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Tyre Tips - Light Commercial Vans, Trucks and Buses


Tyre Tip 7 - Truck Tyre Failure

When we talk zipper failures, most males conjure up images of embarrassing episodes such as standing around at meetings or parties with their trouser fly undone. Or even a close encounter of a painful kind, whereby certain parts of one's anatomy becomes entangled in a zipper!

 

But, if we use the correct terminology as applied to a truck tyre failure, which is circumferential fatigue rupture, then this whole subject becomes much more serious. In fact, it can become deadly.

 

Click to EnlargeA "zipper" failure in a truck steel radial tyre is a condition where the tyre basically blows out with extreme force in the mid to upper sidewall area, exposing an even line of severed casing steel cords along a considerable length. The rupture begins in one cord location, and then progresses along the sidewall as one after the other carcass cords fail due to the almost instantaneous transfer of forces. This gives the appearance of an open zipper, hence the use of the name to describe this sometimes-catastrophic event.

 

Usually, this type of failure is caused by the tyre being run severely underinflated at some stage of it's career, and in nearly all instances is one of a pair of dual tyres where loss of inflation has occurred. Excess flexing in the sidewall of the tyre leads to fatigue failure in the steel cords. Normally, steel casing cords in a tyre are under tension when correctly inflated, but when the tyre is substantially underinflated, the cords are compressed. This leads to severe bending of the cords and ultimate failure if continued to run in this situation. Remember, a typical truck radial rotates around 310 times each kilometre of travel, so you can imagine the many thousands of revolutions this tyre would do if left in this underinflated state.

 

Even if this underinflation is rectified at some point during its operation; there is a high probability that the cords have been weakened by this flex fatigue. Thus when the tyre is reinflated, the cords don't have the strength to contain the pressure, and the tyre blows out in an explosive manner as the casing cords fail one by one almost instantaneously.

 

This blast of air (and associated shrapnel) that is released can cause serious injury and even death. American tyre industry authorities are so concerned about the potential hazard to tyre handlers; they have released posters and videos graphically demonstrating the huge explosive forces released during a failure of this kind. One of their methods of getting the message across was to quote the fact that a standard size truck tyre contained sufficient explosive force to lift a typical passenger vehicle three metres off the ground!

 

The primary cause of many tyre maladies such as these zipper failures and also belt separations is loss of inflation pressure. This can stem from poor maintenance, punctures, incorrect repairs, leaking valves, bead seating, bent rim flanges, etc., etc. So any tyre that is presented in your workshop or even beside the road that has experienced significant underinflation, or run flat, or requires repairs, for whatever reason should be treated with suspicion. They should be examined closely to establish if there is a fatigue factor, or if any damage to the sidewall plies has occurred. A good hands-on examination probing any sidewall variations, distortions, wrinkles, discoloration, repairs prior to any re-inflation would be good insurance. (Even flexing and listening for grating sounds within the tyre can be part of the inspection -- provided the tyre is not fitted, of course!). Get the services of a qualified experienced tyre person to undertake this.

 

It is strongly recommended that any tyre inflation should always be done in a safety cage. If not available for any reason, such as fitting beside the road, some other appropriate protection should be arranged. All service personnel need to ensure they move out of the trajectory area regardless of whether the tyre is in a cage or not. The air blast alone can cause injury and hearing loss.