The Undercut and the Overcut: What Decides Which One Works

Two cars are running a second and a half apart, and neither can pass on track. One of them will be ahead after the pit stops, and it is not necessarily the one in front now. Which of them comes out ahead is decided less by pace than by timing — and the timing can be worked out in advance.

Why the pit stop itself cancels out

A pit stop costs time: the slow drive down the pit lane, the stationary time, the drive out. That loss is large, but both cars pay it. If the two stops take the same time, the pit loss changes nothing between them. What changes the order is what each car does on track during the laps when one has stopped and the other has not.

That window is usually only one or two laps long. Everything in pit strategy comes down to who is faster inside it.

The undercut

The undercut is the following car pitting first. It rejoins on fresh tyres while the car ahead is still circulating on worn ones. If the fresh tyres are quicker straight away, the following car gains time on every part of that lap — and when the leader stops a lap later, it rejoins behind.

A worked example: the following car is 1.5 seconds behind and pits. On its first lap out, on new tyres that are not yet fully up to temperature, it is one second faster than the leader manages on old tyres. The leader then pits. On the leader's own out-lap, the car that stopped first is now on warm, fresh tyres and gains another second. Two seconds gained against a gap of 1.5: the positions are reversed, without an overtake.

The overcut

The overcut is the opposite: staying out while the other car pits, and using those laps to build a margin. It works when the car that stays out can go faster than the car that has just stopped — which sounds unlikely, since it is on older tyres, but happens regularly.

• New tyres take time to reach temperature, so the out-lap is slow — especially where tyres cannot be pre-heated
• The car that stays out is released into clear air once the car ahead has pitted, and can finally run at its own pace
• The car that pits rejoins in traffic and loses time behind slower cars
• Tyre degradation is low, so worn tyres give away very little

The numbers that decide it

Whether to go early or late depends on a small set of quantities, all of which can be estimated from practice running.

• Tyre delta — how much faster a new set is than the current set at this point in the stint
• Warm-up — how many corners or laps a new set needs before it delivers that advantage
• Degradation — how quickly the current set is losing pace; high degradation favours stopping early
• The gap — an undercut has to gain more than the gap between the cars, inside the window
• Traffic — where each car will rejoin, and what it will be stuck behind

High degradation and fast warm-up make the undercut powerful. Low degradation and slow warm-up favour the overcut. Most races sit somewhere between the two, which is why the answer has to be calculated rather than assumed.

Pit strategy is not about the stop. It is about the laps either side of it.

What changes in endurance racing

In GT and endurance racing the same logic applies, but it rarely operates alone. Stint length may be set by the fuel tank rather than by the tyres. Regulations in many series set a minimum pit stop or refuelling time, which removes the advantage of a quick stop. Driver changes, maximum driving times and the need to double-stint tyres all constrain when a stop can happen.

The result is that a pure undercut is available less often, and the planning question becomes broader: which stop sequence keeps the car in clear air, on tyres in their working range, with the right driver in the car at the right time.

Deciding before the race, not during it

The decision to stop is made in seconds, usually with incomplete information. It is made well when the thinking has been done beforehand.

• Estimate tyre delta, warm-up and degradation from long runs in practice
• Work out the gap at which an undercut succeeds, and the gap at which it does not
• Check where the car would rejoin at each likely stop lap
• Agree the triggers in advance: if the gap is below this figure on this lap, we stop

With those figures on the pit wall, the call becomes a comparison between a number on the timing screen and a number agreed before the start. That is a far easier decision to get right than one made from instinct with a car approaching the pit entry.