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  #1  
Old 07-05-2002, 05:50 AM
badgerboy badgerboy is offline
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Balancing on bikes

Hallo all, this is my very first SDMB post, woohoo!
Cant beleive no-one has mentioned this yet, but doesnt a bike balance when it's moving on account of the curved profile of the tyres? If the bike leans, the rubber on the outside of the tyre makes contact with the ground, but has a smaller circumference than the middle of the tyre and so pushes the bike vertical once more. Please point out the glaring error that i'm missing, teeming millions, 'cos i can't see it myself.
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  #2  
Old 07-05-2002, 06:45 AM
C K Dexter Haven C K Dexter Haven is offline
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Welcome to the Straight Dope Message Board, badgerboy, glad to have you with us.

It's helpful when you start a thread if you provide a link to the Staff Report that you are commenting on. It may be obvious today, when that Staff Report is showing on this week's list... but in a few days, when it's buried amongst the hundreds of other Staff Reports, it will be harder to know what you're talking about. So, here's the link to the bicycle balancing staff report.

I'll let Karen answer your question, but I'm not sure why the tire shape would be different if standing or moving? Also, the smaller circumference would tend to push to bicycle away from vertical, wouldn't it? -- once the bike starts to lean, it would lean more, I think. But that's just my wild-arse opinion, early in the morning.


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Old 07-05-2002, 07:59 AM
RingoStarr RingoStarr is offline
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In the film "HELP!" you might see some Beatles riding on snow bikes with skis where the wheels should be.
No gyroscopic wotnots and doodahs here. But when you're going 20mph it's easier to stay upright than when standing still.

Thoughts?
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  #4  
Old 07-05-2002, 09:49 AM
Steve Kurt Steve Kurt is offline
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Hi gang,
I'm not a regular here, but just read The Straight Dope daily. I do have considerable experience and knowledge about bikes and riding, and thought I'd throw out a couple of ideas.
The subjects of why bikes balance has two or more components. One is the question of stability, which is a measure of how easy it is to balance the bike. A second is the question of why or how the bike and the rider manage to keep balanced on those two places where the tire contacts the ground.
For stability, the gyroscopic forces do help. Anyone can take a simple wheel and roll it along the ground, and observe that when it starts to lean left, it also turns left, thereby bringing its point of contact with the ground back underneath its center of gravity. It's widely observed that a bike with big heavy tires is more stable than a bike with very light racing tires.
As reported, trail also improves stability. This has been known by framebuilders for years. Racing bikes are designed to have less trail, producing quicker and more responsive handling. Touring bikes are designed with more trail for greater stability.
The ultimate question of how a bike balances is really quite simple. First, the question has to be re-worded "how does a rider balance a bike". The next step is to consider what it means to balance. In this case, it means to keep the center of gravity of the bike and rider located directly above the patches of tire that contact the ground.
So far, this is pretty obvious. The less obvious part is that the rider can't just move the center of gravity around (or at least not easily). It's much easier to move the tire contact patches underneath the center of gravity, and this is what is done when you steer the bike. When you start leaning to the right, you turn the front wheel to the right, and it then moves to the right. This brings the contact patches back under your center of gravity, and you are (briefly) balanced again. Of course, the job is never done, and you have to continually make small corrections to stay up.
As proof of this concept, just go to a bike track (a.k.a. velodrome), and watch the races. In one version of track races, the two riders want to force the other rider to go first for the early stage of the race. The race can evolve into a contest of who can go slowest, and the racers can end up performing "track stands" for minutes at a time. A track stand is a matter of pedaling forward and backward with the front wheel turned at an angle, and keeping the tire contact patches under their center of gravity. Track stands require practice to perfect, but do prove that gyroscopic forces and specific bike frame geometry are not required to balance a bike.

Steve Kurt
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Old 07-05-2002, 04:31 PM
AskNott AskNott is offline
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I was intrigued by the stuff in the column about the odd research bikes. The link provided showed only one odd-looking bike, with no explanation for what appeared to be a tire made of Danish pastry. I'd like to see the one with contra-rotating anti-gyros, and I'd like to see the unridable bike. The good professor apparently did a lot of work, and I wonder if the university makes it available.
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Old 07-05-2002, 04:44 PM
PBear PBear is offline
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Read the Report with interest. (BTW, the link Dex supplied above actually points to the Jackal article; some admin type might wanna fix it.) But, although I candidly confess to only a high school physics education, I have spent a lot of time in pedals. With some trepidation, then, I’m going to venture to disagree with even the corrected analysis.

First, as KL acknowledges, the gyroscopic effect has little to do with balance. A conclusion proven, IMHO, by something mush simpler than Professor Klein’s counter-gyroscoped bike. Bear in mind that a bicycle can be kept balanced at extremely slow speeds, i.e., just barely moving. (Whereas standing on pedals, i.e., balancing a nonmoving bike, is a skill mostly limited to racers and messengers.) Obviously, some other factor is at work.

On the other hand, I don’t think trailing is the answer. It explains a different kind of stability than that originally raised, viz, “Why is it easier to balance on a moving bike than a non-moving one?” Trailing has nothing to do with this. Rather, it explains why a freely-pivoting front wheel stays true to the line of travel instead of wobbling all over the place. Indeed, so stable that an experienced rider (say, ten years old) can ride “hands free.” But not, and maybe this is what KL had in mind, until traveling at a fairly good clip, i.e., somewhat faster than that needed to stay balanced with “hands on.”

So, what does keep a bike up? I don’t know what to call it, but the only thing with which I can think to compare this (better analogies welcome) is balancing a broomstick on a finger tip. Holding your hand still, it’s practically impossible, yet a fairly simple trick if you’re allowed to move. And takes only a small amount of movement, all within a circle, say, six inches across.

That, I would submit, is why a moving bike is easier to balance, and why it doesn’t need to be moving very fast. What you’re doing, as Steve says, is getting and keeping the tires’ contact points under the center of gravity. Necessarily a dynamic process and therefore one which (for all but the most expert riders) requires movement.

Nor need this be forward movement. That arises only because of the freewheel nature of conventional drive trains, which permit bikes to coast without pedaling. By contrast, non-freewheeling bikes, e.g., stunt bikes and some racers, indeed can be balanced in place, i.e., by moving a few inches forward-and-back, just like the broomstick.
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Old 07-08-2002, 06:56 PM
galt galt is offline
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PBear, your explanation of why it's easier to balance a moving bike than a nonmoving bike is not an explanation at all. Just saying that it's a "dynamic process" and "requires movement" doesn't explain anything. Here is my stab at it:

Balancing the bike, as pointed out earlier, is an exercise in keeping the contact patch directly below the center of gravity (I'll point out how this is an oversimplification later, but it suffices for now). This involves moving the contact patch side-to-side (since the contact patch is long enough front-to-back that you don't have to worry about falling over in those directions). When a bike is moving quickly, you can make a very small adjustment to the handlebars and have the wheels move sideways relative to the CG very quickly. This is demonstrated by those stationary trainers that let your front wheel float freely on rollers, and you stay balanced by letting it track side-to-side as you turn the handlebars slightly. (example: http://www.graberproducts.com/cache/373/373.GIF)

The slower the bike is moving, the longer these corrections take, and the more care it takes to balance. Once you reach a lower bound in speed, it's easier to completely change your approach and use a "trackstand" method of balancing, which involves keeping the wheel turned to one side and going forward and backward to move the front wheel from side to side. (as an aside, you can do this even with a freewheel bike if you know what you're doing. If you have even a very small bump to use to oppose forward motion, you can use it to go backwards and keep your balance, and even on perfectly flat ground, I can "bounce" off my brakes to go backwards if I really work at getting the timing right. You do look like a complete dork doing this at a stoplight, though )

As for the "keep the contact patch under the center of gravity" oversimplification, you're really trying to put it where gravity and inertia are trying to push you. When you're riding in a straight line, the right place to put the contact patch is directly below the CG, but when you're turning, you want your contact patch to move towards the outside of the turn, so you don't fall over due to inertia (picture a plumb bob dangling from the end of your handlebar -- it'll point in the direction you want the contact patch to be in relation to the CG).

Which leads me to the real reason trail is good: If you have positive trail, as bikes do, turning the handlebars to the right moves the contact point of the front wheel to the left, which makes the bike behave as if you're leaning, and counters your forward inertia which is soon to be slightly-to-the-left inertia. Think about a bike with negative trail, and you can see that by turning the handlebars to the right, you're moving the contact patch to the right too, which works against you and makes you fall to the left.
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Old 07-08-2002, 08:44 PM
wsflani wsflani is offline
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I've never spent a minute in a physics class, but here's a way to experience negative trail on most bikes. Turn the handle bars backwards and ride it. I used to do it all the time when I was a kid. It works best on bikes with coaster brakes, as there's no front brake caliper to hit the forks and interfer with turning the handle bars. Maybe I've had a few too many kick balls to the head, but it seems to me that someone wasted a lot of time "designing" a bike with negative trail.
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Old 07-09-2002, 02:21 AM
galt galt is offline
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Quote:
Originally posted by wsflani
I've never spent a minute in a physics class, but here's a way to experience negative trail on most bikes. Turn the handle bars backwards and ride it.
I think you misunderstand the concept of trail. Trail is where the contact patch of the tire is behind the axis of rotation of the handlebars. Intuitively, the fact that the tire sits out in front of the bike due to the "rake" of the fork might make you think that it contacts the ground in front of the axis of rotation, but if you look at a bike, you'll see that this isn't true. Normal bikes have positive trail, and by turning the handlebars backwards, you actually increase their trail. This is because the trail isn't caused by the shape of the fork, but by the angle of the head tube (the axis of handlebar rotation).
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Old 07-10-2002, 11:20 AM
CzarNicholas CzarNicholas is offline
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1) Trail has nothing to do with the balancing of a bicycle. This is obvious because trail does not increase with velocity, yet stability does. If trail were the answer any bicycle with trail would be as easy to balance when stationary as when moving.

The draisene (grandfather of the bicycle) had fixed wheels on both ends; it had no trail yet it was stable. Trail does not add stability _per_se_ but it does minimize the adverse effect that steering inputs otherwise would have on that stability.

2) You can’t cancel gyroscopic force. Not unless you can revoke Newton’s first law. Gyroscopic effect is inertia that is forced to move in a circle. And it has a plane, not a direction. Adding a second spinning wheel cannot and will not cancel the force of the first. In fact, if the planes of spin are coincidental or parallel, the effect will be compounded, regardless of the direction of the spinning.

Consider a tandem-rotor helicopter such as the CH-47 Chinook. If the second spinning wheel trick were effective, the gyroscopic force of each rotor (yes, they counter-rotate) would cancel that of the other. Helicopters are like Frisbees® in that the primary mechanism of their stability is the gyroscopic effect imparted by spinning. And since tandem and twin-rotor helicopters do fly quite handily, the forces obviously _do_not_ cancel.

3) The “track stand” incorporates torque and torsion and a number of other dynamic factors in combination to offset the loss of gyroscopic stability, none of which disproves the efficacy of -- or need for -- gyroscopic force.

4) The Beatles’ skis were about as wide as a gymnast’s balance beam. No extraordinary dynamics are necessary, just normal human balance.

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  #11  
Old 07-10-2002, 12:51 PM
Jeffro H Jeffro H is offline
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Its centrifugal force

Bicycles make it easy to convert forward motion into centrifugal force.

When your balance tells you that you and the bike are falling left, for example, you turn the front wheel left to correct. This puts the path of your wheel of your bike on a wide left arc. All of yours and the bike's mass (centered roughly around waist level), wants to continue moving straight. The resulting centrifugal force "twists" the top of the bike back upright.

Or another way, you could call it simple inertia. The wheels and their contact with the ground move the bottom of the bike left, and inertia makes the rest of the bike go straight (which effectively is to the right, relative to the wheels)
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Old 07-10-2002, 03:09 PM
C K Dexter Haven C K Dexter Haven is offline
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Old 07-10-2002, 03:37 PM
Chronos Chronos is offline
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Quote:
2) You can’t cancel gyroscopic force. Not unless you can revoke Newton’s first law.
...or unless you put on another wheel rotating in the opposite direction. Strictly speaking, you could say that there are now two gyroscopic forces acting on the frame, but they'll be pushing in opposite directions at all times.

A dual-rotor helicopter does not, in fact, have any gyroscopic forces acting on it. The primary mechanism of a helicopter's stability is that the thing providing lift is above the center of gravity of the vehicle. A helicopter doesn't flip over for the same reason that a suitcase doesn't flip over when you're holding it by the handle.

Certainly, the gyroscopic effect does contribute, and it helps, but it's hardly the most important effect, as experiments prove.
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Old 07-10-2002, 03:38 PM
galt galt is offline
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Quote:
Originally posted by CzarNicholas
1) Trail has nothing to do with the balancing of a bicycle. This is obvious because trail does not increase with velocity, yet stability does.
Your logic is flawed. Trail does not increase with velocity, but the effect of the trail does, because the faster you go, the quicker a minute lean (because of trail) translates into the front wheel turning and tracking sideways to balance you out.

I don't believe that trail is wholly responsible for keeping you up, but it is a big factor affecting the stability of your bike. Trail gives the bike a self-correcting property which helps you stay up, and without it, you certainly would not be able to ride hands-free.
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3) The “track stand” incorporates torque and torsion and a number of other dynamic factors in combination to offset the loss of gyroscopic stability, none of which disproves the efficacy of -- or need for -- gyroscopic force.
The only reason torque and torsion have anything to do with a trackstand is that they serve to move the front wheel back and forth. You can do a trackstand by putting your ass on the seat, turning the handlebars, and putting your feet on top of the front wheel to control its movement from side to side. A trackstand is simply the problem of riding a unicycle, but in one dimension instead of two.
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Old 07-10-2002, 03:44 PM
galt galt is offline
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Quote:
Originally posted by Chronos
A dual-rotor helicopter does not, in fact, have any gyroscopic forces acting on it.
And before you get jumped on for this, yes, there are gyroscopic forces acting on the parts linking the two rotors together, but not on the helicopter as a system.

If you were to put two giant flywheels on opposite ends of a steel bar, spinning in opposite directions, and then rotate the entire mess around an axis perpendicular to the steel bar, the system as a whole would not resist rotation, but the two flywheels' gyroscopic forces would try to bend the bar.
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Old 07-11-2002, 12:26 AM
CzarNicholas CzarNicholas is offline
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No, Galt, I’m afraid it is your logic that is faulty. In fact, it is self-defeating. Trail is not a dynamic property, so what physics is at play that causes its effect to be greater at speed?

Answer? None, because trail does not create stability, at rest or at any speed.

But we’re getting off into the weeds here by mingling separate phenomena and separate dynamic principles. What initially keeps a bicycle upright is another matter from what allows the rider to change its direction without destroying that equilibrium.

If you roll a car tire down a hill, will it not remain upright until it runs out of steam? Of course it will, and completely without the benefit of trail (well, okay, it does have a little trail on account of the incline). That’s a demonstration of gyroscopic stability. And that is what tends to keep the bicycle upright.

The gyroscopic force of the wheel increases with speed. And as speed increases the rider's handlebar inputs decrease. This happens in part because the rider's input is dampened by the greater stability of the faster-spinning wheel -- the same force nets a smaller displacement of the wheel – and in part because he doesn’t want road rash. The radical inputs you make when you’re slow and squirrely would toss you on your keester at 40 m.p.h. It's the same trail but the gyroscopic forces and the stability both have increased.

And as I already noted, the draisene was stable but completely without trail. It had no steering mechanism so it didn’t need it. The need for trail only arose when steering was incorporated into the design.

Rake and trail don’t directly add stability; they serve to minimize the loss that otherwise would occur when you turn the front wheel (I know I wasn’t supposed to mention rake but I have a bicycle with a trailing-axle fork that refuses to be ridden hands-off). This is an entirely separate phenomenon from that which keeps you upright in the first place.

As for the track stand, what force do you suppose your leg is applying to the frame of your bicycle while you have your foot atop the front wheel? It’s called torque. It doesn’t matter whether it’s applied by the front brake or backpressure on a fixed gearset or your foot or a log or the banking of a velodrome, torque is torque. And the torque causes flexing in your frame, fork, headset and wheels. This is called torsion.

Chronos, you are correct that the fuselage of a helicopter is essentially a pendulum dangling beneath a rotorhead, but there always is a net effect from the spinning rotors. All helicopters tend to drift in flight in reaction to all those forces. It’s called ‘translating tendency’ and is compensated for by mast tilt, control rigging and pilot input. The pilot instinctively nulls it out in forward flight but, at a hover, it is more noticable.

But the magic is in the flying part, and that’s the rotating wing, a.k.a. the rotor. The rotor does the flying and the fuselage is just along for the ride (mostly). The rotor system must have oodles and oodles of gyroscopic stability because that’s all that keeps it from slicing and dicing the fuselage and all its occupants. In fact, if its r.p.m. is permitted to decay excessively, the rotor can lose enough of its ‘coning’ to allow it to contact the fuselage, usually with disastrous consequences. In the mid-1980’s there was a spate of mishaps with Robinson R-22s chopping off their own tail rotors because the pilot let r.p.m. bleed off, then pitched the nose up.

My point is that in any helicopter the gyroscopic forces created by the rotor system are indispensable. No helicopter can fly without them. Period. And some helicopters use identical, counter-rotating, concentrically mounted rotors. Those helicopters simply could not fly if the gyroscopic forces cancelled out.

And a bicycle has to have its gyroscopes. Is not the First Law of Cycling "wheels that don't roll, fall down?" But if you want to steer it, you gotta have trail.
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Old 07-11-2002, 03:12 AM
galt galt is offline
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Quote:
Originally posted by CzarNicholas
No, Galt, I’m afraid it is your logic that is faulty. In fact, it is self-defeating. Trail is not a dynamic property, so what physics is at play that causes its effect to be greater at speed?
You're spouting nonsense. Do this little experiment. Hold your bicycle perfectly still and upright. Holding it by the seat, lean it slightly to one side and note how the trail causes the handlebars to turn a little. Now, imagine how a small amount of handlebar turning has a greater effect on the bicycle's lateral motion (and thus balance) at high speed than at low speed.

Quote:
And as I already noted, the draisene was stable but completely without trail. It had no steering mechanism so it didn’t need it. The need for trail only arose when steering was incorporated into the design.
If gyroscopic forces are what keeps a bicycle upright, why is it that you fall over if you get your wheel stuck in a rut and can't track side-to-side to keep yourself up? Why couldn't you put a wheel guide on your bike and ride straight down a single railroad track? Even better, let's see you ride a bike in a straight line with the handlebars locked so they won't turn. Shouldn't the gyroscopic force hold you up? Why doesn't it?
Quote:
As for the track stand, what force do you suppose your leg is applying to the frame of your bicycle while you have your foot atop the front wheel? It’s called torque. It doesn’t matter whether it’s applied by the front brake or backpressure on a fixed gearset or your foot or a log or the banking of a velodrome, torque is torque. And the torque causes flexing in your frame, fork, headset and wheels. This is called torsion.
Don't try to apply mysticism to physics and make torque and torsion into magical forces. When i put my foot on the top of my front wheel and move it back and forth, I'm lowering the torques and torsions applied to the bike to the point where they're negligible. If you want to pick nits that don't matter, just sitting on your bike involves torque. But that's completely irrelevant to balance. A trackstand is simply the act of moving your bike back and forth underneath you in order to balance. It's no different than how you balance a pogo stick if you want to get right down to it.

Quote:
My point is that in any helicopter the gyroscopic forces created by the rotor system are indispensable. No helicopter can fly without them. Period. And some helicopters use identical, counter-rotating, concentrically mounted rotors. Those helicopters simply could not fly if the gyroscopic forces cancelled out.
You're just plain wrong here. See this page for an explanation of why gyroscopic forces cancel each other out in a system. Note that this page even mentions what I said before: the link between the two gyroscopes still has forces acting on it, but the system as a whole does not. Where do you get the idea that this isn't true?

So far you've failed to back up anything you've said, and it sounds like you want to just wave your hands and throw out terms like "dynamic property" without defining them and mumble about torque and torsion and hope we buy it. I don't.
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Old 07-11-2002, 03:21 AM
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CzarNicholas, there is ample evidence to suggest that gyroscopic forces are not necessary for balancing a bicycle. A Razor scooter has far less gyroscopic force than a regular bicycle, yet it is no more difficult to balance. And as someone mentioned, ice bikes with no front wheels are not hard to balance.

Which is not to say that gyroscopic forces don't play any role in bicycle dynamics. It does improve stability. I believe the counter-rotating wheel bike could not be ridden hands-off. And small wheeled bikes tend to be twitchy and difficult to ride hands-off, at least in my experience.

As for the gyroscopic forces on helicopter blades, read the replies again carefully. There are gyroscopic forces on each rotor, but on a two-rotor helicopter the gyroscopic forces on one rotor cancles that of the other. Besides, it's centrifugal force that keeps the rotor in shape, not gyroscopic.
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Old 07-11-2002, 09:25 AM
Bindlestiff Bindlestiff is offline
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I just have to chime in here.

A bicycle in motion is balanced primarily by the rider's input to the handlebars. If the rider presses forward on the right bar (counter-clockwise torque), the front wheel is steered to the left forcing the bottom of the tires to the left. There is nothing to force the rest of the bike and rider to the left so inertia causes them to go straight. With the majority of the bike and rider going straight and the contact patches of the tires going left the bike has no choice but to lean to the right. To restore balance, the tires must be brought back to their position under the bike. This is accomplished with rearward pressure on the right bar (clockwise torque). This is an on-going, dynamic process.

When a bike with positive trail is leaned to the right, gravity applies clockwise torque, which, if the bike is moving, tends to restore balance. The greater the trail, the greater the clockwise torque - increased stability. Because of this effect of trail it is possible to ride no-handed by shifting one's body weight.

In executing a turn on a bike, a rider initiates the turn by applying torque to the steering (handlebars) in a direction opposite the direction of the turn. This causes the bike to lean in the direction of the turn. Throughout the turn torque is constantly applied to the steering by a combination of trail's effect and rider input. Finishing the turn entails applying torque to the steering in the direction of the turn to bring the tires back under the bike.

During the turning the vector sum of gravity and inertia (centrifugal force) must pass through the contact patches of the tires or the bike will spin out or high-side depending on the direction of error.
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Old 07-11-2002, 01:16 PM
Nametag Nametag is offline
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Does anyone have any numbers on the torques and accelerations involved? I calculate the angular momentum of a rolling bicycle at less than 3 Nms, and I don't see how conserving that tiny angular momentum is going to compensate for the wobbling of a 100 kg rider.
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Old 07-11-2002, 01:52 PM
CzarNicholas CzarNicholas is offline
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Galt, I'm sorry if you're put off by the technical jargon. It's an unintended consequence from 20 years as a military and commercial aviator.

By the way, I tried your experiment. I held my bicycle straight up, then tilted it a little. And sure enough, the wheel turned, just like you said. Then I let go.

Darndest thing, would you believe it fell over? I can't figure why, shouldn't the trail have saved it?

Then I picked it up and pushed it. It rolled straight and true for a few feet, slowed a bit, began turning left, and then fell over again. So what dynamics is at work that allows trail to function at speed but diminishes as the bicycle slows? I'm so confused!

If you still don’t get it, let Professor Bloomfield explain it to you:
http://howthingswork.virginia.edu/

As for ruts, they don’t change the laws of physics. In my experience, a rider who looses control in ruts does so because he fails to steer straight down them. He turns the front wheel causing the tire to contact the walls of the rut, slowing the wheel’s rotation and diminishing gyroscopic stability. Not to mention that the knobbie now wants to climb up the side of the rut, wresting control from the rider.

As for the track stand, I have seen track riders come to a complete stop – no back-and-forth – for several seconds. I have seen MTBers remain absolutely motionless for the better part of a minute (fat tires simplify the matter). They might twitch the handlebars, and they might shift their weight about, but the tires remain fixed to the same location on the ground. Oh, and you can find information elsewhere on the Internet stating that on 25 November, 1977, one David Steed kept a bicycle upright and stationary for 9 hours and 15 minutes. Stationary. Motionless. Without rolling. Which sort of refutes the contention that rolling is at all necessary.

As for the link you provided, it refers to “sum angular momentum of the system,” also known as gyroscopic precession. Although precession always occurs with it, it is not synonymous with the basic gyroscopic force.

Considered as a system, when an outside force is applied to any spinning gyroscope, the change manifests itself 90 degrees in the direction of rotation. This is gyroscopic precession, a.k.a. “sum angular momentum.” This is why a bicycle wheel feels like it’s twisting funny if you hold it between your hands and have someone spin it up, then try to turn it. It isn't turning in the direction in which you pushed, but another 90 degrees around in the direction of rotation. This is also why a top falls over in a gradually decaying spiral when it looses its momentum (before it hits the ground).

If you mount two wheels on the same axle and spin them in oppsite directions, it is the gyroscopic precession that cancels. Precession is directional because the input was directional. Gyroscopic force _is_not_ directional; it acts in a plane and cannot be cancelled.

And, by the way, any definition of torque will include some phrase akin to “twisting force.” Sitting on your bicycle _does_not_ apply torque until you start turning something.

Chronos, you’re looking at my finger and not at the moon. I mentioned coning of rotor blades because it is part and parcel to the physics that is necessary to preventing the rotor from doing a veg-a-matic routine on the fuselage. You can’t create centrifugal force without also creating gyroscopic force; they both are manifestations of inertia being forced to travel in a circle. Like gyroscopic force and gyroscopic precesion, they are joined at the hip.

This is lapsing into a long-winded aerodynamics lecture, but here goes. All winged aircraft owe their stability to a relative equilibrium of aerodynamic forces. Consider a simple airplane flying straight and level. What makes it turn, for instance, is causing one wing to generate more lift than the other. This tilts the vertical component of lift, disrupting the equilibrium that existed when you were straight and level. The weight that had been in direct opposition to lift now needs a counterbalance. Equilibrium is restored when the airplane turns, its now-angular lift component balanced by an equally-angular centrifugal force.

A helicopter has wings too, they just happen to rotate. Although pilots sometimes refer to it as a “rotor disk”, it obviously is not a disk. Neither do these wings create lift in some magical, uniform disk-shape; they obey the same physics as airplane wings and only create lift directly above each individual rotor.

All aircraft -- airplanes, gliders, helicopters, gyrocopters, balloons, blimps, dirigibles, zeppelins, hang gliders, parasails and parachutes alike (with the exceptions of military fighters and aerobatic airplanes) -- are designed to seek this equilibrium. With the particular exceptions noted, positive dynamic stability is a good thing.

Particularly at a hover a helicopter has little aerodynamic equilibrium apart from that created by the big fan on top. In a manner of speaking, the fuselage is ‘borrowing’ its stability from that generated by the rotor system. In forward flight it gets considerable slipstreaming but even then it still is just tagging along with that oh-so-stable rotor system.

You probably have seen amphibious helicopters, usually with pontoons mounted on the skids. Have you noticed that you never see one shut down the engine(s) while on the water? They can’t. Or start it either. The dynamic forces generated by the slow-turning rotors would cause them to rock so hard they’d tip over.

All helicopters rock about when starting or stopping their rotors, particularly those with 2-bladed rotor systems (Hueys, Cobras, Jet Rangers, etc.). But when the rotors are up to speed they create a gyroscopic force that resists that wobbling and, in resisting, (somewhat) dampens it. And the wobbling is violent enough that it could capsize any amphibious helicopter (including the enormous Chinook).

Land-based helicopters aren’t immune to this transitory imbalance, either. Some helicopters – such as the Hughes/Schweitzer 269 & 300 -- suspend their skids from shock absorber-like devices known as an Oleo struts. As their rotors are being engaged, these “shock absorbers” can amplify the rotor-induced rocking in a harmonic phenomenon known as ‘ground resonance.’ If allowed to proceed unchecked, ground resonance will shake the aircraft apart before the rotor reaches operational speed. The pilot’s one consolation is that when the rotor blades come loose, he’ll be safe in the eye of the hurricane.

The pilot relies on that gyroscope for bigger reasons, too. He counts on it being a nice, stable flying disk that he can, through manipulation of aerodynamic forces, cause to change velocity and direction. Fortunately for him, because of its gyroscopic force, it wants to remain horizontal, just like a well-thrown Frisbee®.

All gyroscopes resist change; that is their fundamental property (and the reason they are an essential component in guidance systems). The helicopter pilot uses to his advantage the fact that his rotor system -- owing to gyroscopic forces -- wants remain horizontal.

If the rotor system did not have its own nominal "steady state," the whole wretched aircraft would be uncontrollable. And that goes for Chinooks and Kamans and Kamovs with their counter-rotating rotor systems as well. In fact, their rotor systems, like those of all large helicopters, have such a powerful gyroscopic force that the pilot requires hydraulically-boosted controls to be able to budge it.

Sure, a helicopter can fly without gyroscopic force. So can a Frisbee®. Oh, sorry, that’s not called flying, it’s called FALLING.

I apologize for the helicopter tirade but the issue at hand was gyroscopes, and I can think of no more powerful a gyroscope on earth than the spinning rotor system on a CH-53.

Razor blades and ski bikes? You’ve left out roller blades and slalom water skis and ice skates and the Beatles’ skiing chairs. These devices keep the user so close to the ground that no extraordinary dynamics is required; the normal human sense of balance does the trick. The moment arm from the ground to the bottom or the user’s feet is very short. And the moment arm to the user’s center of gravity is, in comparison, extremely long. Not that far from the forces present when you’re walking, actually.
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  #22  
Old 07-11-2002, 04:14 PM
Chronos Chronos is offline
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Quote:
By the way, I tried your experiment. I held my bicycle straight up, then tilted it a little. And sure enough, the wheel turned, just like you said. Then I let go.
Darndest thing, would you believe it fell over? I can't figure why, shouldn't the trail have saved it?
No, because trail only works when the bike is moving. The idea is that you want your base to be below the center of gravity. Suppose the center of gravity on a bike moves so it's no longer over the base... In the technical jargon, this is called "leaning to one side". Due to trail, this causes the steering axis to turn. If the bike is standing still, then the turning of the steering axis doesn't do anything, and the bike just falls over. If, however, the bike is moving forward, then a turned steering axis will cause the bike to move to the side. In fact, on a well-designed (i. e., ridable) bike, the bike will move in the direction of the turn, thus putting the base back underneath the center of gravity.

Quote:
Gyroscopic force _is_not_ directional; it acts in a plane and cannot be cancelled.
This would be very interesting, if true. However, there is no such thing as a force "acting in a plane". The only sense in which a force can "act in a plane" is that the force acts in a line, and the line is in a plane. Every force acts in a line, and every force can be cancelled. Furthermore, the only effect which can be reasonably referred to as "gyroscopic force" is gyroscopic precession, which, as you note, is cancelled by a counterrotating gyroscope.

If you don't know a subject, there's no shame in admitting that you don't know it, and allowing yourself to be educated. Pretending that you know it and making up a bunch of jargon will never get you very far, especially not on a board like this one.
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  #23  
Old 07-11-2002, 04:32 PM
galt galt is offline
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Quote:
Originally posted by CzarNicholas
Galt, I'm sorry if you're put off by the technical jargon. It's an unintended consequence from 20 years as a military and commercial aviator.
Appeal to authority noted. I'm not put off by technical jargon - I'm put off by the incorrect use of it, especially when it's used by someone who's just using jargon for its own sake in hopes of sounding like he knows what he's talking about.
Quote:
By the way, I tried your experiment. I held my bicycle straight up, then tilted it a little. And sure enough, the wheel turned, just like you said. Then I let go.

Darndest thing, would you believe it fell over? I can't figure why, shouldn't the trail have saved it?
As I told you before, the slower the bike is moving, the less of an effect the trail has. If you turn the handlebars of a stationary bike, the bike does not start traveling on a curved path, so it does not move the contact point relative to the mass enough to keep it from falling over. When the bike isn't moving at all, trail doesn't help any.
Quote:
So what dynamics is at work that allows trail to function at speed but diminishes as the bicycle slows? I'm so confused!
You sure are. I've explained it twice now.
Quote:
As for ruts, they don’t change the laws of physics. In my experience, a rider who looses control in ruts does so because he fails to steer straight down them. He turns the front wheel causing the tire to contact the walls of the rut, slowing the wheel’s rotation and diminishing gyroscopic stability.
I see. So this implies that you should be able to ride an unsteerable bike in a straight line without falling to the side. Is that your claim? I'd like to see you try it.
Quote:
As for the track stand, I have seen track riders come to a complete stop – no back-and-forth – for several seconds.
You haven't been paying attention to the shifting of the mass of their bodies relative to the contact point between the bicycle and the ground, then. Shifting the mass while keeping the contact point stationary accomplishes the same thing as moving the contact point while keeping (most of) the mass stationary.
Quote:
Oh, and you can find information elsewhere on the Internet stating that on 25 November, 1977, one David Steed kept a bicycle upright and stationary for 9 hours and 15 minutes. Stationary. Motionless. Without rolling. Which sort of refutes the contention that rolling is at all necessary.
Funny, that would also imply that torque and torsion are unnecessary, too, but somehow I doubt he was really motionless. See my previous point.
Quote:
As for the link you provided, it refers to “sum angular momentum of the system,” also known as gyroscopic precession.
You're mixing concepts. Gyroscopic precession is the result of the angular momentum of the system. They're not the same thing. "Precession" does not simply refer to the force being transferred 90 degrees off from where it's applied -- it refers to the effect of this when the force is applied in a consistent way such that circular motion is achieved. This is why the procession rate is measured in Hertz.
Quote:
If you mount two wheels on the same axle and spin them in oppsite directions, it is the gyroscopic precession that cancels. Precession is directional because the input was directional. Gyroscopic force _is_not_ directional; it acts in a plane and cannot be cancelled.
Ok, then what's the effect of this ever-present gyroscopic force which doesn't get canceled? You've already conceded that what you're referring to as gyroscopic precession gets canceled, so if the remaining "gyroscopic force" doesn't contribute "gyroscopic precession", what does it contribute?
Quote:
And, by the way, any definition of torque will include some phrase akin to “twisting force.” Sitting on your bicycle _does_not_ apply torque until you start turning something.
Sure it does. All over the place. For example, unless my ass is positioned directly above the seat post, I'm applying a torque to the clamp holding the seat at the proper angle, regardless of whether or not it's actually moving. Every joint on your frame is likely undergoing some amount of torque.
Quote:
Chronos, you’re looking at my finger and not at the moon.
Your obviously extensive experience with helicopters does not change your misunderstanding of gyroscopic forces.
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  #24  
Old 07-11-2002, 08:42 PM
Bindlestiff Bindlestiff is offline
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Galt,

you've hit on a great idea! We need to build a bicycle with a fixed front fork - unsteerable and see if CzarNicholas can ride it.

High-wire performers do it. Of course they need a twenty-foot pole to keep their balance - but then they are not royalty.
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  #25  
Old 07-11-2002, 11:30 PM
scr4 scr4 is online now
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Quote:
Originally posted by CzarNicholas
Razor blades and ski bikes? You've left out roller blades and slalom water skis and ice skates and the Beatles' skiing chairs. These devices keep the user so close to the ground that no extraordinary dynamics is required; the normal human sense of balance does the trick. The moment arm from the ground to the bottom or the user's feet is very short. And the moment arm to the user's center of gravity is, in comparison, extremely long. Not that far from the forces present when you're walking, actually.
The distance between your feet to the ground has nothing to do with it (why would it?). In any case, the Razor scooter has the rider's feet about 2 inches off the ground. On a bicycle, it's about 4 inches. I fail to see why that makes any difference. And if you looked at the link I provided, you will see that several of the ice bikes in those photos are normal bicycles with the front wheels replaced with runner blades. The geometry is exactly the same as the original bicycles.

Skates and roller blades are different. On those, the user is free to lift a foot and put it down anywhere. Balance is achieved by placing the contact point carefully below the center of gravity. Or more precicely, the average position of the contact points is below the center of gravity. On a bicycle, you can't lift a wheel and put it down in a different spot. The only way to move the contact point is to turn the handlebar, and let the forward motion move the front wheel to the side. The trail makes this action semi-automatic. So you see why trail works better at higher speeds.
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  #26  
Old 07-12-2002, 01:00 AM
PBear PBear is offline
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Good news. It turns out David Jones’ article, The Stability of the Bicycle, Physics Today (1970) is available online c/o Joel Fajans, UC Berkeley. Unfortunately, it’s only pdf of a scan, and not a great scan at that, but better than nothing. Easier to read, I found, when printed out (as opposed to trying to read onscreen). Also, be warned it's a big download (almost 9 MB)

FWIW, in the course of my travels, I came across many articles on the subject, of which these were (for me, at least) the most informative:

An Introduction to Bicycle Geometry and Handling, Karl Anderson (emphasizing side-to-side corrections, but also giving significant weight to trail)

Ask Lou, Physics Central and How Things Work, Louis Bloomfield (slightly different articles, to the same general effect; both emphasize trail, but give weight to side-to-sode corrections)

Exploratoreum (San Francisco) (discussion of Balancing and Steering starts about halfway down the page; continues here) (emphasizing side-to-side corrections)

Forces and Balance in a Bicycle (emphasizing side-to-side corrections)

Balancing a Bike, Herb Weiner (emphasizing trail)

rec.bicycles FAQ, especially 9.35 on gyroscopic forces (debunked); see also 9.15 (discussing, among other things, turns) and 9.16 (trackstands)

How you steer a bicycle, Joel Fajans (good discussion of countersteering); see also Physics & Bicycling

PhysLink.com, Matthew Allen (emphasizing gyroscopic effect, but not discussing Jones)
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  #27  
Old 07-12-2002, 06:16 PM
staubej staubej is offline
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why make it difficult?

Hi folks,
this is actually my 2nd post. The first time this was addressed i got so frustrated at Lingle's response, i had to get in here and post. I see i was ignored the first time, so i thought i'd write again. Oh, i'm not mad or anything, it's just that it seems everyone seems to take this much farther than it should be. A moving bike balances easier because the rider can readily and precisely move the contact patch from, and to, either side of the center of gravity (CG), causing the bike to fall sideways and turn (i think we all agree it turns by leaning). The only way to move the CG on a stationary bike is to pick it up and move it. Not very easy.
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  #28  
Old 07-12-2002, 07:01 PM
PBear PBear is offline
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Hey Staubej. It’s not a matter of making it difficult. It is a difficult puzzle. The center-of-gravity theory, which you may note I expressed myself above, explains only part of the story. Mind, I think/agree it’s the part most directly relevant to the original moving/nonmoving bicycle question. For that matter, it was the starting point of Jones’ article. But, as he demonstrated there and others have argued here, there’s a lot more to overall bicycle stability. Which I, at least, find even more interesting than the original question. But, you know, to each his/her own.
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  #29  
Old 07-12-2002, 08:36 PM
PBear PBear is offline
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BTW, staubej, I did a search for your prior post and couldn't find it. Only your simultaneous almost-identical post (also referring to an earlier post) in the motorcycle thread. Are you sure it was this board? Or did you perhaps change user name?
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  #30  
Old 07-13-2002, 08:03 AM
staubej staubej is offline
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Hi PBear,
i too, searched for my other post, w/o any success. It was months ago when this question was first raised and Lingle went into a diatribe about gyroscopic effects. I would agree there is more going on w/ stability, but i was mearly commenting on getting the thing to turn. Thanks for reading!
-john
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  #31  
Old 07-13-2002, 08:32 AM
CzarNicholas CzarNicholas is offline
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Folks, here's what NASA has to say on the matter:

"Any rotating wheel or body tends to stay in its plane of rotation (due to Newton's laws). That is why a bicycle stops wobbling when you get up speed and why a spinning top stays upright. The spinning bicycle wheel or top gives stability to the system."

(http://kids.msfc.nasa.gov/Teachers/PastEmail.asp?whichpage=28)

Makes you wonder how they ever managed to put a man on the moon, don't it.
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  #32  
Old 07-13-2002, 11:08 AM
PBear PBear is offline
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CzarNick: The NASA statement is correct, but note that it emphasizes speed. Even at moderate to slow speeds, gyroscopism contributes, just not enough to greatly affect the much larger mass of the rider. Nor, of course, does NASA purport to be giving an explanation of all aspects of bicycle stability. It's merely illuminating (actually, suggesting that teachers illuminate) an abstract physics principle by reference to something to which kids can relate in their everyday experience.

Staubej: No worries. Was just puzzled. Now that I think about it, I think I've seen references to lost threads due to a board crash, though I wasn't here at the time. Maybe that's the explanation.
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  #33  
Old 07-13-2002, 01:27 PM
waterj2 waterj2 is offline
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Yeah, there were a couple months worth of stuff (everything from Dec. 9th to whenever the board crash was in February) that is lost for good. The first thread on the first bicycle stability column was part of that.

As for the bicycling stuff, I'm just amused to see someone attempting to lecture Chronos on elementary mechanics. It's especially amusing because the "lecture" come to completely incorrect conclusions.

For example, the track stand is accomplished not by any torsion applied to the frame, but by keeping the contact patches under the center of gravity. Notice how in a track stand, the front wheel is generally turned to the side? That's because the goal is to be able to move the contact patches from side to side, to counteract the movement of the center of gravity.

Ever tried to ride a bike as slow as possible? You end up steering from side to side rapidly to try to keep the contact patches under you. If I ride my bike in a light snow, I can see the path that the bike follows, and it tends to gently wander a couple inches from side to side. Why is this? Because as I inadvertantly lean a little bit to the side, the front wheel moves slightly towards that side. This puts the contact patches back under me. I can't say that I've ever tried going out to where the trolley tracks run in the middle of the road and riding in the groove, but I doubt that it would be possible.
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  #34  
Old 12-29-2002, 01:45 PM
Geno Geno is offline
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CzarNicholas, you stated:
>>>As for ruts, they don’t change the laws of physics. In my experience, a rider who looses control in ruts does so because he fails to steer straight down them. He turns the front wheel causing the tire to contact the walls of the rut, slowing the wheel’s rotation and diminishing gyroscopic stability. Not to mention that the knobbie now wants to climb up the side of the rut, wresting control from the rider.>>>

I disagree. Clear your mind and consider this: A bicycle rider who loses control in a rut does so because he has lost control of lateral movement of the bicycle beneath his center of gravity. Gyroscopic effect has nothing at all to do with balance on a bycycle. With both wheels in a rut, I will crash and you will crash. Think about how a unicyclist maintains balance while stationary. It is accomplished by short forward-and-backward movements of the wheel, which affords front-to-back and lateral adjustments of the unicycle beneath the center of gravity. A bicyclist doesn't have to worry about front-to-back stability because the two tires provide that stability. Look here:

http://boards.straightdope.com/sdmb/...hreadid=148800
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  #35  
Old 01-02-2003, 01:30 AM
Zagadka Zagadka is offline
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Erm, similar thing with balancing on a skateboard. :-p
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  #36  
Old 01-04-2003, 07:35 AM
Irishman Irishman is offline
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CzarNicholas said:
Quote:
As for ruts, they don’t change the laws of physics. In my experience, a rider who looses control in ruts does so because he fails to steer straight down them. He turns the front wheel causing the tire to contact the walls of the rut, slowing the wheel’s rotation and diminishing gyroscopic stability. Not to mention that the knobbie now wants to climb up the side of the rut, wresting control from the rider.
But the reason he turns the wheels slightly is because his weight has shifted slightly to the side, and the wheel turn is an automatic balance adjustment. Thus the sides of the wheels rub the sides of the rut, and cause the aforementioned wipeout. That is precisely the point - the rider has to turn the wheel to maintain the contact patches beneath his c.g.

Also regarding gyroscopic force, you appear to be saying what everyone else is calling gyroscopic force is really gyroscopic procession, and then state there is a completely different force that is gyroscopic force. Your description of gyroscopic force appears to be centripetal force, but then you state that they are not the same thing. So what is gyroscopic force?
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  #37  
Old 01-04-2003, 09:58 PM
Fuel Fuel is offline
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Re: Balancing on bikes

Quote:
Originally posted by badgerboy
Hallo all, this is my very first SDMB post, woohoo!
Cant beleive no-one has mentioned this yet, but doesnt a bike balance when it's moving on account of the curved profile of the tyres? If the bike leans, the rubber on the outside of the tyre makes contact with the ground, but has a smaller circumference than the middle of the tyre and so pushes the bike vertical once more. Please point out the glaring error that i'm missing, teeming millions, 'cos i can't see it myself.
the curved part of the tire does not push the bike upwards. it does create a tighter path and smoother area of contact than a square tire though. this helps the rider balance the bike but does not maintain balance itself.

the main balancer on a moving bike is matching speed with the corresponding lean and handle bar turn. those three variables all must be in harmony. gyroscopic force does not help you stay balanced.... it is a small force that must be dealt with by the three previously mentioned variables, but does not contribute to an easier way to balance.

the faster the bike goes, the more control a rider has in his small handlebar turns and leans. control meaning, each turn of the handle bars of a given measure will do more *work* on the system, as speed increases.

starting at equilibrium going 100 mph..... if you lean with a force of 5 ft-lbs (which is how any fall will begin..... with a lean), you will be able to counteract this force easier, with a handlebar twist, than at 50 mph, because the tire is coming into contact with twice as much road.... this extra speed and road contact makes each handlebar twist more meaningful to the bikes' previous counteractions. each lean/twist produces more G's the faster and faster you go, which creates a lower center of gravity of the system, (majority of mass (your body) is at top of bike, but the more G's exerted, the more this "topheaviness" is lessened).

so any lean at a higher speed will result in a more stable, "self-equalizing" system. (when counteracted with either a speed change or handle bar turn obviously)

to summarize: the imperfection that makes you fall to one side or other is a lean. at 0mph the lean is devastating because there is no way to draw center of gravity to bottom point of tire. at 100 mph you are able to counteract the lean with a G force change, drawing the center of gravity back to the bottom of the tire. the easiest way to do this is to speed up so that you are pulling more G's, so that as far as the system is concerned, your COG is right were it should be.

at 0 mph, if you lean, you will fall unless someone pushes on the top of your head straight into the center of the bike & rider. this would smash the bike into the ground so that you wouldn't fall, which is same thing as pulling some G's around a corner.
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  #38  
Old 01-05-2003, 09:51 AM
Irishman Irishman is offline
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Fuel, reasonable description, but would be better using realistic speeds.

Bindlestiff said:
Quote:
In executing a turn on a bike, a rider initiates the turn by applying torque to the steering (handlebars) in a direction opposite the direction of the turn. This causes the bike to lean in the direction of the turn. Throughout the turn torque is constantly applied to the steering by a combination of trail's effect and rider input. Finishing the turn entails applying torque to the steering in the direction of the turn to bring the tires back under the bike.
Respectfully, I must disagree. While this technique may be required for motorcycles, bicycles do not require the original push on the handlebars to be in the direction opposite you wish to turn. All that is required to initiate the turn is to lean in the direction you wish to turn and rotate the handlebars to match the lean. While it is possible that using this technique would make the turn easier or sharper, I have not been able to notice this myself. What is important for the turn is to have your weight shift into the turn, so the combination of weight and inertia reacts through the contact patches.
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  #39  
Old 07-06-2003, 02:30 AM
nodope4us nodope4us is offline
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Well, I am not to bright in the physic's department, I am however the nephew of a ex-pro motorcycle rider. I asked him how the guys on the street bikes lean into turns and not fall over and how they stand the motorcycles back up. He told me that they have to pull it down into the turns and then control how fast the bikes stand up. It explains why you see guys get thrown off the bikes when exiting a turn. If anyone can help explain how and why this works and the connection between this and the initial question
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