Magnesium in frame to be cycling's metal of the future

Susan Watts,Technology Correspondent
Wednesday 05 August 1992 23:02 BST

THE SHED resembled an abattoir for bicycles. The rows of frames hung from the ceiling like carcasses on the way to the butcher.

These did not look like conventional bicycle frames. The shape was strange - an elongated version of the traditional triangle - and they had an odd, dull sheen. This was one of the first mass-produced batches of frames made from magnesium, a metal probably best known for its spectacular spitting in Bunsen burners during bored moments of chemistry lessons.

Magnesium may not seem the obvious material for bicycle manufacture, although engineers in the car and aircraft industries often use it for strong, flexible metal components.

Almost a decade ago Frank Kirk, an aeronautical engineer, had the bright idea that a magnesium bicycle frame could be made lighter and stronger than conventional aluminium or steel tube frames. He found he could make a magnesium frame only two-thirds the weight of an aluminium frame and one-fifth that of a frame made of steel.

The key difference between his frames and any other is that they are cast as a single component - from molten magnesium. This avoids the need for jointed tubes, which have to be welded for strength.

But the Chelmsford-based company he spawned, Kirk Precision, has been dogged by financial and technical problems. Mr Kirk approached two British companies in a vain attempt to raise finance. Things began to look up in 1989 when Norsk Hydro, one of the world's largest magnesium producers, bought his company. The Norwegians spent more than pounds 4m equipping the factory and the company hoped that production would start in March last year. In fact, the first bikes did not emerge until October and full production only started this summer. Without the backing of such a large company, Kirk Precision would probably not have survived.

Each frame is made from the magnesium extracted from one and a half cubic metres of sea water - just under 2.5kg (5.5lbs) of metal. This may seem hard to believe, but magnesium is one of nature's most abundant metals - a cubic metre of seawater contains 1.3kg (2.8lb) of magnesium. This magnesium is mixed with a little aluminium to give the frame a 'softer' feel and a tiny amount of zinc to help the metal flow more easily in the mould.

A Dutch team used prototype Kirk racing frames in its bikes for the 1990 Tour de France and one of Japan's leading cycling magazines voted the mountain bike its 1991 off-road bike of the year. But mass production has been dogged by problems.

Nobody has attempted to mould such large chunks of magnesium before. Kirk Precision owns a 7,000- tonne die-casting machine, one of the world's largest, that operates at extremely high pressures and temperatures. It is very difficult to ensure that conditions inside the mould are exactly right - allowing the metal to fill the mould smoothly without creating cracks or faults in the frame.

The metal must fill the frame within 45 thousands of a second, and maintain its temperature of around 650C (1,200F). The magnesium atomises inside the mould then solidifies as it cools. If it cools too rapidly the metal will stick to the mould's surfaces. Only 10 per cent of the early frames were usable. Months of fine-tuning finally paid off at the beginning of July. The company is now producing about 425 'raw' and 100 finished frames a day - 85 per cent of them at the standard it requires. Kirk-frame mountain bikes sell under the Dawes label in the UK for between pounds 350 and pounds 600 and are available here now. The racing and town bikes cost about pounds 350 and should be in the shops from next January. The frames on the bikes for sale in the shops in Britain have been designed to weigh about the same as an ordinary cycle but die-casting gives the company the chance to vary the thickness of the frame - strengthening it in the spots where the stresses are greatest and thinning it down where the frame can stand it.

The Lotus bike which helped Chris Boardman win Britain's first gold medal in the Olympic Games has helped to revive interest in bicycle technology. But this machine, with its expensive hollow carbon fibre frame, is designed for the elite cyclist. Kirk Precision is aiming for the mass market.

(Photograph omitted)

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