For aficionados of HPV racing, the Windcheetah SL (nicknamed the Speedy) is a familiar sight. It is a recumbent tricycle designed by Mike Burrows and made to order by Burrows Engineering in Norwich. To date eight have been built and raced regularly both in HPV races and against standard racing tricycles. The SL suffix denotes that it is street legal and Mike Burrows now wants to sell the machines for everyday use. David Weightman, senior lecturer in Industrial Design at Coventry Polytechnic, assesses the Windcheetah SL and its potential market.
The underside of Windcheetah 008 and Mr. A. Pegg
I first saw Windcheetahs at the HPV races on the Isle of Wight at Faster this year. (See Bicycle Magazine, June issue.) There were six of them, all seemingly piloted by people in purple skin suits and fluorescent furry helmets with dangerous grins. They were always together and always moving, even when not racing. Clearly these things were in a different league from the cosseted thoroughbred faired HPVs. Was seeing the Windcheetah pack whistling along below waist level a premonition of the post-petroleum future - Mad Max with cleats?
All this apart, the Windcheetah looked serious and was very competitive. In the fast downhill run and in the short circuit races, Windcheetahs were highly placed and the competition between them and the more aerodynamically efficient but less manoeuvrable faired machines was intriguing to watch. The number of Windcheetahs in some events made for very close and exciting racing particularly on the circuits. The quality of the road surfaces and the variety of events were such that any handling limitations and mechanical defects would come to light but the only incident was a punctured Windcheetah colliding with the kerb.
Before considering the Windcheetah in detail, let's take a general look at HPV racing and how the sport has affected vehicle design. In the early days, straight line, high speed events predominated. At 40-50mph, the biggest resistance to motion is aerodynamic drag, calculated by multiplying the vehicle's frontal profile area by the drag coefficient (Cd) of its shape. Cd is a measure of aerodynamic efficiency or how easily the shape slips through the air. (Those sufficiently impure in thought to read car adverts will have noticed the recent spate of comparative advertising of Cd figures). To go as quickly as possible, HPV designers reclined the rider to reduce the vehicle frontal profile area to a minimum and then provided fairings or bodywork to achieve lower drag coefficients. As recumbent riders find balancing difficult, many designers opted for the stability of three wheels, in spite of the penalties of weight and mechanical complexity. The two wheels are usually at the front with a driven rear wheel to keep the drive train simple. For the straight line events, limited steering was required so the two wheels could be placed close together, to keep the vehicle narrow.
The stability of any tricycle in a corner is determined by its geometry - essentially the further apart the wheels are from each other, the less easily it overturns. Stability is also dependent on the location of the centre of gravity of vehicle and rider - the lower the rider the better. Cornering stability has become more important as HPV racing has come to emphasise road and circuit races, a shift prompted by the desire to relate HPV design to actual road conditions. The long, low, narrow HPV tricycles with their limited cornering ability have become less competitive against wider track tricycles like the Windcheetah. The lines have also been drawn again between two and three wheelers. Currently the fastest HPV in the UK is the Nosey Ferret team's Bluebell, an Avatar semi-recumbent bicycle with large but aerodynamically efficient bodywork. Though Bluebell is comparatively tall, any bicycle is inherently lighter and narrower than a tricycle and can be leant into comers. Bluebell's engine, Tim Gartside, reckons that the more upright riding position adopted on the Avatar enables him to apply more thrust to the pedals than in the recumbent position. So Bluebell's adversaries have to resort to praying for wind (the crosswind stability of such a large area of bodywork is not high) whilst they comb Covent Garden for more Australian solicitors.
In HPV design there are no universal answers. For races with crowded comers on windy days, recumbent tricycles like the Windcheetah are ideal. The low centre of gravity and wide front track give good cornering stability and the lack of bodywork enables the rider to lean into comers. Under those conditions, a number of Bluebells together in one race would be as exciting a prospect as watching Zeppelins mate.
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