- #1
ILoveParticlePhysics
- 9
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You can try this at home!
Why do things with wheels get more balanced as velocity increases?
Click For Summary
Discussion Overview
The discussion revolves around the phenomenon of balance in wheeled vehicles, particularly bicycles, as their velocity increases. Participants explore the mechanics of balance, self-stability, and the effects of steering geometry and gyroscopic forces on maintaining equilibrium while in motion.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
- Debate/contested
Main Points Raised
- Some participants note that balancing on a bike becomes easier at higher speeds, with one sharing personal experience of maintaining balance even at walking speeds.
- One participant explains that the self-stability of a bike is primarily due to its steering geometry, particularly the concept of trail, which causes the front wheel to steer inwards when the bike is leaned.
- Another participant discusses gyroscopic precession and its role in resisting changes in steering angle, suggesting that at moderate speeds, it helps prevent over-correction.
- There is a challenge regarding the description of the trail concept, with one participant questioning whether the intercept point of the steering axis is indeed ahead or behind the contact patch of the tire.
- A participant describes an experiment with a cruiser bike where the front wheel was turned backwards, noting that this configuration allowed for stability at lower speeds compared to the standard setup.
Areas of Agreement / Disagreement
Participants express differing views on the mechanics of balance and stability, particularly regarding the role of trail and gyroscopic effects. There is no consensus on the exact mechanics or the implications of the configurations discussed.
Contextual Notes
Some claims rely on specific definitions of terms like "trail" and "gyroscopic precession," which may not be universally agreed upon. The discussion includes personal observations and theoretical explanations that may not fully account for all variables involved in bicycle stability.
- #2
Nugatory
Mentor
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There’s a pretty good explanation at http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/bicycle.html
- #3
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I was on my bike yesterday (I was on a path shared with pedestrians, so I was often going at walking speed) and I noticed that it was quite easy to balance as long as I kept moving, no matter how slowly; but not if I stopped altogether. The slower I went, the more the bike zig-zagged, which I guess is the idea of using a series of circular paths to maintain balance.
- #4
rcgldr
Homework Helper
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The article itself stated that gyroscopic reactions play very little role in the self stability of a bike (bicycle or motorcycle). The key factor for a typical bike is a steering geometry that tends to steer the front wheel inwards if the bike is leaned. The most common method for this is called trail, if you extend an imaginary axis from the steering axis of the front wheel, it intercepts the ground ahead of the contact patch of the tire. When a bike is leaned, there is Newton third law pair of forces, the tire exerts a downwards force onto the pavement, the pavement exerts an upwards force on the leaned tire, behind the pivot axis, which causes the front tire to steer inwards. Depending on the amount of trail, there is some minimum speed for self-stability.Nugatory said:
In order to do a normal turn, the bike needs to be first leaned inwards, and this is done by steering slightly outwards so that the bike leans inwards. Counter-steering is also used to adjust lean angle.
Gyroscopic precession is a reaction to the torque related to changes in lean angle. If the lean angle is constant, gyroscopic precession is zero. Angular momentum of the front tire resists changes in the steering angle, and acts as a dampening (opposing) torque to the trail related self-stability of a bike. At moderate speeds, this prevents constant over correction. At high speeds (100+ mph), the angular momentum is so great that the bike ceases to self-correct. Mathematical formulas for infinitely thin tires predict an extremely slow inwards fall, called capsize mode, but for real tires under real circumstances, the perceived reaction is that a bike simply holds the current lean angle rather than self-correct to vertical, and requires conscious counter-steer with a lot of effort in order to change lean angle.
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- #5
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Behind the contact patch, surely? The front forks curve forwards. Or am I misunderstanding something?rcgldr said:
- #6
A.T.
Science Advisor
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- #7
A.T.
Science Advisor
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ILoveParticlePhysics said:
- #8
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Right - forgot about the slope of the steering column. Thanks.A.T. said:
- #9
A.T.
Science Advisor
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Are you not born to wild?Ibix said:
- #10
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I did have one of these as a kid, but no, I don't think I'm sufficiently cool to ride that...A.T. said:
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- #11
rcgldr
Homework Helper
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I'm trying to find a bicycle self-stability video on an old cruiser style bike where the handlebars were removed, and a front wheel that could be turned backwards, so the forks curved backwards. Unlike most road bikes, old cruiser type bikes have enough clearance for the front wheel to be turned backwards. The result is a large amount of trail, the contact patch is way behind where an imaginary extended steering axis intercepts the ground. In this configuration, the bike was stable at very slow speed, about 1.5 to 2 mph, before it fell. With the front wheel turned forwards, forks curved forwards, the trail was reduced and the bike needed to be going between 5 to 7 mph to be self stable.