Why doesn't a running bicycle fall over?

The moving bicycle wheels provide a gyroscopic action that makes it laterally stable. The gyroscopic force is more pronounced at greater speeds when the wheels are turning paster, which is why it is more stable the faster you go, but inexperienced cyclists are likely to wobble and meander at low speeds. .

– Best wishes - Majikthise. .

The role of gyroscopic force is minor in keeping a bicycle upright. If you visualize a straight line traveling through center of the steering column of the bicycle you will find that the contact point of the front tire is behind this imaginary line. This is called trail and is essential for bicycle stability. The contact point being behind the axis is responsible for the stability of the bicycle; when the bicycle leans the front wheel naturally turns the direction of the lean partially correcting the imbalance. The contact point would be even farther behind the axis with a straight fork; too much in fact which would make a bike that handles poorly; forks are raked so the optimum amount of trail is achieved. A bicycle with no trail would have no tendency to correct and could only be ridden with great difficulty. The gyroscopic effect of spinning wheels usually is too small to have much effect on the stability of the bicycle but comes into play when a cyclist rides no-hands. Since the gyroscopic effect of the front wheel is small riding no-hands requires keen balance to ride in a straight line.

@filletofspam . .The function of “trail” is primarily to ensure that the bicycle naturally tends to steer in a straight line. Without “trail” there would be a strong tendency for the front wheel to twist round suddenly, especially if riding one-handed while giving hand signals etc., or even more so if riding no-handed. . Without doubt the wheels provide a VERY SIGNIFICANT gyroscopic force when traveling even at a fairly moderate speed. . This assertion is easily tested: . Remove any bicycle wheel from a bicycle dand hold tit by it’s axel - you can easily twist it laterally if it is not spinning. Now spin the wheel and try twisting the wheel laterally. You will undoubtedly find that the difference in force required is remarkably large, ….. . ….. and I have no doubt whatsoever that it is the gyroscopic force (not the trail) that increases lateral stability thus stopping the bicycle falling over, and it is the “trail” that improves the steering / forward motion stability. .

– Best wishes - Majikthise. .

“… bicycle dand hold tit by it’s axel …” (damn those distractions) ….. I meant “bicycle and hold it by its axle …”. .

While the gyroscopic effect of wheels seems impressive when you hold a spinning bicycle wheel remember that the bicycle has a high center of gravity and the amount of leverage on the wheel from the rider will easily overcome the gyroscopic effect. The contribution that the gyroscopic effect contributes is small compared to that of the steering geometry. Bicycles with the lightest wheels and tires are somewhat more responsive than the same bicycle fitted with wheels and tires several times as massive but the change is much smaller than would be predicted if the gyroscopic effect was the largest contributor to bicycle stability.

@filletofspam: You make some interesting points and I take your opinion seriously on account of your long standing excellence in science based answers, so I will perhaps give this some further thought from your point of view. However, the bicycle’s steering geometry is the same whether the bicycle is moving forwards or not, and when the bicycle is stationary, it does nothing to improve the stability in opposition to a lateral twisting moment normal to the bicycle’s long axis. We have learned to maintain or balance against such turning moments using minor adjustments to body position, and it is easy to see how little assistance we need in order to maintain a zero angular momentum when standing on a narrow beam - we instinctively stretch the arms out and any tendency of the body to rotate clockwise due to the centre of gravity becoming skewed outside the vertical lines passing through the support points at the feet is immediately counteracted by a counter-clockwise motion of the extended arms thus creating a force that returns the body to a stable vertical position. I have no doubt that the torque due to the motion of the arms (that maintains balance) is negligible compared to the force due to the gyroscopic action of the moving wheels on the bicycle, and that it provides a very significant contribution to the lateral stability of the moving bicycle as opposed to the stationary bike carrying the same rider. If one observes a rider moving extremely slowly it is evident that the rider attempts to preserve a zero net angular velocity by twisting the body and elbows in a fashion similar to the use of arms or balance pole when walking a narrow beam, or tight rope, however, the magnitude of those twisting movements reduces dramatically as the velocity of the bicycle increases. I am currently inclined to think that the gyroscopic effect of the moving wheels is much more significant than you are suggesting, but I will give it some further thought - particularly since gyroscope action is in many respects counter-intuitive. I must say, I find this by far the most interesting question that I have found here on FunAddvice.

I’d like to take credit for keen insights but my views here were honed by long discussions on rec.bicycles.tech and especially arguments by an engineer named Jobst Brandt. Mr. Brandt in particular has made a career of challenging conventional wisdom and slaying sacred cows. There is a lot of incorrect conventional wisdom in the realm of cycling.

Moving objects are the fundamental balance to life….. because there is a difference between the distance and speed, something upright is less likely to fall unless it is in still motion…..