Does a bike-stalling speed exist? [closed]

Question

Planes can stall. A plane maintains lift through speed. If the speed drops below a threshold, the plane stalls and begins a downspiral.

Now that many 1x modern bikes are discarding front derailleurs, it’s becoming almost common to see lavishly large rear sprockets. When the teeth number 50-52, it starts to look like a chainring.

At the same time I read, with a bit of amusement mixed with bewilderment, cyclists wondering whether they can swap the (single) 34-tooth chainring on such bikes with a 28-tooth chainring.

This makes me wonder how far the gear ratio can go.

If the ratio can approach 1:3 (front : rear), can a bike be usefully fitted (leaving out derailleur feasibility from this discussion) with a 1:4—or a 1:5—ratio? Will the cyclist at some point not be at too high a risk of stalling. How far would manufacturers go? ("Your Honor, my client cracked his cranium riding; he didn’t expect the maker to assume he’d be an acrobat, not a cyclist.")

But if we ask simply about the minimum speed for a cyclist to maintain balance, then each of us will recall the folks who can seemingly balance on the spot indefinitely while waiting for the lights to turn green. Perhaps you yourself can do it.

Hence the “minimal speed that still permits maintaining balance” is a simple Zero.

A twin question to the minimal speed question is why anyone would want, say, a 1:5 gear ratio. We could speculate. On a stationary bike it’s possible to vary the workout to a precise “n-minutes in HRZ 5, followed by m-minutes cooling down to HRZ 3”. On a moving bike the slopes dictate the workout, and a very wide gear ratio might help design a specific workout despite the terrain.

Answer 1

The trick to pulling a track stand is that you are not in fact stationary. You rock forward and backward slightly. So it's not accurate to say that the minimum speed on a bike is 0. In any case, very few people can sustain a track stand indefinitely. It's fair to say that skill is a factor in holding a track stand, and I'll speculate that more skilled riders will have a lower stall speed than less skilled ones.

This video has a lot more on the subject. Caster effect can help but isn't needed to keep a bike upright (Artistic cycling bikes have minimal caster). And it makes the point that "understanding how bicycles work is still an active area of research."

Anyone who has slogged up a very steep grade on a bike has probably experienced stall speed. It's not a matter of running out of gears, it's a matter of making so little forward progress that you can't keep the bike balanced dynamically. As the video says, a bike acts as an inverted pendulum, like a broomstick balanced on your hand, and you need to keep moving the bike underneath you to stay upright. Because you can't move the bike side-to-side directly (barring bunny hops), you can only steer it under you as you move forward. Stall speed, as I understand it, is the speed at which you can't get it underneath you quickly enough to stay upright.

Answer 2

There is another kind of "stall" speed, which is to do with the maximum grade you can effectively climb.

As the road pitches up, your rear wheel takes more weight and the front wheel has less.

The less weight on the front wheel,the less bite/grip it has on the road surface, and steering "authority" reduces

There is a positive gradient at which all your weight is balanced over the rear wheel's contact patch, and this has no weight on the front wheel at all. At this grade, you cannot steer effectively which means balancing can only come from body position. This could be construed as a Stall Gradient if you like.

You can get off the saddle, move your hips toward the stem, lean forward over the front wheel, but all this just moves the limit.

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Example - I know of a short climb at around 40%, and that's at the limit for me. Probably not a great answer, but another way to view the problem.

Answer 3

Best control of a two wheeled bike includes some forward momentum. By design (trail)--and physics (caster)-- the front wheel of a bicycle will want to maintain a straight line when rolling. That's a big reason why riding "no handed" is relatively easy. However, one must maintain a certain forward momentum for the caster effect. This, one could argue that a bicycle stall speed is the velocity at which the caster effect is defeated by other forces and the front wheel moves related to those other forces rather that caster.

That said, one of these other "forces" is steering input of the rider and another his or her center of gravity and it's relationship to the balance point of the bicycle. Many, many years ago there was a movie, I believe, called "Quicksilver," about a young man trying to succeed a bicycle racing. One of the memorable scenes was during a training ride where the protagonist was forced to stop at a red light. Being clipped in (via clips-toe holds with straps that nearly bound your foot to the pedal), he successfully remained stopped, upright, with feet still clipped into the pedals for an inordinately long time until the light turned green. After viewing that, my buddies and I rarely took a ride without emulating that feat. To this day, when forced to halt for a light, my feet remain on the pedals as long as I can. At times it must've been an impressive amount of time based on the honks and thumbs up and outright applause I've witnessed from other drivers sharing the intersection. Lol. Believe me, I've had some epic fails too when I've failed to maintain balance while also failing to get unclipped in time to save my dumbass. Point is, a stall can be avoided by controlling other forces like steering input and center of gravity over the balance point that can act in ways that make the bike remain upright while at rest with a rider on. Balancing a bike at an intersection takes a lot of manipulation of the handle bars/front wheel in response to sensed changes in the balance of the rider+bike system.

So, two conclusions that only bring to mind more questions. A stall is the point at which the caster effect peters out. The velocity of which this happens varies based on the geometry and size of the bike, width and diameter of the wheels. A stall can be avoided by incorporating other forces to bear on an unmoving bike-rder system. I'll edit as time permits to discover more about caster and old movies.

Answer 4

A bicycle "stalling speed" does not exist because bicycles do not stall like an airplane. Stalling is a reduction in the lift coefficient generated by a foil as the angle of attack increases. There is no lift, no foil and no angle of attack on a bicycle.

The original post asks "Does a bike stalling speed exist?"

The post tires to equate stalling an airplane with losing balance on a bicycle

In an airplane "stall" is defined as:

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.

Later

Stalls in fixed-wing flight are often experienced as a sudden reduction in lift as the pilot increases the wing's angle of attack and exceeds its critical angle of attack (which may be due to slowing down below stall speed in level flight). A stall does not mean that the engine(s) have stopped working, or that the aircraft has stopped moving—the effect is the same even in an unpowered glider aircraft. Stall (fluid dynamics)

The question should be "What is the minimum speed needed for a bicycle to remain upright?"
As pointed out in other answers a track stand requires a slight rocking motion - forward speed is zero but there is still movement in absolute terms.

Another way to ask the question is "How long can a track stand be maintained?"

Those proficient at the track stand can maintain the position indefinitely.
Track Stand

Answer 5

I think there are two speeds to quote here - uphill and flatland.

I have a "basement speed" of around ~3.7 km/h on my recumbent, measured by riding up a steep driveway. Incidentally, I can ride up a slope that is too steep to start on.

My Basement climbing speed on a diamond-frame road bike is unknown - I haven't been able to look at a speedo while on a steep-enough climb.

On the flat, most cyclists can pull off a slow drift that is below 1 km/h and can trackstand on the spot for a few seconds. So nominally the basement speed would be 0, but over what duration.

Answer 6

The stall I've felt going uphill was when my foot couldn't get over top dead centre before I ran out of momentum.

That really is a minimum speed for the circumstances, but depends on the gear. This is most likely when a steep hill gets even steeper briefly, and overall the hill doesn't have to be long. This is like a car engine stalling - not enough torque. Being clipped in makes it less likely, though you're more likely to fall hard if you fail to unclip. It's different to just running out of power, which is certainly possible, spinning the back wheel, or lifting the front wheel thus losing control.

On the flat, track-standing say not have zero instantaneous speed, but it does have zero average speed. It's not something I can do, but even without that, balancing is possible well below 1km/h, so I suggest that on the flat there is no such speed.

Answer 7

Stall Speed

We define Stall Speed as the lowest speed at which a given cyclist can ride on a flat terrain without losing balance.

• There does exist a minimum speed—the Stall Speed—but that speed is an epsilon—it is a very low value for seasoned cyclists.
• The low speed before stalling depends on the technical ability of the cyclist—the more talented the cyclist, the lower the speed.
• A beginning cyclist or a child who has just learned how to cycle will have a substantially larger Stall Speed than a seasoned cyclist or a more skilled child.
• Reducing the size of the chainring to an arbitrary lower one makes perfect sense. It makes perfect sense for cyclists to go to 1:3—or even 1:4 or 1:5 gear ratios if they like.
• A cyclist may want to ride very slowly on flat terrain for a number of reasons. Accompanying slower humans or slower pets, or getting a lot of movement without necessarily raising their heartrate too much are some of the possible reasons.
• Cyclists may choose an unusually low gear ratio to climb very steep and very tall mountains—mountains that are normally beyond their ability. This would make it possible to climb without running out of breath, without taking breaks, and without necessarily otherwise trying to reduce the weight of the bike itself.

Credit goes to Jeff, Adam Rice, and MaplePanda.

Stall Gradient

Criggie introduces a related and very important concept: the Stall Gradient. You have surely witnessed beginning cyclists and children walking their bikes up very mild slopes. They walk rather than ride because that slope is steeper than their "Stall Gradient".

The Stall Gradient is closely related to riding on a flat path. When kids struggle to balance, it is because their balance ability is still so modest it imposes a very high "Stall Speed". But they do not dare yet cycle at that speed. They don't realize they need a particular, high, speed to just stay balanced.

When they progress, they no longer need to ride fast to remain balanced. But even a seasoned cyclist will have some Stall Speed and some Stall Gradient.

Some additional points:

• If the limiting factor is that the cyclist is so slow they are unable to stay upright on the bike, we'd say the "Stall Gradient" is reached, but say the cyclist has no trouble staying upright, what is the limiting factor? No lesser than Mark Cavendish asked a while back: Discussing the 27% climb in today's stage with @martinvelits & wondered what's the steepest gradient you could possibly ride on a road bike?
• "Oddly, power is not a limiting factor to tackling the steepest of climbs." The steepest gradient depends on the limiting factor. If it is is for the center of gravity (CG) to lie between the two wheels' contact points, it's 48%. If the rider sits far forward to move the CG, it's 87%. If it's the coefficient of friction, and the tyre is made of pure rubber, it's 80%. Otherwise it's closer to 60%.
• Of course in practice the numbers never reach anywhere near these limits. Maintaining even just 16% is challenging.
• Even at the Tour de France the number don't go that high. Alpe D’Huez is 13%. Col du Portet is a "punishing" 8.6%. And Mont Ventoux has an 8.5% average gradient.
• To summarize, the limit for serious cyclists may depend on human power, on center of mass, or on friction. But none of these explain why a muscular and healthy/fit person can sometimes be seen unmounting and walking a bike up a 5-7% grade.

Future Questions

There are still a few unclear issues:

• If two cyclists are climbing the same mountain at the same ultra-low speed (side-by-side), but one is pedaling at a cadence of 60 and the other at a cadence of 105, does the higher cadence make it easier to remain balanced on the bike? Is it more challenging for the 60-rpm cyclist to remain balanced and hence we are likely to observe that cyclist wiggling the handlebar to stay balanced? If this is the case, that would be yet another reason why someone may want an unusual gear ratio. I myself feel that I am about to stall below some cadence (perhaps 40, I'll experiment).
• Suppose you treat yourself to a nice bike and then discover that you never use, and never anticipate needing the top (say) four speeds on that bike, does it then not make perfect sense for you to be proactive and to reduce the chainring(s) so that your gear shifts "occupy the full spectrum" of the bike you have? If you don't and find years later that these four gears remained unused, then you would have "wasted" the gears and you were riding just an (n-4)-gear bike, rather than an n-gear bike.
• What is the relationship between the Stall Speed and the Stall Gradient? Is it, for example, simply the case that a Stall Gradient is the one at which the power output from the cyclist is insufficient to reach the Stall Speed?