Wire Failure – from Ipods to Tyres

FAILURE – not a pretty word is it? Yet I had two failures this week, both due to FATIGUE.

My steam iron cord failed just where it comes out of the rubber tube at the end of the handle, and my I-pod just where the ear bud cord comes out of the plastic.

Why there? The iron had a long rubber tube, and a spring shaped thingy as well wound around it. Yet it still “failed” there. The wires inside were charred, but very fine. The I-pod wires were just fine, and broken, if you get my drift.

Why do they make them out of such fine wire, you were going to ask. Well if they made these flexible leads out of a thicker wire, they would not last long at all. Witness when you want to bend a coat-hanger till it breaks. It doesn’t take long, and it gets quite hot to hold where you’re bending it.

But like the iron and the I-pod, it will always break first where it is being flexed the most. This needs a bit of explanation.

So it is with tyres. If nothing else destroys it, such as road damage, the tyre will fail where it flexes the most. Wrong! It fails first where the greatest differences between flexibility (the tyre sidewall) and rigidity (the bead/lower sidewall); or upper sidewall to tread and belt area exists. It fails due to fatigue because by then, it will have rotated and flexed at these parts of the tyre, on average 30 to 45 million times for a passenger tyre, and 100 to 130 million times for a truck tyre.  Truck tyres go further because they’re inflated harder, so don’t flex as much, though they may have worn out three tread lives by then- tyre speak for been retreaded twice.

After that, the carcass of the tyre is not worth retreading because it is approaching the unreliable stage due to fatigue. Reliability is highly prized- and highly priced you might say.

Passenger tyres go at least twice as far as they used to 30 years ago, so a large chunk of the fatigue life built into the tyre is consumed in the first tread life. This is why retreading of passenger tyres has declined to such an extent. It is also why the motorist should look after his tyre pressures. The flatter or more overloaded the tyre, the more the tyre deflects as it rotates, and eats into its reserves against the ultimate failure- fatigue.

Incidentally, the wires in the steel belts of tyres are cables of wire made up if strands of fine wire, just like in the steam iron. The wires in the bead, which locks the tyre on the rim, don’t flex, so they are more like a coathanger wire. The iron had lasted quite well really- at least the fatigue beat the corrosion inside the steam chamber!