Driving tests

What happens when a truck tyre blows?

Heavy vehicle tyres are filled to a much higher pressure than car tyres. Because of the potential injury to the tyre technician if the tyre blows while it’s being filled, truck tyres are filled in a cage.

A tyre inflation cage is an essential piece of equipment.

If the tyre blows while being filled, fragments of rubber and metal can be ejected towards the person filling the tyre, fatally injuring them.

Truck tyres blow due to either mechanical failure or overheating.

Mechanical failure

Mechanical failure of a tyre is not as spectacular as overheating failure. When filling a tyre it happens if the tyre is over-pressurised because of a fault in the compressor or the gauge, or the technician deliberately over-inflates it to try to get it seated on the rim correctly.

If the tyre has had an impact, the sidewall could be damaged, creating a weak point which opens a bit like a zip.

Driving with under-pressurised tyres (i.e. less than 80% of the recommended pressure), over-pressurised tyres, or overloading the tyre causes significant wear. The envelope of the tyre can deteriorate through wear, exposing the plies/belts to contamination by air or humidity, weakening them.

The tyre can also come off the rim if the rim is damaged, has abnormal wear on the edge, or is deformed in some way. If the tyre isn’t seated properly on the rim, it can come off during .

Finally, if there was a manufacturing defect, this could cause the tyre to fail.

Chemical overheating

As tyre temperature increases, so does the pressure of the air inside the tyre. If the temperature rises far enough, different chemical reactions can occur inside the tyre, such as thermo-oxidation and pyrolysis. In fact, some companies, such as Pyrolyx, use pyrolysis to dispose of and recycle tyres.

If the temperature inside the tyre reaches 185 degrees Celsius an irreversible chemical decomposition takes place: the rubber degrades and a violent explosion can occur out of the tyre’s sidewall with a force of 1000 PSI or more.

What can cause the tyre to overheat?

Heat can be transferred to the tyre from the brakes – excessive use of the brakes, e.g. on long descents, can cause brake fade, but also generates temperatures in the hundreds of Celsius which acts like a radiant heater for the tyre.

If the vehicle catches fire, this will heat up the tyres which then cause an additional hazard for firefighters tackling the blaze.

Welding the rim with the tyre still mounted will heat up the air inside the tyre.

Overloading the tyre causes additional deformation as the tyre rolls, heating it up; the same applies for under-inflated tyres. If one tyre in a dual wheel setup is under-inflated, more load is put on the other tyre.

Items trapped between dual wheels, or if the two wheels are touching, could cause heat buildup, as will happen if part of the body, chassis or components are touching the tyre.

Using a tyre at excessive speeds will heat it up.

Finally, an electrical arc such as lightning can cause this.

What happens when the tyre finally blows?

As mentioned previously, a shockwave of more than 1000 PSI can be ejected from the tyre, along with bits of metal and rubber. This can destroy body work, cables, hoses and airbag suspension, and could weaken or puncture a tyre next to it as the explosion is almost always through the sidewall.

If the driver doesn’t notice, the tyre will start to fall apart, leaving large chunks of rubber bouncing down the road. Bits of the tyre flail around underneath the body, damaging parts further. Retreads can become separated.

Retreads have come off this tyre

The tyre sidewall eventually completely rips and comes away from the wheel.

The tyre carcass separates from the wheel

If a tyre blows at the rear of a trailer, the driver might not feel it, but they might hear it as it will sound like a rifle shot. The other tyres will take the weight for a short distance until the driver can pull over safely.

Both tyres on this dual axle have shredded themselves. You can see damage to the mudguard.

If a front tyre explodes, the cab will drop and it will pull towards the side of the tyre that’s blown. The driver should avoid braking heavily.

How do you prevent tyre blowouts?

You can’t completely eliminate the risk, but the best way to minimise it is to do a proper pre-trip inspection. Check all the tyres, including between duals, for noticeable cuts and bulges. Replace the tyre immediately if you find one.

Keep the tyre pressures correct for your load and the ambient temperature. Low pressures cause tyres to overheat and delaminate more quickly, while high pressures cause additional wear in the centre of the tread and inflexibility in the sidewall when hitting bumps.

driver training courses

Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

Posted in Advice
When was the last time you checked your Road Code knowledge?

Try some tests for free!

Road Code car quiz

Road Code motorbike quiz

Road Code heavy vehicle quiz