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What Is The Traction Circle? – Fully Explained

One of the most important parts of racing is the element of traction. This is what keeps your car stuck to the road and prevents you from spinning off the track. But there are a lot of things that affect your level of grip, and one way to think about it is by using the traction circle.

The traction circle is a mathematical way of illustrating how much grip a particular car/tire has on the road, and how it can be used.

Although this may sound complex, it is really a fairly simple way to illustrate the importance of maintaining traction. However, before you can start maximizing the positive effect of your traction circle, you must first understand what traction is and what can have an effect on it.

What Is Traction?

Traction is the amount of grip that a car has and is a fundamental part of the everyday driving experience as well as racing. It is not something that your car possesses as such, and instead is something that is generated by a few different factors working together. Loosely speaking, however, it is what keeps your car on the road when you are going around a corner at speed.

Generating Traction

Grip is a measure of how strongly one thing can hold onto another, and the grip of a car to the road surface is usually experienced through the tires. These will be made of rubber compounds, and they act as the medium through which friction can be generated, keeping the car stuck to the road surface. Tires are not the only things that affect traction however, as we will find out below.

Without traction, the car can go into a slide and you can end up off the track. This usually comes in the form of oversteer, when the back-end spins round much more than the amount by which the driver commands, and understeer, when the car turns less than the amount commanded by the driver. Both can prove to be disastrous in a racing situation and in day to day driving.

So, traction is clearly important, but what kind of things affect the amount of it that you experience?

What Affects Traction?

The Road Surface

There are lots of things that affect the amount of traction you will be able to make use of when driving, and one of the most important factors is the road surface itself. If you are driving on a dry tarmac surface, you will experience a lot of grip, and you will actually find it easier to take corners at higher speeds, and it will be easier to slow the car down faster due to the enhanced grip.

Conversely, if you are driving on a dirt track, such as in rally racing, the tiny particles of dirt and mud that sit on the road surface create opportunities for the tires to slip, and this causes a reduction in traction. Similarly, if the track surface is wet, water will get into the tire treads, effectively reducing their surface area and decreasing the amount of grip they have.

The Type Of Tire

Aside from the road surface, the type of tire that your car is using will also affect the level of grip it experiences. Hard compound tires have less surface area interacting with the road surface giving them less grip than soft compound tires, which have more rubber in contact with the track. Soft tires will be less durable than hard tires however, and so grip is not everything.

How You Drive

In terms of driving techniques, increasing your speed will have an effect on your level of traction, as we will explain further below when we discuss the traction circle. How much of the brake you apply, and how much you turn the steering wheel will also affect how much traction you have, and going faster through tight corners leads to the biggest loss in traction.

The Weight Of The Vehicle

Finally, factors like the size, shape and weight of the car will also affect your traction. Heavier cars will have more downforce and thus can experience more grip but will obviously experience tradeoffs with their power. Conversely, lighter cars may experience less traction, thus things like spoilers are added to increase the amount of downforce they can exert to gain more traction.

All of these factors are important, but considering the traction circle is a good way to visualize why these factors affect the way you drive your car.

What Is The Traction Circle?

A Simple Diagram

The traction circle is a way of thinking about how much traction a car has available in terms of mathematical equations and a simple circular diagram. The circle has a circumference which is effectively the traction limit of the car’s tires, and then the circle is split into four quadrants by a vertical and a horizontal line.

Split Into 4

The horizontal line, the x-axis, represents the lateral – or cornering – grip of the car, while the vertical line, the y-axis represents the longitudinal – or accelerating/braking – grip of the car. The top half of the circle is utilized when the car is accelerating, and the bottom half when braking. The left side represents the car turning left, and the right half represents a right turn.

The diagram below illustrates the traction circle in a simplified way. The diagram on the left is the traction circle as it would be interpreted with the accelerating/braking traction being represented by the vertical line, and the cornering traction being represented by the horizontal line. The outer boundary of the circle, i.e. the circumference, is the traction limit of the car.

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Lots Of Lateral, Not Much Longitudinal

In the middle diagram, arrow C represents the car being pushed to its traction limit. Arrow A represents the amount of cornering grip being utilized, while Arrow B illustrates the amount of acceleration grip being used. Clearly, the car does not have very much in the way of accelerating grip when it is taking a sharp corner, which we will look at in more detail below.

Lots Of Braking, Not Much Turning

The third diagram shows the same traction limit being reached, but this time in the “y” direction as the car is near the maximum level of braking traction. Unlike the middle diagram, the car is now forced to use much less cornering traction. Arrow A again is referring to the amount of cornering traction being used; Arrow B is representing the braking traction and C the total traction.

What these diagrams illustrate is the fact that the traction experienced by a car is made up of a combination of the accelerating/braking traction and the cornering traction being used at that time. They also illustrate that, due to the fact that there exists a limit, in each case represented by arrow C touching the circle’s circumference, only certain combinations are possible.

More Speed Means Less Turning

It should seem fairly intuitive that the faster you go the harder it will be to make tight turns. The same thing applies for the brakes as well, as you will find if you are braking very hard into or during a tight corner and you try to turn the wheel, your wheels will not get enough traction in the lateral direction as they are already being pushed to their limits in the longitudinal direction.

This is represented in the third diagram, and no matter where you drag the point around the circumference of the circle, it will be made up of a balance of turning traction and accelerating/braking traction. You can find the theoretical force involved through Pythagoras’ equation, with the length of C2 being equal to the length of A2 plus B2, C equaling the square root of A2 plus B2.

How Can You Use The Traction Circle To Your Advantage?

Pushing It To The Limit

The traction circle functions as a good way to illustrate the idea of the limits in both directions of traction. Thus, it can be used as a tool to reinforce some basic driving techniques, with the idea to try and never end up in a position of maximum traction utilization in either the longitudinal or lateral direction when you shouldn’t be.

Accelerating On The Straight

This might seem very vague, but it essentially means that if you are on a straight, for example diagram (1) below, and there are no immediate turns, you will want all of your traction to be in the forward, or accelerating, direction. This will give you the most grip underneath the tires to allow you to get to top speed quickly, without using any of the lateral traction.

Finding The Balance

Then, as you approach the corner, you will want to find a bit more of a balance between both the x and y directions of traction. Then, when you apply the brake (2), the direction of traction will move towards the bottom of the circle, limiting the amount by which you can turn the wheel. Easing off the brake does the opposite (3) and allows you to turn sharper into the apex of the corner (4).

Accelerating Once More

Finally, as you leave the corner, you straighten up the wheel (5), freeing up traction that can be used to accelerate away. Thus, when you go into a corner, the traction circle may look something like the diagram below, with the single arrow representing the overall direction of traction as you enter, go through, and leave the corner.

In an ideal scenario, the arrows will be at the edges of the circle, at the limits of traction. Beyond this range, traction will be lost between the tires and the track surface.

Final thoughts

The traction circle presents an interesting way to think about how grip is used by the car’s tires. Although it can be utilized in a strictly mathematical manner to calculate exact forces, it is best used as a tool to illustrate the mechanics behind proper cornering technique, and to show what happens at the limits of a vehicle’s traction capabilities.