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Comment by afandian | Hacker News Reader

Can you ELI5 why I can balance ride with no hands on a moving bike but not balance on a stationary bike? It feels like there's something pushing toward a stable state, and I always thought that was gyroscopic.

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It's the combination of steering and forward momentum the gives the negative feedback. Bike tips left -> steering turns left -> circular path -> (from moving frame of reference) centrifugal force tips the bike back upright.

Without forward movement you miss the centrifugal force that tips the bike back up.

Yeah, if you think about it. If you are moving forward, and your steering tips left, the force being applied from the ground is now on the right side of the front wheel. This force is only being applied while you have forward momentum.

It is primarily the forward sweep/caster of the front wheel forks which makes the wheel turn into any lean and the forward motion of the bike with the turned wheel tries to bring the bike back under the overall balance point. With the steering axis/forks in a perfectly vertical position where the wheel's contact point and steering axis are perfectly aligned, the wheel will not really respond to leans and you would have to actively turn it to balance. And with a negative caster the wheel would want to actively turn away from any leans making the bike fight against balancing.

While the bike is stationary there's limited options for moving the bike relative to your body. While the bike is moving, you can make small steering adjustments which move the bike left or right relative to your body, which helps re-balance the body-bike stack. The faster forward you're moving, the faster these steering adjustments take effect.

Thank you. That makes sense. It does feel like an external force helping me to balance, but expectation is such a big part of perception.

If you practice you can do it no-hands on a stationary bike as well. Taken me a few weeks to learn, but cool to stop at a red light and do a no-hand trackstand ;)

I’ll try it! Maybe have another go with the unicycle whilst I’m at it.

It's easier if you can find a sliiiight incline to do it against. So for instance if the road slopes a bit to the right (more elevated on the left side), I turn my front wheel to the left, "up" the small incline. Then I can push the pedal to go a cm forward, or release it a bit to roll a cm backwards. So it kinda simulates a unicycle, except you use the slight incline to do the backpedaling you would do on a unicycle. The better you get at it, the flatter you can go, and when you find the balance you can lift your hands. Good luck!

Here's me doing it: https://gopro.com/v/QoMmEVLp7pJyX

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