# How does the distance affect the balance of the bike?

## Why are bicycles so stable?

The faster a bike rolls, the harder it is to tip over. When stationary, with only two points of support and a high center of gravity, it will fall over immediately - while in motion, its structure and physical effects help to stabilize the wheel. Above a certain speed, the driver hardly needs the ability to balance to achieve dynamic equilibrium - he just shouldn't disturb it with incorrect movements.

Even without a driver: from 21 km / h it runs by itself

At the beginning, Dad has to hold on or support wheels have to stabilize the slow rolling. Every tilting is stopped from the outside, Dad pushes in the opposite direction again. Experienced cyclists straighten their vehicle again and again by turning briefly. This initiates a small curve and the centrifugal force helps the balancing body to straighten the wheel again. Slow driving is therefore a constant slight tilting, counter-steering and straightening up again - depending on ability, this is shown in a more or less strong serpentine line. However, once a bike has reached a speed of around 6 meters per second (or 21 km / h), it almost stays upright on its own. Good bikes run in a straight line on their own. They can even be steered hands-free; you can change the direction of travel simply by moving your body. But what makes a bike so stable when it is in motion?

The gyroscopic forces of the rotating tires, which steer against tipping, are responsible. The so-called gyroscopic effect ensures that the wheel axle, which is deflected in the event of a malfunction, always wanders back to its original position. These physical principles have been known for a long time, and as early as 1899 the Englishman Francis Whipple compiled a collection of equations from solid state dynamics for the bicycle.

This basically described the rolling wheel. However, the shape and geometry of the frame also influence the running stability - balance bikes and high bikes from the 19th century and today's recumbent bikes, for example, are less stable than the classic "normal bike" of the last 150 years. Over the decades, researchers have set up various, sometimes even contradicting, formulas and models. But for bicycle manufacturers to this day it was a matter of experience and trial and error if they wanted to produce new, stable frame designs. Only since 2007 has a new mathematical model from the Netherlands combined all the decisive parameters and allows the computer to precisely simulate how bicycles with different frames behave at different speeds.

### geometry

The same in green: bicycles always look roughly the same

According to Whipple, a bicycle has a very simple structure. Four rigid individual parts, all side-symmetrical, are movably connected to each other: one axially symmetrical tire is rigid, but rotatable about its axis of rotation, fixed in the frame, the second equally rigid, but rotatable in the front fork. The fork and frame, in turn, are movably joined together by a pivot bearing.

The geometry of the frame and the distribution of the mass can vary - but rarely do because the typical structure works well. Bicycle manufacturers usually adhere to three tried and tested parameters: the distance between the two wheel axles (wheelbase), the typical overall shape of the frame and the angle at which the fork points down from there (determines the caster). Points such as wheel diameter, seating position or front wheel lowering have less influence on drivability. The geometry commonly used today is based on empirical values ​​and is superior to that of the earlier high bikes, where the center of gravity was far too high due to the large main wheel and which made it easier to tip over. Recumbents are also less stable, but have found their niche because of their other advantages.

### Gyroscopic physics and dynamic equilibrium

If the speed is high enough, the rotating tires can also be viewed as gyroscopes. The front wheel is especially important because it can swerve from the main plane of the wheel and dictate the finest changes in direction. The rear wheel is fixed in the frame by an axle and above all has a supportive effect.

Both wheels each represent a symmetrical top in which the axis of rotation and angular momentum axis coincide with the axis of symmetry of the figure. If this top tilts to one side, a torque acts perpendicular to the angular momentum. The result is a change in angular momentum, so that the gyro - in order to maintain the angular momentum - performs a precession movement. For the front wheel, this means: The rigidly connected fork and handlebar assembly rotates in the direction in which the front wheel tilts, and the steering angle stabilizes the wheel again. The rear wheel does not perform any precession movement, but increases the gyroscopic effect and thus supports the front wheel.

Ideally, on a slippery track, a bike should be able to stay on its own at high speed, before it becomes too slow again, the righting gyroscopic forces are no longer sufficient and it stumbles. In reality, however, disruptions occur even when driving fast, for example due to uneven roads. The freehand cyclist can take corrective action by shifting his body's center of gravity and thereby tilting the bike slightly to one side. This triggers a change in the angular momentum and a steering angle

If the speed is too high, however, the front wheel can hardly turn to the side any more; like the rear wheel, it is more or less fixed in the frame. Changes in direction then take place primarily via a shift in the center of gravity and the resulting cornering.

### Predictability

Anyone who changes the parameters of the frame geometry influences how big the effect of the gyroscopic forces and how easy or difficult a bike is to steer. Researchers at the University of Delft have now developed a model that incorporates 25 design parameters that influence the handling characteristics of a bike. This results in a computer model that accurately reproduces the behavior of existing bicycles at different speeds.

Manufacturers can use the simulation to try out completely new frame geometries without having to waste time and material. This enables bicycles that are individually adapted and optimized for your own body and needs.