Why do things with wheels get more balanced as velocity increases?

[ILoveParticlePhysics]

You can try this at home!

 

[Nugatory]

There’s a pretty good explanation at http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/bicycle.html

 

[PeroK]

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.

 

[rcgldr]

There’s a pretty good explanation at http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/bicycle.html

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.

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.

 

[Ibix]

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.

Behind the contact patch, surely? The front forks curve forwards. Or am I misunderstanding something?

 

[A.T.]

Behind the contact patch, surely? The front forks curve forwards. Or am I misunderstanding something?

https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

 

[A.T.]

Why do things with wheels get more balanced as velocity increases?

 

[Ibix]

https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

View attachment 283959

Right - forgot about the slope of the steering column. Thanks.

 

[A.T.]

Right - forgot about the slope of the steering column.

Are you not born to wild?

 

[Ibix]

Are you not born to wild?

View attachment 283971

I did have one of these as a kid, but no, I don't think I'm sufficiently cool to ride that...

 

[rcgldr]

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.