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In many motor sports, there are often curbs placed along the parts of the track where the driving line runs close to the edge of the track. This allows the driver to drift off the track slightly when necessary, without hitting the gravel / grass and slowing down.

What is the purpose of this, as opposed to just keeping the original track and expecting drivers to remain within it -- or indeed widening the track at the points where the curbs are?

My initial thoughts are that the curbs exist to allow the driver a little extra room if he accidentally runs too tight or wide on a corner -- and instead of slowing down heavily due to gravel / grass, he only slows down a little on the curb. However, from my experience, most drivers run over the curbs on all of the bends anyway, and intentionally so, to make the most of this extra room. Therefore, it is not extra room for "mistakes" -- any mistake will still cause the driver to run into the gravel / grass, because the driver has changed his interpretation of the track width and still runs right to the edge of the curbs.

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You asked

Why are curbs used in motorsport?

Short Answer

Curbs are used primarily as boundaries in motorsports.

Background

There are several different types of curbs depending the circuit as well as the highest paying series that is going to leverage the circuit as a venue for one of it's events.

The FIA or Fédération Internationale de l'Automobile is governance body that most international motorsports adheres to. A race series will often look to the FIA as a sanctioning body for fees.

Part of the FIA's responsibilities to the series include circuit adherence to specific rules that define safety for drivers, cars, racing circuits and emergency response personal.

Circuits are graded by the FIA. The grading is associated with particular types of racing. A lower grade may have safety standards that comply with the needs of a GT car. Licenses Grade 1 applies to Formula 1 racing which requires higher safety standards due to the extreme speeds.

Longer Answer

Kerbs are primarily considered a component of circuit safety. Kerbs have many different designs. Some are known for their height and others for their width as well as placement. Many are designed to slow a driver down. Exit kerbs will frequently have a washboard design reduce adhesion should a driver place a wheel on the surface. This helps to ensure drivers will stay within a boundary that is designated.

The kerb will serve as a buffer between the track surface and the adjoining surroundings. Drivers exceeding the boundaries can damage their vehicles on some kerbing.

Kerbing design of particular corners can define that corner for decades creating awe at how drivers of the years approach the corner and deal with kerbing. Some corners known to destroy components and others are known for their smooth transition allowing drivers to use the area to their benefit.

Controversy

Particular kerbs at certain circuits have controversy surrounding them. This is not uncommon. In this case former Ferrari driver Felipe Massa destroyed part of his right front suspension hitting a kerb which resulted in a crash during his qualifying for the race.

A quote from this article states

The barriers have also been moved back over the past two decades to give drivers more breathing room if they get the corner wrong.

However, organisers have recently installed a bunch of speed humps in the run-off area on the inside of the Raidillon curve to deter drivers from constantly trying to straighten out the corner too much.

This is the result of the change to the kerbing. Needless to say this was changed before the next race.

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I read a post by Ross Bentley in which he outlines a few points about the use of curbs in motorsport. He mentions 3 types of curbs:
* one on the edge of the track at the entrance to a corner (many tracks don’t have these)
* apex curbs
* exit curbing

Apex curbs are most often meant to be used. If they weren’t there, drivers would drive through the dirt inside the corner to increase the radius of the turn as much as possible (straightening the corner out). Over time, it would create a rut, break away the edge of the pavement, and increase maintenance costs. So they build curbing in the area where drivers place their car’s inside tires.

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    But what is to stop a driver driving through the dirt inside the curb? – Karnivaurus Jan 12 '16 at 17:39
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    Rules in most race series dictate that all 4 wheels must remain within the boundaries of the racing circuit. – Citizen Jan 22 '16 at 1:16
  • It's race dependent I would suppose as well as @Citizen mentions. – whyzar Jan 22 '16 at 16:53
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    @Citizen but if there's a rule to prevent drivers from going outside the track like that, why couldn't this same rule apply to a corner without apex curb? If they touched the dirt in that case, they'd be punished similarly to "not having all 4 wheels on the track". – julealgon Jul 7 '18 at 14:12
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In addition to these brilliant answers, I would like to add a few other benefits/uses of kerbs.

One benefit is that it gives the drivers an easy opportunity to get a feel for the width of their vehicle. For example, the first time a racing driver gets behind the wheel of their car, they will often have to sit very low in the cockpit and may not be able to easily judge where the edges of their car is. Just touching the kerb slightly will cause a vibration in the vehicle, and the driver will be able to instantly know within inches how far they are from the edge of the track.

Of course, drivers will still be able to judge their vehicle width even without kerbs, but this just makes it a lot easier and quicker to learn. An example of where some drivers weren't comfortable without kerbs was the Race of Champions in 2019. In this event, the drivers were required to swap vehicles (of vastly different widths and driving positions) regularly and drive on a very narrow track. Some drivers said this was made a lot more difficult by the lack of kerbs on the track, which made it harder to judge how close they were to the barrier.

Another key benefit of kerbs are that they give the drivers some margin for error. Let's take the apex kerb as an example. Accurately hitting the apex (within inches) lap after lap is a very tough challenge and there are various reasons why a driver may end up going tighter than the apex (e.g. over-steer, being squeezed by another driver on the outside etc.). The kerbs allow you to go a few feet tighter when necessary, with the only impacts usually being a slightly more uncomfortable ride and/or a slight loss of lap time.

One final benefit is fairly simple - the kerbs can be used as a reference for the drivers. On many modern circuits, there is an awful lot of run-off area, and therefore without kerbs, the only reference for corners would be the white lines. Spotting the apex would then become a big challenge, as the track and run-off would merge into one and the white lines would only become noticeable when very close to the corner. Whereas, a kerb is usually painted in distinct colours and most importantly is sat above the surface of the circuit. Therefore, it can usually be seen by the drivers even when a few hundred metres away meaning they can judge their turn in points much more easily.

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