Munich. Thermal stripping processes used in aluminium-alloy wheel refurbishment impair safety, according to experts from TÜV SÜD Automotive. In-depth testing by the organisation showed that heating reduces wheel hardness by almost 40 per cent and shortens the component's lifespan by up to 90 per cent. The tests were conducted in response to the growing number of wheel refurbishment services offered, many of which use these processes in stripping and refinishing wheels.
Scratched car wheels? No problem! Drivers annoyed by faults or blemishes on their wheels can find help from a wide range of wheel refurbishment services or "wheel doctors" at the click of a mouse. Aluminium-alloy wheels, both new and used, represent a vast market, and wheel refinishing is a popular way of giving old wheels a new lease of life – while boosting profits. Aware that the number of wheel refurbishment services in Europe is on the increase, the experts at TÜV SÜD Automotive conducted a series of tests.
The results showed that heat is extremely destructive for aluminium wheels. Stefan Dittmar, head of the "Wheels" team at TÜV SÜD Automotive, explains, "In conventional thermal procedures, the wheels are heated to extremely high temperatures for hours on end. This impairs their stability and slashes the wheel's lifespan by up to 90 per cent, as we have now proved in new tests." In 2010 TÜV SÜD already issued a warning that heat should be avoided in rim refinishing processes; as long ago as 2004, the proper methods for treating rims were described in a guide for car workshops published by the German Technical Committee for Motor Vehicle Engineering (FKT).
Hardness reduced by 40 per cent
What kind of tests were carried out? The TÜV SÜD experts designed their tests on the basis of thermal methods available on the market, in which wheel rims are heated to over 200 degrees Celsius over a period of several hours to strip off the old coating before new granulate is applied. The tests were conducted on a number of alloy wheels in OEM finish, made – like the majority of today's aluminium car wheels – from the heat-treated material GK-Al-Si7-Mg T6. After being heated for three hours at 200 degrees Celsius, the wheels already showed a five per cent loss in hardness; at higher temperatures the effect was even more dramatic, with wheels losing 40 per cent of hardness when heated to 250 degrees Celsius for three hours. Shorter exposure to heat is also not the answer; "Wheel hardness already falls by 25 per cent after only 30 minutes at 250 degrees Celsius", warns Dittmar.
Safety no longer guaranteed
The lifespan of the wheels declines in step with their hardness. After heat-testing the specimens were subjected to rotational bending fatigue tests, which showed that the thermal treatment had reduced their life by up to 90 per cent so that they no longer complied with statutory minimum requirements (as set forth in Art. 36 of the German Road Traffic Licensing Regulation (StVZO) and directive ECE-R 124): "These thermal treatments are carried out at the expense of road safety. Wheels treated in this way may develop cracks, and spokes may even break", warns wheel expert Dittmar.