Below are some important principles to consider that will serve as a preamble to your customisation program
What are the forces involved when a racquet strikes the ball?
The collision that occurs when the racquet strikes the ball is called an “eccentric impact”. It is an “eccentric impact” because the two mass centers of the ball and the racquet balance point do not move along the same line to this point of collision. The two resultant forces of this impact are “Torque”, and “Impulse Reaction”.
Torque – The bending force resulting from impact, that causes the hand to bend back and then catapult forward. The stronger this catapulting force, the wosrse the whipsawing stress cycle resulting from the stroke, and the greater the risk of injury
Impulse Reaction – The push or pull on the axis of rotation (hand) resulting from impact. A positive Impulse reaction is better (vs negative) as this results in a push on the hand adding more speed to the racquet during impact, and causing less stress on the arm. Impacts above the center of percussion cause a negative impulse reaction, thus racquets with a high sweet spot are more favourable.
Even when you hit directly on the sweetspot (which is not an “area” but a single point on the centerline of the racquet, also known as the center of percussion), there is no impulse reaction but there is still “Torque”. It is the Torque force that leads to Torsion.
Torsion – The screwdriver twisting force about the racquet’s centerline resulting from impact. This twist winds up the racquet to release in a sudden handle twist in the opposite direction once the ball leaves. Torsion is the cross product of moment and torque. Moment multiplied by Torque = Torsion. Thus high moment and high torque contribute to high torsion.
The importance of “Momentum” and “Moment”
Moment – The measure of how heavy the racquet feels to hold up parallel to the ground. It is weight of the racquet multiplied by its lever arm (distance from the axis of rotation to the balance point of the racquet). It is Moment, not simply static weight of the racquet that is important, and the lower Moment, the better.
Moment is key for two reasons:
1. A racquet with high moment is bad because it is difficult to hold up and maneuver for volleys, and hard to generate rapid racquet head acceleration on groundstrokes; especially for juniors and ladies.
2. Moment multiplied by torque gives the torsion, which is the screwdriver twist about the racquet handle’s centerline resulting from impacts (even impact on the centerline or “sweetspot”). High torsion causes significant repetitive stress on the wrist and arm. Moment is a thus a critical variable in any calculation for the forces that cause injuries like (but not limited to) “wrist crunch”, “elbow crunch”, “shoulder pull” and “shoulder crunch”.
Momentum and not energy, and not force, is what counts in a collision (Momentum = Mass x Velocity).
Using the analogy from baseball, imagine using a lightweight softball bat to try and hit a home run out of Yankee Stadium. A lightweight bat cannot hit a hard baseball very far because it doesn’t bring enough mass to the collision, and therefore its momentum on impact is low. Theoretically, you might try to swing this wimpy bat a little faster but this will be bad for accuracy because it is harder to time violent swing precisely.
The same is true in tennis, as even if you succeed in increasing the tip speed (swing speed) of the racquet enough to offset the it’s lack of mass, the shot will be very hard to place accurately.
In order to return an “heavy” ball – one with lots of pace and spin, you need a heavy racquet that will bring enough mass to the collision to create enough of its own rotational intertia so that it will not pushed around so much by the force of the impact. Most touring pros have already realised that a more massive (heavier) racquet will crush majestically through the ball instead of bouncing off, which makes it more comfortable on impact and more accurate.
In today’s game, string technology and stroke mechanics are allowing players to hit balls harder, with more spin and more consistency. The evolution of how points are being played means that it has never been more important to consider the proper weight distribution of your racquet in order to play more efficient tennis and reduce the impact forces that cause stress to the wrist and arm.
Energy Loss and Shock Loading
Shock is the difference between the initial and final kinetic energy of the racquet.
Before impact you put energy into the racquet to get it up to speed for the collision, and during the impact you put in a little more energy to aim the shot. After the ball leaves, the racquet mass center (balance point) moves at a slower speed, and this means a loss of its kenetic energy. The ball gets some of this lost evergy, and the rest becomes internal energy, wasted in bending the frame. If the frame is stiff and light, this frame bending energy will not be absorbed by the material of the frame but will have to be dumped into the arm holding onto the racquet. Dont place any reliance on string damping gadgets to save your arm – these have too little mass to do much besides reduce residual string vibration. The most effective vibration damper is a large particulate handle end weight, thus the best way to prevent shock is by proper racquet weight distribution – with head light balance and heavy overall. You need the weight to absorb the lost kinetic energy of the impact (so it doesn’t go to your arm), and a head light balance to yield low Moment, thereby reducing impulse reaction, torque and torsion.
What is an “Efficient” Racquet?
The energy required to produce a certain ball speed is called “Work”. Work measures the efficiency of the racquet so low “work” is good. High work is bad because it means that the player needs to swing harder to generate the same result. Work quantifies a racquet’s power – the less work the player has to put in to get the required ball speed in the allotted time for the stroke, the more powerful the racquet. Of course, the player might put in alot of effort to increase the tip speed (swing speed) of the racquet and generate the same result, but the power then comes from the player, not the racquet.
A racquet with a low work rate also offers increased control, as in order to produce the same ball speed, the racquet need not be swung as violently, which means less chance of mistiming the ball and a greater margin of error for its user.
Head heavy racquets on the other hand (especially light-weight ones), require more work to hit the ball fast – they are also harder on the wrist, elbow and shoulder.
The most efficient racquets will also have a high “Polar Moment”. This is the racquet’s rotational inertia about its longitudinal axis or it’s resistance to a screwdriver twist. This increases Torsional Stability which reduces rotational stress on the arm, and allows users to more easily counter the “heavier” balls hit at the top levels of the game.
It is a tragedy that in junior and club tennis (and sometimes even in the pros!), players are failing to recognise the importance of having the proper equipment for their game. Top players are blissfully unaware that each racket in their bag may be different (in weight,swingweight and balance), even though they are the same model racket. At the lower levels, popular lightweight, head-heavy, stiff-framed racquets are causing a multitude of injuries to their unsuspecting owners. This tragedy is a product of duplicitous marketing campaigns by leading racquet manufacturers. These messages are then unfortunately being encouraged by stringers and even professional coaches across the country. However, in visiting SmarterTennis you are taking a great step in learning how to avoid these growing pitfalls and widespread misinformation.
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