[Dom Ferris]

Hi,
There is a lot of discussion on tire aging and when to replace them.

I've read a number of articles on tire aging, types, etc, and thought I'd share it with you all so here it goes.

Failure Mechanisms: There are basically two ways a rubber part fails; through aging and fatigue. These are separate mechanisms, but interrelated.

Aging: With time and temperature, elastomers age from the time the curative is added to the mix. The rate of aging, for a given compound is a function of temperature; generally doubling for each 16 deg. F increase in temperature. The consequence of aging for most elastomers (natural rubber is an exception) is that the elastomer gets harder (technically - it has a marching modulus). Eventually, you'd get Bakelite. Well before that you get cracking

Fatigue: When an elastomer (or laminate) is bent, it incurs some damage due to the strain imposed. A high degree of bending (run low or flat) fatigues at an exponentially higher rate (basic rate * strain is accelerated by a power between 3 and 5). Engineers will recognize a variant of Minor's law.

Other Factors: Elastomers in general don't like ozone or UV and both of these tend to accelerate hardening and cracking. Modern compounds are very much improved in these areas.

Implications: Keep your tires cool - tires in Maine. should last longer than in Florida.; all else being equal. Properly inflated tires (not under-inflated will last longer). Slight over-inflation may extend life; but my butt is too delicate to consider this option.

??Park with UV shields (covers): less important! Covers may be helpful as long as they don't prevent air circulation and prematurely age the tires.

???Use nitrogen or dry air: - this has no benefit I can see until interior cracks appear (and it's too late by then) - then it might be more benign than oxygen or water within the laminate. Eventually, any of these will result in delamination (blowout).

The biggest benefits will come from proper inflation and avoiding extreme temperatures.

Note: We typically rate critical parts for 5-7 years service; but these are usually significantly over-designed. The FAA has picked six years for elastomer aircraft parts and my tires will get replaced on that schedule.

I hope some find this helpful.