Stan Wagon, a mathematician at Macalester College in St. Paul, Minn., has a bicycle with square wheels. It’s a weird contraption, but he can ride it perfectly smoothly.

His secret is the shape of the road over which the wheels roll.



A square wheel can roll smoothly, keeping the axle moving in a straight line and at a constant velocity, if it travels over evenly spaced bumps of just the right shape. This special shape is called an inverted catenary.



More here.