What is the undercut in F1?

The mechanics

Imagine two cars running nose-to-tail, both due to pit between laps 18 and 22. The chaser pits on lap 18. The leader stays out on worn tyres for another lap, then pits on lap 19.

During lap 19, the leader is circulating on tyres that are about to come off the car. The chaser, now on new rubber, is running 1-2 seconds per lap faster^[2]. That difference, applied to a single lap, is roughly equal to the time it took the chaser to drive through the pit lane on lap 18.

When the leader rejoins after their lap-19 stop, the chaser is now ahead. Same number of stops, same number of laps, different exit order. The undercut has worked.

What makes it work

Three things have to be true at once:

1. The chaser is close enough on track. Within 1.5 to 2 seconds at the moment of stopping is typical. Beyond that, the gain on the in-lap and out-lap is not enough to bridge.
2. The tyre on the rival is past its peak. If both cars are on fresh enough tyres, the leader can match the chaser's pace and the undercut fails.
3. The chaser's fresh tyres warm up quickly. This is where compound choice matters. A soft compound on a warm day comes alive in one or two laps. A hard compound on a cold day might take five, by which time the leader has already pitted.

What kills it

• Cold tyre warm-up. If new rubber takes three to four laps to come into its window, the undercut is dead. The chaser's first out-lap is slower than the leader's worn-tyre lap, and the gap grows.
• Track temperature low. Same problem at the macro level.
• Pit lane time loss high. Monaco's pit lane delta of roughly 23 seconds raises the bar for an undercut to pay back, because the chaser is losing more time at the stop itself^[1].
• The leader pits on the same lap (covering). If the rival's pit wall sees the undercut coming and pits the same lap, both cars switch to fresh rubber simultaneously and the order on track is preserved.

When teams choose the undercut

Race engineers call the undercut when:

• The car ahead is in clear air and pacing well.
• The team is confident in their pit crew that day (no recent slow stops).
• Tyre data from practice supports good warm-up on the next compound.
• The undercut is reversible: if it fails, the position is not lost permanently because the rival pits anyway and the cost is just clean-air time.

Why fans see it called every race

The undercut is the default offensive move because it is low-risk relative to overtaking on track. Modern F1 cars produce so much dirty air that following a similar-pace rival in clean air for 10 laps and then passing them by stopping a lap earlier is more reliable than attempting a pass on track. Until the regulations make wheel-to-wheel overtaking easier, pit strategy will keep doing the work that the cars cannot do.