In the early 1970s, the tyres of conventional class 86 locomotives suffered from both shelling and spalling. Shelling occured as a result of high dynamic loads caused by the axle-hung traction motors, and spalling occurs as a result of wheel slide which causes heat-affected zones on the tread surface. These are Martensitic and contain cracks which eventually cause pieces to drop out of the tyre tread surface. Shelling may be prevented by a reduction in the dynamic forces present in the wheel tread, but spalling will only be prevented in conventional tyre steel by eliminating wheel-slide.

In early 1973, a wide-ranging investigation including various modifications was put in place. As part of this programme the Testing Section were asked to undertake a series of running and braking tests to determine the causes.

The problem was thought to be due to the malfunctioning of the blending of the electric (rheostatic) brakes which resulted in the friction brakes being applied at high speed rather than when the rheo became less effective at lower speeds. The result of this application was to generate high temperatures on the wheel tread which changed the molecular composition of the steel making it brittle. Prolonged repetition of the malfunction resulted in tyre shelling.

Included in the programme was the monitoring of the tyres of a selected locomotive over a prolonged period, running under test conditions with reduced brake pressures, isolated rheostatic brake etc. We undertook a series of tests running between 9th and 18th April 1973 between Crewe and Willesden and also Crewe, Birmingham, Speke (where we were stoned by the local yobs once or twice) with the class 86 locomotive under test being hauled by another locomotive (E3092 or E3110). At the end of the runs the train was turned on a triangle.

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