Overlays are an excellent way to visualize what a driver is doing while they’re out on track. Sensors are outfitted on the car to record each of the driver’s primary inputs; channels tracking the angle of the steering wheel, the selected gear, the throttle percentage, and brake pressure are sent through the airwaves from the car on track to the engineers in pit lane, all in live time.

Together with sensors that record car speed, distance, and time, a graph can be created to show the driver’s inputs over the course of a full lap that reads from left to right like a picture book. These overlays can be incredibly useful, and are used in all levels of motorsport from karting to IndyCar.

In recent years, broadcasters have begun showing such overlays to help describe to fans what drivers are doing inside the car to try and extract as much lap time as possible. But while it may look like a series of squiggly lines to inexperienced eyes, data like this can actually offer valuable insight to drivers, engineers, and of course fans about the relative performance between two cars. A special thank you to Arrow McLaren for supplying this overlay from a test day at Sonoma a few seasons ago. We’ll call the blue trace Driver A, and the red one Driver B.

These overlays can highlight where time is being lost or gained when comparing multiple laps that a driver has done, or when comparing fastest laps between a driver against their teammate, as shown here. Overlays can be used by engineers to help coach drivers, determine the balance characteristics of a setup, or offer a real-life target for simulation results to match in order to develop mathematical models. Drivers and engineers are constantly going through these overlays corner by corner to see where the most performance can be gained. So important are these overlays that drivers will even have a tablet with them when they sit in the cockpit between outings so they can review the data and ensure they are making the most out of the precious track time they have in the buildup to a race. There is a lot of information to be learned from even the most basic of overlays.

T11: Understeer to oversteer

One of the easiest ways to identify the balance of a car is to look at the steering trace through a corner. Naturally, a car that tends to understeer will require the driver to add more steering in order to compensate. A car with oversteer will require less steering, but it is common for the driver to have to make corrections with the steering wheel as they deal with the instability. In T11, Driver A looks to have understeer at the mid corner and then oversteer from mid to exit when compared with Driver B. This is actually a very common behavior, typically referred to as “understeer to oversteer.”

In this situation, the root issue tends to be the initial bout of understeer. This is because at the apex, Driver A has much more steering lock than Driver B while he’s trying to get the car to turn. At this point, he tries to add comparable throttle to Driver B, but he still has a lot more steering lock. The result is that the rear steps out, caused by trying to do too much turning and accelerating at the same time (this is actually the same rear sliding that happens when drivers intentionally do donuts). Because the back of the car steps out, Driver A has to make two big corrections, both to the detriment of lap time. He enters the corner 0.04s ahead of Driver B but exits the corner about 0.06s behind.

The cause of understeer can be any number of things, and it’s up to the engineer to determine the best cure. Engineers will have access to a whole host of other channels that can give further information on the car’s balance, and these can help determine whether a change to the aerodynamics, mechanical setup, or driving style is the best way forward. Drivers are able to induce understeer into the car with how they drive: they can carry too much speed at the entry, delay turning in initially before feeding in a large amount of steering angle, pick up the throttle too early mid-corner, or a whole host of other techniques. However, in T11 the understeer does not appear to be driver induced, as Driver A actually brakes earlier than Driver B, has a slower minimum speed at the apex, and a similar steering trace on initial turn-in. From this quick glimpse, it would appear as though Driver A is dealing with an understeer balance.

T2: Entry vs exit

Through T2, the most noticeable difference between Driver A and Driver B is where they are achieving their respective minimum speeds. In this regard, the two drivers have taken different approaches compared to one another. Essentially, Driver B has slowed down to his minimum speed sooner in order to get back to throttle earlier, while Driver A carries more speed deeper into the corner but is later in getting back to throttle. The result for Driver B: trading a time loss on entry for a time gain on exit compared to Driver A.

Deciding this tradeoff comes down to how much time is being lost in one phase of the corner versus how much time is then made back up later on. In this example, Driver B loses about 0.10s from entry to mid-corner (going from 0.06s ahead to 0.05s behind), but because he is able to get back to throttle sooner, he gains 0.21s from mid corner to exit — a net gain overall.

A decision like this tradeoff shown has to be made for every corner on the track, and is highly influenced by what lies beyond the exit of the corner at hand. For a corner leading onto a long straightaway, prioritizing the exit by getting to throttle earlier is critical for overall lap time. For a corner that leads directly into another corner (or a series of corners), compromises have to be made somewhere on braking, throttle, and racing line. Doing so will inevitably prioritize the entries and/or exits of certain corners over others, but when going through driver overlay data, finding the compromise that minimizes overall lap time will always be the priority for the driver.

T3: Rolling out of the throttle

T3 is a two-part corner with practically no braking, just changes to throttle input and steering through a left- and then right-hand section. Once again, Driver A and Driver B have taken different approaches based on the balance of their respective cars. Driver A lifts slightly later, but more drastically while Driver B gradually lifts as he tries to carry more speed through the entry. Both are having to play with various amounts of throttle all throughout the corner; Driver B doing a single late lift while Driver A does two smaller lifts before getting back to throttle earlier.

Through this section, Driver B is gaining all of his time through the left-hand portion before he begins to fully lift out of the throttle. Driver B’s speed trace up until that point shows just how much more entry speed he carries through the left-hander. Despite Driver A getting back to throttle earlier, Driver B has carried enough speed that by the time they both return to full throttle, they have a similar minimum speed. Therefore, Driver B gains 0.08s on entry and loses none on exit.

It’s also evident once again that Driver A has more understeer through the right-hand portion of the corner, as he has to add more steering than Driver B to get around the same corner despite similar steering through the left-hand portion. This may also explain why Driver A is unable to carry the same amount of speed through the left-hand portion: as explained earlier, carrying too much speed on entry induces mid corner understeer, and if Driver A is already at the limit for his setup then carrying any more speed would cause him to understeer off the track.

T4: Brake shape

The primary difference between Driver A and Driver B in T4 is their brake trace. Both drivers achieve the same minimum speed at the same point, but the brake trace highlights an interesting difference. Driver A’s peak brake spike happens earlier than Driver B’s and is notably less pressure; vitally he’s able to begin releasing the brake sooner. As a consequence, Driver A gains 0.07s on entry, and then he keeps that time from mid-corner to exit by getting to throttle sooner as well.

This can seem counterintuitive to those who think drivers are always trying to brake as late and as hard possible. In reality, the timing of brake application and the peak brake pressure are only part of the equation. Brake shapes can be “peaky,” as Driver B has done in T6, or more “mounded,” as Driver A has done. In either case, once peak brake pressure is reached, there is a huge amount of lap time to be gained in releasing the brake as quickly as possible. This is because combined entry, the phase of the corner when the driver is both slowing down and turning in, is critical for lap time. As each tire only has a finite amount of grip to be allocated to either turning or decelerating, getting off the brakes allows the driver to add steering, and eventually get around the corner faster.

T6: Driving around a balance

While the time delta through T6 is negligible between the two drivers, it is an interesting case study in how there are different ways to achieve the same lap time. From the steering trace, it is a reasonable assumption that the drivers took a similar racing line through T6, the long duration left-hander. However, their speed, brake, and throttle traces suggest that not everything was done the same. Driver A brakes about 50 feet earlier than Driver B but gets off the brake about 40 feet earlier. Driver A is also able to continually feed in throttle thanks to his lower entry speed, while Driver B is having to play with the pedal as he battles to get back to full throttle.

The result is that Driver A gives up about 0.12s on entry but then gains it all back on exit. This is an excellent example of how two different driving styles can achieve the same result with two different car balances. Driver A, who has been shown to be dealing with an understeering car, has prioritized the exit heavily in T6 while Driver B has tried to repeat the trick from T3 by carrying a large amount of entry speed to the mid corner while still matching Driver A from mid to exit. It doesn’t exactly work out, as Driver B has carried so much entry speed that he has to stay at his minimum speed for longer in order to keep it on the track. By the time he’s able to get to full throttle, Driver A is almost 4 mph faster and carries that advantage for the rest of the straight away, which is quite long.

In the end, the two approaches cancel each other out, so how do drivers determine which technique is best to use?

When there is more than one way to achieve the same lap time, the approach that drivers choose will often be the option that wins in some secondary comparison: maybe one approach uses less fuel or saves the tires better. In this case, I would hazard a guess that Driver A has chosen his approach as a way of driving around the understeer in the car. He may not be able to carry the same entry speed that Driver B can through such a long-duration corner, but by putting more priority on the exits he is able to nullify any losses the understeer may have caused him if he tried to drive the car the same way that Driver B has.

T7: Gear selection

Gear selection between the two drivers has been identical up until this point. In T7 however, Driver B downshifts to first gear while Driver A only goes down to second gear. Lower gears offer more torque, meaning better acceleration, but they are also more susceptible to wheelspin. In order to get down to first gear, Driver B is forced to slow down his car much more than Driver A, as evidence by the 2.6 mph speed difference between their minimum speeds. This leads to a massive time gain for Driver A through T7; he gains 0.20s from entry to the mid-corner.

Another consequence of choosing a lower gear is that Driver B is struggling to put the power down on corner exit. The gear is so “buzzy” that Driver B is breaking traction — so much so that he actually has to come back off the throttle, which kills his momentum through the subsequent section. Compare this with Driver A who, by choosing second gear, may not accelerate as quickly, but he is able to keep from over-slowing and can return to full throttle almost immediately, gaining another 0.05s from mid to exit.

Ultimately, Driver A finishes the lap 0.09s faster than Driver B. Knowing the competitive nature of drivers, you can bet they both immediately went to the data to see where they were faster and where they could be better, all in the name of improving for the next outing. No driver is above seeing a teammate take a different approach to a corner, discover in the data that it’s faster, then use that approach themselves to set a better lap time.

In truth, this is just a small glimpse of the data that is available to drivers and engineers. Each IndyCar is mounted with hundreds if not thousands of sensors, all sending different channels of data back to pit lane for the engineers to analyze as quickly as they can. Data can range from onboard video (a much better way to compare racing line when coaching drivers) to suspension travels to engine outputs to aerodynamic loads.

The rate at which IndyCar teams can collect and analyze data during a session is an incredibly impressive achievement, one that unfortunately is hidden away from fans in the name of maintaining a competitive advantage. Still, there is so much that can be learned from just a handful of channels. Taking the best sections from these two drivers would give a lap time that is 0.28s quicker than the lap Driver A did. There is always more performance to be found, and so drivers and engineers will always be striving to go quicker.