The undercut and overcut, explained

In real racing

Overtaking on track is hard. On many circuits the car behind can be faster and still spend lap after lap stuck a second back, unable to make a move stick. So teams look for another way past, and the pit stops offer one. This is the first part of my five-part series on racecraft, and it starts in the pit lane.

What pit loss is

Every stop costs time, and not just the time spent stationary. A car in the pit lane is held to a speed limit, while a car on the circuit races past at full speed. Add the seconds stopped in the box, and the slower run back up to racing speed, and you have the pit loss: how far behind a car rejoins compared with where it would have been if it had stayed out.

It varies hugely. A short pit lane with a generous speed limit might cost fifteen seconds; a long one at a tight limit can cost thirty or more. The numbers depend on the circuit, the series and its rules, so treat any single figure with suspicion. The easiest way to picture it is a ghost: the same car, staying on the circuit, at the same pace.

Why fresh tyres are faster

A racing tyre is at its best when it’s new and slowly loses grip as it wears. By the end of a stint a car might be a second or more a lap slower than it would be on a fresh set. That difference, the tyre delta, is what the whole game is about. (Part two goes deep on compounds and wear.)

Two laps matter most. The in-lap is the last lap before a stop, driven on the oldest tyres the car will use. The out-lap is the first lap after it, starting from the pit exit on brand-new rubber. When two cars stop a lap apart, one car’s out-lap is run at the same time as the other’s last lap on old tyres. The difference between those two laps is the swing.

How the undercut works

Say one car is running 1.5 seconds behind another, with both sets of tyres fading. The car behind pits first. Both cars will pay roughly the same pit loss in the end, so it cancels out. What doesn’t cancel is the lap or two in between: the car that stopped first laps on new tyres while its rival laps on worn ones, then does its in-lap on the oldest tyres of all.

If that swing is bigger than the gap between them, the car that stopped first comes out of the exchange ahead. Nobody needed to overtake on track. That’s the undercut, and it’s usually what the car behind is trying when it pits earlier than expected.

It has a defence. If the car in front reacts and pits on the very next lap, it limits the damage. Teams call this covering the stop. The catch is that covering means stopping earlier than planned, so the leading car may then have to make its new tyres last longer than it wanted to.

When the overcut works instead

The overcut is the opposite play: stay out longer and gain while the rival is on fresh tyres. It sounds backwards, but it works more often than you’d think. The usual reasons:

• Tyres that are slow to warm up. New tyres come out of the pits below their working temperature. On a cold day, or with a hard compound, the out-lap can be slower than a lap on warm old tyres.
• Traffic on rejoin. If the early stopper comes out behind slower cars, the new tyres are wasted queueing.
• Old tyres that are still good. If wear is gentle, the car staying out loses little by waiting.
• Clear air. Once its rival has stopped, the car staying out often has empty road and can push.

Most of the time, the decision comes down to a few numbers set against each other. Here they are as a balance.

The undercut and overcut, lap by lap

On a timing screen, both moves look like lap times crossing. Here’s the same pair of cars both ways. Watch the bottom row: the net gap, with both stops counted.

In Little Engines

Little Engines is a race broadcast simulator: 22 computer-driven cars race for real, and you watch it all unfold with a director, a commentary booth and a timing tower. Nobody scripts the pit stops, so the undercut and overcut happen when the race makes them possible. Here’s how I built that, and where it is simpler than the real thing.

Pit loss on every circuit

Pit loss in Little Engines is worked out per circuit from the pit lane itself: the time to drive the lane at its speed limit against the stretch of circuit it bypasses, the stop, and the run back up to speed. A circuit without a drawn lane gets an average figure, and no stop costs less than seven seconds. Races are short, so the losses are smaller than in most real series: at Pukka Raceway a stop costs around eleven or twelve seconds.

Time in the box depends on how worn the tyres are, from about three seconds on a barely used set up to eight on a finished one, plus extra time to fix a damaged front wing. The pre-race circuit guide shows each venue’s pit loss, and the booth mentions it as the field’s strategies are previewed.

How the computer drivers decide

Before the lights go out, every driver has a planned strategy, built from the same tyre-wear and pit-loss maths. Most of the field takes the fewest stops that are competitive. In the standard and long race lengths, the rules ask each car to use two dry compounds.

During the race, the pit wall treats the plan as a target, not a timetable. Two simple rules turn it into racecraft:

Those rules are deliberately small, and they don’t cover everything a real strategist would think about. There’s no reflex to cover a rival’s stop: a car being undercut doesn’t automatically pit next lap in response, unless its own plan says so.

Warm-up does exist. With incidents switched on (the default is Normal), new tyres leave the box below their working temperature and take most of the out-lap to come in, costing grip until they do. That makes the overcut genuinely possible. The strategy maths doesn’t try to predict warm-up, though, so the computer drivers don’t pick the overcut because of cold tyres. They try it when the gap and the wear say so, and the warm-up decides whether it pays. Traffic at the pit exit is real too: a car rejoining alongside another eases in more slowly, and a car leaving its box waits if another is coming down the lane.

What you see on the broadcast

All of this is told rather than hidden. When a car stops, the pit card follows it from pit entry to stationary to pit exit to rejoined, with the tyres coming off and going on and the place it’s expected to come out in, estimated from the gaps and the circuit’s pit loss. The timing tower marks it PIT, then OUT LAP.

If the director is following a car when it stops, the picture stays with the stop until the car has rejoined, and a couple of seconds more to see it settle in. Only the race being decided, or an incident at the front, takes the camera away sooner.

The voices join in. If you’re following a car and the one just behind it pits, close enough to threaten, its engineer comes on the radio to warn of the undercut. If it works anyway, the driver lets the team know. The booth calls what each stop is for as it happens, but it waits until both cars have stopped before naming the result, so it never calls an undercut that didn’t happen. When a pit exchange changes the order, or an attempt falls short, a card shows the gap before the stops against the gap now, with a verdict: undercut works, overcut works or undercut falls short.

That’s the bit I’m proudest of. Nobody told the race to have an undercut on lap nine. Two computer drivers were close, one pit wall saw a chance, and the broadcast noticed. Next in the series: racing tyre strategy, where the tyre delta behind all of this comes from.