us. A simple “Really Really Hard”, “Really Hard”, “Medium”, “Really Soft”, “Really Really Soft” rating should suffice, my opinion anyway....