Stability

When talking about the stability of a disc there are many opinions, and different players can have very different experiences of how a particular disc flies. Below we scratch the surface of the physics of disc stability and tie it to the terms you meet in disc golf. We try to answer the questions:

• What are turn and fade?
• How do turn and fade depend on speed?
• What does the disc's stability mean under different wind conditions?

Introduction to spin and banking

The stability of a disc is a term for how much the disc banks around an axis that is parallel to the disc's direction of flight. In disc golf the disc's banking is most often described with two concepts; turn and fade. Banking is in all cases dependent on the direction of the disc's spin, which is either clockwise or counter-clockwise. So we distinguish between spin, which denotes the disc rotating around itself when you throw it, and banking, which is changes in the disc's plane of flight so that it tips to the side as it flies. The two terms are illustrated below, where you can see that spin is the disc turning around itself, and that banking is indicated by the black arrows on the side of a disc flying forward.

The two principles are translated in everyday speech into something more tangible, and people talk about overstable, neutral and understable discs. Since a forehand and a backhand throw respectively spin the disc either counter-clockwise or clockwise, we speak below from the perspective of a right-handed player throwing a backhand. For the further description of stability, we briefly recap the disc's flight numbers and the associated concepts in the image below.

The disc's banking translated into turn and fade

A disc that banks a lot to the right at the start of a right-handed backhand throw is commonly referred to as “understable”. The understable disc will thus have a large turn, which is one of the disc's flight numbers. A large turn is seen by the disc banking and thereby turning to the right, provided it's thrown flat. A disc that banks a lot to the left on a right-handed backhand throw is called “overstable”, and this disc typically has no, or only very little, turn, but instead a large fade, which is likewise one of the disc's flight numbers.

When we talk about turn and fade, we also talk about banking in relation to the disc's speed. On a right-handed backhand throw, turn is a banking to the right in the fastest part of the disc's flight, that is the first part of the flight, as the disc loses speed due to air resistance. Fade is the leftward banking in the slower part of the disc's flight, that is the last part of the flight before the disc hits the ground. Both turn and fade are therefore dependent on the disc's speed, and physically this is because of the speed of the air over the disc while it flies.

The disc's speed, the wind and the relative air speed over the disc

If we again talk about the disc's flight numbers and focus on "speed", then speed is defined as the air speed the disc must be subjected to in order to experience the turn and fade that the disc's flight numbers prescribe. The speed of the air over the disc thus has a big influence on the disc's banking, and we have to consider the relative air speed, which you could say is the "air speed the disc experiences". We can't consider only how fast the disc flies the moment it's thrown, since the wind also matters a great deal. To make the relative air speed over the disc easier to understand, below are two illustrations showing a disc flying into a headwind and a tailwind respectively. From these we can imagine that a disc thrown into a headwind experiences a higher relative air speed than the same disc thrown at the same speed but with a tailwind.

Illustration of a disc thrown into a headwind

Illustration of a disc thrown with a tailwind

Based on the two images and the understanding of the relative speed, we can begin to understand why our discs behave very differently when we throw them with a tailwind and into a headwind. The simple reason is that in a headwind the disc experiences a higher relative speed, and the disc therefore "thinks" it's flying faster than it is. When the disc experiences a higher air speed, it will also turn more, since we remember that turn is defined as a banking to the right at higher air speed over the disc. The opposite applies for the same disc in a tailwind: the disc experiences a lower air speed and thereby turns less.

When we throw a disc in the wind on the course, the disc's speed, turn and fade thus have a big influence on how the disc behaves in the air and, ultimately, on whether we make a good or a bad throw. If we look at a flight curve for a disc thrown in different wind directions, we can therefore experience something resembling the 3 lines in the image below.

Summary

We've gone through how a disc is affected by the wind and how the disc's speed greatly influences how it behaves in the air. The disc's flight numbers give an indication of the experience we have when we throw the disc, but it should also be mentioned that flight numbers aren't necessarily the same across manufacturers. Therefore the best way to get to know a disc is, of course, to get out and throw it.

A disc with high speed is designed to fly fast, and we therefore see that a driver, if it isn't thrown at high speed, will turn very early, since it never experiences the high speed at which its turn was meant to show.

A disc's stability thus depends to a large degree on how hard it's thrown. If you throw a disc with flight numbers: 2 5 0 2, you may well experience a turn, since the disc is designed not to turn when you throw it at a level corresponding to speed=2. In the same way, a disc with flight numbers: 12 5 -3 2 can be seen not to get a turn if it isn't thrown quite hard, and thus never reaches a speed corresponding to speed=12.

Finally, it should be mentioned that there are further factors at play — such as the disc's plastic type, how worn it is and, of course, the thrower's technique. With good technique you can achieve greater consistency in your throws and therefore also experience the disc doing as expected more often.