Straight Dope Message Board > Main Why is it easier to balance on a moving bike than a non-moving one (revisited)?
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#51
Yesterday, 07:11 AM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 67,280
Quote:
 But how do you push sideways againt the ground, through the tires?
My preferred answer to this would be to hop on a bike and say "like so" while doing it. I'm not sure where the confusion is arising, so I can't give a more detailed answer in text.

But perhaps a thought experiment would help. Imagine an object like a bicycle, but with no moving parts: The steering fork doesn't rotate at all, and the wheels don't turn. It's perfectly bilaterally symmetrical. You hop onto the bike while it's exactly upright, and do your best to balance. We both agree that you won't be able to balance for very long... but I maintain that you will at least be able to exert sufficient control to determine which direction you fall over. Well, the same motions you make to accomplish that, on a "bicycle" that definitely doesn't countersteer, you can also make on a real bicycle.
#52
Yesterday, 01:04 PM
 scr4 Member Join Date: Aug 1999 Location: Alabama Posts: 12,840
Quote:
 Originally Posted by Chronos My preferred answer to this would be to hop on a bike and say "like so" while doing it. I'm not sure where the confusion is arising, so I can't give a more detailed answer in text.
Then describe the simplest model for a bicycle+rider that can have this ability. If we model it as a rigid object with an additional movable weight in the middle (at the CG), would you agree that this system can only exert a small and short-lived force, equal to the mass x acceleration of the movable weight?

Quote:
 But perhaps a thought experiment would help. Imagine an object like a bicycle, but with no moving parts: The steering fork doesn't rotate at all, and the wheels don't turn. It's perfectly bilaterally symmetrical. You hop onto the bike while it's exactly upright, and do your best to balance. We both agree that you won't be able to balance for very long... but I maintain that you will at least be able to exert sufficient control to determine which direction you fall over. Well, the same motions you make to accomplish that, on a "bicycle" that definitely doesn't countersteer, you can also make on a real bicycle.
So your claim is that this lateral force, which you agree is insufficient to maintain balance, is still sufficient to initiate a turn?
#53
Yesterday, 02:31 PM
 Machine Elf Guest Join Date: Feb 2007
I'm still tired from the discussion we had on steering two-wheeled singletrack vehicles last fall. Rather than contributing anything new here, I'll just link to my summary from that last discussion.

TL,DR: Countersteering is the only way to steer a two-wheeled (or two-skied/skated) singletrack vehicle. The countersteer may happen directly (with hands on bars), or indirectly (hands-off, shift your weight, bike leans the other way, steering geometry effects a countersteer). On (lightweight) bicycles, the countersteer is often extremely subtle (a fraction of a degree) and can be induced with tiny forces on the handlebar (measured in ounces), so it's easy to miss.

Dr. Strangelove, I will very much look forward to your rotary encoder measurements.
#54
Yesterday, 05:15 PM
 Dr. Strangelove Guest Join Date: Dec 2010
Quote:
 Originally Posted by Machine Elf Dr. Strangelove, I will very much look forward to your rotary encoder measurements.
Ahh great, now I'm on the hook!

I hope the signal is visible through the noise. The encoder is 600 pulses/rot, which with the quadrature encoding means I get 2400 ticks/rot. So in principle, it's good down to 0.15 degrees. We'll see if that bears out in practice, and if I can manage to get a stiff enough coupling to maintain that accuracy. The project might take some time if it turns out that I have to 3D print some brackets or such.
#55
Yesterday, 11:31 PM
 Irishman Guest Join Date: Dec 1999
Having had my own experiences with riding too close to an object and not being able to turn away from it, I now understand how countersteering is necessary.

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