A vehicle that's moving forwards has kinetic energy (proportional to the square of its speed). If it wants to come to a standstill, it has to lose that kinetic energy. Neglecting air resistance, the kinetic energy has to be lost at the wheel-pavement interface.
For each individual wheel, decelerating causes the vehicle to experience a forward turning moment relative to that wheel's axle. In vehicles with multiple ranks of wheel, that transfers weight away from the rear wheels towards the front; with just one rank that can't happen: the tendency is for the rider to flip over the wheel and smack face-first into the dirt.
To avoid this, the rider leans backwards when stopping. By transferring their centre of gravity so it's behind the wheel rather than over it, they create a backward moment about the wheel: where this would normally mean they fall off backwards, the trick is to balance the forward and backward moments exactly so the rider remains on the vehicle as it stops.
The faster the deceleration, the further back the rider must lean to compensate. Once the rider is horizontally behind the vehicle's wheel (even assuming they were a good enough rider to achieve that), they've reached the limit. It's impossible to decelerate faster than that without coming off the vehicle forwards.
The faster a vehicle is going, the faster it might need to decelerate in an emergency.
Unicycles travel quite slowly, which is why — even though their emergency braking capability is actually considerably poorer than an equivalent bicycle — this doesn't matter much. However, that Uno goes at 25mph, which means it has four times the kinetic energy of a pretty fast 12½mph unicyclist, twenty-five times the kinetic energy of a much more typical 5mph unicycle.
Googling, I can find no record of anyone ever having taken any vehicle with a single rank of wheels to a higher speed than 25mph. If one ever got to 50mph, that would be fully a hundred times harder to stop in an emergency than the typical unicycle!
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timeasmymeasure
no subject
Date: 2008-08-07 02:43 pm (UTC)For each individual wheel, decelerating causes the vehicle to experience a forward turning moment relative to that wheel's axle. In vehicles with multiple ranks of wheel, that transfers weight away from the rear wheels towards the front; with just one rank that can't happen: the tendency is for the rider to flip over the wheel and smack face-first into the dirt.
To avoid this, the rider leans backwards when stopping. By transferring their centre of gravity so it's behind the wheel rather than over it, they create a backward moment about the wheel: where this would normally mean they fall off backwards, the trick is to balance the forward and backward moments exactly so the rider remains on the vehicle as it stops.
The faster the deceleration, the further back the rider must lean to compensate. Once the rider is horizontally behind the vehicle's wheel (even assuming they were a good enough rider to achieve that), they've reached the limit. It's impossible to decelerate faster than that without coming off the vehicle forwards.
The faster a vehicle is going, the faster it might need to decelerate in an emergency.
Unicycles travel quite slowly, which is why — even though their emergency braking capability is actually considerably poorer than an equivalent bicycle — this doesn't matter much. However, that Uno goes at 25mph, which means it has four times the kinetic energy of a pretty fast 12½mph unicyclist, twenty-five times the kinetic energy of a much more typical 5mph unicycle.
Googling, I can find no record of anyone ever having taken any vehicle with a single rank of wheels to a higher speed than 25mph. If one ever got to 50mph, that would be fully a hundred times harder to stop in an emergency than the typical unicycle!