# How and why does spinning a bullet make it stable? **URL:** \ **Category:** Factual Questions\ **Created:** [May 15, 2013, 12:47pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396 "2013-05-15T12:47:43Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [May 16, 2013, 3:11am UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/41 "2013-05-16T03:11:45Z")
> [@Crafter\_Man](#): > > Many satellites that orbit the Earth also use spin stabilization. This used to be common - and this is why many early satellites were cylindrical ([like](http://en.wikipedia.org/wiki/GOES_4), [these](http://www.boeing.com/boeing/defense-space/space/bss/factsheets/376/376fleet.page)). I think it’s extremely rare these days - most modern satellites are 3-axis stabilized. But some rockets are still spin-stabilized, especially sub-orbital rockets. [This video clip](http://www.youtube.com/watch?v=TTfgOYb1Fn8) from a NASA sounding rocket gives you a good sense of how fast it spins. ---
**Author:** ![Knorf](https://avatars.discourse-cdn.com/v4/letter/k/58956e/32.png) [@Knorf](https://boards.straightdope.com/u/Knorf)\ **Post date:** [May 16, 2013, 7:00am UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/42 "2013-05-16T07:00:36Z")
> [@TriPolar](#): > > Which in no way contradicts our statements. > > ETA: To be more specific, those attributes exist in regular bicycles. They may not be necessary for a bicycle to remain stable, but they contribute to that stability when they are there. You can calculate how much they affect stability, as noted on the paper. It’s trivial. The bike’s tendency to self-correct is why it stays up; you take that away, it will crash. Taking away trail and the gyroscopic effect do not materially affect the stability of the bike. ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [May 16, 2013, 9:01am UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/43 "2013-05-16T09:01:40Z")
Getting back to the OP: gyroscopic stability is caused by conservation of angular momentum, which is a physical law. I’ll try to explain it without getting into the mathematical definition of angular momentum. When an object spins, it tends to keep spinning at the same speed and in the same orientation (I’m oversimplifying here, but please bear with me). Only the application of an external force can cause the speed or orientation of the spin to change. This is why a spinning top stays vertical - if the top were to fall over the angular momentum would change, and if it’s spinning fast enough the force of gravity isn’t enough to make this happen. Similarly, the spin of a bullet tends to cause it to point in the same direction throughout its flight - it would take an external force to cause the direction of spin (and thus the orientation of the bullet) to change. I said above that I was oversimplifying things. It’s not actually the speed of the spinning object that’s conserved - the angular momentum has to do with both the rotational speed and the distribution of mass around the axis of spin. You can see this when watching a figure skater spinning on ice. When the skater pulls in her arms and legs, her spin speeds up, and when she sticks out her arms and legs her spin slows down. ---
**Author:** ![Dervorin](https://avatars.discourse-cdn.com/v4/letter/d/eb8c5e/32.png) [@Dervorin](https://boards.straightdope.com/u/Dervorin)\ **Post date:** [May 16, 2013, 10:24am UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/44 "2013-05-16T10:24:07Z")
> [@FeAudrey](#): > > No love for footballs here? Also thrown with spin for stable-path reasons. You mean, of course, [hand-eggs](http://4.bp.blogspot.com/-u_EoVz2CPPc/T7jzJqF0RkI/AAAAAAAADak/AklIgH7tRx8/s1600/football-handegg.jpg), which are indeed thrown with spin. Footballs, on the other hand, are kicked with spin to allow them to swing through the air, immortalised by the phrase [“bend it like Beckham”](http://www.forbes.com/2010/07/22/forbes-india-best-football-movies_2.html). ---
**Author:** ![Bullitt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bullitt/32/5725_2.png) [@Bullitt](https://boards.straightdope.com/u/Bullitt)\ **Post date:** [May 16, 2013, 1:54pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/45 "2013-05-16T13:54:00Z")
[Rotational Inertia, or Moment of Inertia](http://en.wikipedia.org/wiki/Moment_of_inertia) ---
**Author:** ![Noel\_Prosequi](https://avatars.discourse-cdn.com/v4/letter/n/b19c9b/32.png) [@Noel\_Prosequi](https://boards.straightdope.com/u/Noel_Prosequi)\ **Post date:** [May 16, 2013, 1:56pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/46 "2013-05-16T13:56:45Z")
Rugby footballs are passed with spin, too, ideally. Short passes in close play not so much, but long passes from the five-eight or No 10 to a winger are spun. Completely different technique from a quarterback throwing a long pass to a receiver, but the principle is the same. ---
**Author:** ![Bullitt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bullitt/32/5725_2.png) [@Bullitt](https://boards.straightdope.com/u/Bullitt)\ **Post date:** [May 16, 2013, 5:01pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/47 "2013-05-16T17:01:10Z")
Another comparison is the baseball pitch. Specifically, the knuckleball pitch that has no spin drifts unpredictably back and forth, up and down on its way from the pitching mound to home plate. Contrast that to a pitched ball that has spin: after it leaves the pitcher’s hand, the ball only has a force acting in one direction as determined by the spin direction - the only forces acting on the ball are gravity and air resistance. The air resistance is not equally distributed around the ball due to the spin’s direction. The no-spin knuckleball’s direction is erratic. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [May 16, 2013, 6:21pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/48 "2013-05-16T18:21:33Z")
> [@Knorf](#): > > You can calculate how much they affect stability, as noted on the paper. It’s trivial. The bike’s tendency to self-correct is why it stays up; you take that away, it will crash. Taking away trail and the gyroscopic effect do not materially affect the stability of the bike. You’re half right. “Turning into the fall”, as the guy in the video says, is the critical factor in bicycle stability, but the gyroscopic affect is why bicycles turn into the fall. If you tilt the top of a forward spinning wheel to the right (as if a bike were leaning to the right) the gyroscopic affect steers the wheel to the right, correcting the lean. The only reason the “bike” in the video works is because they attached lead weights forward of the steering axis. Instead of the gyroscopic affect steering the wheel, the lead weight causes the front wheel to steer into the fall. From what I can see, they basically removed the stabilizing affects that do exist on a real bicycle, replaced them with artificial ones that do not exist on a real bicycle, and tried to use this to prove how bicycles work. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [May 16, 2013, 9:10pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/49 "2013-05-16T21:10:11Z")
> [@Jeff\_Lichtman](#): > > …I said above that I was oversimplifying things. It’s not actually the speed of the spinning object that’s conserved - the angular momentum has to do with both the rotational speed and the distribution of mass around the axis of spin. You can see this when watching a figure skater spinning on ice. When the skater pulls in her arms and legs, her spin speeds up, and when she sticks out her arms and legs her spin slows down. Indeed, I used to mess with my coworkers as I spun around seated in my office chair. When I was spinning as fast as I could go, I asked them if they knew why when I stuck my legs out, my spinning would slow down, but as soon as I tucked them back in, I’d speed up again. No one ever guessed right, but I never expected much from a room full of graphic designers. Nevertheless, whether they understood conservation of angular momentum or not, we all had fun seeing which one would get motion sickness first. Blaaaaarrrfff! ---
**Author:** ![MichaelEmouse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/michaelemouse/32/101_2.png) [@MichaelEmouse](https://boards.straightdope.com/u/MichaelEmouse)\ **Post date:** [May 17, 2013, 3:19pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/50 "2013-05-17T15:19:46Z")
> [@Jeff\_Lichtman](#): > > Getting back to the OP: gyroscopic stability is caused by conservation of angular momentum, which is a physical law. I’ll try to explain it without getting into the mathematical definition of angular momentum. When an object spins, it tends to keep spinning at the same speed and in the same orientation (I’m oversimplifying here, but please bear with me). Only the application of an external force can cause the speed or orientation of the spin to change. This is why a spinning top stays vertical - if the top were to fall over the angular momentum would change, and if it’s spinning fast enough the force of gravity isn’t enough to make this happen. Similarly, the spin of a bullet tends to cause it to point in the same direction throughout its flight - it would take an external force to cause the direction of spin (and thus the orientation of the bullet) to change. This really got me to understand. As Newton would have said : “Stuff going a particular way wants to keep going that way.” I’m sure he came up with someone more elegant like a set of laws or something. Does this mean that the faster a projectile goes, the more it has to be spun? What I’m used to seeing is that faster projectiles are a looser twists whereas slower projectiles has a tighter twist. Is this simply to minimize friction or is it optimal to spin slower projectiles more and faster projectiles less? Does the spin tend to be constant over a travel time of 1-2 seconds and over sevral hundred meters? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [May 17, 2013, 4:33pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/51 "2013-05-17T16:33:59Z")
> [@MichaelEmouse](#): > > Does this mean that the faster a projectile goes, the more it has to be spun? What I’m used to seeing is that faster projectiles are a looser twists whereas slower projectiles has a tighter twist. Is this simply to minimize friction or is it optimal to spin slower projectiles more and faster projectiles less? Bullets are generally designed with a maximum spin rate. If you make the bullet go twice as fast and don’t change the twist rate of the barrel, the bullet is going to end up with double the spin rate as well (the same amount of spin over the barrel length, but in half of the time due to the doubled velocity). To avoid this, what you generally see in faster bullets is a slower twist rate, which keeps the rpm of the bullet in the same general range for both the faster and slower bullet. ---
**Author:** ![Jeff\_Lichtman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jeff_lichtman/32/1531_2.png) [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)\ **Post date:** [May 17, 2013, 8:27pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/52 "2013-05-17T20:27:27Z")
> [@MichaelEmouse](#): > > Does this mean that the faster a projectile goes, the more it has to be spun? No, I don’t think so. Angular momentum resists lateral forces on the projectile, but the force of air friction on the projectile is not lateral - it’s parallel to the direction of travel. That is, air friction doesn’t push the nose of the bullet left or right or up or down - it pushes straight back against the nose of the bullet. > [@MichaelEmouse](#): > > What I’m used to seeing is that faster projectiles are a looser twists whereas slower projectiles has a tighter twist. Is this simply to minimize friction or is it optimal to spin slower projectiles more and faster projectiles less? I’m trying to work out what you mean by “looser” and “tighter” twists. Are you referring to the rifling in the barrel - the twist that gives the bullet its spin? I’m no expert on firearms, but it wouldn’t surprise me that firearms with lower muzzle velocities have steeper pitch in their rifling. It would be necessary to make the bullet spin at the same rate when it comes out of the barrel. That it, it’s not that slower bullets need more spin, it’s that they need steeper rifling to achieve the same spin. > [@MichaelEmouse](#): > > Does the spin tend to be constant over a travel time of 1-2 seconds and over sevral hundred meters? Friction will cause the spin of the bullet to slow down over time. My guess is that it would slow very little over the course of one or two seconds. ---
**Author:** ![Knorf](https://avatars.discourse-cdn.com/v4/letter/k/58956e/32.png) [@Knorf](https://boards.straightdope.com/u/Knorf)\ **Post date:** [May 17, 2013, 10:28pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/53 "2013-05-17T22:28:39Z")
> [@RedSwinglineOne](#): > > You’re half right. “Turning into the fall”, as the guy in the video says, is the critical factor in bicycle stability, but the gyroscopic affect is why bicycles turn into the fall. That’s not what the [gyroscopic effect (i.e. angular momentum) is](http://en.wikipedia.org/wiki/Angular_momentum). Here’s an easy-to-digest version of the paper that appeared in[Popular Mechanics](http://www.popularmechanics.com/outdoors/sports/physics/physics-of-a-riderless-bike). > [@Popular Mechanics](#): > > There has been a mythology about what makes [a bicycle stable]. People have locked onto two ideas. One of them is the gyroscope effect and the other is the caster effect. What we’ve shown is that those two effects _aren’t actually necessary_. Italics mine. You do not need the gyroscopic effect for a bicycle to be able to self-correct its steering and maintain stability. Anyway, just read the paper, read the [wiki](http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics). Do the math. > [@wikipedia](#): > > If the steering of a bike is locked, it becomes virtually impossible to balance while riding. On the other hand, if the gyroscopic effect of rotating bike wheels is cancelled by adding counter-rotating wheels, it is still easy to balance while riding…It has been proven that neither gyroscopic effects nor positive trail _are sufficient by themselves or necessary for self-stability_… I hope this settles that. If not, feel free to write an article to the journal _Science_ and tell them the paper “[A bicycle can be self-stable without gyroscopic or caster effects](http://bicycle.tudelft.nl/stablebicycle/StableBicyclev34Revised.pdf)” _Science_ 332 (6027): 339–342 is incorrect and that you have an important refutation. You could make your career. Good luck. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [May 20, 2013, 6:21pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/54 "2013-05-20T18:21:30Z")
> [@Knorf](#): > > That’s not what the [gyroscopic effect (i.e. angular momentum) is](http://en.wikipedia.org/wiki/Angular_momentum). Really? Here is how I described it… > [@RedSwinglineOne](#): > > If you tilt the top of a forward spinning wheel to the right (as if a bike were leaning to the right) the gyroscopic affect steers the wheel to the right, correcting the lean. Here is how your linked article described it… > [@](#): > > The front wheel of the bicycle is spinning forward quickly, acting like a gyroscope. Then when you tip the bike to the right, the gyroscope applies the torque, which turns the handlebars to the right and causes the steering, bringing the wheels back under the bicycle and holding it up. … > [@Knorf](#): > > You do not need the gyroscopic effect for a bicycle to be able to self-correct its steering and maintain stability. Nothing in your linked articles, or any of the others I’ve been able to find contradicts my point. Real bicycles have trail. Real bicycles have a gyroscopic affect that does steer into a fall. Just because it is **possible** to build a two wheel contraption that does not use these forces does not mean they aren’t the main forces at play on a real bicycle. ---
**Author:** ![Knorf](https://avatars.discourse-cdn.com/v4/letter/k/58956e/32.png) [@Knorf](https://boards.straightdope.com/u/Knorf)\ **Post date:** [May 20, 2013, 7:15pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/55 "2013-05-20T19:15:12Z")
> [@RedSwinglineOne](#): > > Nothing in your linked articles, or any of the others I’ve been able to find contradicts my point. Real bicycles have trail. Real bicycles have a gyroscopic affect that does steer into a fall. > > Just because it is **possible** to build a two wheel contraption that does not use these forces does not mean they aren’t the main forces at play on a real bicycle. Never said they weren’t. They point is that they’re not _required_, nor the primary explanation for bicycle stability. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [May 21, 2013, 10:46pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/56 "2013-05-21T22:46:21Z")
> [@Knorf](#): > > Never said they weren’t. > > They point is that they’re not _required_, nor the primary explanation for bicycle stability. So what is the primary explanation for bicycle stability? (in the case of a real bicycle) ---
**Author:** ![Knorf](https://avatars.discourse-cdn.com/v4/letter/k/58956e/32.png) [@Knorf](https://boards.straightdope.com/u/Knorf)\ **Post date:** [May 22, 2013, 5:43pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/57 "2013-05-22T17:43:06Z")
> [@RedSwinglineOne](#): > > So what is the primary explanation for bicycle stability? (in the case of a real bicycle) I’ll quote [one of the links I posted that explains the paper](http://www.popularmechanics.com/outdoors/sports/physics/physics-of-a-riderless-bike) one last time. And then I’m done. The person answering is Andy Ruina, a Cornell professor of mechanical engineering and a cyclist. > [@Popular Mechanics](#): > > Question: What did scientists believe about bike self-stabilization up to this point? > > Answer: There has been a mythology about what makes [a riderless bicycle stable]. People have locked onto two ideas. One of them is the gyroscope effect and the other is the caster effect. What we’ve shown is that those two effects aren’t actually necessary. > … > Question: Your group built a bike prototype that included a set of counter-spinning wheels (which canceled out the wheels’ angular momentum, eliminating the gyro effect) and a point of ground contact in front of the steering axis (removing any caster effects). Since the bike could still stay upright on its own, this proved the two effects weren’t necessary? > > Answer: Yes. Jim proved it mathematically and then we put it in practice. If we push our self-stable bicycle straight, it goes straight, but it’s really self-correcting and weaving back and forth like a regular bike. If it falls to the right, it steers to the right. It doesn’t need gyro or caster. > > Question: Your experiments showed your bike can stay up without those two effects because its mass distribution—high in front and low in back—causes the front to fall faster, pulling the wheels into falls and preventing the bike from completely falling over… Yes, the gyroscopic effects and caster trail exist on a real bike. But the bicycle can be designed to self-correct without them. The primary reason for a bicycle’s stability has to do with the distribution of mass causing the front wheel to turn into the fall, and does not require the gyroscopic effect or trail, as was demonstrated mathematically (go the paper for this), and by the bicycle the scientists built. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 22, 2013, 5:54pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/58 "2013-05-22T17:54:54Z")
> [@RedSwinglineOne](#): > > So what is the primary explanation for bicycle stability? (in the case of a real bicycle) In the case of a “real” i.e. conventional bicycle or motorcycle, [trail](http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Trail) is what allows the vehicle to automatically steer into (and recover from) turns. At higher speeds, the gyroscopic stabilizing effect **slows** the rate at which the vehicle will lean over under the influence of gravity, but it doesn’t **stop** it; only trail can do that. If you had a conventional bicycle/motorcycle with zero trail (i.e. a vertical steer tube), it would not be stable. At low speeds, any deviation from upright on this unusual vehicle will result in rapidly falling over if there is no external steering input; at high speeds, gyro-stabilization means any deviation from upright will result in slow (but inevitable) falling over. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [May 23, 2013, 6:12am UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/59 "2013-05-23T06:12:44Z")
… ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [May 23, 2013, 4:44pm UTC](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396/60 "2013-05-23T16:44:46Z")
> [@Knorf](#): > > Yes, the gyroscopic effects and caster trail exist on a real bike. Agreed. Andy Ruina spends 5 minutes in [this video](http://www.youtube.com/v/84Wczsi4vHg?hd=1) explaining how the gyroscopic forces and caster cause a real bike to steer into a fall, and he explains that steering into a fall is the key to two wheel stability. > [@Knorf](#): > > But the bicycle can be designed to self-correct without them. Agreed. Andy then explains how instead of the gyroscopic forces and caster that exist on a real bike, **his bike** uses carefully placed weights to ensure that the steering fork falls faster than the rest of the bike. These weights do not exist on a real bike. > [@Knorf](#): > > The primary reason for a bicycle’s stability has to do with the distribution of mass causing the front wheel to turn into the fall, and does not require the gyroscopic effect or trail, as was demonstrated mathematically (go the paper for this), and by the bicycle the scientists built. I don’t see how you keep coming to this conclusion. Nowhere in the paper does it say that real bicycles don’t use gyro forces or trail. It merely says they are not the only way to achieve the necessary steering affect. From the conclusion of that paper. > [@](#): > > Although we showed that neither front-wheel > spin angular momentum nor trail are necessary for > self-stability, we do not deny that both are often important > contributors. But other parameters are also > important, especially the front-assembly mass distribution, > and all the parameters interact in complex > ways. As a rule we have found that almost any selfstable > bicycle can be made unstable by mis-adjusting > only the trail, or only the front-wheel gyro, or only > the front-assembly center-of-mass position. [Previous page](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396.md?page=2) [Next page](https://boards.straightdope.com/t/how-and-why-does-spinning-a-bullet-make-it-stable/658396.md?page=4)