That’s adult tricycles, of course.
You may think that, having ridden bicycles for years, riding a tricycle would be a no-brainer. But remember when you made the transition from tricycle to bicycle, probably with the aid of training wheels? At first you probably felt pretty unstable—three points on the ground keeps you level; two doesn’t. But at some point you discovered that if you leaned one way or the other the bicycle turned in that direction rather than falling over, and that fact rather quickly got into your muscle memory. Without even being consciously aware of it, you began to rely on weight shifts, rather than the handlebars, for steering.
The reason a bicycle reacts that way is conservation of angular momentum. I’m not even going to try to explain it (it takes calculus) but if you’ve ever taken a physics class you’ve probably seen it demonstrated. A student sits on a revolving chair holding a spinning bicycle wheel. If he tries to change the direction of the axis of the wheel, he will turn himself. A spinning bicycle wheel on a bicycle acts exactly the same way. Its axis tries to stay pointing in the same direction, which keeps the bicycle upright. If you force the axis to move, by shifting your weight, the result will be a rotational force at right angles to the force you put on the axis. Leaning tries to tip the axis vertically; the turn forced is horizontal.
Of course a bicycle does try to tip over if it is stopped—if the wheels aren’t turning. A wheel that is not spinning has no angular momentum—which is what makes the axis resist changing direction.
What happens if we apply the same principles to a tricycle?
A tricycle touches the ground at three points, so it has no propensity to tip over while it is stationary on level ground. But that same three-point stability means that you cannot change the direction of the axis of a spinning wheel simply by shifting your weight. A tricycle has to be steered with the handlebars. Further, the front wheel will resist being turned, and if is turned too sharply when you are going fast, turning the wheel will try to tip the tricycle over. This isn’t usually a problem with a child’s tricycle, because most are direct-driven, with the pedals on the front wheel, and they don’t move very fast. My adult tricycle is a three-speed that easily exceeds 10 mph on a slight downward slope.
Even worse, if you are riding on level ground and hit a side slope, the tricycle will try to turn of its own accord. This leads to real problems with uneven ground, side slopes, and angled curb cuts on bike paths, especially since a tricycle with a rider is very top-heavy.
A tricycle is a bicycle legally, so I can ride on bike paths. I tend to avoid narrow shoulders, for obvious reasons—first because a tricycle is a good deal wider than a bike, and second because most roads are crowned. This means I am riding on a side slope where I must steer very accurately to avoid getting one of the rear side wheels off the pavement. It’s also rather miserable to “walk” a tricycle–the rear wheels keep hitting you in the back of the leg
In short, a tricycle takes some retraining of your reflexes if you are used to riding a bicycle. They are still wonderful exercise for someone like me who no longer has the balance to ride a bicycle.