From: veritasium
Most people do not know how bicycles actually work or how they balance [00:00:00]. It’s often believed that turning a bicycle involves simply pointing the handlebars in the desired direction, similar to driving a car [00:02:00]. However, the mechanics of bicycle balance and steering are more complex, as steering affects not only direction but also balance [00:02:13].
Rider-Controlled Balance and Steering
When a rider wants to turn a bicycle, they instinctively use a technique known as countersteering [00:02:44].
The Role of Countersteering
To make a right turn, a rider must first steer the handlebars to the left [00:02:44]. This action effectively steers the bike out from under the rider, causing them to lean to the right [00:02:26], into the turn [00:02:47]. If one were to try turning right by steering right, they would immediately lose balance and fall to the left [00:01:41]. This countersteering motion is intuitive for cyclists, performed without explicit thought [00:02:50].
Analogies for Balance
The act of balancing a bicycle is comparable to balancing an inverted pendulum, such as a broomstick on one’s hand [00:03:10]. To move the pendulum in a certain direction, one first moves the base in the opposite direction to initiate a lean, then moves with the pendulum [00:03:32]. Similarly, on a unicycle, longitudinal balance (front-to-back) is managed by the pedals, and lateral balance (side-to-side) is achieved by small counter-steer adjustments to shift the wheel’s contact patch [00:04:04].
Steering as a Balancing Mechanism
The primary function of steering on a bicycle is balance, not just turning [00:04:54]. A stationary bicycle is difficult to balance because steering is ineffective [00:05:00]. Balance is maintained by steering the bike to keep it underneath the rider, rather than solely by body positioning [00:05:17]. Even when riding in a straight line, constant, small steering adjustments are made to maintain equilibrium [00:05:25].
Self-Steering Mechanisms for Riderless Stability
A bicycle can remain upright and coast indefinitely without a rider, provided it maintains sufficient speed [00:05:45]. This phenomenon is due to inherent self-steering mechanisms [00:07:27].
Debunking Gyroscopic Dominance
It is a common misconception that bicycle stability is primarily due to the gyroscopic effect of the spinning wheels [00:05:06]. However, experiments demonstrate that this is not the case [00:06:25]. When a bicycle’s handlebars are locked, preventing steering, it becomes nearly impossible to balance, even at high speeds [00:06:29]. This shows that the ability to steer the bike back under oneself is crucial for stability [00:07:21]. The gyroscopic effect plays a role, but it primarily helps steer rather than directly keeping the bike upright [00:08:32].
Three Corrective Steering Mechanisms
Bicycles are designed to steer themselves if they begin to fall, turning the handlebars in the direction of the lean to bring the wheels back underneath [00:07:31]. At least three mechanisms contribute to this corrective steering:
- Caster Effect: Due to the angle of the front fork, the steering axis intersects the ground in front of where the front wheel touches the ground [00:07:43]. If the bike leans to one side, the ground force on the tire causes the wheel to turn in that same direction [00:07:52]. This is similar to how a caster wheel on a shopping cart aligns itself [00:08:02].
- Mass Distribution: The center of mass of the handlebars and front wheel assembly is located slightly in front of the steering axis [00:08:17]. When the bike leans, the weight of these components pushes the front wheel to steer in the direction of the lean [00:08:23].
- Gyroscopic Effect (Indirect): The gyroscopic effect contributes to steering through gyroscopic precession [00:08:32]. When a force is applied to a spinning gyroscope (like a bicycle wheel leaning), the resulting turn occurs 90 degrees from the applied force [00:08:40]. This helps the wheel turn into the lean.
It’s important to note that not all three mechanisms are required for a bike to be stable [00:09:05]. Researchers have built stable bicycles that lack the gyroscopic effect (using counter-rotating wheels) or the caster effect, demonstrating that mass distribution alone can provide stability [00:09:09].
Ongoing Research and Future Developments
Understanding how bicycles work is an active area of research [00:09:37]. There are programs available that allow researchers to input various bicycle parameters to determine the range of speeds at which a bike is self-stable [00:09:40]. This research is leading to innovations such as prototype bikes with smart motors in the handlebars that actively assist steering, enabling stability even at low speeds [00:09:50].