Why is it easier to balance on a moving bike than a non-moving one (revisited)?

Cecil's Columns/Staff Reports
21

I think the Dope Staff should update this article. It’s pretty clear that for humans riding bicycles, gyroscopic forces are mostly insignificant. Evidence includes calculation of the forces, and practical demonstrations of various bicycle-like devices with no gyroscopic effect (ski-bikes and the test bicycle with an extra counter-rotating wheel in front) that are able to be ridden perfectly well with the same technique as standard wheeled bicycles. (The gyroscopic forces are strong enough to help balance a unattached wheel or hoop rolling by itself, but not enough to affect a bicycle with rider.)

The main reason one can balance on a moving bike is that with forward motion and steering, the rider can move the bicycle wheels back under their center of mass (quicker than bicycle falls over). Negative trail, where the geometry is such that the wheel naturally turns to correct a lean, helps the rider balance, but it’s not completely necessary. Evidence for this is that bikes with positive trail can be ridden, but it’s much harder.

22

I think Strangelove is probably right, here. It’s really difficult for humans to consciously notice all the little tiny shifts of balance that happen and the unconscious corrections. And so it’s really easy for humans to take advantage of a momentary lean without realizing it.

I mean, do the physics, Chronos. If a perfectly upright bicycle was rolling forward and the front wheel was steered to the left, the front of the bicycle would move to the left, putting the center of gravity on the right side of the wheels. And there’s no possible source of torque to keep everything from falling over to the right.

23

Exactly!

Too much trail can also be a problem because it tries to over-correct for the lean - usually referred as “wheel flop.” The rider is constantly having to fight against it.

24

As Chronos has noted, I got this wrong - should be “If you push the left handlebar briskly forward …”

25

True, for certain values of “suddenly”.

If you do this just a bit less suddenly, you have initiated a turn to the right.

26

This is exactly the definition of countersteering. Turning the bars initiates a fall and then that fall is “caught” by the lateral acceleration of a turn in the opposite direction.

Oh, but it does. This is a settled question in the field of vehicle dynamics.

The classic experience that drives this home to most people is finding oneself an inch or so from the curb or the edge of the pavement. It’s very hard to move away from that edge. If one could initiate a lean (and thus a turn) without countersteering, it would be easy: just do it. But it’s hard because there’s no room for the required countersteer toward the curb that initiates the turn away from the curb or rut. One must instead edge away slowly, which is regular turning but with very small-amplitude countersteering.

One can countersteer weakly by shifting one’s weight, which is how one initiates a turn when riding hands-free.

Chronos, I agree with Dr. Strangelove and Quercus: your experiment, laudable though it may have been, was likely contaminated by subconscious weight-shift countersteering. And Quercus raises an important point: if countersteering wasn’t responsible for shifting your center of mass out of vertical alignment with your tire contact patch, what was?

I don’t mean to go all Clever Hans on you, but human beings are terrible data acquisition devices. A subconscious hip-shimmy seems more likely than the discovery of an previously unknown steering mechanism for two-wheeled vehicles.

27

I concur. While the article contains a prominent link to an updated article (and a less-wrong one to boot), we’re still left with a staff report that gives the wrong answer. While Karen’s mea culpa in the updated article is just fine, wouldn’t it make more sense to delete this article and append its text to the corrected version?

Failing that, an explicit disclaimer at the top of this article would be helpful. Currently, we’ve just got “please see the update to this article,” which doesn’t really convey that the current article is just wrong. Something like: “Correction: this staff report contains a major error; see this link for a more accurate answer.”

I’m sure someone can come up with a more succinct blurb. But as it stands, this article is less “ignorance fought” than “ignorance promulgated.”

28

Which proves nothing, because there’s also no room for steering away.

So you’re saying that countersteering was the only possible way I could have shifted my weight, and that my shifting my weight was what caused the countersteering. Which one do you assert caused the other?

29

What do you mean by “shifting my weight was what caused the countersteering”? I didn’t think anyone claimed that.

30

EdelweissPirate said it right there: Subconscious weight-shift countersteering.

31

Well, obviously it is possible to affect steering or weight shift without touching the handlebars, otherwise nobody could ride a bicycle hands-off. But weight shift requires external force, so it’s not as simple as “move your upper body to the right to shift your weight to the right.”

I think the only available force is the traction of the tires: when you move your upper body to the right, you are really bending your body, and pushing with your feet/tires towards the right. This is a very weak mechanism, and obviously not a dominant control mechanism for a bicycle - if it were, it would be just as easy to take a bicycle, lock its handlebars, and balance on it (stationary) without using your hands.

ETA: thinking about it more, it’s possible that when riding hands-off, the lateral force (generated by bending your body) is not shifting your weight, but actually generating steering input through trail. (Bend your body to the right, which pushes tires to the right, and trail converts this lateral force into a rightward steering input.) This explains why some bicycles are much easier to ride hands-off than others.

32

It works both ways. Just as how on a fixie bike, your legs can drive the pedals or the pedals can drive your legs.

If you wish to turn right hands-free, you must tilt both your body and the bike to the right. But first you must tilt your body, and due to good old Newton, that means applying force to the bike to the left. Due to trail, this will manifest as a subtle turn to the left.

One way of looking at it is that the angular force along the bike’s axis and the steering input are tied together. How is it that a bike leaning into a turn stays upright? Answer: centrifugal force counteracts the force of gravity on the bike and rider. How is it that the bike experiences centrifugal force? Answer: it’s traveling on a curved path. Why is the bike traveling on a curved path? Answer: the front wheel is steered in that direction.

So there’s a direct relationship between the two, but it’s not limited to already being in a turn. If you wish to make a right turn, you must first get the bike tilted in that direction. And if you’re currently upright, then you must apply a torque along the axis. Applying a torque to the right requires first steering left.

This can be done gently, even imperceptibly: a very tiny input starts the tilt, and after that you lag the amount you need to stay upright. For instance, say you steer left by 0.5°. The bike starts tilting over, and once it’s 10° over it requires a 5° steering input to counteract gravity. But you’ve only steered by 4.5°, and so the bike tilts farther. It hits 20°, which requires a 10° input, but you’re only applying 9.5°. And so on, until you reach the desired angle.

If you wish to turn quickly, this can be done all at once. You apply an aggressive countersteer until you are at the desired level, and then you apply steering appropriate for the turn. This is more difficult since the transition is harder, but the principle is the same. And it’s utterly necessary for anyone doing “serious” bicycling, like BMX.

All of this is symmetrical. If you ride hands-free, then you must first tilt in the appropriate direction to make a turn. And tilting in that direction requires first accelerating the bike in the opposite direction, which will naturally cause some countersteer. But it’s going to be a very subtle effect.

You might imagine that you could do it purely with weight shifting. If you simply locked the handlebars from turning left at all, you could get the bike tilted over without the countersteer. But you still applied a countersteer force; it just happened to be blocked. And most likely you fell on your ass before doing anything, because it would be impossible to ride such a bike in a straight line.

I have a rotary encoder on order. Hopefully it’s precise enough to show the effect. Might be a few weeks before I have the time to set it up, though.

33

Alternatively, you turned the stearing wheal to the left, the front wheel goes to the left, the center of mass ends up on the rights side of the wheels, and you maintained ballance by leaning to the left.

If it takes both actons to execute a turn, and they have to happen simultainously, it can’t matter which one is “first”.
People on heavier bikes may choose to counter-steer to throw the bike over, but it’s not the only meathod of doing so.

34

Actually, the centre of balance of your body is in a different position than the centre of balance of the system, which allows you shift the relationship between the centre of balance of the system, and the centre of balance of the earth.

And if there was indeed “no possible source of torque”, the bicycle would be stationary. The brakes and the motor may both be used when turning a motorcycle, and I suggest that the same is true of a bicycle.

35

I didn’t say it proved anything, only that many people found the experience compelling.

I’m not sure I follow your point about there being no room for steering away. That might be true in the case of a curb, but this happens whether the thing you’re steering away from is a “positive” feature (a curb that sticks up) or a “negative” feature (such as an abrupt drop-off in the pavement). What’s limiting the room for steering away from a negative feature if, as you suggest, no movement toward the feature whatsoever is necessary to steer away from it?

You’re right; we’ve been imprecise in our choice of terminology. For lack of a better term, let’s use “body english” to mean a motion that changes the angle of the bicycle relative to the plane of the road without moving the system’s center of mass. We can then use “weight shift” to mean the state of having the system’s center of mass no longer directly above the tire contact patch.

I’m suggesting that you used subconscious body english to lean the bike. As a result of this motion, trail caused the bike to steer opposite the direction of the intended turn. Momentum then tilted you back the other way (high-sided, in the parlance of our sport) such that the center of mass was no longer directly over the tire contact patch and you began turning normally.

The reason I believe this is what happened is because this is what conventional vehicle dynamics says is happening, and it jibes with my personal understanding of the dynamics. I find Chronos’ knuckle-steer experiment uncompelling, and I gather he feels the same way about my account. Fair enough.

But really, Chronos, if you think you’ve legitimately found something that doesn’t agree with the currently accepted theory, you’ve got a a paper to publish, don’t you?

If you’re not just playing devil’s advocate, then, how is it that you move the center of mass out from directly above the contact patch without countersteering?

36

I understood Quercus to mean that there was no possible source of torque about the bicycle’s axis of travel, not that there was no torque at the brake disc or the rear wheel. That said, I think turning from a steady-state travel is complicated enough without adding in acceleration or deceleration.

37

Oooh! That’s exciting!

I think your post articulated the argument I’m trying to make better than I did. Do keep us posted on your adventures with the encoder!

38

I maintain that it’s not the currently-accepted theory, in that a theory must be well-tested, and so far as I can tell, this whole countersteering thing is barely tested at all. Most of what passes for “proofs” of countersteering is silliness like welding the fork of a bike in place and then finding that it’s impossible to steer. I agree that my own experimental test is simplistic, but it agrees with the intuitive notion of steering, and I’ve yet to see anyone show any more sophisticated experiment that doesn’t.

39

Do you agree with the following?

  1. Normal straight-line progress on a bike is a continuous process of noticing that the bike is starting to tilt left or right, then turning the front wheel to that side to counteract the tilt.
  2. A normal turn requires that the bike lean in the direction of the turn, and that the rider maintain the lean until the turn is complete, at which point the upright position of #1 must be re-established.
40

I’d quibble that the driver doesn’t usually notice the slight tilts in #1, and instead corrects for it through a combination of automatic human balance reflexes and automatic trail stabilization effects, but other than that, sure.