Why are recumbent bikes unstable?

Question

OK, why are recumbent bikes LESS stable that their traditional counterparts? The recumbents that I have ridden are very darty at low speeds, such as when climbing a hill. I found them to be comfortable at cruising speeds.

Answer 1

Simple Newtonian physics.

1) Consider the horizontal plane that is parallel to the ground and intersects the front and back wheels. It is located much higher on an upright bike than for a recumbent bike.

2) Upright bikes distribute weight of the rider along the vertical plane above the seat. This means that there is significant mass at a further distance from the seat and the plane mentioned above. The more mass at a distance, the greater the stability when in motion. I realize it is upside down, but think of a pendulum, as in a grandfather clock. The longer the length of the rod, the longer the time it takes to swing back and forth in a cycle. That is due to the mass being further from the point of attachment.

3) A recumbent has most of the rider's mass at a much closer level to the seat, which itself is closer to the essentially horizontal plane that runs between the front and back wheel. The wheel diameters are also usually smaller on recumbent bikes.

All of this combines to have the effect of lesser stability, in motion or at rest.

In addition, we are most accustomed to be upright during movement. We tend to react quickly to extend our legs or arms to stabilize ourselves, if we sense that we are not balancing well. On a recumbent bike, this is a shorter distance so the rider's reaction time has to be faster to compensate for instability.