The most important concept you don't know about | Friction Circle guide

Updated: Feb 18, 2021

You may think you understand the basics of car handling and racing lines, but are you aware of the important concepts of something called the Friction Circle? No? Find out with our latest video guide.

Check out our other video guides and pro onboard laps here.

As always, you can find the video's full script below in case you would like to read about certain details.

Check out our other video guides and pro onboard laps here.

The Friction Circle - Track Titan guide on the fundamental concept most sim racers don't know about (full script)

You might know about understeering, about oversteering, you know your racing lines - but do you know about something called the Friction Circle?

It's one of the most fundamental concepts of both real-life racing and sim racing. It's crucial to understanding topics such as under and oversteering, and to understand where you can maximize your time on track.  

We're going to be talking about what the Friction Circle is, why it's useful to you, and more importantly, that the Friction Circle is not a static visualization of grip - but how that changes depending on the car you're in, as well as the state of that car on track.

Let's start with a simple illustration of what the Friction Circle is really about. So we have this friction circle here, and you can see we're traveling along the road. Nothing happens, the car is steady, but as soon as we get on the brakes, we see the arrow going upwards. We're trying to put force through the tire but the car wants to keep moving. We're trying to slow it down and if we just brake a little bit, that is fine, the arrow goes about halfway.

But what happens if we go further at some point?

We reach the tyre's limit and once you cross that outside line of the friction circle that's when you would either lock up the tyres, or in most cars cases the ABS would start working for you.  

The same principle can be applied to accelerating, just the other way around. So let's say the car is standing still, we're accelerating gently you can see the arrow moving down just about, but we're nowhere near the grip limit. However, if we then put our foot down in a high-powered car, we're going to break traction, and breaking traction just simply means in visual terms going beyond the confines of the friction circle.

The same principles can be applied to lateral movements on the car, so if we're turning right, the car is leaning to the left, we're turning left, the car is leaning to the right. And same thing, at some point we're just going to exceed the tyres' limits and obviously we want to get to that limit, but don't go beyond it.

When the friction circle really comes in handy is when trying to understand the combination of lateral as well as longitudinal forces. So you can see in this example we're taking a right turn and the arrow is pointing quite far to the left. We're approaching the limit, but we're not quite there, so all good within the grip confines of that tyre - of that friction circle.

However, if we now apply a bit of throttle you can see the arrow coming down that side. The meeting point of those two arrows is actually outside of the friction circle, and that's exactly where you break grip, and why combining lateral and longitudinal forces is usually not a good idea, and you got to be really careful with it.

So now we know what the friction circle is about, and how it represents longitudinal as well as lateral forces being combined, but as we said at the start of the video the friction circle actually changes in size depending on the car that you're in and whether aerodynamic components are a part of that car.

One of the key differences between different types of cars that impact the weight distribution, as a result, the size of the friction circles on either axis is where the engine is positioned.  

Most modern road car manufacturers aim to have an exact 50/50 weight distribution for various reasons, but an extreme example such as a Porsche 911 due to the engine being in the very rear of the car we have a weight distribution of closer to 70 percent on the rear, and 30 at the front. What does that mean? We have a lot more weight on the rear axle and as a result of that - as you can see here - the friction circles when the car is standing still are actually much much bigger on the rear axle because of that additional weight.

So now that we understand that having more or less weight on an axle impacts the size of a friction circle, a similar sort of thinking can be applied to aerodynamic components and that's exactly why high downforce cars such as F1 cars or LMP cars are so fast around corners. 

The aerodynamic components really push the car down, and without having any negative weight impact the car gets pushed down and therefore all four tires increase their friction circle sizes.

Besides the car type that you're in, what also impacts the size and therefore the grip available to you is the state of the car - so whether you're braking, accelerating, cornering to the left or to the right - and actually a really good example to show is simply with a half-filled water bottle. So we can see if the car is standing still - this is about the weight distribution of the car - its quite even, but if we brake, all the weight moves to the front which means we have much more grip on the front wheels and the rear gets light.

Equally when we accelerate - and you can feel it when you're driving in the car in real life you really feel this the back of the car setting down - equally the weight of the car moves backwards which means we have much more traction on the rear, whereas the front gets light.

The most important point to talk about when talking about the friction circle though is that it's not just a pretty diagram. This is something you should put to use as you go out on track, so the next time you're understeering, or you're oversteering, you may be turning in a bit too much, as you lose traction out of a slow corner - think about the friction circle and think about what's going on.

What is the car trying to tell you? Is it leaning on one side? Is it asking for more grip where it doesn't have it?

And that's really the point of this video so you can get the fundamentals of the friction circle before diving into more advanced topics.

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