In 1939, the American bolt-on "Whizzer" gas-engined bicycle kit was introduced, utilizing a 138 cc side-valve four-stroke engine, the first whizzer was of the friction drive variety, but was soon replaced with belt drive. Despite some initial engine reliability issues, the "Whizzer" enjoyed modest popularity during World War II due to fuel and automobile shortages, and was used by war plant workers and others with priority for transportation. After the war, the "Whizzer" became popular with youth who desired faster speeds from their heavy cruiser-framed "Schwinn" bicycles. In 1949, the company introduced a complete production bike, the "Pacemaker". Sales of the Whizzer conversion kits continued until 1962. In the United Kingdom, the motorized bicycle saw a resurgence of popularity and such bolt-on motors as the "Cyclaid" and the "Cyclemaster" motor wheel saw brief periods of immense popularity. The "Cyclemaster", which was a hub motor that could be fitted to an ordinary bike, started at 25 cc (painted black), but later the size went up to 32 cc (painted grey).