| For part 1 in this Car Control Fundamentals lesson series, we’ll explore the physics of driving a vehicle at the limit. We’ll learn what it means to drive at the understeer and oversteer limit as well as why these terms can sometimes be confusing. We’ll also learn the difference between racing line errors and car control errors and why identifying the difference is important. |  1. The Limit Defined 2. The Understeer Limit 3. Slip Angle & Rotation 4. The Oversteer Limit 5. Load Transfer |
Before beginning this series, make sure to go through the first two lessons in the Racing Line Fundamentals series. Just as in the Science of Speed books where we first learned about standard corners in The Perfect Corner before moving on to car control in Perfect Control, and finally coming back to advanced corners in The Perfect Corner 2, we will do the same in our Racing Line and Car Control Fundamentals lessons.
The Physics of Driving at the Limit
To begin this lesson, we will first be examining the limit from the perspective of the blue car from The Ideal Apex lesson. You can see in the image below how the car is at the limit experiencing 1.2 g of lateral acceleration throughout corner exit. This is called the “limit” because it is the maximum lateral acceleration the vehicle’s tires are currently able to produce. Although in vehicle dynamics we often use the term lateral acceleration because it is to the side of a vehicle, in physics, the car is simply experiencing centripetal acceleration.
To begin this lesson, we will first be examining the limit from the perspective of the blue car from The Ideal Apex lesson. You can see in the image below how the car is at the limit experiencing 1.2 g of lateral acceleration throughout corner exit. This is called the “limit” because it is the maximum lateral acceleration the vehicle’s tires are currently able to produce. Although in vehicle dynamics we often use the term lateral acceleration because it is to the side of a vehicle, in physics, the car is simply experiencing centripetal acceleration.
It might be difficult to visualize how there can be acceleration when a car is traveling on a circular arc at a constant speed, but consider what would happen if all friction were removed as the driver passed the apex. Because centripetal acceleration is felt as a force pushing the driver outwards, it might be intuitive to think that removing friction would cause the car to move outward as well, but this is not the case. Due to Newton’s first law, if all external forces were suddenly removed as the car passed the apex, it would continue straight ahead toward the track edge at its current velocity. So the sideways acceleration that a driver feels in a corner is essentially the vehicle’s acceleration away from this straight line that it would travel on if not for the tires’ ability to generate force.
The Understeer and Oversteer Limit
Although the blue car has reached 1.2 g of lateral acceleration at the limit, this does not necessarily mean that all four tires are being used to their maximum. If the front or rear tire pair of a vehicle reaches its maximum force before the other pair, then the vehicle has reached its limit, leaving at least some remaining capacity at the other end. If the front tires reach their maximum first, we call this the understeer limit and if the rear does, this is the oversteer limit. Almost every class of racecar will be set up to understeer at the limit, but the amount of remaining rear capacity will vary depending on the needs of the car as well as the driver. Most will ideally have a setup that maintains the understeer limit throughout entry, but the rear will have a small enough capacity remaining that the driver can still reach the oversteer limit with the throttle during corner exit. So when a driver says they prefer a car with oversteer, they don’t truly want a car that always reaches the limit of the rear tires first, but simply prefer a car with relatively less remaining rear capacity so that it’s easier to achieve oversteer. At the other end of the spectrum, a car that has a lot of remaining rear capacity, which makes it very hard to achieve oversteer, is said to have a lot of understeer.
Although the blue car has reached 1.2 g of lateral acceleration at the limit, this does not necessarily mean that all four tires are being used to their maximum. If the front or rear tire pair of a vehicle reaches its maximum force before the other pair, then the vehicle has reached its limit, leaving at least some remaining capacity at the other end. If the front tires reach their maximum first, we call this the understeer limit and if the rear does, this is the oversteer limit. Almost every class of racecar will be set up to understeer at the limit, but the amount of remaining rear capacity will vary depending on the needs of the car as well as the driver. Most will ideally have a setup that maintains the understeer limit throughout entry, but the rear will have a small enough capacity remaining that the driver can still reach the oversteer limit with the throttle during corner exit. So when a driver says they prefer a car with oversteer, they don’t truly want a car that always reaches the limit of the rear tires first, but simply prefer a car with relatively less remaining rear capacity so that it’s easier to achieve oversteer. At the other end of the spectrum, a car that has a lot of remaining rear capacity, which makes it very hard to achieve oversteer, is said to have a lot of understeer.
Understeer and Oversteer Defined
The use of the terms understeer and oversteer can be confusing sometimes however, as they describe different handling characteristics under and over the limit. The official SAE/ISO definitions primarily deal with sub-limit behavior as it relates to slip angle differences between front and rear tires. This is an important element of car design because as rear tire slip angles increase with lateral force, the angle of the entire vehicle increases, which will therefore turn the front tires without the driver adding any additional steering.
The use of the terms understeer and oversteer can be confusing sometimes however, as they describe different handling characteristics under and over the limit. The official SAE/ISO definitions primarily deal with sub-limit behavior as it relates to slip angle differences between front and rear tires. This is an important element of car design because as rear tire slip angles increase with lateral force, the angle of the entire vehicle increases, which will therefore turn the front tires without the driver adding any additional steering.
A vehicle has neutral steer when these front/rear tire slip angle characteristics match so that the increased vehicle angle with increased lateral force gives the front tires the exact additional slip angle they need to negotiate a given radius corner with the same steered angle. Understeer is when the front tires need more slip angle than the rear and so more steering wheel angle would be needed and oversteer is the opposite where less would be needed. Keep in mind that this is all sub-limit behavior, but it can be important to take into account in vehicle design. For instance, you generally wouldn’t want a passenger car that sub-limit oversteers and starts turning itself more if a driver increases speed in a corner.
The way most drivers use these terms however, and the way they are primarily used throughout these lessons, is to describe what happens when a vehicle goes past the limit. If the front tires reach the limit first and the driver continues turning the steering wheel, they are essentially forcing an increased slip angle at the front tires without an increase in rear tire slip angle and so we call this understeer. When the rear tires go past the limit first, they continue to increase slip angle and therefore vehicle angle more than the front tires need and so we call this oversteer.
So while the front/rear slip angle relationship is the same for understeer and oversteer both under and over the limit and we can therefore use the same definition, these are describing different situations. Under the limit, we are primarily describing part of a vehicle’s response characteristics, whereas over the limit, we are describing what a vehicle will do when it overcomes the front or rear tires first. For instance, you could have a vehicle that naturally understeers below the limit, but the driver could still accidentally or purposefully overwhelm the rear tires and induce oversteer.
So while the front/rear slip angle relationship is the same for understeer and oversteer both under and over the limit and we can therefore use the same definition, these are describing different situations. Under the limit, we are primarily describing part of a vehicle’s response characteristics, whereas over the limit, we are describing what a vehicle will do when it overcomes the front or rear tires first. For instance, you could have a vehicle that naturally understeers below the limit, but the driver could still accidentally or purposefully overwhelm the rear tires and induce oversteer.
Car Control Vs The Racing Line
When we talk about understeer and oversteer, someone might picture a car with the front sliding across the track or with the rear stepping out wide and the driver valiantly trying to recover, but these are usually not just car control errors. They often start with a racing line error. As we learned in Racing Line Fundamentals Part 1, the radius a vehicle is able to achieve is dictated by its speed. What this means is that when at the limit, any additional speed, at any point, will force a vehicle wider than its intended line.
When we talk about understeer and oversteer, someone might picture a car with the front sliding across the track or with the rear stepping out wide and the driver valiantly trying to recover, but these are usually not just car control errors. They often start with a racing line error. As we learned in Racing Line Fundamentals Part 1, the radius a vehicle is able to achieve is dictated by its speed. What this means is that when at the limit, any additional speed, at any point, will force a vehicle wider than its intended line.
Serious racing line errors can often lead to car control errors. For instance, if a driver was attempting an ideal corner entry at the limit, but missed their braking point by even a little bit, the vehicle would be forced outside the ideal line and it will be impossible for them to reach their apex. This does not mean that the vehicle has gone past the limit and understeered however. So far, this is just a racing line error. The driver could recognize their error and still stay right at the understeer limit throughout entry, albeit on a slightly wider line and with a different apex than they would prefer. This is why it’s generally advisable for a driver to not push their corner entry to the very limit. A perfect ideal entry requires perfection, but holding back some allows a margin of error while still being able to reach the ideal apex. This more conservative entry approach will usually cost less time than always trying for perfection, but then often missing the apex.
What we often see among less experienced drivers however, is a car control error made in an effort to fix a line error. The driver might overuse the brakes or turn the steering too much leading to excessive understeer, or simply lose control of the rear leading to excessive oversteer. The key takeaway here is that when we talk about the racing line, we are talking about optimizing a vehicle’s path based on its available tire forces. Car control however, is about driving a vehicle right at the understeer or oversteer limit in order to maximize those forces. These are two separate subjects and it’s important to understand the difference.
| Up Next - The Understeer Limit I hope you enjoyed this first installment in the Car Control Fundamentals lesson series, and if you have any questions, please use the comments section below. Up next, we’ll learn more about understeer before moving on to take a closer look at slip angle. 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 |
