Light years ahead

2 min read

For the sport of motocross, reducing the weight of the motorbike can be the difference between winning and losing. Forcing the bike around a cross-country track of turns, hills and jumps and over dubious and irregular surfaces means the rider has to accelerate and decelerate often and rapidly.

This means the bike has to be as light and as strong as possible. When Yamaha was designing its 2006 YZ model motocross range, it decided to introduce a weight-reducing component: titanium, rather than steel, rear-shock springs.

The YZ bikes are the first in the world to have titanium springs as standard and Yamaha turned to an experienced producer of titanium for automotive applications, TiMet Automotive of Pennsylvania, to supply the metal.

This broke new ground for TiMet, too. Although the company helped supply titanium springs for the Volkswagen Lupo in 2001 and the Ferrari Challenge Stradale in 2004, these only required a few thousands units. Annual sales of the Yamaha YZ range are many times greater.

The company had to supply the high-spec material and work with Yamaha's chosen spring manufacturer, Chuo Spring, to devise a volume- production process to form the metal into springs. Just to complicate matters further, Chuo had never worked with titanium before.

There are five motorbikes in the 2006 YZ range, weighing from 66kg to 100kg. In competition, the bikes line up against other manufacturers' models that are closely matched in engine performance. Finding ways to reduce the weight can therefore give riders an important advantage — and titanium springs are about 30 per cent lighter than steel, weighing some 500g less than the steel springs on the 2005 YZ models.

Yamaha has already introduced titanium into its bikes, using it for intake and exhaust valves to reduce engine weight in previous motocross models, and also introducing titanium parts for the exhaust systems and footpegs.

TiMet supplied the metal for these applications but the spring application required considerable development. For aerospace spring applications, the company supplies beta-C titanium. However, the production techniques for this are very demanding. To achieve its strength, the metal has to go through a 24-hour ageing cycle with heat and cold treatments — far too long for a mass-production motorbike application.

For its other automotive applications, TiMet developed a low-cost beta grade of titanium, whose strength derives from the combination of metals in the alloy rather than from post-treatment. According to Kurt Faller, TiMet's president, the company then had to work with Chuo to develop systems for transforming the titanium from wire into springs.

The first stage, Faller said, was to duplicate exactly the supply conditions for the steel coil that Chuo was used to dealing with. Chuo then had to incorporate a two- to three-hour heat treatment process into its manufacturing systems — somewhat longer than the ageing required for steel, but this time is recovered later in the process as, unlike steel, titanium does not need a surface coating.

The final process is comparatively simple. The titanium is supplied in the same form as steel, in elastically-bound coils of straight wire. Chuo receives these, cuts the bonds in a protective cage and feeds the wire directly into a CNC spring winder. It is a relatively minor tooling change, Faller said.

Although the titanium is eight times more expensive than steel, the improvements in power-to-weight ratio are making the change in material worthwhile, Faller said.

'Yamaha's endorsement of titanium in a range of applications is visible, significant and has continued to grow,' he said. 'We believe others will learn from their positive experience.'