On yer bike

[Badly Shaved Monkey]

Just a quicky.

I've been talking about bike-riding with the kids and found I was not completely confident on this question.

Why is it easy to balance on a bike that is moving and hard to balance on one that is stationary? Is it because of the conservation of angular momentum of the wheels, i.e. they act like gyroscopes?

[Donks]

The Straight Dope (http://www.straightdope.com/mailbag/mangularmo.html)

I want to ride my bicycle I want to ride my bike
I want to ride my bicycle I want ro ride it where I like
-- Blümchen

[Badly Shaved Monkey]

Donks,


Just what the doctor ordered. Thanks

[CurtC]

Karen's article on the Straight Dope site has a major flaw, which she admitted in the forum discussion soon after that article was written.

Although gyroscopic principles have some effect in bicycle stability, it's a small factor. A professor once intended to make an unstable bike by putting on counter-rotating wheels which would cancel out all the gyroscopic effects. To his surprise, the bike was just as easy to ride even with no gyroscoping.

The major thing that makes a bike stable is that when it leans to the left, the leaning makes the front wheel turn into that direction, so the bike's motion tends to the left, and the tires go back under the center of gravity. If you don't have enough speed, that movement under the CG doesn't happen quickly enough, and it's not stable.

The critical measurement here is the distance between an imaginary point on the ground which is projected down from the pivot point of the headset, and the place where the tire contacts the ground. This distance is called the trail, and the greater the trail, the greater the stability. Racing bikes have low trail, and are thus less stable, but are more nimble.

See this thread at the SDMB: http://boards.straightdope.com/sdmb/showthread.php?threadid=123998

[RussDill]

Originally posted by CurtC


The major thing that makes a bike stable is that when it leans to the left, the leaning makes the front wheel turn into that direction, so the bike's motion tends to the left, and the tires go back under the center of gravity. If you don't have enough speed, that movement under the CG doesn't happen quickly enough, and it's not stable.


I figured as much, the gyroscope thing doesn't make much sense to me, and doesn't explain how non-wheeled travel becomes more stable at higher speeds (Ie, surfing or snowboarding).

[shecky]

While there may be a gyroscopic effect at relatively high speeds, it doesn't seem to be a necessary factor in general bicycle balance when you consider it's possible to ride a bicycle at a walking pace, where there'd be essentially no gyroscopic effect. Things like those "razor" type scooters get along nicely with teeny wheels and presumably less gyroscopic effect. There are even ski "bicycles" that manage to stay upright with no wheels at all. Or even ice skates.

[Bikewer]

I have a book called Bike Cult:

http://www.amazon.com/exec/obidos/ASIN/0785764372/qid=1109902369/sr=2-1/ref=pd_bbs_b_2_1/103-2218237-7397409

An encyclopedia of all things bike-related.

He has one section on this, incuding the efforts of an engineer to produce an unstable bicycle. The task proved very difficult indeed, and all manner of wierd combinations of wheel size, radical rake, caster, and so forth proved to be quite rideable!

[Terry]

The canonical "unridable bike" as seen at village fairs all over the place (where place == England when I were nobut a lad) has cranks that have to be turned anticlockwise, and handlebars connected via gearing so that when you turn the bars to the left, the wheel goes to the right. Typical successful strategies for winning the prize for riding the thing 10 yards without falling down include holding the left bar with the right hand and the right bar with the left hand, or going as fast as possible and using no hands on the bars at all. Of course, the carny, who had had countless hours of practice, could ride the thing perfectly well without any such tricks, so it wasn't really unstable.

--Terry.

[Badly Shaved Monkey]

Ah, more complexity!

So, to clarify. As you go faster, the natural speed at which the wheel can be turned into the turn to reposition the CG increases. I think I would be right to say that the fundamental reaos why this is good is that acceleration due to gravity is constant at all speeds so as you begin to topple your velocity at any time from the beginnng of the topple is the same whatever the forward speed of the bike, but at higher speeds you can make the correction quicker.

No one is disputing that the gyroscopic effect is zero, but is it really negligible? Having played a bit on little-wheeled bikes, I wouldn't like to ride those with my hands off the handlebars, it feels as if the front wheel could be snatched away from the axis parallel to the direction of travel by even the smallest bump. Doesn't the "trail" and CG correction argument apply equally to small and big wheels? So, if big wheels feel more directionally stable at speed, wouldn't that difference be due to a greater gyroscopic effect?

[shecky]

I suspect for the average bike, gyroscopic effects are probably negligible at speeds under about 10mph. Fat heavy beach cruiser wheels probably do better than skinny lightweight road bike wheels at the same speed in this respect. I doubt the effect would be much on roller blades, which I can skate well enough on one leg without falling over.

Hands free riding may be enhanced by gyroscopic effects. I generally find that I ride better hands-free at higher speeds than slow speeds. And not at all under about 10mph. But don't discount frame and fork geometry, where it seems steering tube angle, rake and trail probably have big effects.

I'm guessing the rider, perhaps not surprisingly, is the biggest factor in keeping a bicycle upright. I've noticed when riding that I constantly shift weight slightly and correct by steering. The amount is very small. If I didn't, I would soon lean too far to one side and crash.

[Badly Shaved Monkey]

I've had a few more thoughts.

1. Chopper bikes have very small front wheels and a huge trail (presuming I have correctly understood what trail is). It seems obvious that this is more than just a style and that if those tiny wheels were simply vertically beneath the handle bars the rider might soon discover whether his big bushy biker beard is useful for cushioning impact on the road as he hurtles over the handle bars.

2. There must be a relationship between the tendency of wheels to be forcibly misdirected and the average roughness of the surface- you wouldn't be stable on cobblestones on one of those kids' scooters with tiny hard plastic wheels, but you would bump along OK on a bike with 26" wheels.

[Rolfe]

I suspect most of this thread is way above my head.

I always thought that a moving bike was stable because you could steer into any instability and correct it. Wheel size doesn't seem to make so much difference as far as I can see, having more to do with gearing, and the portability of the folded-up machine.

Rolfe.

[CurtC]

Rolfe, I think you're pretty much right, but the rider doesn't have to do it all himself. The bike, if it begins to lean over, naturally turns the front wheel into the direction of the turn, which is self-correcting so the rider needs to do very little. The reason the whell wants to turn that way is the trail - the place where the force is applied on the steering axis (the tire/ground contact point) is offset from the axis, so causes it to turn. A big part of teaching a kid to ride a bike is getting him to allow this to happen without trying to overpower it.

A bike with no gyroscope effect would also become more stable at higher speed, because it will get the tires underneath the CG faster. However, the gyroscope effect, however large it is, would also increase.

Skateboards are in a way similar to a bike, but with much less of the gyroscopic effect. If the rider begins falling to the left, it tilts the board to the left, which turns it back under his weight. But with skateboards, there's a speed above which the system is unstable. I learned about this when I was a teenager - a friend held onto the tailgate of a pickup truck while it accelerated. He got up to maybe 40 km/h, then it reached the point where the skateboard would over-correct. The skateboard went left-right-left-right a few times in about half a second, then left my poor friend with nothing under his feet. I wonder if bicycles might do this, but the gyroscopic stability counteracts it.

I have another learning to ride a bike story. Last year for easter we got my youngest son, who was 3.5 years old, a Razor scooter as a gift. He spent some time with one foot on the scooter, and the other foot basically walking on the ground. But he started going a little farther between steps, and pretty soon he was pushing it for speed, then actually balancing on it like you're supposed to do. At this point, I put him on a bicycle, and he just rode off. We had a tiny little bike for him, and it was funny seeing a 3.5 year old kid riding this tiny bike without training wheels. The Razor taught him the balancing art, in a low-consequence way - if he messed up a little, it didn't hurt, so he learned how to ride very quickly.

[Rolfe]

Originally posted by CurtC
Rolfe, I think you're pretty much right, but the rider doesn't have to do it all himself. The bike, if it begins to lean over, naturally turns the front wheel into the direction of the turn, which is self-correcting so the rider needs to do very little. The reason the whell wants to turn that way is the trail - the place where the force is applied on the steering axis (the tire/ground contact point) is offset from the axis, so causes it to turn. A big part of teaching a kid to ride a bike is getting him to allow this to happen without trying to overpower it.So how come an unridden bike will just veer more and more to one side until it falls over?

Rolfe.

[Badly Shaved Monkey]

Originally posted by Rolfe
So how come an unridden bike will just veer more and more to one side until it falls over?


Sheer inexperience?

Oh, but then again, you might have meant "riderless bike"

:)

[rppa]

Originally posted by Badly Shaved Monkey
No one is disputing that the gyroscopic effect is zero, but is it really negligible? Having played a bit on little-wheeled bikes, I wouldn't like to ride those with my hands off the handlebars, it feels as if the front wheel could be snatched away from the axis parallel to the direction of travel by even the smallest bump. Doesn't the "trail" and CG correction argument apply equally to small and big wheels? So, if big wheels feel more directionally stable at speed, wouldn't that difference be due to a greater gyroscopic effect?

I'm of the generation that was taught the gyroscope explanation by our physics teachers, complete with demonstration. The demonstration includes holding a spinning bicycle tire in your hands, by the axle, and feeling what happens as you attempt to change the plane of rotation. It's a very effective demonstration that there *is* a gyroscope effect and it is far from negligible, at least compared to the strength of your arms.

However, I can't argue with the calculations that show it is negligible as far as the forces needed to stabilize a bicycle. Learning the actual explanation, having had my "understanding" turned upside down as an adult, has been one of the many pleasures of a lifetime of learning. I think those demos also use a much higher spin rate than is typical of bicycle riding.

It is only after learning the importance of leaning that I developed a technique I use now during commuting (walking bikes through train stations), for pushing a bicycle along with one hand on the seat alone. It is easy to use leaning to counter the random fluctuations that occur in the direction of the front fork as you push the bike this way. Prior to learning how bikes actually work, it would never have occurred to me that you could maintain stability this way.

[Rolfe]

Extraneous post, sorry.

[shecky]

I'm curious. I wonder if I remove the handlebars on my bike and place it upside down, resting on the stem and saddle, if it could balance on those two points by spinning the wheels fast enough? Do you think this would be a sufficient experiment to observe gyroscopic balancing effects?

Not having tried it, by intuition says the bike will still fall at reasonable wheel rpms. At higher rpms, there might be less stability from typically unbalanced bicycle wheels gyrating.

Anyone ever tried this?

[shecky]

Originally posted by Rolfe
So how come an unridden bike will just veer more and more to one side untiol it falls over?

Rolfe.

I assume a rolling unridden bike rather than a stationary unridden bike.

My bike, when stationary, will turn into the direction it's leaned into. The more it leans, the more it turns, until it reaches a angle of about 65 degrees. At which point the wheel wants to turn the other way quickly.

My guess is that a rolling unridden bike will likely lean to one side or another, and the wheel will turn in the direction it leans into. Which exaggerates the turning until it reaches a angle that's no longer sustainable and it falls over. I wonder what effect steering angle, fork rake, etc have on this? I have no idea how gyroscopic effects work here.

[rppa]

Originally posted by Rolfe
Extraneous post, sorry.

This post intentionally left blank.

[Soapy Sam]

Surely, if gyroscopic effect contributed significantly to stability, a bike would be very hard to turn?

(I often wondered how Fred Flintstone steered that car of his, which was essentially a double roller bicylinder).

[Badly Shaved Monkey]

Originally posted by rppa
This post intentionally left blank.

I am making no response to this derailment.

[Placebo]

Originally posted by Badly Shaved Monkey
I am making no response to this derailment.
No further comment :D

Regarding the bicycle, most of this is over my head.
But shecky's observation seems to make sense.
The wheel is self correcting, but the balance (vertical angle) is not.
Such that if nobody was riding it, it would unbalance and cause the wheel to turn.

So in other words, if you could push the bike without a rider, but keep the center of the bike upright, it would do just fine.
My common sense says this should work too :)

[rppa]

Perhaps it would be interesting if we can find out something about vehicles in the DARPA challenge. DARPA is a US government research agency, and they have started sponsoring a cross-country autonomous-vehicle road race. The first one was last year (no vehicle finished) and I believe it's planned to be an annual event.

Anyway, as I recall from video I saw, while most vehicles were four-wheeled, a few teams entered autonomous motorcycles. I can't swear to it, but it is my impression these motorcycles had some sort of active control doing the same kind of twisting that a rider would do on the front fork to maintain balance.

[CurtC]

It wouldn't have to do too much - if you push a bicycle with enough speed (maybe 10 mph, 15 km/h), it is self-balancing.

[TeaBag420]

Originally posted by RussDill
I figured as much, the gyroscope thing doesn't make much sense to me, and doesn't explain how non-wheeled travel becomes more stable at higher speeds (Ie, surfing or snowboarding).

It's because people who can go faster are better at it and less likely to fall. If you fall down, you don't GET to the higher speeds. And surfing isn't even a speed event, since it frequently involves GOING BACKWARDS. Size of the wave is much more important than speed, and a surfer or snowboarder is MUCH more stable at low speeds (e.g. 0 mph).

The gyro explanation is right.

[TeaBag420]

Originally posted by shecky
I'm curious. I wonder if I remove the handlebars on my bike and place it upside down, resting on the stem and saddle, if it could balance on those two points by spinning the wheels fast enough? Do you think this would be a sufficient experiment to observe gyroscopic balancing effects?

Not having tried it, by intuition says the bike will still fall at reasonable wheel rpms. At higher rpms, there might be less stability from typically unbalanced bicycle wheels gyrating.

Anyone ever tried this?

You could balance it without spinning the wheels at all. A retard could do it.

[shecky]

Originally posted by rppa
Anyway, as I recall from video I saw, while most vehicles were four-wheeled, a few teams entered autonomous motorcycles. I can't swear to it, but it is my impression these motorcycles had some sort of active control doing the same kind of twisting that a rider would do on the front fork to maintain balance.

ISTR some kind of radio controlled motorcycles. But I've never seen one in action.

On the topic of gyroscopes and two wheeled vehicles, I recall Douglas Self's site (http://www.dself.dsl.pipex.com/MUSEUM/museum.htm), particularly here (http://www.dself.dsl.pipex.com/MUSEUM/TRANSPORT/gyrocars/schilovs.htm) and here (http://www.dself.dsl.pipex.com/MUSEUM/TRANSPORT/gyrocars/gyrocar.htm). Also, these (http://www.dself.dsl.pipex.com/MUSEUM/TRANSPORT/2wheelcar/2wheelcar.htm) gyroscope-less two wheeled "cars" which seem more like overgrown motorcycles to me.

[Mason]

Try riding a bicycle on a treadmill to see if the gyro effect happens. I don't have a treadmill handy, so someone else will have to look into that one for us. ;)

Meanwhile, consider centre of gravity vs. support. When the CoG is over the support, everything is fine. When the CoG is not over the support, things fall. To regain balance, either the CoG must be moved back over the support, or the support must be moved back over the CoG. Simple, yes?

Thanks for the physics lesson, Mase, what's your point?

When standing stationary, we can adjust our weight to keep the CoG over the support (the legs), or we can adjust the support back under the CoG. While sitting on a stationary bike, we can't adjust the support to bring it back under the CoG. Then, we fall.

While moving, if our CoG gets mis-aligned, we can correct it with our next step to bring the support back under the CoG. On a moving bike, we can now do the same while we couldn't do it on the stationary bike. A slight adjustment to the front wheel, and the bike now moves back under the CoG.

On an un-manned bike, the bike will lean to the side that the CoG has shifted. This causes more drag on that side of the front tire, making it turn in that direction. Now, the drag has increased on the lower part of the bike but not on the upper part of the bike. So, the upper part of the bike tends to continue in the original direction while the lower part is now steered in the new direction. This brings the CoG out of line again in the other direction. Repeat the process, and the bike steers itself back towards the original direction.

The faster the bike is moving, the faster these adjustments occur, making it ride almost straight, then wobbling more as it slows, until finally it doesn't have enough momentum to over-adjust and just spirals to the ground.

None of this is possible on a stationary bike, which just falls over as soon as balance is off.

(uneducated guess coming...)

I don't think the gyro effect has much, if anything at all to do with the bike balancing. I'm not even certain that a wheel has a gyro effect when it is spinning at a rate identical to its speed. The part of the wheel that is touching the ground actually has no speed, while the opposite side has twice the speed of the centre of the wheel. I think for a gyro to work, the centre has zero while the edges are equal to each other?

Anyone who can correct that, please feel free.

[Dr Adequate]

A remote-controlled unicycle --- blurring the line between cleverness and stupidity. (http://www.boltontech.org.uk/loony_cycle.htm)

[Bikewer]

The standard training device, rollers, allow stationary riding with the bike not secured to anything, unlike the stationary "trainer" that holds it securely.
You have to learn to balance on the things, which allow you the luxury of crashing in your own living room. Rider input is definitely required. The rollers are connected by belts, so the fronts and rears (and thus your tires) rotate at the same speed.

I think the gyroscopic effect regarding the rotating wheel has more to do with the stability of the wheel, rather than the bicycle. Riding around campus today, I hit one of those seed-pods that fall off sweet-gum trees. The wheel deflected sharply, then self-corrected with nary a bobble.

[shecky]

Originally posted by Dr Adequate
A remote-controlled unicycle --- blurring the line between cleverness and stupidity. (http://www.boltontech.org.uk/loony_cycle.htm)

HA!

I remember the patent for the Segway, viewing it before it had been unleashed on the public, seemed to have one implementation that was like a skateboard with a single wheel in the center. I think this version may have gone a long way to kill the nerd factor associated with the Segway.

[69dodge]

Originally posted by Mason
Try riding a bicycle on a treadmill to see if the gyro effect happens.Why should riding on a treadmill be any different than riding on the road?I'm not even certain that a wheel has a gyro effect when it is spinning at a rate identical to its speed. The part of the wheel that is touching the ground actually has no speed, while the opposite side has twice the speed of the centre of the wheel. I think for a gyro to work, the centre has zero while the edges are equal to each other?No, that's not right. The wheel is spinning. The forward motion of the bike doesn't eliminate the wheel's angular momentum. But, from what I've read, it's not the main source of stability for a bike, as you say.

[Terry]

Originally posted by Bikewer
The standard training device, rollers, allow stationary riding with the bike not secured to anything, unlike the stationary "trainer" that holds it securely.
You have to learn to balance on the things, which allow you the luxury of crashing in your own living room.

Ha! so true...

Riding on rollers feels a lot like riding on ice to me. I think since you aren't moving forwards, the stability given by trail is reduced. The front does move around some when you turn the bars, but I have the impression it is less than would be riding on the road.

--Terry.

[Mason]

Why should riding on a treadmill be any different than riding on the road?

It shouldn't, if balance is because of the gyro effect and not because of the ability to get your wheels back under you. But then, being on the treadmill would also allow the wheels to be adjusted, so I guess it might not show one way or the other.

No, that's not right. The wheel is spinning. The forward motion of the bike doesn't eliminate the wheel's angular momentum. But, from what I've read, it's not the main source of stability for a bike, as you say.

So the direction of the whole system doesn't change the system itself? (just the wheel, I mean, not the bike)

[CurtC]

Ideally, riding on rollers should have the same effects as riding on the road, for both the gyroscopic portion of stability, and the self-correction effect.

However, as Terry pointed out, I can see that depending on where the front roller is placed and its contact point with the tire, the stability will be different. If the roller contacts the tire at the same point the ground contacts it, then the stability should be the same, and if the roller is too far forward (so that it pushes backwards slightly on the front wheel), it will be less stable.

[rppa]

Found a link to the motorcycle from the DARPA Challenge.

It was built by a student team from UC Berkeley. Here's their home page: http://www.roboticinfantry.com/

Agilent also had some press releases about it. Sounds like they were a sponsor of the Berkeley team:

http://www.agilent.com/about/newsroom/features/2004mar01_darpa.html

And here's an excerpt from the Agilent press release on staying upright:
The challenge with the motorcycle design, however, is keeping it upright while stationary and in motion across rough terrain.

To address this challenge, the team used a control moment gyroscope -- a mechanism that provides balance through the application of torque, even through directional and speed changes -- to stabilize the motorcycle, and a Crossbow solid-state gyro and inertial measurement unit that measures speed and pitch to provide the data needed to ensure stabilization at high speeds.

I can't figure out from that exactly what it's doing to stay upright, but one thing to keep in mind is that this thing is going cross country, not on level roads, and has to handle bumps, riding over rocks, hills, slipping gravel, etc. (None of the robots in the Challenge managed to handle everything, they all broke down within a few miles).