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 FOUR WHEELING ACADEMY

SPARE TIRE/TANK CAUTION!

by Harry Lewellyn

This alert must be read by all_who use or are considering using their spare tire as an onboard compressor_tank. In the December 1995 FOURWHEELING ACADEMY (Get it Up), I discussed tire inflation schemes. Following is a partial quote.

I disclaim all responsibility and liability for sharing the insane reserve-tank approach I have used for years. Plumbed via a valve stem connector to my ARB air locker compressor and automatic shutoff switch (not recommended by them), I carry my spare tire at 105 PSI. With quick-connect hose and chucks, I simply zip around the car as if I were at home or at the local station. This technique approaches household/service station speed and convenience. Then, in the leisure of the paved (good road) cruise back to wherever, I refill the "spare/tank." I run the ARB compressor, at short intervals to minimize overheating, and bring the spare/tank back up to maximum pressure. I can also fill the spare/tank directly from any larger compressor (service station or home, 110 VAC unit).

I'll offer only a few words to validate this continuing scary experiment. My BFG T/As are load rated at 50 PSI, cold. I reason they must be designed with at least a two-to-one safety factor taking into account road heat, road hazard and some sort of wear consideration. Very thin-sidewalled 10-speed bicycle tires are typically inflated to 100+ PSI for example, and they too must have a pressure design safety factor.

The tire manufacturers won't share design-limit information, but a few confide you would probably split or break the wheel before a new tire would blow. Five-plus years of personal experience show no failures on several test vehicles. For example, in a combined gas mileage/tire pressure test, I inflated all four tires to 80 PSI (cold) and ran them about 1,000 highway miles with no problems. Ride was terrible and gas mileage up 4%.

Without belaboring the spare/tank thing, consider one spare of equal size to the other four, inflated to 105 PSI. Assuming the other four tires to be totally flat, divide 105 PSI by five and you've got five tires at 21 PSI each. This is plenty of air to get me home or to another pump. In reality, all four tires would not be flat, therefore the average pressure would be even higher yet.

 

The explosive force of compressed gasses (air) is incomprehensible to most of us. Do not use the above approach on exposed spares. This goes for spares within the 4X or carried outside on swing- away racks! Spares underneath the car offer some degree of shielding.

I recently learned of a "spare/tank" within a 4X that exploded. The internally- stored, overinflated spare not only broke out all of the windows, but bent out the top of all four doors and deformed the roof of the Grand Cherokee! A shoe-size piece of rubber came forward and broke the windshield. Fortunately, no one was hurt. The explosion only rang the driver's bell and there were no other occupants. The tire had seen unpaved duty and I suspect, therein lies the problem.

Barring random manufacturing defects, I have great faith new tires are safe at twice the load-rated pressure. I have less confidence once a fire sees duty. We can inspect the outside for flaws and that is good, but visible defects are not the only potential source of failure. Invisible, internal damage can weaken a tire too. Be extremely cautious with overinflated used tires. Further, do not inspect an overinflated tire. Deflate the tire to normal pressure before getting your face close to this potential bomb!

The tire companies conduct burst- pressure tests with water. They've learned air tests rip, shred and scatter the tire. Fluids do not compress and store energy, therefore they safely release the pressure at the first flaw. The small, initial flaw can be studied in more detail rather than trying to collect, reassemble and analyze all the scattered pieces.

Help me help others by spreading this caution. "DO NOT CARELESSLY OVERINFLATE SPARES WITHOUT SAFELY TESTING FIRST." If you must use the spare/tank approach, use only a new tire and crawl before you walk! With the tire out of sight of everyone and everything that could be damaged, progressively raise the tire pressure and let it age a week or two between steps. My first tests were at 120% of load rating, then 130%, and so on.

I'm eager to hear of your experience. Drop me a note if you've had an over- inflated tire blow.

SPARE TIRE/TANK, ALERT II!

Whew! I'm really glad Ken Obenski read the December [above] issue of this newsletter (SPARE TIRE/TANK CAUTION!) and took the time to share his expertise. He's a registered engineer, works in failure analysis, and blew open my eyes regarding tire overinflation hazards. My original fear dates back to a 6-year-old Coyote blowing out a 24-inch bicycle tire. The December newsletter understatement, "The explosive force of compressed gasses (air) is incomprehensible to most of us," is only emphasized by what follows. I expect, if you're like me, by the time you finish his letter (salient points reprinted below), you'll even be afraid to inflate your tires to street pressure!

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.

OK, what have I left out now?

© Harry Lewellyn

Ecological 4-Wheeling Adventures
P.O. Box 12137
Costa Mesa, CA 92627
voice: (949) 645-7733
fax: (949) 645-7738
email: info@eco4wd.com

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