Test Procedure title graphic

TW has developed and built its own equipment for measuring the power potential of a racquet. The physics of the test is based on a concept known as ACOR (the apparent coefficient of restitution). It is the standard for measuring rebound performance and is used by both racquet manufacturers and science researchers.

TW's basic procedure is this:

Racquet Preparation

Each racquet is strung with the same nylon string at the midpoint of manufacturer recommended tension. Thus, each racquet is tested as the manufacturer intends it to be presented to the customer. In so doing, there is no intent to test racquets at the same tension (or more specifically, at the same stringbed stiffness) for the same reason that they are not tested at the same headsize, flex, beam width, string pattern, balance, or swingweight. The whole idea is to test the differences between racquets in their standard configuration. We test the racquet as the racquet, not as a particular customization of it.

Impact locations

We impact a stationary, free standing racquet at 15 locations, each measured in inches from the butt. (The results are the same whether the racquet is moving or stationary, but procedure is enourmously simplified by using a stationary racquet.) The goal in choosing these impact locations is to include both the normal hitting area plus some areas that, one would hope, are rarely hit, but serve to illustrate racquet behavior.

Number of impacts

Each location is impacted 3 times and the average rebound distance and speed is calculated. For any given racquet, most rebounds at a location are within 1 cm of each other, and for impacts near the middle of the strings, they are often within .1-.2 cm. In other words, the behavior of each location on a racquet is very precise indeed. A location's rebound potential is written into its DNA — into its design, construction, and existence.

Impact speed

All impacts occur at the exact same speed. The actual speed chosen is irrelevant but is dictated by the test setup. The results would be the same at any speed.

Rebound speed

Rebound distance and speed are measured and calculated. This is the performance of the racquet. The rebound speed is the performance result of all the design parameters including weight, balance, swingweigt, twistweight, material, flex, balance, headsize, and many more.

Power Potential ratio

The ratio of rebound speed to impact speed is the power potential of the location. Because the impact speed is the same for every location and every racquet, the ratio is a very accurate method of comparison.

Analysis

The entire procedure is very time, labor, and knowledge intensive. But it is worth it. To the best of our knowledge, TW's results are the only comparative performance measurements available in the tennis industry. "Performance" and "measurement" are the important words.

However, a disclaimer is warranted: Because of the difficulty and magnitude of the project, we tested only one racquet of each model. Just as there is variation in specs between racquets of the same model, so will there be variation between power potential measurements. In fact, the variation in power potential is because of the variation in specs. Each racquet is a little bit different.

Consequently, some leeway in the interpretation of the results is necessary. Racquets that are separated by less than 1 percentage unit in power potential may reverse their relative performance results if two other racquets are chosen. Then, too, there is always a small but unavoidable element of experimental error built into every test.

But these facts of life aside, the power potential measurements are the most definitive guide to racquet performance available. They won't necessarily tell you how a racquet will feel (just ask any two players about any two racquets!), but they will tell you what the racquet will do.