Tennis on clay is a very distinct game. More so than “smoother” surfaces like grass or concrete, clay-court tennis is a game of consistency, where points tend to involve long and exhausting baseline rallies. This is due to the fact that shots on clay move slower and bounce higher than on other surfaces, which makes it more challenging to put the ball away.
We tennis players know this stuff from experience, and any engaged spectator knows by watching. It’s pretty obvious, especially at Roland Garros, where attritional rallies are bread and butter and matches last for hours. But what isn’t so obvious is why the ball plays there as it does. What is it about a clay court that makes for clay-court tennis?
Let’s start with the court itself. As Reeves Wiedeman explained last week in the New Yorker’s Sporting Scene blog, there’s a lot more to the terre battue than “red clay”; in fact, a court at Roland Garros is comprised of several different strata, of which red clay is only the finishing crust. From (beneath) the ground up, Wiedeman details, it all starts with,
“a layer of large stones, two feet thick, topped by an inch of gravel, five more of clinker—residue from volcanic rock that can absorb and retain water—and a top strata of limestone. This last layer is the one off which the balls at the French Open are actually bouncing, and the event’s organizers are proud to say that the stone comes from the quarries in Saint-Maximin, outside Paris, that were mined to build most of Haussmann-era Paris. The red clay is ground brick or tile. Each court requires more than a ton of the stuff, which is spread at a depth of only two millimeters.”
If you really get into the physics of tennis, there are all manner of variables and complicated reasons why the ball bounces off the clay as it does—the ball’s angle of incidence before impact with the court, the (Babolat) ball itself, shot speed, the type of spin employed, the extent to which the court squashes the ball (and the ball the court), various frictional forces, and so on. I don’t pretend to understand all of it, so don’t take my explanation as comprehensive.
That said, here are two reasons for the ball’s relatively high, slow bounce at Roland Garros.
One is that limestone. As Rod Cross and Crawford Lindsey explain in Technical Tennis, how high a ball bounces off a court correlates with “the magnitude of the force pushing the ball off the court,” which is in part a factor of how hard the surface is. For example, “[i]f the court is soft, energy will be lost deforming the court surface (or worse, if the ball lands in a pile of dirt, it won’t bounce at all). The surface does not spring back fast enough or efficiently enough to aid the ball in its bounce, so that energy is ‘lost,’ and the ball won’t bounce as high.”
Without being plugged into the context, it’d be reasonable to assume that red clay would play similarly soft, because the ball is connecting with dirt, after all, and that dirt is laid down pretty loosely. Recall, though, that there’s only two millimeters of the stuff down, underneath which is a layer of limestone. Limestone is hard; no wonder it’s used to make cement. It turns out, then, that the courts at Roland Garros are actually even harder than the rubberized “hard courts” at the U.S. Open. (Of course, they’re harder than the grass at Wimbledon, too.) The result is that clay courts—of all the major surfaces, and when properly rolled and kept in good shape—deform least upon impact with the ball, thus causing the ball to bounce highest.
So the court’s hardness has to do with all those high bounces. But what about how slowly the ball plays? Reason two for the red-clay bounce is, less surprisingly, the clay. Here, friction plays a role. As Cross and Lindsey explain, when a ball hits the court at a low angle, “around 20 degrees or less, which is typical of a fast first serve or a low, hard groundstroke, then the ball will slide through the bounce. In that case, the horizontal speed of the ball when it bounces will depend [in part] on whether the court is rough or smooth.”
Red clay is a pretty rough surface; it’s crushed brick, remember. So as the ball impacts and slides across the surface, frictional forces slow it down. What’s more, because the clay is laid loosely over the surface, C. & L. say, “the ball digs a groove in the court and has to push clay ahead of it” as it slides. (This happens regardless of the angle at which the ball hits the court.) These two forces—increased friction, combined with the force required to press on through the clay—act on the ball, causing it to bounce slower than on any other court.
So there you have it. Enjoy the tennis this weekend. It shouldn’t go by too fast.