The Physics Behind the Spin
The mechanism researchers eventually isolated is called snapback. When a strung ball is struck off-center, or with enough force, the individual strings deflect sideways before snapping back to their resting position, and that snapping motion imparts additional rotation to the ball on top of whatever spin the player's swing path already generated. Polyester's stiffer, slicker surface lets the strings slide and snap back more freely than a stickier natural gut or nylon bed, which grips the ball and resists that lateral movement. Physicists Rod Cross and Crawford Lindsley of the University of Sydney ran a detailed study on the effect, testing sixteen different strings across multiple tensions using both clamped and hand-held rackets, and found that polyester strings produced roughly 20 percent more spin on average than nylon strings under comparable conditions. Separate testing by the International Tennis Federation in 2006 reached the same broad conclusion: polyester generated measurably more spin than the synthetic gut strings that had dominated the sport through the 1980s and early 1990s.
