18

When I arrive at a traffic light that is red I usually just stop the bike, stand up and balance it for a minute or two before going on, without ever putting a foot on the ground. Learning to balance a bike like that was easy, I estimate it took less than hour before I could stand indefinitely (well, the static load is quite high so I probably won't stand more than 5-10 minutes due to exhaustion). With some effort and luck I can also release one hand.

BUT, now I have tried to learn to balance the bike sitting down on the saddle and that is basically impossible. Why is that so much harder? Any hints on how to do it?

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    "now I have tried to learn to balance the bike sitting down on the saddle" can you reach the ground with one of your feet while sitting on the saddle? You'll be on a bit of an angle, but depending on your size, the bike's size and it's model, it's often quite doable. – Mast Jun 17 at 8:53
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    @Mast It's not impossible: I do it pretty much every day of my life. It's not dangerous: by construction, you're only doing it when the traffic is stationary and, if it goes wrong, you can just put your foot on the ground. – David Richerby Jun 17 at 9:36
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    To me it's unclear if the OP talking about trackstand or claiming that they can balance without moving at all. – ojs Jun 17 at 12:41
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    @d-b and how do you do the track stand? The standard way, going a bit back and forth, or in place tilting the bike from side to side? – ojs Jun 17 at 20:41
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    @ojs It is not a fixie so I remain in the same spot. – d-b Jun 17 at 21:45

11 Answers 11

27

Just speculation, but I suspect that when we balance standing up, we move our hips laterally to make fine adjustments to the relative position of our center of mass over the wheels. When doing so, we push laterally from our feet on the pedals, which are low enough and far enough below the center of mass to make adjustments relatively easy to accomplish.

With your backside planted on the seat, the main mechanism to laterally adjust your center of mass is to move your shoulders and chest relative to your hips. Now you have a smaller mass on a relatively short lever anchored relatively high up. So a movement of the shoulders mostly results in a countermovement of the hips in the opposite direction. That makes it not only fairly difficult to get much movement of the center of mass relative to the wheel-ground contact, but also difficult to do so in a controlled manner.

11

Your centre of gravity (CoG) aka centre of mass (of you + bike) is balancing over the contact patches of the tires. In fact, over the line connecting the two patches.

Standing up raises your CoG higher up which helps some (try balancing a broom on your palm head-end up vs. handle-end up). A longer pendulum tips more slowly.

But much more importantly, having your butt off the seat lets you shift your weight sideways much further and faster. You're pushing sideways on the pedals (and some handlebars but mostly pedals) to do this.

This applies a sideways force to the ground via your tires, and an equal and opposite net force on your centre of gravity. So you can apply a centering horizontal force on your own CoG, letting you get it back over the balance point on the ground.

It also lets you tip the bike, moving some close-to-the-ground mass one way, and your torso the other.

If you start to tip left, you tip the bike more left, but move your body right. The centre of mass of the whole you+bike system moves to the right, because you were able to push sideways on the ground to make that happen.

If all you could do is lean your shoulders (because your butt is stuck to the seat), that's not helpful because you're moving a high-up part of your body.

The lever arm from pedals to ground is short, far below your CoG. I think this is relevant to being able to generate horizontal force on the ground. Tipping the bike left/right levers you sideways, like moving your palm under a broom handle that you're balancing. (I think).

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    Maybe this question would be better at the physics stack but I think the last paragraph of this answer has what I believe to be the closest point to the truth: The stability of the system is based on the arrangement of the center of gravity and the center of force, and when standing on the pedals, the center of force (the pedals/lever arms) is far below the center of gravity, and when seated, the center of force (the seat) is much closer to the center of gravity, and might even be above it. The CoG for a person is near the hips, so it's a bit lower for a person and bike system. – Todd Wilcox Jun 18 at 17:25
  • @ToddWilcox: yeah, it might be fun to see if someone on physics.SE who also knows how to trail-stand wants to attempt some formalism, like a free body diagram. This is my attempt just doing the physics in my head, and it's not super clear. I'm sure not confident about the "why" of everything. – Peter Cordes Jun 18 at 17:28
  • Another way to look at my above comment is when standing, you can tilt the bike easily left to right without moving your whole body left to right. Your hips can stay in about the same place. When seated, wherever the seat moves that's where your hips have to go so when you tilt the bike, you move the CoG of the whole system off the center line. – Todd Wilcox Jun 18 at 17:28
  • Moving your palm under a broom is a different mechanism than what you are describing before it. If works by moving the contact point while the CoG does not move as much. It is also how trackstands are actually done. – ojs Jun 19 at 11:27
  • @ojs: You mean with cleats, where you hop the bike to move it? It's somewhat possible to trackstand / trailstand without cleats just by shifting your weight, which is what I was trying to describe; I don't have cleats because I'm a filthy casual cyclist. But good point about moving the contact point - that makes it much easier. – Peter Cordes Jun 19 at 15:22
4

I think it's mainly harder because there are more contact points between you and the bike, and the one in the center, your seat, makes the system a lot more rigid. Which makes it harder to shift weight around at the crucial moments which is what balancing is all about. Also when standing up you can do things like push the bike left/right while your body stays roughly in the same position which is pretty much impossible when sitting, because you'd be moving your body at the same time as the bike.

As for hints: start on a small slope without engaging brakes but using pedals to allow the bike to move forward by pushing the pedal and backward again by releasing some pressure. Apart from sticking out elbow/knees this allows more movement to counter the bike (and you) trying to fall. But that's about all you can do so for the rest: practice, practice, practice.

4

Balancing is done by shifting mass in a way that your centre of gravity is opposite to the side on which you are currently falling.

  • When sitting, you can move the upper body and head by bending your spine but this movement is restricted to about 20 cm, and still involves only about half of your mass. Your centre of gravity will move by only half of that distance (10 cm), which gives a correction range of +/- 5 cm.

  • When standing, you can easily move your entire body (except the lower legs and hands which must remain on the pedals/handlebar) by half a meter. Your centre of gravity will move by about 80% of that distance, (40 cm) with a correction range of +/- 20 cm.

If you lean 5cm to the side at the CoG level (10 cm at head level) while sitting down, it will be very hard return to equilibrium. While standing, you can easily return to equilibrium even from a 10 cm tilt.

3

When you're standing, the pedals are effectively the endpoints of two torque arms in the shape of a small "v". It's reasonably easy to adjust the amount of force applied to either by shifting your entire weight via your legs. When you're sitting, your weight comes to bear largely on the seat, creating a longer (single) torque arm that is more difficult to counteract. The principle agent, then, would be applying your upper body weight to either handlebar. Simply leaning in either direction would not oppose the torque arm once it were no longer vertical.

  • I don't think the amount of torque applied to pedals matters at all. You don't go out of balance while pedalling, do you? – Dmitry Grigoryev Jun 17 at 12:09
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    @DmitryGrigoryev Steve is talking about the torque applied relative to the line between the ground contact points of the tires (tipping the bike left or right), not the torque applied relative to the crank axle (rotating the cranks). Torque relative to the bike's contact points is the only way it can be tipped, and it can only be applied through the cranks, handlebars and seat (if seated). – David Richerby Jun 17 at 13:15
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    @DavidRicherby I'm talking about that torque too. When I'm saying "go out of balance", I mean leaning sideways, not jumping over one of the wheels. – Dmitry Grigoryev Jun 17 at 13:34
  • @DmitryGrigoryev OK but the moving situation is completely different. When you're moving and you fall to the left, the bike steers left underneath you, so you don't fall. That doesn't happen if you're not moving forwards fast enough. – David Richerby Jun 17 at 13:35
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    How do you apply different force to left and right pedal without turning the cranks? – ojs Jun 19 at 21:39
2

Also, two other effects:

  1. When standing, it's much easier to apply different levels of force to each pedal. You can even pull up on one pedal if you're clipped in. That will make balancing while standing on the pedals easier.

  2. When seated, a large amount of your mass will move with the bike - if the bike starts tilting left, your bottom half is going to move to the left with the bike. That will make balancing while seated harder.

1

I agree with Andrew's comment, and would add this: typically we trackstand by pedaling into the road crown, and letting the crown push us back. It is much easier to maintain force on the pedal when standing than when sitting.

1

I think this should be on physics stack exchange for an explanation. It's not exactly 'center of gravity' as @Nelson said, but you have to think about where the forces are applied. When sitting, most of the force is applied down onto the seat, and the rest to the handlebars, and this is transferred down to where the tires meet the ground. If you begin to tilt to a side, the bicycle seat is now on that side and most of the force is off to that side, tending to increase the tilt in that direction. If you try to lean to the other side to counter that, the force of your body moving actually initially makes the tilt in that direction worse as the opposite reaction is pushing the bicycle seat that way when you lean the other way.

When standing on the pedals, you can shift weight more easily from side to side, but the main difference is how low your feet are. Now you have a low point with most of your weight on it and a high point to use to tilt the bike: the handlebars. You can apply weight easily to either side using your feet and use your hands to move the top of the bike from side to side. In a sitting position you lose most of the downward pressure on each side from your feet and you are basically unstable. When standing if the bike tilts left you put more pressure on your right foot and pull on the left handlebar to get it vertical again. What move would you use when sitting to accomplish the same thing?

  • "easier to shift weight" is an excellent point. I've been riding a recumbent, and there's no way to move your bodyweight around easily. That is, you can't "butt-steer" and the only part of the body free to move reactively is the head. Welcome to the site - good first answer. – Criggie Jun 17 at 20:39
0

Unlike most answers assume, you can't laterally move your center of gravity when balancing on a bicycle, because there is nothing to brace against. Instead, the balance is maintained by moving the contact patch so that it is under the center of mass or just enough to the side to tilt the bike and ride in desired direction like an inverted pendulum (the "movable weight on a stick" mentioned in a comment). When the bike is moving, this is done with very small steering movements.

In a normal trackstand, the bike is held in place with front wheel at angle so that the contact patch can be moved left and right by going a bit back and forth. This is easier to do when seated, but not what the question is about.

The other way to move the contact patch is to tilt the bike. Because the tires are not a knife edge but have a round profile, they roll a bit laterally when the bike is tilted, just enough to move the contact patch. The center of mass of bike and rider does not move much, because there is nothing to push against. If the bike and rider were very close to balance when the movement started, this can be enough to maintain balance. Other forces that have very small effects are torque from deforming the tire and rotational inertia of the tilted bike. This trick requires that the bike can be tilted without the rider - which is possible only when standing up.

Yet another way to move the contact patch is to twist the handlebars. A video that shows all three techniques in action can be found here.

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    Sorry, but this is wrong. First, there is something to brace against: the bike. There would be nothing to brace against if you were balancing on ice with greased tires, with close-to-zero friction, but on concrete your bike generates enough lateral force to efficiently move your weight. And second, by how much do you think you can move the contact patch in a standstill? With a tube diameter of 2', moving the contact patch by 1 cm requires a tilt of 22,5°. A tilt twice as big will result in a fall for most people, so you're essentially limited to +/- 2cm. – Dmitry Grigoryev Jun 18 at 6:45
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    You are describing a situation where the bike is fixed to something. The question is about balancing a system of bike and rider. You also don't need large movements to maintain balance if your starting position is good and you don't overcompensate. Most people can't do a trackstand at all. – ojs Jun 18 at 6:50
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    But the bike is "fixed" to the concrete by the friction due to the rider's weight: when you move your body to one side, the tires don't slide to the opposite side. – Dmitry Grigoryev Jun 18 at 7:19
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    True. When you move your body, the bike tilts to other side. – ojs Jun 18 at 8:04
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    Unlike (frictionless) sliding, a tilt doesn't exclude a lateral force at the contact patch. And considering the mass of the bike vs. your mass, I'd say that such a tilt is not nearly enough to keep the CoG of the system (bike + rider) in place, so I expect there to be such a lateral force. – Dmitry Grigoryev Jun 18 at 8:13
-1

I think it's easier, since you can move more. Sitting down, you are limited to shifting your upper body abit which EASILY throws you off balance. But if you stand, you can eversoslightly move your hips, you can tilt left and right without shifting your entire body weight to either side.

I can't tell you why or if you are more stable standing up, but is way easier balancing because of what I explained above.

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    Welcome to the site! What you say here makes sense, but this question already has many answers that cover exactly the same ground. We have lots of unanswered questions and it would be great if you could have a go at those, instead! – David Richerby Jun 17 at 9:24
  • @DavidRicherby would do but I know literally nothing about bikes – bv_Martn Jun 17 at 9:29
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If you want to balance sitting down, just spin your cranks backwards, works for me.

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    How does that help? We're looking for answers that give more detail than this. – David Richerby Jun 16 at 21:39
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    Yes, we know that's what you meant. (But still edit your answer to remove the unhelpful "clockwise" description of that). More importantly, how / why does that help at all? You aren't going to get anything moving fast enough for gyroscopic forces to come into play. – Peter Cordes Jun 17 at 3:30

protected by Criggie Jun 17 at 20:34

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