How string tension and type affect spin in tennis

Let the strings slide. Spin comes back with them.

Key facts

1
The ball stays on the strings for about 4 to 5 milliseconds.
2
Strings return about 95% of the energy stored in them, while the ball loses about 45%.
Brody, Cross & Lindsey, The Physics and Technology of Tennis (2002)
3
In a Tennis Warehouse University spin experiment, polyester strings produced roughly 17 to 20% more spin than nylon, and the exact gain may depend on the angle of the swing.
Tennis Warehouse University spin experiment
4
In 1997 Gustavo Kuerten, 20 years old and ranked 66th, won Roland Garros on Luxilon polyester strung well below the old gut tensions.
Tennis.com, "20 years ago in Paris"
5
In one test, 11 of 18 professional players could not feel an 11 lb difference in string tension.
Bower & Cross (2003), cited in Brody, Cross & Lindsey
6
A string bed can lose up to about 15 lb of tension in the first minute and about 20 lb by 20 hits, then loses more slowly.
Cross & Lindsey, Tennis Warehouse University, "How Strings Go Dead"

Why it works

The ball hits, the bed deflects, and the main strings slide sideways across the crosses as the ball grips them. Slick polyester lets them slide further. As the ball leaves, the stretched mains snap back and torque it.

The spin you get depends on two frictions: high friction between string and ball, and low friction between string and string. Your low-to-high path sets the direction of the brush. The strings do the multiplying, late in contact, as the ball decompresses.

Tension is a fine adjustment. Tighter strings deflect less and launch the ball slightly lower, with almost no change in speed. A steeper swing path adds spin and dip, which widens the window for a ball to land in. After 1997, players strung 10 to 15 lb below their old gut tensions.

A bed goes dead long before it breaks. Tension drops from the first minute, and a dead bed multiplies less spin. Competitive players restring every four to eight weeks even when nothing has broken.

The line to say on court
“The strings spin it. You just point them.”

Drill: The dead-string log

15 min · alone

Write the date and tension on your grip tape at every restring.
Each week, hit twenty drop-feed topspin forehands over a rope 1 m above the net that still land inside the baseline. Log the count.
When the count drops two weeks running, restring, and note how many weeks the bed lasted.

Number to beat12 of 20 over the rope and in

Faults and fixes

Restringing only when a string breaks

The tension is gone long before the string is.

FixKeep the log. Competitive players restring every four to eight weeks.

Stringing tight for control

Ball speed barely changes, only the feel does.

FixChange your swing path before you change your tension.

Buying shaped strings for spin

Friction and snapback do the work.

FixUse a slick polyester and a full swing.

Trusting the labelled tension

The bed relaxes from the first minute.

FixTreat the number as a starting point.

Questions players ask

Do lower tensions give more spin?

Tension is a small effect. Tighter strings launch the ball slightly lower with almost no change in speed. String material and your swing path matter more.

Why do polyester strings give more spin?

Polyester is slick against itself, so the main strings slide further and snap back as the ball leaves. In one test that added roughly 17 to 20% more spin than nylon.

My forehand keeps going long. Should I string tighter?

Change the path first. At 30 m/s, 20 revolutions per second of topspin widens the launch window from 4.1 to 6.4 degrees, about 56% more room. Tension is the fine adjustment.

How often should I restring my racquet?

Competitive players restring every four to eight weeks even without a break. Tension falls from the first minute, so a log of a fixed drill tells you when your bed is dead.

What this doesn't cover

Whether string shape adds spin is contested, and friction, snapback and technique dominate. The evidence linking polyester to arm injury is mixed. The labelled tension is a starting point, not the tension in your bed, and the exact figures for tension and ball speed still need checking against the source.

Sources

Tennis Warehouse University: String snap-back and spin
Tennis Warehouse University: Spin and string friction
Cross & Lindsey, Tennis Warehouse University: How strings go dead, parts 1 and 2
Brody, Cross & Lindsey, The Physics and Technology of Tennis (2002)
Haake, Allen et al., Proceedings of the IMechE Part L (2012)
Tennis.com: 20 years ago in Paris

StrikeHow to hit topspin in tennisTo hit topspin, swing low to high so the racquet brushes up the back of the ball with a slightly closed face, instead of driving through it. The path makes the spin, not a wrist roll: the ball has left the strings before a rolled wrist closes the face. Polyester strings add a bonus on top of whatever spin your path makes.StrikeHow to choose a tennis racquet: weight, balance, swingweightChoose a tennis racquet by three numbers: mass, balance and swingweight. Mass is what you carry, balance is where it sits, and swingweight is what your arm feels when you swing. A lighter frame isn't automatically easier to swing. If its mass sits in the head, its swingweight can be the same or higher. Ask for swingweight.StrikeWhy you hit the ball late in tennis, and how to fix itYou usually hit the ball late because you arrived in the wrong place, not because of your swing. The ball stays on the strings for about 4 to 5 milliseconds, too short to feel or correct. Everything you do at contact was decided before the ball arrived. Fix your spacing with your feet so contact lands in front of the lead hip.Know the gameHow tennis racquets and strings changed the gameRacquets changed what players could forgive, and strings changed what they could swing. Bigger heads, up from about 65 square inches on wood, bought forgiveness on off-centre hits. The decisive change was polyester string: Gustavo Kuerten won Roland Garros with it in 1997, and the dipping, heavily spun passes it allowed helped make the net a losing position.