How racing drivers overtake, and how my computer drivers learnt to

In real racing

A pass starts laps before it happens

Overtaking looks like a single moment, but it rarely is. A driver who is quicker than the car ahead still has to turn that pace into a place, and the car ahead is doing everything it can to stop it. So a pass is usually built over several corners.

First comes closing in. The chaser needs to be close enough to strike in the next braking zone, ideally within a car length or two at the end of a straight. That’s where the slipstream helps: tucked into the hole in the air behind the leader, the chaser gains speed and reels the gap in. The catch is the corners. In the turbulent wake the chaser loses grip and its tyres run hot, so it can’t sit there for ever.

Then comes setting it up. The chaser studies where the car ahead is weak. Perhaps it brakes early into the hairpin, or gets a poor exit out of the last corner. A good driver shows a nose, pulls back, and shows it again, making the defender think about the wrong corner. Then, when the gap and the run are right, it commits.

Why most passes happen under braking

On a straight, two similar cars are going roughly the same speed, so getting past means having a much faster car. Braking zones are different. Every driver has a marker where they start braking, and the difference between a safe marker and a brave one is worth several metres. Brake a little later than the car ahead, on a different line, and you arrive at the corner alongside it.

That is the moment that matters: the overlap. Once the attacker’s front wheels are level with some part of the other car before it turns in, the corner belongs to both of them. The defender can’t simply turn in on the racing line, because the other car is there. It has to leave room, and the attacker, on the inside, has the shorter way round.

The cost is the exit. Braking late and diving to the inside means a tighter, slower line out of the corner. The attacker only needs to be ahead at the apex and hold position on the way out. Get the braking wrong, though, and it slides wide, leaving the door open for an instant counter-attack.

The main overtaking moves

Almost every pass is a variation on a handful of moves. The inside line under braking is the classic. A pass round the outside is rarer and braver: when the defender has covered the inside, the attacker carries more speed on a wider arc and hopes to be alongside on the exit, ready to have the inside for the next corner. And the switchback, or cross-over, is the reply to a late lunge: let the other car dive in and run deep, turn in later, cut back underneath and use the better exit to come back past on the straight.

There’s also the drag race. Get a cleaner exit out of a slow corner, pull alongside on the straight, and the move is finished in the next braking zone. And sometimes no move is needed at all: in a pit-stop race, the place can be won in the pit lane instead, which I covered in the undercut and overcut.

Defending, and leaving a car’s width

The driver in front isn’t a passive obstacle. The simplest defence is covering the inside: move across before the braking zone so there’s no room to dive into. It works, but it costs speed. Turning in from the inside means a tighter, slower corner, which can hand the chaser a better exit and a run on the next straight.

The alternative is to hold the racing line, take the fastest corner possible and trust the gap. That leaves the inside open for a brave lunge, but makes any pass harder to complete.

Every series has its own rules, but the common principles are similar. A defender may make one move to cover a line, made early, not in the braking zone, and no weaving. And once an attacker has a real overlap, the defender must leave at least a car’s width of room on track. Squeeze a car that’s alongside off the road and the stewards will usually have something to say.

Sometimes the smartest defence is not defending at all. If the car behind is far quicker, fighting for every corner just cooks the tyres and loses time to the cars further back. Letting it go and getting back to your own race can be worth more than the place.

In Little Engines

Reading the road, every moment

None of this is scripted in Little Engines. All 22 computer drivers race each other for real, and the passes you watch are ones they decided to make. Teaching them was most of the work of making a race worth watching.

Every moment, each computer driver rebuilds its picture of the cars around it from scratch: the gap to each one, how fast it’s closing, whether the cars overlap, who is a threat and who is an opportunity. A rival only counts as an opportunity if the driver has real pace in hand. Closing speed under braking spikes for a moment in every braking zone, so the drivers ignore those spikes and look for a sustained speed advantage instead.

From there, each driver runs in one of a few modes: conserving, following, pressuring, attacking, recovering, defending or conceding. On the broadcast, the timing tower shows a small symbol next to drivers who are attacking, defending, pushing, conserving or recovering, so you can see a fight forming before the cameras find it.

Pressure, then commitment

A pass in Little Engines builds the way a real one does. Once a driver is within about 28 metres of a slower car, it starts building readiness for a move. How fast that builds depends on its aggression, how much quicker it is, and how urgent things are: the last two laps make everyone keener. How much it needs to protect its tyres slows it down and raises the bar. When readiness crosses the threshold, and the pace advantage is big enough, the driver commits.

Plenty can stop it first. Sitting in dirty air heats the tyres, and past a limit the chaser drops back to let them cool, unless it’s on fresher tyres and has grip to spare. A rival that is about to pit isn’t worth a fight, so appetite drops. And under yellow flags or a slow zone, nobody starts a new move.

Braking later, and picking a side

A committed driver does what a real one does: it pulls out and brakes later. It only does that with a rival within about seven metres, and it brakes less deeply into tighter corners, where a lunge is riskier. Aggressive drivers push the braking point deeper.

The side matters too. Drivers prefer the inside of the coming corner relative to the car they are passing, but an inside that is already taken never beats an outside that is clear. If an attempt fails, the next one leans to the other side. Behind the scenes, the drivers weigh 32 possible lines against every car around them at once, so they commit to one line through a group rather than darting at each car in turn.

Not every attempt works. If a move hasn’t stuck after a second and a half, the attacker backs off, recovers and remembers that rival for somewhere between 10 and 28 seconds. It won’t try the same car again in that window unless a much better chance turns up, such as a mistake. That’s what gives battles their rhythm on the broadcast: a lunge, a pause, a second go a lap later.

Each driver is also stronger in some corners and weaker in others, so the same corners tend to produce the passes, much as they would at a real circuit. That isn’t a named move, and I haven’t programmed a switchback as a manoeuvre. Moves like it come out of the drivers reading the road.

Defending, and leaving room

The drivers defend by the same principles as real ones. A defender covers the inside before the braking point, only into a corner and never on a straight, and it picks the side from the shape of the road rather than copying the car behind. It can only cover so many times before its budget runs out, and it never starts a fresh swerve once it’s in the bend.

Once the attacker has an overlap, the defender holds its line and doesn’t add a block. Pressure from behind doesn’t own road space; only a car alongside does. Where the road is wide enough for two, both cars race it, and a committed driver will run two-wide through a gap a timid one would give up.

Defending has a price. The game’s mistake risk rises for a risky overtake and for a driver defending under pressure, and the defender carries the larger share. So a driver being caught by a clearly faster car, where covering would cook its tyres, concedes and lets it through. Lapped cars yield to the leaders under blue flags, and composed, patient drivers yield more cleanly.

What you see on the broadcast

All of this is there for you to watch. The tower symbols show who is attacking and who is defending, and the cars’ rear lights glow green on an attacking car and amber on a defending one, so you can read a fight from the Overhead or Chase camera at a glance. The director treats a building battle as the shot before anyone has made a move, which I’ll come to in part five. And a pass only counts once it has held for a second and a half: two cars swapping back and forth through a corner is a battle, not two passes.