Ball speed: The speed of the incoming ball just before impact with the racquet.
Bounce speed: The speed the ball bounces from the racquet after impact. It is not the same as shot speed. The best way to visualize this is for a ball hitting a stationary racquet. In this case the bounce speed is the shot speed. But what if the racquet is moving? If the racquet is moving 10 mph, the bounce will occur from a racquet already moving 10 mph. In this case, the ball starts at 10 mph before it even bounces. The bounce speed is then added on top of the racquet speed to achieve the shot speed.
The bounce speed is the speed of the ball relative to the racquet. A bug on the racquet sees the ball leaving at the bounce speed because the racquet is itself chasing the ball at its own speed. Observers from court-side see the ball traveling faster. They see the ball traveling relative to their stationary position in their seats. To them, the ball's speed is the bounce speed plus the speed of the surface it bounced from. Bounce speed is the major component of power potential. For each impact location on each racquet, the bounce speed is always a fixed percentage of the impact speed. This percentage is known as the power potential. The bounce speed is the racquet's contribution to the shot. The impact location speed is the swing's (player's) contribution to the shot.
Hittingweight: This is the "effective" weight of the racquet at the impact location. The entire racquet weight is not involved when you hit the ball. Only the "local" weight is involved. The impact location behaves as if it were a localized mass. The racquet behaves (how much its motion is altered upon impact) heaviest at the balance point and lighter toward the tip and the periphery of the stringbed. The hittingweight is calculated using the location coordinates (distance of impact from the center line and from the balance point), swingweight, and twistweight. The higher the hittingweight, the higher the racquet's power potential at the impact location.
Impact speed: The combined speed of the ball speed and the racquet at the impact location. If the ball is traveling at 30 mph and the impact location at 60 mph, then the impact speed is 90 mph.
Impact location: The impact location is measured in inches from the racquet butt and left and right of the center-line running from butt to tip. The location is often expressed as coordinates. For example, an impact 21 inches from the butt and 3 inches to the left of center would be indicated as 21 / -3. This location is approximately the 9 o'clock position on the racquet face. TW measures impacts at 13 stringbed locations on each racquet.
Impact location speed: Every point along the length of the racquet travels at a different speed. The speed continually increases as you move toward the tip. This is because the racquet swings in a big circle, and the tip travels a longer arc than the throat or the handle during the same amount of time. It is therefore traveling faster than throat, which is faster than the handle.
Maximum power location: The maximum power point on the racquet will be somewhere in the throat area of the stringbed, as close as possible to the balance point. This is where the racquet behaves heaviest (where the hittingweight is almost equal to the actual weight of the racquet). However, this is not the location where you will most frequently achieve the maximum shot speed, as explained under "maximum shot speed."
Maximum shot speed location: This is the impact location that will achieve the highest shot speed for a given incoming ball speed and racquet tip speed. As you move toward the throat you have more power but less racquet speed, and as you hit toward the tip, you have more speed but less power. Somewhere along that continuum the power and speed will be at a maximum combination. Depending on the shot, sometimes that will be in the lower half of the stringbed and sometimes in the upper half.
Power potential: A racquet's power potential is determined in the lab by measuring ball velocities and calculating the ratio of the ball's outgoing bounce speed to its impact speed. The impact speed is defined as the combined speed of the impact location and ball just prior to collision.
Impact speed = ball speed + racquet speed
For a given hitting location, the power potential is always a fixed percentage of the impact speed. It is also a different percentage for each location and generally varies from racquet to racquet. If the ball bounces at 20 mph and the impact speed is 40 mph, the power potential is 50%.
Racquet contribution: The shot speed is the sum of the bounce speed plus the impact location speed. Another way of saying this is that the shot speed equals the speed contributed by the racquet plus the speed contributed by the player's swing. So the racquet's contribution to shot speed is the ratio of the bounce speed to the shot speed. If the bounce speed is 20 mph and the shot speed is 80 mph, then the racquet contributed 25% of the final shot speed. The swing of the racquet contributed 75%.
Racquet speed: Technically, each part of the racquet is traveling at a different speed, so when we are talking about racquet speed we are really talking about impact location speed, or if so specified, tip speed. The racquet as a whole is in fact traveling a singular speed, but this is an angular speed and no one refers to this in tennis. For example, since a racquet is swinging in a circle, its speed as a whole is really its speed in revolutions per minute (rpm), or some variant of that (revs per second or degrees per second or radians per second).
Shot speed: The actual speed of the ball leaving the racquet. For conceptual and calculation purposes, this speed is considered as being composed of two parts: the ball's bounce speed plus the speed of the racquet from which it bounced. As such, we say that shot speed equals bounce speed plus impact location speed.
Swingweight: Swingweight is a measure of the racquet's resistance to being moved in a circular motion about an axis — either by the player or by the ball. When the player swings, swingweight manifests as "maneuverability." When the ball hits the racquet, swingweight shows up as stability and comfort. It influences power in two ways: First it influences how fast a player can swing the racquet. Second it influences whether the ball or the racquet "wins" the impact collision. The racquet actually always wins, but swingweight influences how much it wins by (i.e., whether the ball will leave the collision faster or slower and how much the racquet will "feel" the collision).
Tip speed: This is the speed the tip of the racquet is traveling parallel to the ground in the direction of the shot. This is used as a reference point from which all impact location speeds can be calculated. This is not a number most players are familiar with, so when it is needed for calculations, we have provided average speeds that have been measured by scientists for different levels of play and kinds of shots. You simply choose from a drop down menu.
Twistweight: Twistweight is the same as swingweight only about a different axis. Twistweight is a measure of the racquet's resistance to movement about an axis going from the butt to the tip — i.e., rotational or twisting motion. Swingweight measures resistance to motion in a backward/forward, somersault. Swingweight and twistweight are the major components of hittingweight.