The kerb is part of the corner: why exit kerbs change a lap more than apex kerbs

Ask a casual viewer where the kerbs are at any given circuit and they will describe the apex kerbs first — the painted red and white strips on the inside of a corner that drivers attack and clip and sometimes vault over. Apex kerbs are the visible feature of a corner. They photograph well. They appear in every onboard. They are, on most modern circuits, the part of the kerb infrastructure that gets the most attention from designers and the most coverage from broadcast.

They are also the kerbs that matter least to the lap time.

The kerbs that decide the lap are at the exit of the corner, on the outside, where the car is at maximum throttle and maximum tyre stress and lowest available margin for error. Exit kerbs are usually less visually prominent than apex kerbs. They are sometimes flatter, sometimes painted differently, sometimes installed as part of the runoff infrastructure rather than the corner itself. But the lap is decided on the exit kerbs, and a circuit designer who treats them as a finishing detail is making the same mistake as an architect who treats the foundations as a finishing detail.

The two jobs a kerb is doing

To understand why exit kerbs matter more, it helps to be clear about what a kerb is for. There are two distinct jobs, and they are often confused.

The first job is to extend the racing surface beyond the painted edge of the track. A kerb that is flat enough, smooth enough, and grippy enough is essentially additional asphalt that the car can use under controlled conditions. The driver can put two wheels onto it, or in extreme cases all four, and use the extra width to take a wider radius through the corner. This is the apex kerb's job: it lets the driver clip the inside of the corner more aggressively, taking a tighter geometric line than the painted track surface would otherwise allow.

The second job is to penalise the driver who goes beyond what the corner permits. A kerb that is taller, more aggressive, or made of a different material — the so-called sausage kerbs, or the serrated kerbs at the exit of fast corners — exists to make sure that running wide carries a cost. The driver who uses too much of the exit kerb feels the car unsettle. The driver who tries to use the runoff beyond it gets bounced. The kerb is acting as a soft barrier, in the engineering sense: a feature that admits gradual contact but enforces a steeply increasing penalty.

These two functions are in tension. A kerb that is welcoming and flat — good at job one — is bad at job two, because the driver can extend the racing surface indefinitely without penalty, which over time produces track-limit disputes and renders the corner's geometry undefined. A kerb that is aggressive and tall — good at job two — is bad at job one, because the driver cannot use it as part of the racing line and the corner effectively narrows to its painted width.

The resolution to this tension is that the two kerbs should be different. Apex kerbs should be flat, smooth, and welcoming, because their job is to extend the racing surface. Exit kerbs should be progressive, with a flat painted section nearest the track for the racing line and a more aggressive section beyond it for the penalty. This is not an aesthetic decision. It is a design decision about where on the corner the lap is won or lost.

Where the lap actually gets won

Consider what is happening physically at each point in a corner. On entry, the driver is braking heavily, downshifting, beginning to turn in. The car is decelerating and the tyres are sharing their grip between braking and lateral cornering. Lap time at this point is determined by braking distance, braking pressure, and the speed at which the driver releases the brake to begin trail-braking. The kerbs play almost no role here, because the driver is not yet at the edge of the track laterally.

At the apex, the driver has rolled off the brake and is at the minimum speed of the corner. The car is at maximum lateral load. The line through the apex is the geometric optimum — the point of maximum curvature — and the driver has the option to extend the racing surface inward by using the apex kerb. This produces a marginally larger effective corner radius, a marginally higher minimum speed, and a marginal lap time improvement. The improvement is real, but it is small. The car is at minimum speed; the time spent at this point is short; the gain from a slightly tighter apex is in the range of a few hundredths of a second per corner.

At the exit, the situation is completely different. The driver is opening the steering, increasing throttle, and accelerating away from the corner. The acceleration phase is long — typically two to four seconds, depending on the corner type — and the speed at which the car exits determines the speed at every point on the following straight. A 5 km/h advantage at the exit of a medium-speed corner is a 5 km/h advantage all the way to the next braking zone. On a 600-metre straight, that is roughly a tenth of a second. On a 1,200-metre straight, it is two tenths.

The exit speed is determined by how aggressively the driver can use the throttle on exit, which is determined by how much grip the rear tyres have, which is determined by the line the car is on — and the line is determined by the available width on the exit. If the exit width includes a flat, useful kerb that the driver can run over with two wheels, the effective corner exits much wider than the painted track does. The car can be straighter sooner, the throttle can be applied earlier, and the entire following straight benefits.

This is why exit kerbs change the lap. They extend the corner exit beyond the painted edge. They allow the racing line to be straighter at the point where the throttle is being applied. They turn a corner into a corner-plus-launchpad, where the launchpad is the most lap-time-relevant section.

Cases where the principle is visible

The principle becomes obvious as soon as you start looking at specific corners. Take the exit of Pouhon at Spa-Francorchamps — a fast double-left that drops downhill toward the Fagnes complex. The exit of the second part of Pouhon has a long, flat kerb on the outside that drivers run aggressively on every lap. The kerb is not a decoration. It is the corner's exit. The painted track ends somewhere around 9 metres of useful width; the kerb extends it to roughly 13. The cars that take Pouhon well are the cars that use the full extent of the exit kerb on every lap, and the time lost by a driver who fails to use it is in the range of two-tenths per lap — a margin that compounds quickly over a 44-lap race.

Turn 1 at the Circuit of the Americas is another instructive case. The corner is famous for its dramatic uphill approach and 35-metre climb to the apex. What gets less attention is the exit — a long, gradually-curving downhill descent toward Turn 2 where the exit kerb is critical. The exit kerb at COTA T1 is one of the longest in modern Formula 1, extending for nearly 100 metres along the outside of the corner. Drivers begin using it at the moment of maximum lateral load and continue using it well into the run toward T2. The kerb is, in effect, part of the racing surface for a substantial fraction of the lap.

The reverse case — what happens when exit kerbs are designed badly — is visible at corners where the kerb has been replaced with a sausage kerb after a safety incident. The sausage kerb solves a specific problem: drivers were using too much runoff and risking pickup of debris, or in extreme cases launching their cars on the way back onto the track. But the sausage kerb's solution is to penalise the use of the exit. Drivers stop using the kerb at all, which means the effective racing surface narrows back to the painted track, which means the corner exits more slowly, which means lap times go up. The corner is safer but slower, and the cars are doing less of the racing the corner was designed to produce.

The 130R exit at Suzuka was modified after Allan McNish's 2002 crash by a redesign of the runoff and the exit kerb. The corner is still fast — taken essentially flat in a modern F1 car — but the kerb arrangement determines how much of that flat-out commitment is rewarded. A flatter kerb encourages a wider line and a higher minimum speed; a more aggressive kerb forces a tighter line and a lower minimum speed. The trade-off has been adjusted multiple times over the years, and each adjustment has changed the lap time at 130R by detectable amounts.

The corollary: which corners deserve exit kerbs

Not every corner needs an exit kerb. Slow corners benefit less from exit width because the acceleration phase is shorter and the speed differential into the next braking zone is smaller. A hairpin at the end of a short straight gains very little from an aggressive exit kerb, because the car will be braking again before the exit speed has paid off. The acceleration phase is too short for the kerb's contribution to matter.

The corners that most benefit from exit kerbs are medium-fast corners leading onto long straights. The exit of Maggotts-Becketts at Silverstone, leading onto the Hangar Straight, is a corner where every centimetre of exit width converts directly into top speed at the end of a 770-metre run. The exit of Eau Rouge-Raidillon, leading onto the Kemmel Straight, is another. The exit of Turn 9 at Suzuka, the second half of the Esses, leading onto the run to the Dunlop Curve. Every one of these corners has a kerb arrangement that has been carefully tuned because the lap time at the next braking zone depends on it.

The corollary for circuit designers is that exit kerb placement should not be uniform around the circuit. The kerbs at the slow corners can be aggressive and short — their job is mostly to penalise track limits. The kerbs at the medium-fast corners leading onto long straights should be flat, long, and welcoming — their job is to extend the corner exit. The kerbs at the fast corners that don't lead onto major straights should be somewhere in between. Treating the kerb design as a uniform decoration applied around the whole lap misses the opportunity to use kerb placement as part of the corner-by-corner geometry.

What a designer should do

Most circuit-design tools — and most circuit-design discussions — treat kerbs as a finishing detail applied after the layout is decided. The layout is sketched, the corners are placed, the widths are set, and then someone adds kerbs as a graphic overlay. This is a mistake. The kerb is part of the corner. A 12-metre track with a 4-metre flat exit kerb is, for racing purposes, a 16-metre track. A 12-metre track with a 1-metre serrated exit kerb is a 12-metre track. The designer who treats those two cases as equivalent has missed something material about how the corner will race.

When you design a corner, decide first what the corner is for. Is it a setup for an overtaking move on the following straight? Then the exit width matters, and the kerb should be flat and extensive. Is it a slow corner where track limits need policing? Then the exit kerb should be short and aggressive. Is it a fast corner where the racing line is essentially fixed? Then the kerb arrangement can be minimal, because the cars aren't going to use the exit width anyway.

When you trace a corner in RaceTrackDesigner and place kerbs along it, ask the same question the real designer should be asking: what is this kerb doing? Extending the racing line, or enforcing it? The answer should differ from one corner to the next — and the corner-by-corner variation in kerb design is one of the things that separates a thoughtfully-designed circuit from one that was laid out by template.

The apex kerb is the photograph. The exit kerb is the lap. Designers who keep that distinction in mind produce circuits that race better than the ones that don't, and the difference shows up exactly where you would expect it to show up: in the speed at the next braking zone.