Originally Posted By: GC4lunch
The quantity of misinformation in this thread is mind-boggling. The European speed system is a rating that is the same as the Temperature rating in the American Uniform Tire Quality Grading mandated by the NHTSA. It is a measure of one thing, and one thing only: the maximum temperature to which a properly inflated tire, loaded within its load limitations can attain before it will explode. The rating would be more descriptive if it were called the "guaranteed explosion temperature rating," but that terminology might hurt tire sales......
I'm sorry, but that is not correct.
Both the speed rating and the UTQG Temperature ratings are not tied to temperature. Both tests are about the maximum speed a tire can attain before failure - and failure doesn't mean a loss of inflation pressure. It means loss of casing integrity. Typically, the failure is a "belt-leaving-belt" separation - and the air chamber is still intact.
Originally Posted By: GC4lunch
..... The S, T, H, V, Z speed ratings simply indicate how many rotations per time period the properly inflated, not overloaded, tire can sustain before it will explode; the speed ratings mean nothing more or less than that......
In some repects that is correct, but more accurately, the speed rating is the speed that the tire can attain before failure. The tests are step speed tests, where the tire is subjected to increasing speed in a series of steps. You could look at that as either time or revolutions, but that's because the test has that defined, but the way this is always reported is a speed. Temperature is NOT the criteria used to determine the speed rating.
Originally Posted By: GC4lunch
.... Obviously a stiffer sidewall will flex less than a more flexible sidewall, for instance, and rayon carcass plies will generally perform batter than polyester carcass plies........
I disagree. There are a number of different materials that could be used for the plies, but they don't impact that speed rating. The casing is the same for an S rated tire and an H rated tires. The difference is the cap plies.
Originally Posted By: GC4lunch
....... But there are no hard and fast rules. For example: the Continental ExtremeContact DW tire has a single polyester ply carcass construction, and has a relatively flexible sidewall therefore, and a relatively soft ride; but the 'DW is a Z-rated tire, albeit a tire that never, ever, should be driven underinflated, because it heats up very rapidly when driven at high speed when underinflated......
I think you will find that the larger tires will have 2 plies regardless of the tire line - and by larger, I mean load carrying capacity - and Load Index is a short cut to that value.
Originally Posted By: GC4lunch
.......There is no rule -- none -- about the number of cap plies a tire must have for a specific speed rating. None.
You're right. There is no "rule", but in order to pass the higher speed ratings, the tire has to have more cap plies. You could call it a "Rule of Thumb", but it's a generalization about how things are currently done.