Tyres occasionally, even rarely, ‘blowout’.
When examined, the reason for the ‘failure’ can be tracked down to the failure of a component due to a fracture, split, or separation of the components.
Emblazoned on the wall in code, there is a symbol which tells the car engineer how much load the tyre can carry. But will it burst the moment you overload, or over-inflate it? The answer is ‘No’. It has a margin of safety built into the casing strength to allow it to cope with the normal abuses that it will encounter during its life. These can be as varied as rocks sticking out of an unmade road, collisions with concrete road islands, kerbing, potholes, broken road edges, and the like.
All these are normal conditions, and the tyre is expected to cope with them.
So how much margin of safety is there. The tyre wall states ‘maximum pressure of 250 kPa (36 p.s.i.) It certainly won’t fail (burst) at 37 p.s.i., will it, and knowing the accuracy of service station gauges, it is just as well.
You’d be pleased to know that the pressure that an over-inflated tyre will burst, is at least 150 p.s.i., and more likely, closer to 200 p.s.i. In fact, what bursts first is the metal rim, which buckles under the huge pressure being exerted on the rim flange.
As part of the design process, tyre engineers periodically test a tyre by blowing it up on a massively thick, reinforced rim, so that they can check their computer design programs. They do this in a heavily reinforced steel cage for safety’s sake.
But cleverly, they use water, not air. They get wet, but not killed, by the blast. Another test, required to qualify a tyre under the Australian Design Rules, has a plunger driven into the tread area of an inflated tyre. Some resemblance to running over a protruding bolt head on a wooden bridge could be construed by this laboratory test.
Unfortunately, there have been many fatalities in the tyre fitting industry caused by exploding tyres. The worst case scenario is the farmer who connects his compressor to his tractor tyre, the phone rings, and he forgets to take it off again. When it bursts, it does so explosively.
The margin of safety built into the tyre is dictated by the tyre engineer, in anticipation of the type of service that the tyre will undergo. For example, an off-road design will have a higher margin than an on-road. Hitting a pothole at speed may have a vastly different effect to hitting it slowly. The margin is calculated to give satisfactory service, whilst keeping the cost under control. Otherwise the tyre comes back as a claim on the manufacturer.
Compare this fact with a crane on a building site. Cranes occasionally collapse too. Each crane has its ‘S.W.L.’ (Safe Working Load) in large letters at each lifting point on the boom. Also the operator has a load cell readout in the cabin, telling him the actual load at that point. Occasionally crane operators have been known to ‘push the envelope’ If a structural components fails, it can be due to either exceeding its design limit, fatigue, or lack of prescribed maintenance. Exactly the same thing applies to tyres, and they’re working in a much more dynamic environment.