Although I don't know it's full history, I do know the lawnmower began as a flatland trick in '80s and was quickly taken to kick-turn wedge ramps soon after, and eventually as a lip trick on quarterpipes. The most significant thing worth mentioning about this trick is that it's best done with brakes, but I'm sure not having brakes wouldn't stop you from learning it, but it'll be a lot harder and you probably won't be able to get them as consistent as the lawnmower master, Clint Reynolds.
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After doing a BMX 'zine for ten years (1987 - 1996) FATBMX made it on the internet. The year was 1998. Here to stay.