Some of the great mysteries of science hide in plain sight—and, in this case, roll on two wheels. As hands-free cyclists know, a bicycle in motion can recover from wobbles with no steering input, or even with no rider at all. But physicists have never agreed on how this self-balancing act works. Some argue the rotating wheels act like a gyroscope to help the bicycle correct itself; others believe the “trail” (the angle between a bike’s steering axis and its point of contact with the ground) forces the bike into a stabilizing turn. Now American and Dutch engineers have built a bicycle that defies both theories.
The team devised a bike with a minimal trail and with discs that rotate in the opposite direction of its wheels to cancel the gyroscope effect. Despite lacking either form of self-stabilization, the cycle can cruise along riderless. “It shows that what everyone has ...
