69 RA: neither soft nor stiff — the honest stiffness

RA is one of the most misused numbers in tennis. It measures frame stiffness at impact — how much the frame flexes when the ball hits. Both extremes of stiffness cost the player something. Here's why we fixed the whole adult range at 69.

What RA actually drives. Stiffness governs yield (energy returned to the ball), dwell time (how long the ball stays on the strings) and comfort. Stiffer = more return and a short, crisp contact; softer = a longer, more muffled contact. It is not, on its own, “power” or “comfort” — it's a trade-off.

What both extremes cost. Raising stiffness manufactures power on paper: the ball leaves a platform that barely flexes. The price is a harsh strike for the arm and a vague feel. Lowering it manufactures comfort: contact becomes muffled, but the ball lands short the moment you brush. Either way, a single number stands in for a trade-off that has three terms — yield, dwell time, comfort.

Why 69. It's the honest floor of modern yield. Modern tennis is tangential: you brush to create spin, and that costs linear energy. You need enough frame yield to keep depth — but not the rigidity that kills feel and comfort. 69 sits exactly there: enough response for the modern stroke, without the harshness. Not soft, not stiff. That holds for an adult arm and a built stroke. The junior line is fixed at 55 RA: on 23 to 26-inch frames, where racquet-head speed is lower and the joint still growing, the trade-off shifts, and the frame must give back in flex what it does not give in speed.

69 is a starting point, not a finished setup: mass, balance, swingweight and handle are decided after it, in the configurator.

So where does comfort come from? Not from a soft frame. It comes from the right setup (mass and balance that let you relax), the right string (that absorbs and snaps back), and a properly fitted handle. We remove what holds you back; we don't add fake softness.

Beyond RA: the two numbers that say more. RA is a single point. The stiffness profile (how stiffness is distributed along the frame) and torsional stiffness (resistance to twist) tell you far more about how a frame really behaves. We measure these profiles on the bench, frame by frame.

How RA is measured, and why the published figure is not the one you play

RA is taken on a bare, unstrung frame: a load is applied to the frame and the resulting deflection is measured. It is a measurement of a frame, not of a racquet.

But you never play a bare frame. The string bed pulls constantly on the hoop, and frame plus string does not behave like frame alone. The published figure therefore places the mould, not the object in your hand.

A second, more prosaic reservation: comparing two RA figures from two different brands assumes they were taken the same way. Nothing guarantees that. A gap of two or three points between two spec sheets says very little; a gap of ten does.

What stiffness does not change

• It does not decide trajectory. Face angle and spin decide that.

• It does not decide spin. The stroke produces it, the string transmits it.

• It does not decide stability at contact. Twistweight and recoilweight do, and they are tuned by weighting, not by choosing a frame.

So a stiffer frame does not "give" power in the usual sense. It returns a slightly larger share of energy you supplied — and sends a slightly larger share back up the arm.

The case of a sensitive arm

If discomfort is already there, dropping RA is rarely the first move, and it is the most expensive one: it means changing frames.

Two levers act more and cost far less. The string first: between a multifilament and a stiff monofilament, the difference in what travels back is of the same order as between two families of frame. Then tension, which costs nothing beyond a restring you were going to have done anyway.

The full order, from free to expensive, is set out in tennis elbow and your racquet.

Going further

Comfort comes from the setup and the string, not from a soft frame: the weight, balance and swingweight guide covers the first, the science of strings the second, and racquet physics places RA among the other parameters. Head size covers the other half of the frame choice.