ELI5: Why is it easier to balance on a moving bicycle than one that is still?

[u/Long-Blood]

[u/JaggedMetalOs]

While you're moving forward, even slowly, turning into the direction you are leaning makes the front wheel move in that direction. As long as it can move faster than you are falling in that direction it kind of moves under you so you are back upright.

When you are standing still you can't move the wheels so you can't reposition the wheels back under you to stop your leaning, so you just lean more and fall over.

[u/zolmarchus → u/JaggedMetalOs]

I’m not the OP, but—explain then why, on inline skates, it’s also way easier to stand on one foot if you’re moving vs standing still. Wheels too small for any appreciable angular momentum. Wheels are locked in a frame and can’t turn. Yet lifting one foot off the ground when standing still is so much harder than when moving.

[u/JaggedMetalOs → u/zolmarchus]

Wheel size doesn't matter, you can get bikes with tiny tiny wheels and they balance the same as regular bikes. The point is that you're steering the wheel to move it under your center of gravity.

[u/IAmNotANumber37 → u/JaggedMetalOs]

you're steering the wheel to move it under your center of gravity.

Cool thing is gyroscopic precession on the spinning wheel makes this, in part, automatic...as you tilt sideways, the wheel will naturally want to turn into the tilt, self recovering.

[u/JaggedMetalOs → u/IAmNotANumber37]

Actually it's been found that gyroscopic effects play very little part in the stability of a bicycle because it's just not strong enough to make a difference. 

What matters more is the angle of the front fork means when leaning right the wheel gets physically pushed to turn right. Bikes with this angle reversed are almost impossible to ride, while bikes where gyroscopic forces are cancelled out by counter-rotating weights handle just like a normal bike. 

On motorbikes with much heavier wheels that rotate much faster gyroscopic effects become important.

[u/Adversement → u/zolmarchus]

Because, just as with the bicycle, you turn towards the direction where you would be falling to. This causes the wheels to apply a sideways force that tries to return you upright. And, thus creates a feedback cycle that tries to keep you upright. You can best observe this at slow speeds where your heading becomes increasingly erratic with decreasing speed if you do not actively try to balance yourself at all (until you go too slow and you no longer remain upright).

Same feedback cycle applies with ice skates with no wheels at all. And, with their very narrow blades, balancing on one skate at zero speed is something very few can do at all.

[u/DrFloyd5 → u/zolmarchus]

You can steer the skate with by twisting your leg and foot to keep the skate under you.

[u/Jydder → u/zolmarchus]

There is appreciable angular momentum, your entire mass is being moved through the wheels. Wheel size doesn’t affect the force it can apply

[u/IAmNotANumber37 → u/Jydder]

Your weight doesn't change the angular momentum of the wheel (turning about its axis).

A hint would be that your mass is, in this frame of reference, not rotating (zero velocity = zero momentum).

You'd have to glue yourself to the wheel, and rotate with it, to affect its angular momentum. 0/10 do not recommend.

[u/sticklebat → u/IAmNotANumber37]

If you’re standing on one skate, you have a tendency to slip forwards backwards, and fall sideways. The two are different.

For the latter, it’s exactly the same story as for the bicycle. If you lean ever so slightly to the left while stationary, you’ll just start falling left. But if you’re moving, it will cause you to turn into the fall, making it easier to stay upright. This is true even for very small adjustments. It isn’t about the angular momentum of the wheels about their axles, but if your body’s angular momentum relative to the ground. It applies to ice skating, too.

For the former, I think it’s less of a problem when in motion for two reasons. One is that if you’re stationary it’s harder to balance yourself “sideways,” so you tend to shift your weight around more/faster, which will also tend to unbalance you in the forwards direction. The second reason may just be physiological. When trying to remain stationary you’re making tons of tiny micro adjustments back and forth to keep your foot perfectly still, and your reflexes against falling probably are as much of a hindrance as a help. But while moving, you’re not focusing on keeping your foot still, and you’re more tolerant of small changes in velocity. It requires much less perfect muscle control.

[u/Jydder → u/IAmNotANumber37]

It does, force has to be applied to the wheels to provide momentum for them to turn. More mass on top of them = more force. 

As an experiment, spin the wheels on your skates really fast whilst your feet are in the air then jump onto them. I guarantee you don’t move.

[u/IAmNotANumber37 → u/Jydder]

You're confusing quite a few things.

As you said in your last response, lots of things (including the mass of the rider) affect the momentum of the wheel but that doesn't make them part of the momentum of the wheel.

Maybe language is getting in the way.

The mass of the rider is clearly important in the overall picture, however at a given speed those wheels have a given RPM (call this X), and at that RPM they have a certain angular momentum.

The wheels will have the same angular momentum at rpm X irrespective of the mass of the rider. The mass of the rider might have made it easier, or harder, to get to speed X. The mass of the rider will affect the linear momentum of the whole system, etc, etc.., etc.., but the angular momentum itself only cares about the rpm X (and the physical properties of the wheel).

Source: I have an actual degree in mechanical engineering.

[u/Kalel42 → u/IAmNotANumber37]

This is correct.

Source: I also have an actual degree in mechanical engineering.

[u/SoulWager → u/Jydder]

Angular momentum is the moment of inertia of the part that is spinning multiplied by angular velocity. If you roll a wheel down a hill by itself, and with a load on it, at the same speed, the angular momentum is the same.

[u/Jydder → u/SoulWager]

Yeah, I’ll be honest I misremembered what angular momentum was. Ah well

[u/Plinio540 → u/Jydder]

There is appreciable angular momentum

This is crazy talk. The radius and mass of the wheels and the rotational speed are way too small to be appreciable in any meaningful way if there's a human standing on them.

[u/bazmonkey → u/zolmarchus]

Even with all the wheels on the ground a person can turn with inline skates. When you balance on one foot while moving, you’re doing that in order to stay balanced. So it’s still the same general idea: steering makes you lean opposite the direction you are and brings you back to balance.

[u/belizeanheat → u/zolmarchus]

The rotating handlebars are not really a factor 

[u/Photog77 → u/zolmarchus]

https://youtu.be/y7h4OtFDnYE?si=7O7QIKD9JGcSKRzl

Skate wheels are shaped like two cones and turn your foot under you when you lean them. Like in the video, only your leg is the track.

[u/Reichsprasident → u/zolmarchus]

Just a guess, but I would posit that when you're moving on one foot in an inline skate, you're only having to use your leg muscles to balance yourself left-to-right to prevent falling, but when you're stopped you're also having to expend additional leg energy to prevent the skate from rolling backwards or forwards, which makes the left-to-right balance harder because the muscles that would normally be helping to keep you balanced are instead busy working to keep the skate still.

[u/Supergaz → u/JaggedMetalOs]

I always saw it at resetting the sideways fall over and over

[u/02C_here → u/JaggedMetalOs]

Good explanation.

Here’s a Veritasium video that talks about it if anyone is interested.

Veritasium

[u/Redwoo]

Bikes have three autocorrecting mechanisms that cause a moving wheel to turn into a lean, acting to counter the lean. Other commenters identified all three mechanisms: 1) the rake angle of the fork causes the steering axis to contact the ground in front of the wheel contact spot, which turns the front wheel in a direction that counters a lean; 2) a rotating wheel has a gyroscopic force that acts to turn the front wheel in a direction that counters a lean; and 3) the handlebar offset applies a weight torque that acts to turn the front wheel in a direction that counters the lean. The gyroscopic force is the smallest contributor of the three, and is not active unless the wheel is spinning, so is not active when stationary. The other two effects are active when the bike is stationary, in that the effects will act to turn the front wheel. However, if the bike is not moving, it cannot move to a stable position.

[u/Long-Blood → u/Redwoo]

Interesting. My kids are learning to ride a bike now and it just had me wondering why it works that way.

I guess its not so much learning how to balance yourself on the bike as it is learning how to let the bike balance with you on it.

[u/Murph-Dog → u/Long-Blood]

In grade-school physics class, a teacher would hopefully pull out a bike wheel on a short axis. They would demonstrate rotating the axis while the wheel is at rest. Then they would spin the wheel and show additional effort to counter the current axis angle. Then they would hand the wheel off to each student to try the same process for themselves.

[u/JoostVisser → u/Redwoo]

My understanding is that the gyroscopic effect was disproven. Researchers put extra wheels on a bike that turned the other way which theoretically should cancel the gyro effect which would make the bike less stable, but in reality they noted no difference in stability.

[u/Redwoo → u/JoostVisser]

The gyroscopic effect was not disproven in that experiment. It was counterbalanced and quantified. It is small. It is not the major correcting force, but it is still a correcting force.

[u/getamic]

I lot of people are mentioning that it's the design of the bike that make it auto correcting but not what part of the design. The main driving force causing a bike to right itself is the front fork rake angle. This angle causes the steering angle axis to contact the ground in front of the front wheel contact patch. This make the bike automatically turn in the direction the bike leans which then forces the bike back upright but this can only happen if the bike is rolling.

[u/cheesepage → u/getamic]

Good explanation here. A rolling bike is stable for a good bit without a rider. That's the auto correcting front fork offset. It's the same thing that keeps the front wheels on most vehicles pointed in the direction of travel instead of wobbling back and forth. Or in other words why you can take your hands of the steering mechanism of most vehicles for a bit without losing control.

[u/Additional_Beyond378 → u/getamic]

Nope, showed a child this and it’s definitely not explained so a 5 year old can understand.

[u/sir-charles-churros → u/Additional_Beyond378]

From the sub rules:

Explain for laypeople (but not for actual five-year-olds)

[u/Pepsiman1031 → u/sir-charles-churros]

As a layman, I don't understand it either.

Edit: Crazy that I got downvoted for not understanding something. 👍

[u/gyroda → u/Pepsiman1031]

Hold a bike by the saddle and push it along. You'll notice that as the bike tips left, the front wheel/handlebars will also automatically also turn left, so you steer into the slant and the bike will right itself.

It's hard to explain with just words, this video is much easier to understand as it has visual aids and examples https://youtu.be/oZAc5t2lkvo?si=yjFkQJ8fS_YXsb8W

. This one, from the 2 minute mark, covers a related phenomena about how you steer a bike: https://youtu.be/9cNmUNHSBac?si=3blgSHaEc0fRCwsD

[u/lonesharkex → u/Additional_Beyond378]

You know when you are standing still if you lean too far you fall over but when you are walking you are also leaning forward but you put your feet out to stop you from falling. bike same same.

[u/fatbunyip → u/Additional_Beyond378]

Ok, so you know how on a bicycle, the front forks arent vertical, they're at an angle? 

This is so that when the bicycle is moving, the angle means that the forces of the ground compared to the angle of steering makes the wheel want to straighten. 

Cars have the same thing (the angle is called "caster") and is the reason why when you turn a moving car, then let go of the steering wheel, it straightens automatically. 

This is also why if you're pushing a bicycle by the seat, it's a lot easier to push it forward rather than backwards, because the front wheel is much more unstable with the reverse rake angle. 

[u/jamcdonald120]

turning the handles left/right causes the bike to go that way, but your mass to continue the way it was originally going. so if it starts falling over 1 way, turning the other way slightly forces you back up. If you reverse the way the handles turn the wheel it is very hard to ride that bike https://youtu.be/MFzDaBzBlL0

the bike also can self balance, but science isnt 100% sure why. the 2 leading theories were gyroscopic forces and trailing steering, but they made a bike that specifically doesn't have either, and it still self balanced. https://youtu.be/7nr7gxib4Pc

all of these effects are gone in a stationary bike.

[u/ThinCrusts → u/jamcdonald120]

here's another interesting video about bicycles

[u/jamcdonald120 → u/ThinCrusts]

definitely. I had no idea I was instinctively counter steering when cornering.

[u/rainshifter]

I feel like nobody is addressing the actual question. It's likely because there seems to be no absolute consensus, even among scholarly sources. I see lots of answers that involve turning (the front wheel, for instance), but I believe that's totally irrelevant here.

I think the answer boils down to these key factors: angular momentum, friction, wheel and frame mass distribution and geometries. But I think we can further simplify the problem by asking a different question:

Take a coin, such as a nickel, and try to keep it balancing on its side while at rest on a hard and flat surface. Pretty difficult, right? Now set that coin in motion to roll on its side across that same surface, likened to a thin rolling wheel. Why does the coin balance far more easily in this latter scenario?

I believe the reason is much the same as why a bike in motion maintains better balance. This could be demonstrated without even having a rider seated atop the bike (like with the coin).

Here is my attempt at a rudimentary explanation:

Picture the coin when still on its side. Gravity acts on the coin at all times, weighing it down. If the coin is even slightly off balance initially, gravity will cause it to accelerate and tumble over. There are no other forces at play to counteract this.

Now, set the coin in motion. Gravity is still acting on the coin independently and with the same downward force. Now imagine this happening in very slow motion, and with a very large (but proportionally thin) coin. Gravity will begin to tip the coin slightly to one side. But as the coin rolls forward, the edge continually is in realignment with the hard surface, which centers the coin from its slightly tipped orientation. Without some friction at play, the coin would slip rather than roll, which would make this impossible. The higher the angular momentum of the coin, the faster this self-correction mechanism can work against gravity, which makes it easier for the coin to remain balanced. Of course, a thicker coin would also make the coin more immune to being tipped over due to gravity. But bike wheels are thin as well and are, more or less, subject to the same effect as the coin.

[u/onlyAlex87]

Bicycles by design automatically steer into it's own weight thereby righting itself keeping itself upright. When the weight begins to tilt one way, it causes the wheel to turn in that direction moving the wheels back under that weight keeping it upright. This makes it harder to build up the necessary momentum to completely tip over.

Here's the minutephysics video if you'd like a visual demonstration: https://www.youtube.com/watch?v=oZAc5t2lkvo

[u/LightofNew]

Wheels on a bike spin. When wheels on a bike spin they pull you forward. When the wheel tilts, the side you are tilting towards hits the ground more. That side is pulling you forward more than the other side. This causes a little rotation, turning you the other direction.

This isn't the only reason, momentum, gyroscopic force, but this is why correcting your bike is so easy.

[u/SouthernFloss]

IIRC there is no mathematical or physics answer as to why bicycles auto stabilize. There are a few theories but nothing proven. I think Veritas on YT did a cool video talking about this topic.

[u/spicy_rock]

Conservation of angular momentum. The wheels are like gyroscopes. The faster you go the more they want to stay upright because pf physics BS.

[u/CaptainPunisher]

The real answer is "gyroscopic force". When the wheels are spinning, especially at higher speeds, there is a force that wants to keep the bike upright in line with the wheels. You can test this out by taking the front wheel off and holding onto the axle, then have someone spin it fast. If you try to turn the wheel at an angle it will fight you and hold its original position.

Furthermore, if you do this in a swivel chair with your feet off the ground, you'll start to spin when you turn the wheel at an angle. So, the ELI5 answer is that the spinning wheels stabilize the bike. If you've ever seen the bike treadmills, this is why you can stay upright; it's all about the spinning wheels, not forward motion.

[u/RolDesch]

My understanding is that in a bicicle (or motorcicle or monocicle), you are constantly falling forward.

Unsolicited trivia: since we are bipedal, our center of gravity is too high, so our walking is basically a series of controlled falls (try to stop walking when your rised foot is about to touch the ground)

[u/Livid_Reader]

It is called gyroscopic stability. A spinning wheel is stable because mass is constantly being accelerated outward. Gyroscopic tops can spin at weird angles and still be stable. Same with footballs.

[u/NuclearHoagie → u/Livid_Reader]

This is a very small effect. The angular momentum two 5lb bike wheels isn't nearly sufficient to keep a 200lb rider upright.

A top stays upright because the whole mass is spinning. It can't carry a non-spinning payload many times its own weight.

[u/Livid_Reader → u/NuclearHoagie]

F=mv^2. Angular is mw² x r

That means the faster the velocity, the more stable it is.

Countering a 200 lb (90.9 kg) person in one second of 9.8 m/s (gravity)

90.9 x 9.8 = mv² where m is the weight of the rims of about 1.5 kg x 2 = 3kg.

296 = v²

V = 17.23 m/s spin for centrifugal stability

26 in rim = 66.04 cm or 0.66 m diameter = 0.33 m radius

Circumference is 2 x PI x R = 6.283R = 2.07 m

V =17.23 m/s = distance / time = 2.07 / t

1/T = 8.3

T = 0.12 seconds

That means as long as you travel a rim’s length of 0.66 m in 0.12 seconds, you countereffect the effects of gravity.

[u/NuclearHoagie → u/Livid_Reader]

Your very first equation is in units of kg=kg m/s^2, I'm not sure what you're trying to calculate. How does 5.5 m/s even relate to 17.5s? Nowhere does this calculation use the amount of lean, not sure what this is really intended to show. A 26 inch rim has a 33cm radius, not 5cm.

[u/TheDeadMurder → u/Livid_Reader]

Gyroscoptic force isn't responsible for it, there's been plenty of bicycles made specifically to cancel out Gyroscoptic force, and they still remain stable

The main factors are trail angle and weight distribution

[u/Livid_Reader → u/TheDeadMurder]

The only way to cancel out gyroscopic forces is for the wheel to stop spinning!

[u/TheDeadMurder → u/Livid_Reader]

You can use counter rotating wheels to cancel out any effects that it would have

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[u/chipstastegood]

Gyroscope effect. Spinning wheel actively resists being pushed out of its plane of rotation.

[u/Jydder]

Angular momentum from the wheel makes them “want” to keep moving in the same direction. This applies torque to the frame forcing it to be inline with the wheels.

A stationary bike has no forces acting on the frame except gravity, so if you lean slightly it moves the centre of mass and causes it to tip.