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Erskine-JAPwas a motorcycle produced from 1955 to circa 1960.
This was a typical speedway machine built by Mike Erskine, in small numbers. It was fitted with the usual JAP engine, countershaft, rigid frame and short-travel forks, in contrast to the usual Jackson-Rotrax.
I have learned the Mike switched from building speedway bike frames to racing cars already in 1951. Also, the Rotrax frames can be seen as copies of the Erskine frames, because Mike's frames were very successful. The differences are very small in the principle they were built. Erskine built the frames the way his customers' wanted, so there were frames of different sizes, to fit specific riders. As far as I know, Rotrax has only made two different frames, the Mark I and, later, Mark II.
The British Motorcycle Directory - Over 1,100 Marques from 1888 - by Roy Bacon and Ken Hallworth. Pub: The Crowood Press 2004 ISBN 1 86126 674 X