String Tension and Effect on Racket Stability

In the past few weeks, more than a few very good players have asked me to add more lead to their racket hoops. 

All wanted a little more stability, especially when dealing with heavy balls, or when returning hard first serves.

Most of these folks already have their swingweights in the 320s to 340s region. So I thought it was decent enough.

After reviewing some of their shots in question, it seemed to me that the problem was a little more string related than racket.

Most were using stiff frames of RA68 and above. All were strung with full poly above 50 lbs. To me, that was the problem.

Let me explain...

Playing tennis is managing a series of collisions between stringbeds and a tennis ball. When both the racket and stringbed is very stiff, and the ball is new, hard and bouncy, where does the energy dissipate?

Some of the impact energy would be absorbed by the racket, stringbed or tennis ball. But not all. So the racket, deflects backwards or twists and turns. And the ball compresses too.

I have posted this slow motion video of Federer's racket wobbling on impact. Here it is again. Take a look at 1:16 in the video below. That's the racket flexing and absorbing the energy on impact. 

Stiff frames with tight strings would always be less stable than softer frames, or when strung looser.

To prove the point to an unbelieving player, we beefed up his stiff racket (with tight strings) to 365 swingweight, total weight about 360 grams and balance 32.5 cm. Then got him to try returning first serves from his coach.

Same twisting. Same instability.

Eventually, he moved a gauge up to a slightly thicker string to compensate for a slight drop in tension and obtained his desired stability.

Some may claim the thicker string added weight as well.

Yes it did.

Merely 2 grams. 






     

"Breaking Into" Rackets

Some months back, a junior competitive player asked for advice on how to continue playing with an arm injury.

Then, he was using a Babolat strung very tightly with full poly. Switching to a hybrid or even full multi did not help.

I felt a more flexy racket was necessary. After testing different sticks, he made his choice and stuck with the new racket since. According to published specs, the RA was around the low-60s.

During a recent restring, he started lamenting how the accuracy was still poor compared to his previous stiff Babolat.

I was perplexed.

He has been happily using the four new stick for about four months. Even managed to beat some team-mates he regularly lost to. Why the sudden nostalgia about his old Babolats?

He rang up his coach, passed me the phone and we spoke.

Top on my mind was whether he was "arming or swinging" (link) the new stick. This technique has huge implications on accuracy, especially when one is switching from a stiff flex to something softer. I also asked about his timing.

All seemed well. 

To ascertain, I even dropped in on one of his training sessions to see for myself how he was hitting. And there he was, leaking some forehand shots just a tad right. First serve consistency was not as good as what I recalled.

I borrowed and played a tie-break each with all four of his sticks. Then realised that the flex has changed! The rackets has all broken-in and the graphite has softened!

The pieces started falling into place. Like when he mentioned that he felt he was not able to "load" the strings and "pocket" the ball as much as before. And the reduced spin. And the "not enough time" to shape the ball.

It was finally confirmed after we put the rackets into a Babolat RDC machine. The RA stiffness for all four of them now measured 59, 60, 60 and 61 compared to 63, 64, 63, 65 when new.

To address this, I proposed to:
(1) add some lead tape to increase mass, which would "stiffen" the dynamic flex, &/or
(2) lower the tension of his existing strings, &/or
(3) change to a thinner gauge or softer string.

As it was near an important competition, he selected option (2). It was a quick-fix that addressed most of his problems. His accuracy returned while the arm pain stayed away.

Perfect!




     

Racket Flex and String Tension Preference

Many users of very flexible frames like to string very tight. 

The most common reason was to improve accuracy and that they enjoyed the feel of the racket flex. When strung at normal tensions, these folks claimed the frame stopped flexing.

Out of curiousity, my friends and I tried stringing some very flexy frames (RA 50s) to investigate the flex response.

Tight (75 lbs)
Both the full poly and the full synthetic gut felt like a board. 

The stringbed was so stiff and tight we did not feel any string pocketing at all. 

However, impacts did not feel harsh as we could clearly feel the racket flexing on impact. 

Contrary to the beliefs about stringing it tight for greater accuracy, all our shots suffered some loss of directional control. Some forehands leaked to the right and backhands leaked left. It felt like our timings were slightly late and off.

We tried "swinging" the racket and "arming" the racket. (link) Well connected sweetspot hits felt nicer but it was still boardy, extremely low powered and inaccurate.

Neither could we generate any decent spin. Not even slices.

Serves were pathetic. No power, no pace, no spin, no height and plenty of double faults!


Normal (55 lbs)
This was near our usual tensions so everything felt familiar.

It was comfortable, spinny, accurate although still a tad low powered.

It was interesting to note that when we had sufficient time to prepare, step into the shot before "swinging" the frame, there was a very powerful delivery. The string and racket flex worked in unison.

Even when we were late, on the run and half-muscling through the shot, we were able to partially load the strings before flexing the racket.

After hitting with this, we discovered what and why we did not like about the racket flex earlier with the tight tension. 

Whenever it flexed, power was absorbed by the frame. The energy that was incoming from the ball, and the energy which we threw into the forward swing, was heavily dissipated by the racket flex. 

And before the flex could rebound, the ball has left the strings. Hence the very low power and poor directional control.

Swinging worked much better.


Loose (35 lbs)
For this tension, we chose a stiff poly string and strung it full bed.

The first thing we noticed was the extra huge sweetspot. It was so forgiving we thought we were using an oversized racket!

The second thing that impressed us was the abundance of power and spin. A huge increase over the normal tension.

Along with this was the ability to load and pocket the strings. With the added advantage that the harshness of any off-centred shots would be safely cushioned by the racket flex.

At this tension, the usual downsides were the high rebound angle of the stringbed and some light string buzzing. However, this had nothing to do with the racket flex. It was a common problem across all types of racket frames.

Most distinctly, we did not feel much of the racket flex.


Conclusion
Among the three, we preferred the one lowest tensioned. 

However, for practical play, mainly to mitigate the high rebound angle, we would string somewhere around the mid-40s lbs, depending on strings.

What we unanimously agreed was:
(1) flexible frames felt superior to very stiff frames,
(2) flexible frames are very demanding to wield properly, and
(3) stringing flexible frames loosely made them much easier to use for recreational players as it was much better to load the strings than to flex the racket.




     

Measuring String Left in Reel

Most who favour a particular brand of strings end up buying reels. 

However, many of my friends still get caught not knowing how much string there is left in the reel.

Depending on the brand, gauge and type of string, the more common strings weigh between 15 grams to 20 grams per set of 40 ft. It is easy to use weight as a proxy to estimate how much string there is left.

Below is a partially used reel of poly a friend handed to me. It weighs 331 grams.

He wanted 3 rackets strung with full poly, so I cut out 3 sets of 40 ft each, total 120 ft of string from the reel.

That left me with 275 grams.

The simple calculations that follow:

120 ft of string weighs 56 grams (331-275)

Each set of 40 ft weighs ~19 grams (56/3)

This full reel of strings only should weigh ~308 grams (56/120 x 660)

So when you next buy a new reel, measure and record the weight of the entire reel before using it. You will have a better idea how much string there is left.