The following description of the Calthorpe comes from an original sales brochure of that period.
The Calthorpe "Speedway Special" is the outcome of a wide experience on British Dirt Tracks. The whole machine has been specially designed and built for the new sport of "Dracing", and the successful results that have been achieved on this mount - even against more powerful rivals - enable us to offer it with every confidence to the latest adherents to the sport as well as to the most experienced Speedway riders.
The 3? h.p. Special Super Sports engine has been designed and is built solely in the Calthorpe factory, giving over 20 BHP at 6,500 rpm. Its capacity is 348cc (74 x 81mm bore and stroke). The enclosed Overhead Valve gear runs on roller bearings and is automatically lubricated by oil mist from the timing case. Valves are made of special, practically unbreakable valve steel. A domed racing piston is fitted. Double row roller big end and main bearing, pulley side bearing, are provided, whilst on the timing side a hard Phosphor Bronze bush bearing is used, running entirely submerged in oil. The Magneto is a B.T.H. with variable ignition. The Carburetter is a Racing Amac with adjustable pilot jet.
The cam gear is so designed that a minimum stress is laid on the valve springs and push rods, with the consequence that light springs are used and the risk of failure is practically eliminated.
The adjustable countershaft and clutch only are fitted. Lubrication to all parts of the engine throughout is by an improved and easily adjustable Mechanical Pump.
The frame is specially strutted to withstand the rigors of the track, has an adjustable foot rest, knee iron and special fixed forks.
There are no brakes, the wheels and tyres are 28in x 2?in, while the saddle height is 25 inches. The finish is nickel plate all over. Further details and arrangements for a free demonstration from:
The Calthorpe Motorcycle Co.
Barn Street, Birmingham.