Lab note · 4 min read · 2026-09-20
We built a padel engine to play our puzzles. It has opinions about the smash we did not expect.
The Rally is a small physics engine: a ball with gravity, a 20 by 10 court with 3 metres of glass and a metre of mesh above it, four players with reactions and speeds and error, and a coach who plays every option in his head before you choose. Build one of those and it starts telling you things.
The smash is two shots
The first thing the engine refused to do was let a smash from behind the service line be a winner. With the ball coming down at pace from six metres out, the geometry gives you two choices: hit it flat enough to clear the net and it lands deep and comes off the back glass at chest height to a player who has been waiting; hit it down hard enough to bounce high and it does not clear the net. The pros know this, which is why the smash from deep is a bandeja with ambition and nothing more. From inside the service box it is a different shot: the ball comes down steeply, bounces at 1.6 times its arrival speed in our model (the topspin and the angle do that in real life too), climbs, and clears the fence. Por cuatro. The engine calls it a winner about as often as a real one at club level, which is to say not often enough for anyone to build a game around.
The wall smash needs a number
The contra-pared, hitting your own back glass so hard the ball returns over the net, was the shot that broke the engine most often. Aim it by feel and the ball hits the glass too high, drops short, or leaves the court through the side. What fixed it was doing what a player learns over years without knowing it: running the flight for every launch angle between 24 and 60 degrees and three speeds, and keeping the one that clears the net by the most after the glass. The shot is not brave. It is a calculation you do with your body, and when the calculation is off by five degrees it is a gift.
Four metres, not three
The back wall is three metres of glass and a metre of mesh; the side walls stand four metres for the first two metres from the back, then three. We had the side at three metres everywhere, and smashes were leaving the court over the side glass at 3.4 metres in the corner where, on a real court, they would rattle the fence and stay in. One line of geometry and the engine got a whole category of points right: the ball that kicks up off a deep bounce and hits the corner mesh, dies, and gives the defender the strangest easy ball in padel.
The door is a duel
The door drop, the sliced dejada that bounces and runs out through the opening by the net post, is the most beautiful point in the sport and, in the first version of the engine, a certainty. Every wide drop that bounced went out. Real ones do not: a defender standing near the post gets a racket on it two times in three. So the engine now decides at the moment the drop is hit how far the nearest defender has to travel, and gives them their chance. A blocked door drop pops up short about three times in ten, and then the attacker has a sitter and one job. The point that was a certainty is now a sequence of two decisions and a reaction, which is what it is on a court.
Long points make everyone worse
From about the seventh shot the engine adds a degree of error per shot to everyone, slows reactions, and speeds the point up a little. That came from watching club players: the first six shots of a point are the plan, and everything after that is legs. The pair that wins the long point is not the pair with the better shots. It is the pair that made fewer decisions, because they were standing in the right place and the ball kept coming to them.
The coach cheats, honestly
Before every call of yours, the coach replays each option thirty-two times with realistic error and keeps the win rate. He also holds the finisher for the last call of a chain, because a chain that ends on the first ball teaches nothing. When he judges you wrong, he is not applying a rule; he is telling you what happened the thirty-two times he tried it. Chess engines changed how humans understand chess by being unsentimental about positions everyone thought they understood. A padel engine is a much smaller thing, but it is unsentimental in the same way, and it has already taken the smash down a peg.
You can argue with it: The Rally is the engine with you in it, the chains are the engine's puzzles, and how to play lists every shot it knows.