Mr. Smartypants
Early in the 20th century, until about 1910, motorcycles were more popular than cars because they were a lot cheaper. The Model T changed that. Anyway, there were a lot of experimental designs, and I'm sure someone had the idea of retractable wheels.
There was a company just a few years ago that made retractable wheels for a Honda Gold Wing. They mounted on the back, outside (or in front of) the saddlebags. When you slowed down under about 20 mph, they came down by themselves so you didn't worry about falling over at a stoplight. I'm not sure if they still make it.
parochial school graduate
I read a history of old bikes once and there was one way back that had wheels you put down with a lever. The reason was it was almost a car and you sat in it instead of straddling it. Sorry I do not remember but it had liquid cooling and brass or copper tubes all around it to cool the engine.
curmudgeon55
Not the motorcycle maker-- there is a aftermarket maker of 'training wheels' that have a small tire-12 inch- on each side of the motorcycle that is about to level of the upper part of bottom of rear tire(about 6 inches) and is mounted with adjustable triangulated frames. The one rig I saw was on a Sporty ridden by lady with very weak legs due to medical condition- she said the local Harley dealer had the catalog for it and had put it on for here after last hospital visit- we compared notes on our radiation treatments for cancer and I know resulting weakness side affects.
Tim D
http://peraves.wordpress.com/
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You can buy a trike.