[Tom Wells]

Argess,

Very nice, comprehensive look at rubber hardness.

Alas, after being involved in tire construction and testing for 7+ years, hardness is not the only factor in tire safety. I definitely agree it's one factor.

Another consideration, perhaps invisible from outside the tire - but not always - is internal adhesion between the various layers of rubber, and between the rubber and its reinforcing tire cord.

Occasionally there can appear a bit of a bulge, or surface irregularity that gives a clue, but often there's no change visible to the casual observer.

As you accurately point out in so many words, the chemical composition of a tire changes over time. You might say it ages. As it does, adhesion can be reduced between the components.

As aging occurs, and it often reaches significant effects around the 5 to 6 year mark, the tire becomes structurally weaker. The rate and the extent of the loss of strength is dependent on a myriad of factors such as its design, materials, original chemical composition, any trauma that may have happened over its life, temperature conditions, and probably ten or twenty other factors.

Agreed, these cars are usually lighter in relation to their tires rated load capacity so the effect of the weakening may take longer to appear. Conversely, speed, acceleration and braking forces may hasten failure, even on a lighter car.

So, as Clint once famously said "How lucky do you feel?"

I change mine every 6 years. Anything more than that is a big dice roll - maybe it adds to the thrill?

Tom