In this lesson, we're going to explore one of the most important but misunderstood concepts in motorsport: slip angle. If you understand slip angle, everything else you do in your driving will make more sense, and you will find it much easier to find the limit and, most importantly, stay on it. The problem is that there are many wrong explanations about slip angle out there, so let's fix that.

What Slip Angle Is NOT

Before we define what slip angle actually is, let's clarify what it is not:

• Slip angle is not a driving style

• Slip angle is not a technique

• Slip angle is not a state of the balance of the car

What Slip Angle Actually Is

Slip angle is simply the measurement of the distance in degrees of angle between where your tires are pointing compared to where they are going. This difference in angle creates the lateral force that turns the car.

For example, in a 40 degree drift, the rear tires of the car are at a 40 degree slip angle while the front tires are closer to zero slip angle with consistent adjustments on the steering to help adjust that drift angle of the rear tires.

The Optimal Slip Angle

Each tire will have a specific slip angle where it will generate the strongest lateral forces or maximum grip. We call this the optimal slip angle. This is important: a fast racing driver is capable of staying as close as possible to the optimal slip angle of all four tires during the whole corner from entry to mid-corner to exit.

Every technique and every concept you learn—trail braking, engine braking, steering traces—we do all of that just to maintain the optimal slip angle during the whole corner so that you can get as much lateral force from the tires and carry the most speed.

The Physics Behind Slip Angle

The tire is made of a flexible rubber that stretches and compresses. When we turn the front tires, that part of the tire that is still in contact with the ground resists that rotation and bends because it is glued to the ground and it wants to keep going in the direction it was going before. This distortion at the contact patch between the tire and the road is what creates the force that will turn the tire like a compressed spring trying to release its energy.

Slipping Versus Sliding: Understanding Different Levels of Slip Angle

How much slip angle is too much? Let's explore what happens at different levels of slip angle from under the limits to optimal to slightly past optimal and finally to excessive slip angle.

The Slip Angle Graph

This is a graph of slip angle versus lateral load. Here you can see that the force extracted from the tire rises as we increase the slip angle as we start bending and distorting the tires at their contact patch.

Under the Limit

In this first point, we are under the limit of the tires because they were designed to deform more than this and generate more lateral forces than this, so we're just not using all the grip.

At the Optimal Slip Angle

At this point, the optimal slip angle, we reach the tire peak deformation and maximum lateral force that it can offer.

Past the Optimal Slip Angle

Past this point, when it comes to the front tires, we are already starting to feel the force feedback on the steering becoming weaker, which is our way to feel that we are abusing the tires. The tire continues to deform, but the deformation no longer turns efficiently into grip. The carcass of the tire starts getting more and more stressed, so instead of producing higher lateral forces, the tire actually saturates and begins to lose its ability to generate more force. Instead, we feel a loss in lateral grip. At this stage, some parts of the contact patch start to slide relative to the road.

Excessive Slip Angle

If we increase the slip angle even further, even more parts of that contact patch start to slide and we transition to a full slide or scrub which overheats and destroys the tire, all while we feel less and less lateral grip. We can also call too much slip angle something like a slide angle or a drift angle because the deformation at this point is now mostly just bad friction.

Summary of Tire Deformation

In summary, the tire even deforms more overall as you pass the optimal slip angle, but that deformation shifts from being primarily elastic with efficient grip generation to being dominated by sliding, resulting in a loss of lateral force while the tire begins to skip or scrub across the surface.

The Perfect Slip Angle: It Depends on the Tire

Street Tires

Street tires designed for everyday driving and meant to have a long lifespan have their optimal slip angle at around 8 to 12 degrees. The build up of the lateral force in these tires is more progressive, the feedback is more forgiving, and when we go past the optimal slip angle the grip decreases smoothly, minimizing the chance of a sudden loss of control. You can see that the area in which the grip is close to its maximum is relatively large, which means a more predictable handling behavior.

Semi-Slicks or Competition Tires

Semi-slicks or competition tires designed for trackdays and high performance driving have their optimal slip angle at around 5 to 7 degrees. They have stiffer sidewalls and softer compounds. They generate higher levels of grip at lower slip angle compared to street tires. Here's the thing though: when you go past the optimal slip angle on these tires, there's a faster drop in grip compared to the street tires, which demands more precise control.

Slick Tires

These slick tires meant for professional racing have a slip angle at around 2 to 4 degrees. They have the stiffest construction but the softest compounds. This grip builds up very quickly, reaching the peak very quickly, and the grip drops very quickly past the optimal slip angle, which requires the most precise driving technique.

How Do We Feel the Slip Angle?

The closer to the optimal slip angle, the stronger the resistance of the force feedback of the steering wheel. This is obviously true only for the front tires. The force feedback on your steering wheel is the most important way you will feel the car when driving at a high level, especially in the simulator.

Pro Tip: Peak Force Feedback vs. Peak Lateral Force

The peak lateral force you get from the tires doesn't exactly match the peak resistance from the steering. The peak force feedback comes ever so slightly before the actual maximum lateral forces you will get from the tire, which means you can force an extra few degrees of steering past the peak feedback to reach that limit. Some drivers call this over-driving the front, but be careful not to overdo it. We are talking about really subtle stuff here.

Slip Angle in Different Scenarios

Understeer

In understeer, the slip angle of the front tires is way higher than their optimal slip angle and also higher than the rear tires slip angle. Because of that, the front tires are not capable of generating enough lateral force while the rear tires are generating more lateral resistance to that rotation that the front tires are asking, which makes the car go straighter.

Oversteer

In oversteer, the slip angle of the rear tires is way higher than their optimal slip angle and also higher than the front tires slip angle. Because of that, the front generates a high amount of rotation that the rear tires cannot keep up with, so they keep sliding towards where they were going because of inertia—they keep going forward while the front tires are torquing the whole car laterally, causing the car to slide and spin.

Neutral Steer

In neutral steer, things can get slightly complicated because neutral steer means the slip angle of both rear and front tires are exactly the same and the forces generated are pretty similar. In neutral steer, the car is sliding equally with just some minimal adjustments in direction coming from the front tires from the steering to maintain that state.

Neutral steer is interesting because it can be good or bad depending on how much all four tires are sliding. Technically, you can be in neutral steer while being on the ideal slip angle of both front and rear tires, but you can also be just sliding too much on both ends, which looks amazing but is just slower.

In an ideal scenario with neutral steer, you have actually a little bit more forces being generated by the front tires because otherwise the rotation would never happen.

Example: A Near Perfect Corner

This is an example of a near perfect corner done by one of our coaches, Kane, driving the Mazda MX-5 at Okayama in iRacing. He turns in and immediately enters the optimal slip angle and ideal neutral steer, uses the light hands technique, the weight transfer adjustments from the brake release and the engine braking. All that to control and maintain this optimal state and generate the maximum lateral load possible to carry the highest speed possible.

Again, like I said, every technique that he's applying here is serving a purpose: it's trying to put the car on the optimal slip angle as early as possible.

Corner Entry Analysis

So here on entry, for example, you can see him pretty much turning this much. That's it. That's it. He went from here—steering angle zero degrees—to here. That's what, five degrees? And then that already, with the engine braking—he just went to second gear, he's still trail braking, lots of weight on the front tires—that's already enough to put the car on the optimal slip angle by here.

By here, the car is literally already on the limit of the tires. Now he's already countersteering a tiny bit. Of course, this is some micro countersteer, pretty much with the wheels straight, and then now a little bit to the left. But the car is still gaining rotation. Why is that? Because the front tires are still pointing to the inside because of the total slip angle of the car. So that means the tires are still creating a force to the inside, even though the steering is pointing ever so slightly to the left, because the final slip angle between the tire and the road is still pointing to the right because the whole platform is slipping.

Mid-Corner Analysis

Now we can see he maintains that. He's releasing the brakes to adjust how much the car is rotating. And now right here, the car has low rpm, so the engine braking is not rotating the car as much. Now he can add more steering because he's now forcing a little bit more of that front tires to maintain that perfect slip angle that was perfect on entry. Now it's kind of losing a little bit because of the engine braking that's going down, so he's now compensating to stay on that optimal slip angle mid-corner.

Corner Exit Analysis

And then he gets back on power. Now the wheel spin—the micro wheel spin—that's coming from accelerating aggressively. Even though this is a low power car, the grip is already pretty much on the limit, so if you accelerate a lot you can spin. Now he's going to adjust that by having also light hands on exit. You can see a little bit of adjustment here on the steering. All of that, all those adjustments he's doing, it's just to stay on the optimal slip angle.

Strategic Use of Slip Angle

A fast driver knows exactly how much slip angle they should exploit depending on the situation. You can technically get the same amount of grip in two opposite scenarios—being slightly under the limit versus being over the limit—and get the same forces or the same lap times, and you can technically choose to be in either of them if you have enough experience.

Tire Saving Scenario

For example, in a tire saving scenario, you might want to be slightly under the limit just to get some good lateral grip and lap times without generating too much slip.

Cold Tire Scenario

Or you have cold tires and you want to get them up to temperature a little bit because the track is too cold. Well, you could technically overdrive the car in a controlled way to get the temperatures to go up.

iRacing Specific Considerations

In iRacing, for example, the tires are very sensitive to scrubbing or being over the slip angle as they overheat their surface temperature too quickly and lose grip, so over-driving in iRacing is not the best way to find consistently fast lap times. Rather, being right on top of the optimal slip angle so that you can generate peak forces without overheating the tires is the way to go.

How Front and Rear Tires Interact

A good way to think about slip angle is that the front tires always dictate the motion while the rear tires always react to the fronts. So the front tires initiate a rotation before the rear tires get into their own slip angle while trying to resist that rotation and creating lateral forces themselves in the process.

The Point of No Return

That's why this happens: when you get into a big enough slide and you try to correct it but you get into full steering lock, you reach the point of no return when the slip angle between the front tires and the track is still causing a force towards the inside, which still forces the car to gain more and more rotation, getting you into a full spin.

A spin is only saveable when the countersteer of the front tires allows the front to create a force to the other side of the spin direction.

Conclusion

In the end, slip angle is the foundation of motorsports, and everything we do in terms of driving technique is done so that we can achieve its optimal values. On the next lesson, we will talk about neutral steer—the true limit, the true point where you get the most forces from the tires, and you will spend the rest of your life chasing.