When talking about the stability of a disc, there are many opinions and different players may have very different experiences of how a particular disc flies. Below, we scratch a little at the surface of the physics around disc stability and link it to the various expressions you encounter in disc golf. We're trying to answer the questions:
- What is turn and fade?
- How are turn and fade dependent on speed?
- What does the stability of the disc mean under different wind conditions?
Introducing spin and tilt
Stability of a disc is a term for how much the disc tilts about an axis parallel to the direction of the flight of the disc. In disc golf, the tilt of the disc is most often described with two terms; turn and fade. Tilt is in all cases dependent on the direction of the disc's spin which is either clockwise or counterclockwise. We thus separate spin, which means that the disc spins around itself when you throw it, and tilt, which is changes in the disc's flight plane so that it turns to the side when it flies. The two terms are depicted below, where you can see that spin is the disc that revolves around itself, and that tilt is indicated by the black arrows on the side of a disc that flies forward.
The two principles are translated in everyday speech into something more tangible and we talk about overstable, neutral and understable discs. Since you spin the disc either counterclockwise or clockwise with the front and back throws, respectively, we are talking below from a right-handed player who throws a backhand throw. For the further description of stability, we briefly repeat the disc's flight numbers and the associated concepts in the image below.
The Disc's tilt is translated into turn and fade
A disc that tilts a lot to the right at the start of a right-handed backhand throw is referred to in everyday speech as ""understable"". The understable disc will thus have a large turn, which is one of the disc's flight numbers. A big turn is seen by the disc tilting and thereby turning to the right, if it is thrown flat. A disc that tilts a lot to the left at a right-handed backhand throw is called ""overstable"" and this disc typically has no or very little turn, but instead a large fade, which is also one of the disc's flight numbers.
When we talk about turn and fade, we are also talking about tilt in relation to the speed of the disc. By throwing in the right-hand backhand, the turn is a tilt to the right in the fastest part of the disc's flight, ie the first part of the flight, as the disc loses speed due to the wind resistance. Turn is the left-hand tilt in the slower part of the disc's flight, ie the last part of the flight before the disc hits the ground. Both turn and fade are thus dependent on the speed of the disc, and physically this is explained by the speed of the air over the disc while it is flying.
Disc Speed, Wind, and Relative Disc Speed
If we again talk about the disc's flight number and focus on ""speed"", then speed is defined as the air speed the disc must be exposed to in order to experience the turn and fade that the disc's flight numbers prescribe. The speed of the air over the disc is thus of great importance for the tilt of the disc, and we have to relate to the relative air speed, which can be said to be the ""speed that the disc experiences"". We can not only relate to how fast the disc flies as it is thrown, as the wind also has a great significance. To facilitate the understanding of the relative air speed over the disc, below are two illustrations, which show a disc flying in headwinds and tailwinds, respectively. Based on these, we can imagine that a disc that is thrown in headwind experiences a higher relative air speed than the same disc that is thrown at the same speed but in tailwind.
Illustration of a disc thrown in headwind
Illustration of a disc thrown in 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 in tailwinds and in headwinds. The simple reason is that in headwinds the disc experiences a higher relative speed and the disc ""thinks"" therefore that it flies faster than it does. When the disc experiences a higher air speed, it will also turn more, as we remember that the turn is defined as a tilt to the right at a higher air speed over the disc. The opposite is true for the same disc in tailwind, that the disc experiences a lower air speed and thereby turns less.
When we throw a disc in the wind on the court, the speed, turn and fade of the disc are of great importance for how the disc behaves in the air and ultimately whether we make a good or a bad throw. If we look at a flight curve for a disc that has been thrown at different wind directions, we can thus experience something reminiscent of the 3 lines in the picture below.
We have reviewed how a disc is affected by the wind and how the speed of the disc has a great effect on 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 are not necessarily the same across manufacturers. Therefore, the best way to get to know a disc is, of course, to go out and throw with it.
A high speed disc is designed to fly fast, and we therefore see that a driver, if not thrown at high speed, will fade very early as it never experiences the high speed at which its turn should manifest.
The stability of a disc thus depends to a large extent on how hard it is thrown. If you throw a disc with flight numbers: 2 5 0 2 you may well experience a turn, as the disc is designed not to turn when you throw the equivalent of speed = 2. In the same way, a disc with flight numbers: 12 5 -3 2 can not be seen to get a turn if it is not thrown quite hard, and thus never achieves a speed corresponding to speed = 12.
Finally, it should be mentioned that there are additional factors that come into play, it can be the plastic type of the disc, how worn it is and of course the technique of the thrower. With good technique, you can achieve higher monotony in your throws and therefore also experience that the disc more often does as expected.