ω is positive and α is zero. “Slowing down” means that ω and α have opposite signs, not that α is negative.

First definition Angular velocity is positive when the rotation is counterclockwise and negative when it is clockwise. Second definition: Angular velocity is positive when the angular displacement is increasing and negative when the angular displacement is decreasing.

Linear/tangential velocity, in a circlular path, increases with the increase in radius and decreases with the decrease in radius. Hence, the angular velocity remains the same no matter what the change in radius is(W=V/r).

Riding a bicycle is possible because of angular momentum. The angular momentum of the wheels is a vector that remains constant unless the external torque is imbalanced. When the wheels are not turning, the bicycle and rider are in an unstable equilibrium.

The answer is that you slip and fall and no gyroscopic force of the wheels or anything else prevents this. The answer is that if you try to turn (lean) even a little bit too much the sand flows under the bike wheel and doesn’t provide enough friction to keep the bike up. You fall over.

The accepted view: Bicycles are stable because of the gyroscopic effect of the spinning front wheel or because the front wheel “trails” behind the steering axis, or both. This “trail” gives the force of the ground on the front wheel a lever arm to cause steering in a way that can help restore balance.

Unless torque, or twisting force, is applied from outside the system to change the wheels’ angular momentum, that momentum and the direction of the momentum remain constant. It’s easy for you to move them, but hard for an outside force to do the same, and so the bike is easy to keep balanced but doesn’t topple easily.