| For part 2 in this Car Control Fundamentals lesson series, we’ll explore the physics of driving a vehicle at the Understeer limit. We’ll learn different ways that a driver might try to sense the limit as well as how they can test for the limit by using driver inputs to check the vehicle’s response. We’ll then explore this testing process further by looking at an understeer training exercise from the Academy lesson series. Finally, we’ll finish up by learning how driving smoothly is achieved in different ways as a vehicle approaches and then reaches the limit. |  1. The Limit Defined 2. The Understeer Limit 3. Slip Angle & Rotation 4. The Oversteer Limit 5. Load Transfer |
How a Driver Senses the Limit
In the Racing Line Fundamentals lessons, we looked at several examples where a car was always perfectly at its 1.2 g limit. While using a vehicle’s theoretical optimum is useful when learning how the lateral vs longitudinal forces determine the ideal line, now we’ll begin looking at how a real world driver can work toward getting as close to this ideal as possible by examining the limit from their perspective.
In the Racing Line Fundamentals lessons, we looked at several examples where a car was always perfectly at its 1.2 g limit. While using a vehicle’s theoretical optimum is useful when learning how the lateral vs longitudinal forces determine the ideal line, now we’ll begin looking at how a real world driver can work toward getting as close to this ideal as possible by examining the limit from their perspective.
We use the term cue to describe all the different types of information that a driver has available, and there are several cues that a driver might try to use to sense the limit. For this lesson, we will primarily focus on the driving cues related to the understeer limit where the vehicle has reached the limit of the front tire pair first, leaving at least some capacity at the rear.
| The first driving cue we will discuss is tire noise. As a typical street tire approaches the limit, it will often start to produce a squealing sound, but this will vary from tire to tire and under different conditions. The tread pattern, carcass, surface, etc… all combine to produce different sounds. In comparison however, most track tires with little to no tread often make almost no discernable sound until well past the limit. So while the tire noise cue is typically not very useful for track-oriented tires, it can sometimes be helpful for a driver on street tires, as hearing tire squeal means they are at least somewhere near the limit. It doesn’t offer precise enough information to Identify when the tire actually reaches the limit however. While there is often a change in pitch as more and more of the contact patch starts to slide, a tire doesn’t produce a specific sound at the point it reaches maximum force. We’ll take a deeper look at the contact patch and tire slip in the next lesson. |  Street Tire  Track Tire |
The next driving cue we will look at is steering wheel forces. A well-designed steering system allows the forces at the front tires to be felt through the steering wheel, and these forces are directly proportional to the amount and direction of force at the contact patch in relation to the steering axis. The result is that as a driver starts to turn and lateral forces at the front tires begin building, there will be a relatively linear increase in resistance felt through the steering wheel. Then however, depending on steering geometry, this force may begin to taper and drop as the front tires near the limit and more and more of the rear of the contact patch begins to slide. As with tire noise, while in certain circumstances this steering force drop can allow a driver to identify that they are somewhere near the limit, it doesn’t offer precise enough feedback to determine when they actually reach the understeer limit. For a deeper look at steering wheel forces and their role in car control, see our article here.
The last driving cue we will discuss is vehicle movement, which includes both position on track as well as rotation. A driver primarily senses this movement through vision, but also by the forces felt through their body. While this movement cue is the primary way that a driver senses the limit, it’s important to understand that, on its own, sensing vehicle movement will not be enough.
| For example, even if a very skilled driver were riding as a passenger around a skidpad, they would be unable to determine if the car was precisely at the limit or not. While as a passenger they would be able to see and feel the movement of the vehicle just as well as the driver, only by gauging how this movement is responding to input changes can someone accurately find the limit. This process is called testing for the limit. |  A skidpad is a special circular track used for vehicle testing and training. |
How a Driver can Test for the Understeer Limit
To test for the limit, a driver makes an input change while checking the vehicle’s response. Continuing with the skidpad scenario as an example, in order to test for the understeer limit, the driver could increase steering while paying attention to the vehicle’s movement. If the front tires respond to increased steering by moving inward and tightening the vehicle’s line, then it was not yet at the understeer limit, as the front tires still had some capacity remaining. If on the other hand, the vehicle did not tighten its line, then the front tires had already reached the limit and continuing past this will increase induced drag, causing it to slow down. In order to avoid this, the driver would need to reduce steering until the front tires just start to move outward again, indicating they are once again below the limit. At this point, the driver can test for the limit again by increasing steering once more.
To test for the limit, a driver makes an input change while checking the vehicle’s response. Continuing with the skidpad scenario as an example, in order to test for the understeer limit, the driver could increase steering while paying attention to the vehicle’s movement. If the front tires respond to increased steering by moving inward and tightening the vehicle’s line, then it was not yet at the understeer limit, as the front tires still had some capacity remaining. If on the other hand, the vehicle did not tighten its line, then the front tires had already reached the limit and continuing past this will increase induced drag, causing it to slow down. In order to avoid this, the driver would need to reduce steering until the front tires just start to move outward again, indicating they are once again below the limit. At this point, the driver can test for the limit again by increasing steering once more.
The end result of this testing procedure is that driving at the understeer limit becomes a process of repeatedly going past and then dropping below the limit as a driver constantly searches for the point the front tires just barely stop increasing force. The reason this search must be constant is that the point of maximum force is not static and even a tire-testing machine programmed to keep a tire at the limit in a controlled environment produces some movement back and forth across the peak. Below is a video demonstration of a car at the understeer limit from an exercise in the first Academy lesson. There is purposefully no sound in the video as a primary goal of this exercise is to focus on vehicle movement, so all sound and force feedback are turned off.
In the video, notice the back and forth steering wheel motions used to keep the vehicle as close to the understeer limit as possible. Also notice the constant throttle (green bar) movement as well however, because other driver inputs are also part of testing for the limit. In this case, if more throttle increases speed but the vehicle is able to maintain its current line, then it is not at the understeer limit yet. Likewise, if increased speed forces the vehicle onto a wider line, then it was already at the understeer limit. A driver will typically use a combination of two inputs to test for the limit depending on which portion of a corner they are in with steering and throttle being used in combination during corner exit, and steering and brakes during corner entry. Although we’ve primarily been discussing the understeer limit in this lesson, understand that these same type of driver input testing motions are also used to sense the oversteer limit as well, and this will be covered in an upcoming lesson.
Lastly, keep in mind that although it’s useful to learn how this input testing process works as a driver will then understand the exact mechanisms behind driving at the limit, this needs to primarily become a subconscious action. For instance, it’s important to realize that the steering and throttle inputs seen in the skidpad exercise video are not individual purposeful testing motions. The goal of that exercise is to maximize speed around a skidpad and this requires a driver push themself increasingly closer to the understeer limit in order to improve their times. At a certain level this causes them to start naturally developing these testing motions as their sensitivity to changes in vehicle movement improves. When introducing this exercise to more experienced, higher-level drivers, many were not aware this input testing process was taking place as they drove, despite already doing it quite well.
How to Drive Smoothly at the Limit
Being smooth is an important element of car control, but a driver must achieve this in different ways as a vehicle approaches and then reaches the limit. For example, although in the skidpad exercise video we see constant steering wheel movements, the associated changes in vehicle movement are subtle and can be hard to see.
Being smooth is an important element of car control, but a driver must achieve this in different ways as a vehicle approaches and then reaches the limit. For example, although in the skidpad exercise video we see constant steering wheel movements, the associated changes in vehicle movement are subtle and can be hard to see.
| If a driver were below the limit however, those same back and forth steering motions would cause the car to move side to side noticeably. The reason for this difference is the progressive nature of tire response. This diagram shows how a typical tire has an initial linear increase in force, but then begins to taper off to the peak before steadily dropping. The result of this is that a steering wheel movement done below the limit in the linear and early transitional area has a much greater effect on tire force and therefore vehicle movement then that same steering movement done near the limit. |  |
This is why those new to performance driving are often taught to focus on smoothing out their inputs, as they are not near the limit yet and keeping the vehicle stable as they start to experiment with faster speeds still requires relatively steady inputs, just as it does in regular street driving. Once a driver starts approaching the limit however, a continued focus on smooth inputs can become counterproductive because it inhibits testing for the limit. As another part of the Academy lesson discussed earlier, drivers are also asked to complete the same skidpad exercise as before, except they should instead try to keep their inputs as smooth as possible. You can see a demonstration of this alternate exercise in the video below.
In the video, we can see how not only the car’s average speed is lower because the ability to test for the limit has been reduced, but it also tends to wander more because corrections can’t be made as quickly as needed. In order to keep their tires producing maximum force, a driver must constantly test and correct by making appropriate steering, throttle, or brake input changes and those changes often need to be made very quickly in order to keep a vehicle stable at the limit.
| Up Next - Slip Angle Explained I hope you enjoyed this second installment in the Car Control Fundamentals lesson series, and if you have any questions, please use the comments section below. Up next, we’ll move on to take a closer look at slip angle and how it relates to vehicle rotation. After that, we’ll dive into oversteer and then finish up our final lesson by learning about the role load transfer plays in car control. If you are interested in a complete guide to the physics of racing, we also offer The Science of Speed book series, available through our bookstore or at popular retailers such as Amazon. Adam Brouillard |  1. The Limit Defined 2. The Understeer Limit 3. Slip Angle & Rotation 4. The Oversteer Limit 5. Load Transfer |
