The Intamin straddle coasters are very different to the Vekoma motorbikes in terms of vehicle design, though. On Juvelen you’re still sitting upright and held in by a lapbar, with the handlebars mostly for show (you can grip onto the lapbar like any other coaster and ride just fine):
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Whereas the Vekoma motorbikes have you lean forward on your stomach and with your feet planted on the sides of the vehicle, with a heavy bar securing you from behind:
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The latter is more equivalent to riding an actual motorbike and thus much more restrictive. I would hazard a guess that the 1.4m requirement is a manufacturer reccomendation given both Booster Bike and Velocity has it.