Published: May 23, 2013; updated: September 8, 2018
Today we are going to talk about cutting curbs: why do it, when to do it, and just as importantly, when not to do it.
Basic Tradeoffs - Apex Curbs
Normally, cutting inside curb in a corner flattens the corner. Therefore, the turn can be taken faster in theory.
The flip side of curb cutting is it is harder on suspension parts and it can unsettle the car to the point where the benefit of a flatter line is negated.
Cutting the apex curb increases the effective radius of the corner. As such, there is less lateral grip required to go through the corner at the same speed. Or, more usefully, given the same amount of grip the corner can be driven through faster.
How much faster is it to use curbs than not? This naturally depends on the track. On the rather extreme end of the spectrum, I found that in a stock-powered Miata using all possible curbs at Road Atlanta (6 total, 4 apex curbs and 2 exit curbs) is worth about 2 seconds/lap on a 1:50 lap. This is with driving both curb and curb-free lines with the same, moderate aggression level at roughly 9/10ths pace. Applying maximum aggression to the curb line and increasing the pace to 10/10ths lowers the laptime by a further 2 seconds - this gain is not possible if one restricts oneself to pavement only, because being aggressive inevitably results in the car ending up on curbs.
On more typical tracks that are not "curb-heavy" one can expect roughly half a second per lap per significant use of a curb, up to a full second per use for curbs in slow corners followed by long straights. Examples of corners that qualify:
- Barber turn 9 - a one second gain in turn 9 with the use of apex curb.
- Watkins Glen turn 1 - half a second gain due to using the apex curb, easily a full second gain in using the exit curb, especially in low power cars, because the speed advantage is carried over the enormously long esses and back straight.
- VIR turns 6-7 - half a second gain for using one or both apex curbs as necessary; up to a second gain in using the apex curb in turn 16b.
- NJMP Thunderbolt turn 3 without the chicane - half a second is possible by cutting most of the apex curb.
Driving over a curb provides audible and sensory feedback to the driver, letting the driver know that they used the entire track width in the given corner.
Novice and intermediate level drivers can rely on this feedback to validate car positioning in a corner. In other words, feeling the curb is how the driver knows that they are on the apex rather than 1 foot off of it.
Advanced level drivers are typically able, through experience, visually gauge where the car is on the track with much higher precision. They are also able to predict how much of a curb the car will need to use long before the car gets to said curb. Therefore, cutting curbs to check car position becomes unnecessary.
The stiffer a car's suspension, the harder the curbs are on it. Hard bumps and landings are tough on wheel bearings and hubs in particular, and can crack wheels of insufficient strength at the spokes.
Curbs themselves vary in height and profile; more vertical curbs are proportionally more likely to bend wheels, while curbs with sharp edges are similarly likely to puncture tires. Even curbs that feel flat often have defined square or triangle profile - Watkins Glen curbing in turn 1 as well as FIA curbing are perfect examples of curbs with abrupt edges.
Driving over the curbs themselves produces vibrations in the chassis which can loosen various fasteners, those in the exhaust system being fairly common victims.
When advanced drivers use curbs, they normally accept the additional wear that they put on the car, and normally only use curbs when necessary, such as to improve time on an already good lap.
Unsettling The Car
Because curbs are not flat, going up on them and then back down produces moments when the contact patch of tires going over the curb is reduced, ultimately to zero if the tires become airborne.
In most corners it is desirable to be on power from slightly before the apex. Eventually going over taller and sharper curbs requires the driver to lift when going over the curb to keep the car under control. Typically, lifting over curbs costs more time than flattening the corner saves. Look at it this way: a flatter line permits taking the corner faster, but if you are not accelerating, you end up driving slower. The rule of thumb, therefore, is this: if you lift solely because you drive over a curb, do not use the curb and instead accelerate. "The driver who spends the most time on throttle, is generally the fastest."
In momentum-type corners such as Road Atlanta turn 3, curbs can be cut to increase the speed of the car through the corner (specifically, minimum speed). Because it is quicker to carry more speed through the momentum-type corners than to accelerate, curbs can be used very aggressively to the point of the car launching into the air - as long as minimum speed keeps increasing. This makes curb usage in the mentioned Road Atlanta turn 3 extremely hard on the car and also extremely tempting, not to mention fun.
Using outside, also known as exit, curbs is typically an aggressive move, designed to obtain absolute best speed through the given corner. It is not advised for novice level drivers; intermediate level drivers can carefully experiment with using exit curbs if they wish to attain advanced level of driving.
Apex and exit corners affect the car differently because of where the weight is dynamically located:
In a corner weight transfers from inside to outside tires. Going over apex curbs further unloads the inside tires and loads the outside tires. As the line through the track is flattened, this typically means there is additional grip available, and weight transfer to the outside makes use of this added grip.
Also, the apex curbs are typically cut with front tires first, and if the outside front tire exceeds the grip limit in the process due to weight transfer the car will understeer back on track surface.
In contrast, when driving over the exit curbs it is the loaded tires that go from higher grip pavement to (typically) lower grip curb surface - often paint or concrete, with "staircase" type bumps. If the car is still turning and the outside rear tire experiences this loss of grip, that rear tire can slide right out which is likely to induce immediate spin of the car. A frequent consequence is the car hitting the inside wall nose first shortly thereafter. In many if not most cases of wall impacts at corner exits, the cause is dropping the outside rear tire off pavement.
If the car loses grip on outside front tire before outside rear, it will experience understeer. Since the car is already on the exit curb, any understeer at all will tend to put the car at least two wheels off track. If the driver attemps to keep the car on pavement by adding steering input, the situation described just above will play out with the rear outside tire going over curb while the car has steering input.
For these reasons, in novice groups the use of exit curbs is generally discouraged, and in intermediate groups the use of curbs is recommended with caution and in corners with ample runoff room.
The way to use exit curbs safely is to unwind steering way before getting to the curb, with the goal of the car being almost parallel to track surface by the time it goes on an exit curb. Then, if the car should exceed the grip available on the curb, it will continue traveling in a straight line and hopefully will be able return to paved surface without hitting anything.
In any event, aiming and planning ahead of time to use the exit curb is an extremely aggressive move. The curb is the runoff area; if the driver intends to use the curb for "normal" line through a corner, they are leaving no runoff in that corner, and overdriving it is expected to produce an immediate two off at least.
Widening The Track
In rare cases curbs extend all the way from one corner to the next. It is then possible to drive on the curbs to effectively make the track wider.
An example is NJMP Thunderbolt turns 1-2. The exit curb continues unbroken from the exit of turn 1 until the entry to turn 2, and driving on it can make the entry into turn 2 about 1.5 feet wider.
Another example is Sebring turn 7. Using both apex curbs in that turn effectively makes the track about 5 feet wider.
As with any intentional use of exit curbs, doing this is an aggressive move that should only be used by experienced drivers.
Window Of Lowest Time
Another reason to cut curbs is doing so provides a window, or a range, of lines on track that are almost identical in overall performance to the absolute fastest line.
All drivers have some variance in their lines. More skilled and experienced drivers have less variance than less skilled and experienced ones, but the variance is still there. Since the optimal line is usually adjacent to the curb, if a driver is never using curbs, they hit their optimal line on their best lap and on most of the laps they are off the optimal by not necessarily large, but almost always measurable amount. A driver using curbs ends up having the optimal line in the middle of their range of lines and, just as importantly, when they turn in a bit too early (and thus take more of the curb) this frequently has minimal cost compared to being away from the curb on the pavement. This dramatically increases the number of laps that are almost at the driver's best performance.