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.
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.
Link fixed, thanks PBear! – Dex
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?
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
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.
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.
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.
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.
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).
- 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.
- 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.
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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.
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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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His Imperial Majesry
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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)
Blessed be those who go around in circles, for they shall be known as Big Wheels.
…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.
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.
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.
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.
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.
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.
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?
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.
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.
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.
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.
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.