Why do we lose balance in a bike when at a standstill?

[Gnosis]

The main reason is that when you are moving, steering allows you to move your point of support around. In particular, you need to keep the support vertically in line with your centre of gravity. When you are stopped, you can no longer do this. It's like standing on one foot. If you are not allowed to hop, and you start falling sideways, you cannot recover. But if you can hop to relocate the foot with respect to the c.of.g, you will recover balance.

Krab’s explanation is correct. The scenario is akin to balancing an upside down broom in one’s hand; so long as you can move your hand around as required, the broom can remain in an essentially balanced upright state. Likewise when steering the bicycle, even at low speeds, steering allows redirection of the bicycle to allow balancing corrections. Zero bicycle velocity fails to provide an means to correct the bicycle's balance.

 

[haruspex]

The question of what makes a bike rideable has been properly researched at least twice - once in Loughborough, UK, back in the 60s, and again independently (apparently in ignorance of the earlier work) in the US a year or so back.
For each theory, the investigators built a bicycle which lacked the theoretically key element (e.g. contrarotating wheel to cancel any gyroscopic effect).

Result: gyroscopic effects are useful, but the critical item is the steering geometry.
If you take a line down through the steering column to where it hits the road, you'll see it is in front of the point of contact of tyre with road. As a result, if the bicycle leans to the left the front wheel turns to the left. You can observe this with a stationary bike, though of course it doesn't help you stay upright unless moving forward.
This is why bicycles with small wheels are still rideable.
The gyroscopic effect does the same, but not as strongly in standard designs.

 

[rcgldr]

5) Our (Delft+Cornell) TMS bike and related calculations show that gyroscopic and trail effects are not necessary for bike balance.

True, but the TMS bike located some mass in front of and above the front wheel to produce an effect similar to trail.

Still wondering why the mathematical model for the Delft bicycle predicted capsize (near neutral stability) speed at 8 m/s when the actual bike being modeled ended up being "very stable" at 8.33 m/s (30 kph).

links to articles:
http://home.tudelft.nl/index.php?id=13322&L=1

link to pdf file with diagram showing capsize (near neutral stability just above 0) speed at 8 m/s or higher, figure 1.3 on page 4:
http://home.tudelft.nl/fileadmin/UD/MenC/Support/Internet/TU_Website/TU_Delft_portal/Onderzoek/Wetenschapsprojecten/Bicycle_Research/Dynamics_and_Stability/doc/Koo06.pdf

video is the last one on the page, the 30kph run.
http://bicycle.tudelft.nl/schwab/Bicycle/index.htm

 

[arendschwab]

True, but the TMS bike located some mass in front of and above the front wheel to produce an effect similar to trail.

Still wondering why the mathematical model for the Delft bicycle predicted capsize (near neutral stability) speed at 8 m/s when the actual bike being modeled ended up being "very stable" at 8.33 m/s (30 kph).

Hi,

Arend Schwab one of the co-authors of the Science paper and PhD adviser to Jodi Kooijman here.

The oscillatory weave mode is very stable, which is clearly visible in the video where we see the lateral oscillation die out quickly. The capsize mode (falling over like a ship with no steering involved) is very mildly unstable, an eigenvalue of say +0.1, then for things to double it takes a long time, exp(0.1*T)=2 so aprox T=7 seconds, which is a long time indeed and that is why you don't see this capsize happen in the video. Due to the change in heading after the lateral perturbation, it would have rolled of the treadmill by then anyway.

Arend Schwab

 

[rcgldr]

The capsize mode (falling over like a ship with no steering involved) is very mildly unstable, an eigenvalue of say +0.1, then for things to double it takes a long time, exp(0.1*T)=2 so aprox T=7 seconds, which is a long time indeed and that is why you don't see this capsize happen in the video. Due to the change in heading after the lateral perturbation, it would have rolled of the treadmill by then anyway.

OK, but in the video at 30 kph (8.33 m/s), the bike quickly returns to vertical after being disturbed (the direction changes, but that heppens even when in stable mode due to the distrubance).

I'm thinking that once in capsize mode, the bike would tend to hold the lean angle induced by the disturbance unless the trail / caster effect is still dominant when the bike is disturbed in that manner (tapping the bike sideways just behind the seat).

For motorcyles at sufficient speed, they tend to hold a lean angle as opposed to tending to straighten up. This could be a very mildly unstable capsise mode, one where the time for the bike to fall inwards is so long that it's not perceptible to the rider. The width of the front tire profile could be producing just enough outwards torque when leaned (contact patch on side of tire) to counter the slight inwards torque of capsize mode to prevent a motorcycle from falling inwards.

 

[haruspex]

The question of what makes a bike rideable has been properly researched at least twice - once in Loughborough, UK, back in the 60s, and again independently (apparently in ignorance of the earlier work) in the US a year or so back.

I have been justly taken to task by Andy Ruina of the Cornell team for suggesting they were unaware of the earlier work in the UK. Andy suggests I'm thinking of DEH Jones around 1970 at Imperial, London; possibly, though I recall it as a team at Loughborough ca. 1965.
More importantly, the Cornell work takes matters further than Jones did, finding that the whole answer is rather more complex.
My sincere apologies to Andy.

 

[danmay]

I think the bicycle stability article (Physics Today 1970) by David E.H. Jones has summed up most of this issue. It was posted by another user earlier in this thread, but I can't seem to find it now going back. So here are the links to the same article:

http://www.phys.lsu.edu/faculty/gonzalez/Teaching/Phys7221/vol59no9p51_56.pdf and http://socrates.berkeley.edu/~fajans/Teaching/MoreBikeFiles/JonesBikeBW.pdf.