Race tyres for the road, in a roundabout way

‘WHAT we are selling to the public is a racing tyre. What the racers are using is a standard tyre. So the tyres are the same.’

Those are the words of Helmut Dahne, PR man at Metzeler and the fastest man ever around the old Nurburgring circuit. The 57-year-old clearly knows a lot about tyres – you don’t race for 30-odd years and not. But it seems incredible to claim the road and race tyres are the same thing. Yet we saw them coming off the production line together when we visited the firm’s factory at Brueberg, Germany, recently.

The thing is, we tend to think of tyres as that bit of rubber touching the road. In fact, there is a lot more to a tyre. The tread is just the bit of it you can actually see. The real work goes on under the tread. The carcass in many ways is the bit that does the work, or at least allows the tread to get on with gripping the road. And when Dahne says the road and race tyres are the same, he means they share the same carcass.

Radials first appeared during the mid-1950s on cars and have since become by far the most common type of tyre on the roads. Making the transition to bikes, however, has proved a much more complicated affair, and it wasn’t until the early 1980s that the first such realistic attempts began to appear.

The most obvious visual difference between car and bike tyres is that the former is curved in one plane only – around its circumference – whereas a bike tyre, because it needs to lean into a corner, is double-curved and falls away towards either rim. This gives the tyre varying diameters depending on how hard the motorcycle is cranked over.

The simple definition of a radial tyre is that it has belts crossing from side to side (1) secured by another layer of ” zero degree ” belts around its circumference (2). Bending that belt in two planes initially proved to be a problem.

What tends to happen is that as the bike leans, the belt folds up on the smaller-diameter shoulders of the tyre, causing awkward tread deformation.

But since conventional crossplys already work perfectly well, you have to ask why firm’s such as Metzeler have even bothered with radials.

The answer lies in demands from Japanese bike manufacturers for a better tyre. Racing technology in recent years has led to stiffer frames and the use of steeper steering head angles on road bikes.

The upside is that the bikes turn faster. The downside is that they become twitchy and are more prone to tankslappers. Fitting steering dampers is one way around that problem. But radials offer another way to go.

Crossplys, due to their stiffer structure and the fact that the belts run in an X-shaped crossover pattern from side to side as opposed to directly across the middle, have a significant stability advantage in cornering. But in a straight line, radials have the advantage. Their construction gives them a softer sidewall, allowing them to dampen the twitching much better.

Originally, Metzeler only made radial tyres for the rear and found that fitting them virtually eliminated the front-end problems. But, thinking it wouldn’t work so well at the leading end, the company was reluctant to build a radial front.

Crossplys use a minimum of two overlaying plys. That naturally builds in strength to the sidewall and creates lateral stability. Radials use a single ply base, which keeps weight down. But by putting a long turn-up on the single ply in a radial carcass (3) Metzeler created a two-ply sidewall on the tyre.

Despite all that, they still don’t quite match up to crossplys for lateral stability. It was close enough for the rear, but the front wheel needs to offer totally precise steering, something a soft sidewall can’t give.

The designers continued to develop the sidewall strength, and in 1995 Gregorio Lavilla used a slick radial front in the German Pro-Superbike championship. He loved it and that convinced Metzeler to put it into production.

The decision to use steel for the belts, rather than Kevlar as preferred by rivals, is how Metzeler got the radial front to work. The steel belt gives the overall carcass enough stiffness to rival a crossply, and by slimming the wires in the cable – and using fewer winds around the tyre – they got the weight the same as a Kevlar belt.

A weakness of radial tyres would have been to grow in circumference at speed – potentially very damaging. So Metzeler searched out a special steel cable and put belts around the circumference. The cable is made into a rubber-coated ribbon with two or three cables attached side-by-side which, in turn, is wound on to the tyre in a spiral. The spiral means it isn’t quite at a perfect zero-degree angle, but is close enough for the difference to be negligible.

When a tyre spins, the main effect of inertia is to make the tyre’s diameter expand. Crossply construction allows the plies to move, creating heat and also allowing a lot of growth – up to 30mm at 125mph. Metzeler claims its steel-belted radials grow by a maximum of 2mm at 186.5mph.

With the belt running around the radial tyre, there’s no movement in the ply – while it prevents the tyre from growing. That keeps the profile the same at all speeds, which prolongs tyre life and keeps handling consistent.

When a tyre grows, the warmer rubber wears faster. By keeping the tyre cooler Metzeler can use softer compounds for ultimate grip on road-legal tyres with a reasonable life.

Dahne underlines this, saying: ” The tread on the Z3 Racing road tyre is the same as the RS3 race tyre – it’s a full racing compound. ” So while your bike might be a race replica, your tyres are the real thing.