If you're in the market for a new tennis racquet, then one of the specifications you'll see listed by retailers is the ‘stiffness' of a racquet.
Is this something you need to pay attention to or just another arbitrary number? Does it impact how a tennis racquet plays, if so, how by much?
As usual, there are a lot of old wives tales and anecdotal points of view on racquet ‘flex'. While some of them can be helpful, this often leads to misinformation and players making ill-informed purchases. So let's take a look.
What is Stiffness in a Tennis Racquet?
Racquet stiffness is a measurement of how much a racquet bends when it makes contact with the tennis ball.
The more the ball distorts the frame, the more flex a racquet has, therefore, the lower the stiffness rating.
Stiffness ratings are quoted as a number, called an RA rating. This is usually between 50 and 70 on most retail racquets. The lower number indicates a more flexible racquet, and the higher number a stiffer racquet.
So a racquet rated RA 50 will bend more than a racquet at RA 70 at impact, assuming they are identical in all other specs so the same weight, head size etc.
How is Tennis Racquet Stiffness Measured?
Measuring racquet stiffness is primarily done using the Babolat Racquet Diagnostic Centre, which is pictured above.
There are other machines around from Prince and Gamma, but Babolat is the most widely used and trusted machine out there. It is capable of measuring stiffness, weight, swing weight, balance and stringbed deflection.
These machines run for around $4,500 so it's not something your average club player will have access to.
As players, we're wholly reliant on manufacturers (who provide unstrung stiffness ratings) and retailers like Tennis Warehouse (who provide strung stiffness ratings).
The RDC machines calculate stiffness by applying force on the hoop of the frame while it's locked in at the handle and resting on a bar near the throat area which acts as a lever.
This causes the frame to flex slightly, and the RDC machines record the measurement. On most racquets aimed at intermediate and advanced players, this falls somewhere between 50 and 70 but can go as low as 40 and as high as 85.
You can see an example of an RDC machine, and it's how it works in the video below:
How is Racquet Stiffness (RA) Calculated?
Interestingly the racquet stiffness measurement is not some commonly used formula that is seen across many other products or walks of life.
You won't see the term RA used anywhere outside racquet sports and it's said to be the acronym for racquet analysis. A name coined in the early days of measuring racquet stiffness by applying a specific force to a frame and steel bar placed at 327mm to act as the opposing force.
Babolat treats it as proprietary technology, so they don't reveal the calculation or formula used. This, in turn, does highlight one of the flaws of RA which we'll look at further down this guide.
Are There Any Other Methods of Measuring Stiffness?
One other measurement is used by Wilson, something they called Stiffness Index (SI). This is a number they provide in mm.
For example, the Wilson Clash 100 Tour, one of the most flexible frames on the market and has an SI of 11.2mm which they claim is 115% more flexible than the leading racket. However, this is based on internal testing and we don't know what the other leading racquet is.
You can see an example of Wilson's stiffness measuring technique in the video below
This method is an interesting one as it clamps the racquet it at the handle and doesn't use a metal bar halfway up the frame like Babolat do to counter the force.
Wilson's method is more realistic for how a racquet reacts during match play as the clamp is essentially replacing the hand. Again though, it's just a number and they don't tend to publish it on every single one of their racquets either which makes comparison difficult.
Can I Measure Racquet Stiffness Myself?
While you cant measure racquet flex to get an RA number at home, you can get an idea of the flex by lying it flat against a surface and pressing down on the throat to see what give it has.
This is obviously only useful if you have two (or more) racquets to compare and it's not exactly scientific but you will see the difference in flex by applying pressure with your hand.
You could also set up a makeshift system like the Wilson method above to clamp a racquet to a table and hang a 2.8kg weight from the hoop, measuring how far in millimetres it flexes.
How Does Stiffness Impact Racquet Performance?
The stiffness of a racquet impacts a racquet's playability in three areas:
Without going deep into the physics of it, the reason stiffness impacts the power of the racquet is due to energy transfer.
When you make contact with the tennis ball, the frame bends. The more it flexible a racquet is, the more it bends and the more energy it absorbs. That decreases power as it's taking energy away from the ball.
A stiffer frame flexes less at impact, keeping more energy in the ball, leading to more power in your shots.
One of the myths you will see peddled at club level is that flexible frames are powerful because the racquet works like a slingshot, propelling the ball when it returns to its normal state.
However, the truth is a tennis ball will only stay on the strings for a fraction of a second, which is not long enough for the racquet to recover and return energy to the ball.
So, all other things being equal, a stiffer frame bends less and keeps more energy into the ball than one with more flex.
Whereas power is easily explained by physics, control is a more subjective area and varies from player to player.
The general theory here is that the more flexible racquet, the more control it will give because it has less power potential.
Does that mean that a player who can't find the court should switch to a more flexible frame and see instant results? No. Control comes primarily from a players technique and skill level; a racquet can only make subtle changes to the placement of the ball.
Where it gets even more subjective is that a stiffer racquet will provide a player with a more consistent hitting experience and a higher margin for error on off-centre hits.
Stiffer racquets are more stable at contact and off-centre hits. This stability will produce a better quality of shot that will typically cross the net with more pace and depth. This can lead to higher perceived control as you're able to stay in points longer.
A more flexible frame can give players the perception they're hitting dead spots on the string bed, where the ball has less pace and depth. This can allow an opponent to mop up on your short balls, ending points quickly.
In terms of feel which also relates to control, players generally find that stiffer racquets provide a crisp or lively feel, due to the increase in power a more rigid frame offers.
Flexible frames are often described as providing a plush feel as they feel more connected with the ball when especially on touch shots such as volleys or drop shots.
It's worth noting that the type of strings you're using also has a significant impact on a racquet's feel and control. Above and beyond that of stiffness from my own experiences.
Like control, comfort is an area where physics and personal feelings collide.
In terms of comfort, because they bend and absorb more shock on impact, flexible racquets are generally considered more arm-friendly.
Stiffer frames pass harsher vibrations to the hitting arm, and many consider them a contributing factor to arm or elbow issues.
As a result, senior players or those with elbow and wrist issues tend to choose a flexible frame, exchanging lesser power for improved arm health.
The problem here is the science behind it isn't clear cut and many players may find the exact opposite is true.
When a tennis ball makes contact, the racquet vibrates. The stiffness of the racquet impacts the amplitude, frequency, and how much those vibrations are dampened.
A stiffer frame has a lower amplitude, but a higher frequency and more damping. So the vibrations are small, they happen at a faster rate but also stop more quickly. More flexible rackets have more significant vibrations that occur at a slower pace but last for longer.
So which is better? This is where it's somewhat murky. There is a strong argument to say that a stiffer racquet could well be more arm friendly because the vibrations are smaller and stop sooner.
I play with a 70RA racquet and have never had any arm or elbow problems, but I can feel the change in comfort depending on the string I use, that leads me to think the type of tennis string you have in will affect the comfort far more than the racquet stiffness itself.
Similarly, stringbed tension can increase or decrease comfort with higher tensions reducing comfort and lower tensions offering up slightly more comfort.
I've also seen players refer to frames with an RA in the low 60s as firm and crisp, while others describe something in the 70s as having a soft response. So that does suggest regardless of any stiffness rating, the only measurement that matters is how a racquet feels and performs in a player's hand. And there's only one way to figure that out – by demoing it.
How Do You Categorise RA Ratings?
You can use the chart below but do bear in mind the above in terms of power, control and comfort are subjective and have other variables that can affect them.
|64 and below||Flexible||Low||High||High|
|70 and above||Stiff||High||Low||Low|
Should I Choose a Stiff or Flexible Racquet?
As I keep stressing, demoing a racquet is an absolute must, and you can't say with any certainty you should choose one over the other without first playing with it.
As a general guide, players with fast swing speeds — i.e. those who don't require lots of help from the racquet when it comes to hitting with pace, might find a flexible racquet could be a good choice. They can take big cuts on the ball with fewer worries of overhitting beyond the baseline.
Players with slower swing speeds who don't hit the ball as cleanly may prefer a stiff frame as the added power helps them get shots over the net more consistently.
But there is no golden rule here. I play with fairly fast swings, but my racquet is 70RA. Strings and string tension play a far bigger role in my ability to keep the ball within the lines.
In terms of arm issues, this is against a tricky one, most players tend to get more arm problems from stiff racquets so if you have tennis elbow, I'd test a flexible frame first. But don't rule out slightly stiffer models, for example, ProKennex offers a very solid range of arm friendly racquets that aren't exactly low on the RA scale.
How Useful is RA Rating?
With RA rating being a well-kept secret for how it's calculated then it can only really be used as a quick point of comparison when choosing a tennis racquet.
The number allows you to quickly benchmark which frames are considered stiff or flexible, demo them and see which you prefer, assuming all other specs are similar or equal.
One problem with the number itself, however, is consistency. Not every manufacturer uses a Babolat RDC machine, and they are not all calibrated the same way.
If you look at the RA ratings on Tennis Warehouse assign to racquets, you will also see they differ from those listed by the actual racquet manufacturers.
This is because TW posts strung specs, whereas other retailers and manufacturers post unstrung specs. When a racquet is strung, the stiffness (RA) typically drops 4-6 points, the weight goes up about 15 grams, swing weight goes up, and the racquet becomes more head heavy. So try to stick to one when comparing – either unstrung or strung specs, rather than mixing the two.
One other area the Babolat Machine lets itself down is that it cannot measure in multiple areas of the frame. So if a frame flexes more in the hoop more than it flexes in the throat, you will not know that from it's RA rating.
That's why, in my opinion, while RA is a nice number to know, it doesn't always tell the whole story. The best way to know if a frame is suited to your game is to demo it.
Examples of Stiff Racquets
Below are some of the stiffest racquets on the market right now,
|Racquet||RA Rating (Strung)|
|Prince O3 Speedport Black||70|
|Volkl V-Sense 7||71|
|Wilson Blade Team||72|
|Wilson Burn 100S||73|
|Volkl V-Feel 9||74|
|Head Titanium TI.S6||75|
|Dunlop Srixon CS 8.0||76|
Examples of Flexible Racquets
And here some of the most flexible ones you can currently buy.
|Racquet||RA Rating (Strung)|
|Yonex VCORE 98||64|
|Yonex EZONE 98 (305g)||63|
|Head MicroGEL Radical MP||62|
|Head Gravity Tour Graphene 360+||61|
|Prince Textreme Tour 95||60|
|Wilson Clash 100 Tour||55|
|Prince Phantom Pro 100||54|
|Wilson Triad XP5||46|
Frame stiffness is one of the many things that can impact the playability of a tennis racquet and while not the most important consideration, it's something to think about if you are struggling to find comfort in a frame or would like more power (or control).
However, nothing beats testing a racquet on the court for a couple of hours to see how it performs. If you have access to a demo program, check out my guide to getting the most from a demo frame.
Finally, if you're looking for a new tennis racquet, be sure to check out this guide on some of the best tennis racquets for 2020.
Got any questions about racquet stiffness? Let me know in the comments.