The racing line: turn-in, apex and exit.

The parts of a corner

Driver61 calls the racing line the fastest arc through a corner, and says the right one depends on how sharp the corner is, how long the straight after it is, and what car you are driving. It names four reference points:

Braking point

Where you start slowing for the corner.

Turn-in point

Where you start steering. Driver61 says your eyes should already be on the apex here.

Apex

Also called the clipping point: where the car comes closest to the inside edge, often marked by a curb.

Exit point

Where the car arrives back at the outside edge.

Wikipedia describes the usual shape as outside, inside, outside: the line uses the whole width of the track to make the turn's radius as large as possible, and the rubber left by many tires can show where it runs. HP Academy puts it the same way, as the largest-radius arc, which means less steering and more speed kept. It adds that there is no single apex position that works for every kind of corner.

Driver61 gives the trade-off at turn-in: turn in too late and you lose corner speed, turn in too early and you lose exit speed.

Two meanings of "apex"

Ross Bentley, writing with Chuck Tucker in Winding Road (2022), argues the word is used for two different things that do not always fall in the same place:

• The clipping point, where the car is nearest the inside. Bentley traces the term to Piero Taruffi's 1958 book The Technique of Motor Racing and suggests thinking of it as a zone rather than a spot.
• The minimum speed point, where you stop entering the corner and start exiting it. Bentley notes that Alan Johnson called this the "balancing point," where the car is balanced front to rear and has the most lateral grip. A corner can have two clipping points but only one minimum speed point.

Bentley's advice is to put the minimum speed point in the right place and let the clipping points land where they need to.

Geometric line versus late apex

Driver61 describes the geometric line as the widest possible arc, apexing at the middle of the turn, so that in theory speed could stay the same from turn-in to exit. Both Driver61 and HP Academy say this is the fastest way through one corner on its own. Driver61 adds that real circuits rarely let you treat a corner on its own, so the geometric line is usually not the quickest.

Driver61's ideal line turns in and apexes a little later, turns more sharply on the way to the apex, and leaves a straighter path from apex to exit so you can get back to the throttle sooner. Driver61 lists the gains (braking later, less time turning, full throttle sooner) and the costs (a tighter radius and a lower minimum speed). It also says drivers tend to chase entry speed at the expense of exit speed, and recommends patience: get the car turned and pointing out of the corner, then go to the throttle as early as you can.

Corner speed changes the balance. Driver61 says fast corners generally take earlier apexes, slow corners and hairpins later ones, and the two lines differ most in slow corners. Its illustration uses a car with a 150 mph top speed: leaving a 40 mph hairpin there is about 110 mph still to gain, so early throttle is worth a lot, while leaving a 100 mph corner there is only about 50 mph to gain. Wikipedia quotes a similar rule of thumb from Bentley: the faster the corner, the closer to the geometric line; the slower the corner, the later the apex.

The early apex

Wikipedia describes turning in early as a faster entry for a slower exit and says it is widely considered a beginner's mistake. HP Academy explains why: the car reaches the apex under-rotated, still pointing toward the outside, so it runs out of road and the throttle has to wait. HP Academy offers two clues for reviewing a corner afterward:

• If you did not need the full width of the track at the exit, the apex was probably late.
• If you ran out of road, needed lots of steering lock and could not get to the throttle, the apex was probably early.

Which corners matter most

Coaches sort corners by the straights before and after them. Two systems appear in the sources:

The PCA material classifies a corner by comparing the length of the acceleration zone after it with the braking zone before it. A longer acceleration zone means an exit-speed corner and a late apex; a longer braking zone means an entry-speed corner and an earlier apex; equal lengths mean a balanced corner. It says most corners on a typical track are Type I, suggests labeling each corner on a track map, and lists camber, elevation change and crests as features that change how a corner should be driven. The PCA region states that its techniques are advisory only.

Two cautions from coaches. Bentley notes that most drivers prioritize the corner onto the longest straight, but says drivers who work on other corners often beat them. HP Academy says the key corner is often a slow one followed by a long straight, but that the decision should come from data rather than a track map and a few laps. Without a logger, it suggests comparing your speed at a fixed point on the following straight as you try different approaches, while calling that a rough method.

A physics view, and a disagreement

Adam Brouillard of Paradigm Shift Driver Development treats the line as the result of getting the most total tire force pointed in the right direction. In his view the ideal apex depends on how much the car can accelerate compared with how hard it can corner: the more acceleration relative to grip, the later and slower the apex. A low-powered car with no grip to spare has to hold a near-constant-speed arc after the apex; a powerful car needs a later apex so its line can open up as it gains speed.

Brouillard offers a shortcut: if full throttle from the apex carries the car too wide, use a later, slower apex; if it does not reach the track edge, use an earlier, faster one, with limits for cars that are held back by wheelspin. He says running wide at the exit should be fixed with a later apex rather than less throttle. For corner entry he recommends a steadily tightening radius (an Euler spiral), with rear-brake karts as an exception. Wikipedia describes the Euler spiral as a common solution when the apex is mid-corner, close to best but not optimal in every case.

The sources disagree on straights. Brouillard says the length of the straights before and after a corner does not change its ideal line. Driver61 says the longer the following straight, the more an exit-focused line gains, and the PCA material uses straights to define corner types in the first place.

Corners in sequence

When corners come close together, the exit of one is the entry to the next. Wikipedia notes that a driver may give up the ideal line through the first corner to set up the last one for the straight that follows. HP Academy frames every corner as a trade: be as fast as possible at the exit without losing too much in the middle, since extra mid-corner speed reduces rotation and costs exit speed.

• Early apex in a sequence. KartClass says an early apex, though less common, can suit closely spaced corners because it opens up the entry to the next one, and advises choosing the apex that best sets up what follows.
• Double apex. HP Academy describes a "diamond" line for some double-apex corners: more speed early, then rotate the car for a straighter run past the second apex. Brouillard advises against accelerating and then slowing again between the two apexes.
• Chicanes. Brouillard says a later second apex implies a later first apex, and the changeover point should move toward whichever apex is tighter.

Finding the apexes at a new track

HP Academy suggests looking for the most heavily rubbered section of curb, walking the corner to see it from different angles, and watching onboard video of professional drivers. The PCA material suggests asking, for every corner, why the apex is where it is. More on this is on learning a track and data and video, and how the line changes in the rain is on wet weather.