1. An 8x15 inch trailer tire, load range E, 85 psi rating, blew out a sidewall during inflation. Tire tech was on top of the tire because the safety equipment was not working right. It launched him first, and went through the shop roof. Compressor was set for 165 psi with no regulator between the tank and the tire but 150 feet of 1/2 inch pipe.
2. A 15-inch car tire, 32 psi rating, gas station had a locking air chuck (OS HA required). User did not realize that letting go of the chuck would not stop air flow. The hose blew out, knocked him down and lifted the tire over a retaining wall. Severe head injury.
3. Wheelbarrow tire, 4x8, 32 psi rating, aired up with unregulated hose connected to 100 psi. The rim split in half and decapitated owner.
4. Nearly new bicycle tire, mine, 90 psi rating, blew off rim at 70 psi. Stress in bead wires is 300,000 psi! Tire manufacturer says, "Tires blow off the rim all the time," like it was no big deal. Tell that to my knee.
5. Bike tires are thin-walled but also very small cross section. Thin-wall pressure-vessel design is just a ratio of diameter to wall. Bicycle tires are bias ply, so the cords don't separate as the tire swells, like a radial. (Bill Fragale, Phoenix area, also nailed me on this one.)
6. My tire engineer warns against exceeding sidewall rating by more than 10 percent. Orris says the first thing to fail should be the sidewall. Steel rims are formed from one piece of sheet and will not fail, but some aluminum rims might fail by the flange breaking off. The rim fragment will be a bullet.
7. Air at about 125 psi behaves like pure oxygen! Organic materials can spontaneously com- bust in it. When this happens the gasses expand even more and what you have is a one-cylinder engine with no crankshaft and no exhaust valve.
8. Navy has one serious accident a day from high-pressure air. See 7.
9. Desert temperatures and high altitude can raise tire pressure another 20 percent (pvnrt = pvnrt). See 7.
10. Tires have a warning on the sidewall not to exceed a certain pressure to seat beads. Usually it is less than 125 percent.
11. Most of the tire-explosion problems we have seen are from putting tires on the wrong size rims. There are 15.5- and 16.5- inch tires and rims, that almost interchange with 15, or 16, but will release without warning, at or near rated pressure.
12.Just because you get away with something does not mean it's safe. Safety factor is not there for us to use, it's a safety margin to deal with the unpredictable, like: Suppose your gauge is off, or the tire has latent defects, or they overstressed it in mounting, or in your case, you high center on a rock.
13.The energy stored in your 32- inch diameter tire at 100 psi is 13,000 foot pounds, enough to lift the entire truck 4 feet, or launch tire and wheel at 146 feet per second. That's 12 times the muzzle energy of a .44 magnum! How securely is that tire attached to your truck?
In doing the research for the FOURWHEELING ACADEMY on deflating tires, I learned the typical snap-in tubeless valve stem is only rated to 65 PSI maximum. There are others that are rated to 100 PSI maximum. You'd have to dismount the tire to tell the difference. The large-diameter, bottom contour will have straight sides (lighter line) versus the curved sides of the 65 PSI version. The Tire and Rim Association, Inc. (TRA) engineer told me there's more than just a rubber contour difference, too. The metal core also has a flange to make it virtually blow-through fail-safe. There is no pressure rating specified by TRA for the bolt-in type.
With all that, let me radically revise my recommendations about using the spare as a compressor tank. 1) Only use the spare as a tank if it is stored outside and beneath the car. 2) Use a new tire only. 3) Use only an LT (load rating C or better) tire. 4) Per 6 above, don't exceed the sidewall rating by more than 10 percent. 5) Make sure you use high-pressure valve stems. 6) Only use steel rims. 7) Slowly and progressively increase (test) the pressure on your particular tire and rim. First test at rated pressure, then increase in 2 percent increments. Make sure the test-tire is out of sight of everyone and everything that could be damaged. 8) Don't inspect an overinflated tire. 9) Deflate the tire below the sidewall rating before using, removing or working on or around the spare. 10) Have a gauge permanently plumbed into the storage system and make sure it is in plain view. You want to detect a compressor automatic shutoff switch malfunction. 11) Put a blow-off valve in the system.
