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Why do we lose balance in a bike when at a standstill?

  1. Jul 20, 2004 #1
    ive been wondering...

    why is it so much easier to maintain our balance when the bike is moving?

    is it something of centripital (sp?) force, just like when you sit on a spinable chair and spin a bike wheel and it turns you in circles. (does the force of the spinning force you downward?)
     
  2. jcsd
  3. Jul 20, 2004 #2
    not sure if this is exactly right, but conservation of angular momentum is one reason. when the wheels are up and spinning, they act as gyroscopes. their angular momentum vectors point to the left when you go forward. tilting the bike would change the angle of this vector and would require a torque to be exerted on the system. so in general, the wheels will resist changes in their angular momentum when they are spinning, but will not when they are not spinning - just like a gyroscope.
     
  4. Jul 20, 2004 #3
    I originally just said, "Partially angular momentum, partially rotational inertia." Oops, then I reread the question. You lose your angular momentum, the main thing that helps you stay upright at all after you stop is your rotational inertia.
     
    Last edited: Jul 20, 2004
  5. Jul 20, 2004 #4

    krab

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    The main reason is that when you are moving, steering allows you to move your point of support around. In particular, you need to keep the support vertically in line with your centre of gravity. When you are stopped, you can no longer do this. It's like standing on one foot. If you are not allowed to hop, and you start falling sideways, you cannot recover. But if you can hop to relocate the foot with respect to the c.of.g, you will recover balance.
     
  6. Jul 20, 2004 #5

    Janitor

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    I remember thinking about this same issue waaaaaaay back in the days when I had a bicycle. I came to the same conclusion as krab. I could in those days keep my feet off the ground at a very low speed on my bicycle, albeit with a lot of jerky movements of the handlebars. I decided that the gyroscopic action at such a low speed was so slight as to be negligible, compared to the "moving of your point of support" as krab puts it. At high speeds, I don't doubt that gyroscopic action is important, and very subtle movements of the handlebar suffice at high speed.
     
  7. Jul 21, 2004 #6
    Found a good description of the physics of bicycle riding here:

    Bike Physics

    In this article, he atributes balance to centripital force.
     
  8. Jul 21, 2004 #7

    HallsofIvy

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    One nice thing about a bicycle is that when the bicycle leans to the right, for example, the "gyroscopic" force makes the front wheel turn to the right, thus "catching" the lean. That's why you can ride "no hands".

    By the way, the Olympic Games are coming up. Be sure you watch the bike "sprints". I put "sprints" in quotes because sprinting is the last thing they do! The effect of drafting is so great that each rider wants to force the other to lead. They spend an enormous amount of time almost motionless! But of course, they can't can't come to a complete stop because then they couldn't remain upright. (They can "rock" back and forth!)
     
  9. Jul 21, 2004 #8

    BobG

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    DarkEternal hits the primary reason. Yes, you can keep a very slow moving bicycle upright, but it doesn't have any real stability unless the tires are moving fast enough to generate some gyroscopic ability.

    Take the tire off of your bicycle, fashion a handle that sticks out from the hubs on your bicycle tire, and spin the tire as fast as you can. Now turn the tire in different directions. You can feel the effect of the tire's angular momentum. In fact, if you sit on a rotating stool, have someone hand you a rotating bicycle tire, lift your feet, and turn the tire over, you'll start spinning the opposite direction of the tire's new direction. You've formed a system including you and the tire, and between you, the angular momentum will attempt to stay constant (minus the affect of friction, etc). You can control the rate you're spinning by changing the angle of the tire, returning the tire to it's original orientation to bring yourself to a complete stop.
     
  10. Jul 21, 2004 #9
    What a great post. I was thinking of this exact question while mountain biking today...
     
  11. Jul 22, 2004 #10

    krab

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    The gyroscopic effect is important; it allows one to affect a bike's lean by applying steering force. But it is not the effect that explains why it is possible to balance while moving and practically impossible when stationary. There are in fact 3 phases: 1. When stationary, it is hard to balance; 2. When moving too slowly for gyro effects to matter (below a fast walking pace), you tend to meander around while balancing; 3. When moving above a walking pace, it's easy to balance with hardly any meander, one can also ride with no hands.

    Experiments have been done where the gyro effect is cancelled by a counter-rotating wheel. It is still possible to ride such a bike.
     
  12. Aug 5, 2004 #11
    Just seen this post and it reminded me of an experiment where a bicycle was pushed down a slope without a rider (so no moving the handlebars to regain balance). Once the bicycle was pushed to a slow running speed, it was let go to run down the hill on its own. It accelerated at first due to the hill's slope and the bicycle carried on going until it reached a flat piece of ground and slowed down. It was only when it had slowed to a walking pace that the bicycle fell over.

    So, the balance of a bicycle when being ridden is the same as a bicycle travelling at speed without a rider. This brings doubt to Krab's theory of moving the handlbars to get the "moving of your point of support". The bicycle somehow manages this on its own. So it must be more to do with the gyroscopic effect caused by the motion of the wheels.
     
  13. Aug 5, 2004 #12
    The gyroscopic effect is the unique reason for the balance differences between a stationary bike and a moving bike.
    By the way, in a high speed, it is enough just to tilt the bike to change the movement direction. How could explain this? In fact, this is the precession started by the perturbation introduced by your tilt.
     
  14. Aug 5, 2004 #13

    As far as I know the bike turns because of camber thrust. The distance around the tire at it's center and closer to the edges is different. When the tire is leaned the inside of the contact patch doesnt have as far to travel so the tire turns.

    Also what you said about "catching" the lean works slightly different than that. When the bike is traveling in a straight line to get it to lean to the left you have to actually turn the bars to the right. This causes the bike to start to fall to the left because the front tire is going ever so slightly to the right, and the back tire and the weight of the bike are still wanting to go straight. The result is the bike leans to the left and camber thrust causes it to turn that way. The "catching" the lean comes into play when we decrease pressure on the bars enough to stop leaning.

    I don't think the gyroscopic force of the wheels has much if anything to do with how the bike turns at all unless you happen to be doing a wheelie. A bike being ridden is basically always in a controlled fall. Falling to either the right or left. The more foward speed it has the less likely it will fall over. You can see this at motorcycle races all the time. Bikes without riders (meaning the rider fell off) many times continue on riding down the road until it slows enough to fall to one side or the other.
     
    Last edited: Aug 5, 2004
  15. Aug 8, 2004 #14
    Lets consider making turns just by leaning the bike (whithout moving the bars).

    Well, with a little math and according to your thinking, the same difference between the internal and external paths, but using a smaller wheel, would imply in a smaller curvature radius.

    So, if it was true, at the same leaning, a bike with a smaller wheel (a smaller radius but the same tire cross section) would perform better turns.

    But, it seems to be exactly the opposite, which just reinforces the importance played by the gyroscopic effect mentioned before.
     
  16. Aug 8, 2004 #15
    Actually smaller front wheels were used for a few years on some sport bikes and they did indeed turn quicker. They used a 18 inch wheel on the rear and a 16 on the front. So when you say it seem exactally the opposite I miss your reasoning.

    Other things like wheel base, rake angle, and amount of trail also play a big part in how fast a bike turns. The act of turning is solely accomplished by camber thrust as I mentioned above. Don't believe it? Take a tire and roll it, when it gets slow enough it will start to lean then turn in a circle that tightens the more it leans. I'm sure we have all seen this.

    The fact that you can trurn a bike at all without turning the bars should help you see that gyroscopic effect has nothing to do with it. Why dont you weld your stearing straight and then push your bike very slowly as you lean it. Now you will see it turns without any gyroscopic effect.
     
  17. Aug 9, 2004 #16
    I have another idea that may contribute to this post. (maybe not). When a person is sitting on the bike (compare to when bike is rolling without a person) due to the weight of the person the bike will be pushed down hard on the ground making the surface area of tire in contact with ground larger, therefore making it more stationary. And when the wheel of the bike is rolling hard on the ground, a certain part of tire is in collision with the ground that is no perfectly flat, faster the speeds of the bike, harder it will collide with the ground making again making the surface area of tire in contact with ground larger. After all said, I am not really sure about what I said. Can someone criticize or pick up where I left off?
     
  18. Aug 9, 2004 #17

    krab

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    That's strange, first you present data that is completely consistent with what I said in my post, then you say my post is wrong. Maybe you should read it again. "Bicycle manages stability on its own" when going fast enough; just as I said one can ride with no hands (I called it phase 3). Bicycle fell over when speed dropped below walking pace; just as I said (called it phase 2)

    You then cannot explain why it is possible to balance a bike at such a slow speed that gyro effects are negligible (what I called phase 2).

    This is an interesting claim. How does it scale with tire width? I ride both a motorcycle and a bicycle, and the difference in tire width is larger than a factor of 4! Yet they steer pretty much the same, or IOW I've always attributed the difference to the difference in mass; mass of spinning tire and also moment of inertia about steering axis.
     
  19. Aug 9, 2004 #18

    Yes tires are bigger on a motorcycle than a bike....wheelbase is different, rake angle is different, weight and many other things too. They turn "pretty much the same" because all the numbers add up to stability in the given aplication. Try getting that bicycle up to about 100mph and see how the steering compares to a nice stable motorcycle designed to handle at those speeds.

    You can bring up all kinds of scientific words to try to describe how it turns, but in the end the bike leans over on a contact patch that produces camber thrust. All the forces and effects that the bike feels from inertia wanting to push the tire straight, or trying to flip it over and to the outside are all overshadowed by the inside of the tire traveling a shorter path than the middle. Doesn't matter if the tire is skinny or fat, it will still turn the same way. The bike could even be rolling on spheres with an axle and it would still turn "pretty much the same".

    To get back on topic I think you were on the right track as to why the bike doesnt fall over at speed until you started talking about gyroscopic effect. We know when the bike is still and the bike is perfectly balanced that gravity pulls equal on both sides of the bike so it stays upright. Now if it leans even a small degree to the left gravity pulls harder on that side and the bike falls. The more foward momentum the bike has the less effect gravity has at pulling the bike to the left or right. This allows the rider to use less pressure on the bars to make corrections and keep himself balanced (hence less wobbling at speed). As I said the bike is always falling to the left or right, the rider has to make small corrections constantly to remain upright. I'm sure I'm missing some good scientific words here such as an object in motion tends to stay in motion or something like that, maybe someone else can help with that part.
     
    Last edited: Aug 9, 2004
  20. Aug 9, 2004 #19

    BobG

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    I can buy that. But, both phase 1 and 2 are pretty unstable. It's basically a process of constantly catching your fall. Phase 3 is very stable and the stability comes from gyroscopic stability.

    The counter-rotating wheel experiment is kind of interesting, though. What you really have are two equally large angular momentum vectors that are pointing opposite directions. Each has its own stability, so you would think overall stability is either 'twice as good' or, since each angular momentum vector is pointing in the opposite direction, they cancel each other out and you have no gyroscopic stability.

    I would tend to think that overall stability would be increased.

    If you hang one end of the axle of a spinning bicycle tire from a rope loop, the tire won't just sit there in one place spinning away. Gravity will result in torque and a second angular momentum vector. This doesn't reduce the stability of the tire relative to the ground (the tire stays perpendicular to the ground). But it does cause the tire to precess about the rope in the direction of the weaker angular momentum vector provided by gravity. (The tire's been taken off of the bicycle, by the way).

    Having two tires whose angular momentum vectors are 180 degrees apart should result in increased stability with none of the undesirable side effects such as precession.
     
  21. Aug 9, 2004 #20
    why do we maintain balance on a bike when, e.g., the earth's rotation gives it an apparent forward motion of 20 km/hr, but otherwise (with zero torque applied to the petals) it would seem relatively at a standstill?

    Ach Mach!
     
    Last edited: Aug 9, 2004
  22. Aug 10, 2004 #21

    krab

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    Just remembered I wrote something about steering a while ago:

    Motorcycle Dynamics:
    There's been lots of controversy and dim-witted flame wars on rec.motorcycles concerning how a motorcycle steers. I myself have given this lots of thought, done many calculations, tried some experiments, and here in layman's terms is what I think.

    When you initiate a turn by pressing the bars in the opposite direction (i.e. countersteering), the bike responds by leaning. This is due to the `outtracking' effect. Basically, you steer the bottom part of the bike out from under the centre of mass.

    As the bike leans, gyroscopic forces cause the forks to turn into the proper direction. You don't notice this because the handlebar movement is very small. All you notice is that a constant pressure to the left causes a lean to the right and subsequently a turn to the right. In effect, the handlebars end up turning in the direction opposite to that in which you are pushing them. But this depends upon the spinning of the front wheel. That's why it doesn't happen in the same way when driving very slowly. You can verify this by rigging up some kind of long pointer from the handlebar centre. Others have done this. The gyroscopic effect can be verified by playing with spinning bicycle wheels.

    Rake and Trail:
    To understand the importance of motorcycle/bicycle steering geometry, a good place to begin is with a simple caster wheel as found for example on grocery carts. This is a system that has trail but no rake. (Trail is the distance that the point of contact of the tire with the road trails the intersection of the steering axis with the road.) Basically, the wheel pivots to the necessary direction to follow the direction of motion. A similar system does not work on a single-track vehicle because such a vehicle leans when it corners. Try the following. Take such a wheel (say from an office chair), and lean it to one side, pushing it down to mimic the effect of the vehicle's weight. You will notice that it tries to flip outward, turning into the turn. In other words, if a motorcycle had such a geometry, you'd have to counter-steer very strongly just to keep it leaned over in a turn. Now orient the caster the 'wrong' way so that the trail is negative. Lean it and weight it. You'll notice that it tries to turn out of the turn. This suggests that steering would be neutral if trail were zero. This is correct, but zero trail is not an option since we want some of the self-stabilizing caster effect. The way around this is to tilt the steering axis back from vertical. The angle of tilt is called rake. Take a wheel on a steering axis with no trail. Now rake it backwards. Immediately you have trail equal to the tangent of the rake angle multiplied by the wheel's radius. It turns out that for neutral steering, this is too much trail. The optimum is about 2/3 this amount. On a motorcycle, this is achieved by moving the forks ahead of the steering axis. On a bicycle, the forks are curved forward.

    Example: My motorcycle has a rake of 27.5 degrees, and a wheel radius of 13.5 inches. With no fork offset, the trail would be 13.5" x tan 27.5 = 7". A 2.5" offset reduces the trail to 4.5".
     
    Last edited: Aug 10, 2004
  23. Oct 8, 2004 #22

    rcgldr

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    Krab has this mostly right. A motorcycle is a uni-trak vehicle. Just like balancing a broom stick on your hand, you have the lean the broom stick first before you can accelerate it sideways.

    Consciously or not, you have to lean a bicycle, or motorcycle by steering the wheels out from underneath you in order to turn, motorcyclist call this counter steering, you twist the bars right to roll (lean) left and vice versa. Gyroscopic forces, if anything, just make this more difficult, as they resist any effort to adjust lean (both downwards and upwards, more on this below), or turn the front wheel.

    Next is the stability question. The steering geometry is setup so the contact patch is behind where the steering axis line would reach the ground. Hold a bicycle by the rear seat and lean it over. The front wheel will fall into the direction of the lean because gravity is pulling down on the bike, the contact patch is pushing up, and multiplied by the offset from the steering axis, this creates a torque force that turns the front wheel into the direction of the lean. The steering geometry is setup so that within a speed range (more on this later) it self corrects, so that while a two wheeled vehicle is in motion, any lean is countered by the front wheel falling inwards and straightening the vehicle back up.

    Most bikes reduce the amount of self correction, to reduce steering effort. On bicycles, the forks are curved forward reducing the trail as mentioned by Krab. On motorcycles, the there are two triple clamps. The middle clamp is the part that pivots. The outer two clamps that hold the fork are forward of the middle clamp, again to reduce the trail. Move this too far forward and you can get stability issues. One case was the first year Honda 900RR sport bike, it would wobble a bit at racing speeds when encountering irregularities on race tracks. The fix was to move the forks 3/8" back closer to the pivot point with a new set of triple clamps, the result was a more stable bike, but it takes more effort to steer it.

    Unlike the previous posts talking about pushing a bicycle and letting it free run, the speed that it falls at has to do with the amount of trail and the amount of inertial yaw (turn) resitance in the front wheel. If you have a bicycle that you can turn the wheel backwards, so that the forks curve backwards, which greatly increases the trail, the bicycle will pratically come to a stop before falling over. It's almost unbelievable the first time you see this. No gyroscopic forces in this case.

    Maintaining a lean requires constant counter steering pressure (or the rider hanging off to one side, unbalancing the system). Because the steering is setup to self-correct, the front wheel "wants" to turn inwards enough to straighten up a bike. To hold a lean, a bit of opposite pressure on the handle bars is required to overcome the self correction force. This is more apparent on a motorcyle than it is on a bicycle.

    So why can you ride a bike with no hands? This works because the bike counter steers for you if you lean to one side or the other. Say you lean to the right, the bike leans to the left, and the steering geometry self corrects to straighten up the bike. However, at this point, the center of mass is off to one side of the wheels, the system is out of balance, so the bike now falls to the side your leaning on. Fortuantely, the self correcting geometry keeps the bike from falling over as long as you reduce your lean with respect to the bike soon enough. This works on a motorcyle as well, unfortunately, this can be an issue if the driver doesn't understand what's going on, and at high speeds, it doesn't work. There's a limited speed range where the no hands method works; too slow and there's too much yaw interita of the front wheel to react in time; too fast and gyroscopic forces resist any change in the yaw axis at the front tire, or in the roll axis on the bike. You'll lean over, but not the bike, and not much else is going to happen. More on this next.

    At high speeds, gyroscopics forces resist any change in lean, virtually eliminating any self correction. Up to about 70mph or so, this isn't much of an issue, but a motorcyclist taking turns at 100+ mph is going to be in for a thrilling experience if he doesn't know about counster steering (steering left to roll right and vise versa). It takes a lot of force on the handle bars to lean a bike at high speed, and it take almost as much force to straighten up as it does to lean over at high speeds. For high speed control, it's better to think ot the bike like an airplane that you roll by steering the other way. Actually there's almost no perceptible movement of the handle bars, you're really just applying pressure (and a lot of it at high speeds). A comment once made by a motrocyle magazine tester regarding riding a race bike at daytona, where one of the banked turns is exited at close to 180mph, "either the rider knows about counter steering, or he ends up in the infield". He was impressed by the large amount of force on the handle bars it took to straighten up the bike.

    If you're new to motorcycling, you can get acclimated to counter steering by leaning a bike side to side while weaving a bit within a single lane, by twisting the handlebars side to side just a bit. It's best to do this on a freeway, as there's almost no resitance to counter steering at speeds below 50mph. At 65mph or so, there's enough resistance that you'll get the sensation of applying a force instead of actually moving the handlebars.
     
    Last edited: Oct 8, 2004
  24. Oct 8, 2004 #23

    rcgldr

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    No, its a combination of gravity and trail that cause the wheel to turn inwards when a bike is leanded. Gravity pulls down on the bike, the ground pushes up at the contact points, and in the case of the front wheel the contact point is "behind" the pivot axis, and the resulting torque force turns the wheel inwards. You can hold a non-moving bicycle (no gyroscopic forces), and lean it side to side, the front wheel will "fall" into the direction of the lean.

    There are some gyroscopic forces that turn the wheel inward when leaned, but in most cases this is insignificant, with the exception of some bonneville type (they look like torpedos with 2 wheels) bikes with very low center's of mass (some of these transition from counter steering to steering like a tricycle (at around 100mph) back to counter steering again (at around 200mph) as the vehicles go through speed transitions - note very high speeds invovled here combined with very low center of mass, lots of trail for stability, not a normal case). You're dealing with the gyroscopic forces of both front and rear tires with these special bikes. Also the driver is just trying to follow a straight line and keep the bike vertical.
     
    Last edited: Oct 8, 2004
  25. Oct 8, 2004 #24

    rcgldr

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    Actually when stopped, because the front wheel contact patch is not in line with the pivot point, you can hold the rear brake and steer with the front to move the bike around underneath you (the contact patch moves a little, but it's mostly a rolling (leaning) torque force that's being created), to balance yourself. Velodrome track racers can stand still (no forward or backward motion) for long periods of time without moving and maintain balance this way. The reason they do this is that they want the other rider to take off first and follow them, their equivalent of a psych job. Some rock back and forth, but this is more like showing off, these guys don't need to do this.

    Trials riders on lightweight motorcycles swing one leg side to side, sort of like a high wire act, to remain balanced on a stationary motorcyle.
     
    Last edited: Oct 8, 2004
  26. Oct 8, 2004 #25

    rcgldr

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    Race bikes weren't setup like this, but as you mentioned, Suzuki made a few bikes like this with the idea that the smaller wheel would take less effort to turn. It would also reduced unsprung weight. I had one of these bike (GS 1150ES), and didn't really notice much difference. It did have more of a tendecy to straighten up when braking while leaning (the contact patch is on the side of the tire which contributes to this). I'm not sure if the smaller front wheel was the cause of this.
     
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