Dual Tyre Pressure Mismatch at Roadside Check
A tri-axle semi-trailer is stopped at a roadside check with an 11,800 kg axle load on the middle axle (limit 12,000 kg). The left inner tyre of the dual pair reads 620 kPa, while its outer partner reads 830 kPa. The inner tyre has 2.0 mm tread in the centre and a visible sidewall bulge; the outer has 4.5 mm. The manufacturer's load/inflation table for this tyre size gives 2,450 kg at 620 kPa and 2,950 kg at 830 kPa per tyre. The driver has 180 km left to the depot and says the trailer 'tracks fine'. The company policy requires tyre pressures within 10% across a dual pair and a minimum 2.5 mm tread on steer and 2.0 mm on other positions.
1. Calculate the approximate load capacity of the dual pair on that side and decide whether the vehicle may continue.
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The axle load is 11,800 kg, so each side of the axle carries approximately 5,900 kg. The dual pair capacity at the measured pressures is 2,450 kg + 2,950 kg = 5,400 kg, which is below the 5,900 kg side load. The pair is therefore overloaded even though the axle itself is under 12,000 kg.
The pressure mismatch is (830 − 620) / 830 ≈ 25%, well outside the company's 10% limit. The inner tyre also has a sidewall bulge, which indicates carcass damage, and its tread is at the company minimum for non-steer positions rather than above it. 'Tracks fine' is not evidence of tyre integrity. The vehicle should not continue. Fit a serviceable matching spare or replacement tyre, set both pressures to the same correct value, and re-check the axle load.
2. Explain the risks of running the pair and the checks required after replacement.
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The underinflated inner tyre deflects more, heats up, and carries less load than its partner. The outer tyre then carries more than its share, and the pair can overheat or suffer sidewall failure. A bulge suggests structural damage, so the tyre could fail suddenly. Uneven pressures also affect braking, traction, and trailer stability, particularly under load or in a curve.
After replacement, check both tyres for matching size, construction, and load rating. Set pressures cold with a calibrated gauge. Inspect the wheel and hub for cracks, check bearing play and brake drag, and look for axle misalignment or a bent rim that could have caused the original pressure loss. Re-weigh the axle if load distribution may have changed. Record the defect, the replacement, and the pressures. The damaged tyre should be removed from service and inspected by a tyre specialist.