The Physics of Trail Braking

Now we’re starting to move on to the more advanced techniques. This technique or concept is single handedly responsible for dropping more than 10 seconds off of our lap times. Earlier in the month, we had the chance to speak with Kyle Prins and Cory Minto, who are data analysts at Splunk who partner with the McLaren F1 Esports Team. They said that by far, trail braking was the most important thing that professional drivers look at when comparing themselves to other drivers. They spoke with Lando Norris and Oscar Piastri, the two drivers for the McLaren F1 team, about how they examine data to find differences in lap times. The next post will go more in depth to the data, but their first answer? Braking. In essence, proper braking is the most important technique that you can learn when racing. 

So, what is trail braking? Most people think of braking as a very binary input in everyday driving. You’re either braking to stop the car, or you’re not. The way that professional racers think about braking is much different. To explain why they do this, let’s take a look at the physics behind racing, and then we’ll go further in depth into the technique. 

WARNING: This next section is extremely physics dense, and if you aren’t interested in how cars work, then skip to the START HERE text. You have been warned

One of the goals of a corner that we layed out earlier in our blog posts was to take the corner at the highest minimum speed. Now what determines how fast a car can take a corner? It turns out to be friction. Any car can take a corner at an extremely high speed, it’s just that then when they try to turn in, your average car won’t have enough grip and it will continue straight into the barrier.

The lateral force that is induce by turning the wheel is too much for the car’s grip, which results in the video above. So what creates grip? Put simply, grip is a the sum of forces that press down on the car, typically, a combination of the force of gravity in conjunction with the force of the air. Modern F1 cars have extremely advanced aerodynamic systems to create up to 5 Gs of down force. 

But why does greater down force equal greater turning capacity? If you think of a wheel in terms of infinitely small points, the points of contact with the road aren’t moving. It’s weird to think about, but the bottom of the wheel is spinning backwards while the wheel itself is moving forward, which results in the point of contact being a set point. So, the force that allows the car to move laterally is the force of static friction. Friction is calculated by this formula below: the force of friction is equal to mew (a constant) times the normal force.

If you’ve taken a physics class before, you recognize this formula, specifically that the normal force is just a fancy way of saying the amount of force pushing something down. In racing, the normal force is the amount of force pushing the car down into the track. Hence, a greater down force results in a greater force of static friction, which results in a car being able to take a corner at a higher speed. 

So now we’ve established that down force and weight is how a car can take a corner faster. Unfortunately, the physics doesn’t end there. What is left is to examine the physics behind trail braking. To do that, however, we must first establish what trail braking is.

Very simply, trailbraking is slowly releasing the brake pedal as you turn into the corner. That’s it. It’s that simple. Or so one would think. Below are a series of videos of us attempting to learn trail braking into Les Combes after the Kemmel Straight. The mistakes get slightly less egregious as we go on.

As you can see, trail braking is not a simple process. Both your brake and steering input have an extremely small window where you achieve perfect trail braking, and the slightest mistake can result in a wreck, or at the least, a failed lap time. While this has been an extremely challenging technique to learn, it is extremely rewarding. I don’t think anything will ever be able to compare to the satisfaction of taking a corner using trail braking. It feels incredibly smooth while being incredibly fast. Below is a screenshot of the speed, brake pressure, and steering angle going through Les Combes at Spa. The yellow bar through the screen is lined up with when both cars began to turn into the corner, shown in the steering angle at the bottom. At this point in time, the red delta symbol at the top displays a difference of 68 kph. You can see in the top graph that the difference in speeds is relatively similar throughout the corner, which means that we were able to take the corner over 60 kph, or 38 mph faster by using trail braking. The efficacy of this technique speaks for itself.

To perform a proper trail break, you brake all the way into the apex of a corner. However, you are never at 100% brake pressure. Instead, the second you begin turning into the corner, you slowly reduce brake pressure. The goal is to try to maintain the highest possible break pressure without inducing a lockup in the rear. The exact amount that you reduce per second changes depending on the corner and the car, so it is difficult to describe a general feeling to search for. However, there is a general rule of thumb that you can use called the invisible string technique. The invisible string technique wants you to imagine a string tied between your shoe and the bottom of the wheel. As you begin to brake, the string makes the car straight. Then, as you start to turn into the corner, the string lifts your foot slightly off the brake as you turn. This continues until the apex, where you have maximum wheel input and you are fully off the brake. As a quick side note: the invisible string technique is also helpful for determining the correct throttle pressure out of an apex to ensure you don’t spin out. Trail braking requires an immense amount of trial and error, and it took us around 20 hours on the sim to properly learn it.  Here is an example of a successful trail brake.

As you can see from the telemetry graph in the bottom left of the video, I am braking all the way until the apex of the corner. Effectively, this technique allows me to maintain an extremely high speed while still rotating my car through the corner. Looking at the data from above, the lap that performed linear braking struggles to rotate his car through the corner while maintaining a high minimum speed. That lap’s minimum speed is consistently 60+ kph below the trail braking lap. 

Now that you have a basic understanding of trail braking, there’s still a long way to go to improve. A more advanced view of trail braking is to determine how much pressure we put on the brakes, depending on how the car responds. If the car is understeering through the corner, hold more pressure on the brakes, or if the car is oversteering, then release the brakes more. Understeering is when your car turns less than the wheel input, and oversteering is when the car turns more than the wheel input. As you continue to practice taking corners while trail braking, this process will gradually become more natural. 

To examine why trail braking has this incredible effect of increasing the minimum speed, we must dive back into the world of physics. 

Again, if you’re not interested, you can skip this next section. You have now learned the basics of trail braking! While our description of the technique itself seems extremely simple, that’s because it is. Theoretically. On track, trail braking requires hours of practice and finding the feeling of your car before you can consistently perform it at a high level. So off you go! To your sim!

Welcome back, you physics lovers. Now we have established trail braking as a technique. So why does it work, and what was I talking about with rotating through a corner? I thought you just drive through a corner? All of your questions will be answered in the next few paragraphs, so have fun reading, knock yourself out while trying to understand it, and then go watch a few YouTube videos. That’s what we did. 

When you brake, the “weight” of the car is shifted to the front. For proof, think of when you brake in your car. You feel as if you’re being dragged forward. This is the feeling of inertia, which will be touched upon more later. In addition, we can examine the spring deflection of the front tires vs rear tires when in a corner. Below, the driver is braking, and you can clearly see that the front is much lower than the rear, demonstrating the fact that there is more weight on the front tires.

Now, why does this weight on the front result in a tighter turn? Earlier, we established that the car has increased grip due to down force. Now that we have proved that there is a greater weight on the front wheels, it stands to reason that the front wheels now have increased grip and can make a tighter turn. But that’s not all that increased weight in the front does. In physics there is a concept called rotational inertia that is used when you try to spin an object. If you try to roll a pencil between your fingers, it rotates extremely easily. But if you try to spin the pencil by holding the end of the pencil and then rotating the entire pencil, you will notice that it is much slower.

Essentially, the rotational inertia increases depending on how much mass is at a distance from the point of rotation. In racing, our point of racing is the front of the car where our tires are. So, by braking through a turn and forcing more weight onto the front of the car, it becomes easier to rotate the entire car. 

To sum everything up, trail braking through a corner forces more weight onto the front of the car. This creates more grip for the wheels and allows easier rotation of the car, which combined enables the car to carry more speed through a corner. 

That’s it for trail braking! We hope you enjoyed learning more about the physics behind advanced racing. Now all that’s left is to develop the skills to take advantage of those physics. Good luck, and we’ll see you on the sim!