How fast should my ball machine shoot?

We’ve gotten a few questions about how fast the Proton can shoot, so I wanted to write a little about ball machine speed, and post some video of the Proton in action. But first a little theory.

When the ball is sucked into the spinning wheels, it only has a few inches of contact to accelerate to the final launch speed. This can take a lot of force, which has to come from friction between the ball and the wheels. For example, accelerating a tennis ball to 100 kph over 5cm takes an average force of 440N, or almost 100 lbs. For a typical portable machine with ~6” wheels, even if the wheels spin faster than 100kph (70mph) the ball won’t generally go any faster. In fact, as you can see in the test data below, shot speeds tend to become less consistent when you operate right at the machine friction limits.

You can also use the wheels to put spin on the ball, by spinning one faster than the other. The ball velocity is determined by the average wheel speed, and the spin by the difference in wheel speeds. Because the angular inertia of a tennis ball is fairly low, friction doesn’t usually limit the spin. That’s just a function of how fast you can run the wheels.

Here are some measurements of ball speed vs wheel speed taken with the Proton. You can see that they pretty much track 1 to 1 until the wheels reach about 70mph. Beyond that point the ball speed doesn’t get much higher, but the variability is increased.

I’ve had the chance to measure machines from most major manufacturers, and while not all machines can spin fast enough to reach 70 mph, the portable ones that do all show approximately the same 70mph ball speed limit. To go faster, you need bigger wheels. Some club machines we’ve tested can go faster, but not much beyond 80 mph. Also, some of the machines we measured could do decent spin rates, or decent top speed, but not both, because the motors were maxed out at 70mph and there was no wheel speed left to add spin. With the drone motors used in the Proton, it can comfortably shoot 70mph, and also add a Nadal-like 3000rpm of topspin or backspin. Nice!

The other good news here is that 70 mph (113kph) is actually a pretty hot ball for a typical club player, especially if it has heavy spin. A ball without spin launched from low to the ground at 70mph can just barely clear the net and still land in the court. As you’ll see in the video (where the Proton is shooting at 100 kph), this is also enough speed to launch a ball over the back fence, for a later time when we program “tantrum mode” into the app. Or not.

Check out some live shots in the video below. I stop talking at about 1:50 if you read all this and want to skip ahead.

Happy practicing!

Jonah