Anyone who has seen the film Star Wars and, in particular, the attack on the star destroyer at the end of the first episode, can imagine roughly what it is like – in theory – to enter the tread of a tyre. After approaching the tyre at high speed, the craft turns away at the last moment, flies parallel to the surface into one of the many grooves and accelerates. Steep blocks of rubber known as positives project upwards to the left and right, followed at regular intervals – again on both sides – by turn-offs, grooves and voids known as negatives. The larger the proportion of positives, i.e. rubber blocks, the higher the risk of aquaplaning.
Too many joints, grooves and voids reduce useful life, on the other hand, and increase the noise level of the tyre. What this means in practice: it is very rare for the number of grooves to be increased – except if someone is going around deliberately puncturing tyres. But then the tyre has to be changed anyway. The situation is different where the number of positives is concerned: a tyre loses its tread the further it is driven; it quite literally wears itself out in the course of time. This means that the blocks gradually get smaller and smaller, the contrast to the voids decreases and, with this, the ability of the tread to transport rain water from the middle of the tyre surface to the outside. Specific attention therefore has to be paid to this.
There is an excellent rule of thumb here: when half of a 50 cent coin no longer disappears in the tread, a new tyre is required or the existing tyre needs retreading. Since the regulations about tread depth are revised from time to time, here is just an approximate indication of how to avoid problems. Winter tyres should have at least 3 to 4 millimetres of tread, whereas summer tyres should be replaced as of a tread depth of about 1.6 millimetres.
In recent years, the tyre industry has succeeded in lowering the rolling resistance of tyres considerably by using special raw material combinations for the tread – without sacrificing either safety (primarily braking performance on wet roads) or useful life (i.e. abrasion resistance). This has been a breakthrough.