The Foxwell Supercharger

The unusual supercharger that Messrs Irving and Willis chose for their experiment was a product of Daniel Foxwell and Son of Cheadle, Manchester. The main interest for this long-established company was textile machinery and the Foxwell blowers appear to have been a sideline, apparently intended for use with industrial vacuum cleaners. However, it was a compact unit, suitable for the initial purpose and, eventually, relatively easy to install on a motorcycle. This one component sets Clara apart from any other KTT Velocette. While Ivan Rhodes could gather all the other bits he needed for the restoration from various sources, without a replacement supercharger it looked as if the project would stall. Salvation arrived courtesy of Velocette enthusiast Dennis Frost, who had managed to track down some original Foxwell drawings. Fellow enthusiast Chris Wiggins did some updating to the design and his brother Rob produced the casting patterns. Another enthusiast, Harold Beal, offered his services and eventually took over the castings for finish machining, as well as dealing with the other internal parts. The support of the vanes is unusual on the Foxwell unit. A circular, aluminium bronze plate locates within each end cap. Each plate has a circular groove that determines the path of six aluminium segments. Each segment has a circular hole to support a round piece of brass. Each brass piece has a slot to locate the end of a vane, but the design also permits a certain amount of float. It leaves a minimal clearance between the vanes and the housing. Compared to other centrifugal compressors, the unusual bearing design reduced some frictional losses, but created others. While the drawings that Dennis Frost had located had enabled the castings to be made, there were no precise dimensions for the machining of the supercharger. Harold is a retired engineer, but he also has a comprehensive home workshop and a knowledge of CAD (computer aided design). He worked his way through the components until he could prove, in theory, that machining everything to the parameters he had worked out, the supercharger should do as its designers intended. The clearance between the vane and the housing was just one-thousandth of an inch! It took many hours of workshop time to complete the work but, eventually, it was time to put the re-created supercharger to a ‘dry test’. Ivan supplied a Best & Lloyd oil pump, which was the same as that fitted originally to Clara.the rest of the trial jig included a 2hp electric motor to drive the supercharger. Apparently, used in this application, the mechanical operation of the vanes absorbs a lot of power. In Harold’s opinion, the original set-up was probably worth no more than an additional three or four horsepower. The supercharger was run for an hour, dismantled and checked, then run for a further hour. With nothing untoward, it was handed over to Ivan for him to carry on with the rebuild.