Bikes get a rear-wheel drive

Sir Clive Sinclair's latest attempt to push back the boundaries of environmental transport has resulted in the Zeta - the "Zero Emission Transport Accessory". Unlike its precursors, the C5 and the Zike neither of which conspicuously conquered the British roads the Zeta is not a complete vehicle, but a battery-operated power-pack which can be added to most types of bicycle.

Since its introduction in April it has sold about 5,000, about double the number of Zikes sold so far.

The colour-supplement advertisement claims that the Zeta will "take the slog out of cycling". Beneath the claim is a drawing of a woman, ponytail flying in the air, joyfully climbing a steep hill against a savage headwind. Most days I cycle five or ten miles through London. Perhaps with this machine I could broaden my horizons? Perry Cohn warned me not to raise my hopes too high. Mr Cohn was the mechanical engineer at Sinclair Research responsible for the development of the Zeta. I caught him shortly before he left the company to join the McLaren motor-racing team. While he fitted the Zeta, he explained why I was far from the ideal customer.

My weight was the first problem. The Zeta does not replace the cyclist's legs. It is there to give assistance. It claims to provide half the power needed to propel a 12-stone cyclist up a one-in-ten gradient; my 15½ stone was no picnic for a 174-watt motor. Another problem was that my bike, a 12-speed Townsend "Triathlon", was too fast. The Sinclair machine is designed for the more sedate end of the market for those who want to cycle the two or three miles to the office but are deterred by the hill past the town hall.

The machine itself is a 4.5kg box with a shiny blue cover which is screwed to the frame above the back wheel. The box holds a re-chargeable battery attached to a motor powering a revolving belt pressed down on the rear tyre. The motor is operated by a switch on the handlebars. The battery has a range of about ten miles.

Pulling away from the kerb, my first impression was of a shove from a divine hand. For all my bulk, the Zeta had enough power to set me off from a standing start at the pace of a gentle stroll. Four hours later, after a 30-mile trip through London and Kent, my enthusiasm was mixed.

The first of the design faults emerged in Camberwell. As I rattled over a bump, the Zeta's cover, held in place only by a Velcro arrangement, flew off. This happened another four times during the afternoon.

The second fault waited until Bromley. I thought that the battery had expired, until I realised that I had bent the operating lever flat against its mounting bracket, the equivalent of having an accelerator pedal on a car set flush with the floor. I bent it back and rattled on down the road.

The motor certainly helped on the hills, but at other times it tended to slow my progress. The extra weight and the drag of the drive-belt acted like a brake, while the constant rattling of the cover gave the sensation of being pursued by a heavy engineering plant.

My verdict is that the Zeta is of little use to serious cyclists. However, for anyone who enjoys a potter on a bike, it may well be a help and the revised model takes care of most of the problems I encountered.

The revised Sinclair Zeta is available by mail order at £144.95, including battery and recharger (0933 279300). It needs no licence, tax or insurance and can legally be used by anyone over 14.