We spend a lot of time writing about suspension and different tips and tricks to achieving a great set up to make you and your car as fast as possible around the track. However, a suspension set up can often only be as good as the tyres used. This is because the aim of a perfect set up is to deliver excellent dynamic feedback to the driver as well as getting the most out of the tyre in order to generate the highest amount of grip as possible. This is the reason why if you are taking your car out on a track day, one of the most recommended steps is to buy and fit some track day tyres. The softer compound and often reduced amount of tread, increases contact patch surface area and the softer compound increases the grip. How all this works and what you can do to increase grip levels and spec the ideal tyre for your car is outlined below.
An Introduction To Grip
If you have read our “Basics” section you will have already come across our fundamentals on grip and how it is generated. The basic equation is shown below:
The friction force is equal to the coefficient of grip multiplied by the normal force. Normal force is what is applied by the mass of the car as well as any downforce being generated. Therefore, the tyre falls into the category of the co-efficient of grip which has two major areas:
- Ground Surface Conditions
- Tyre Compound and Size
The above equation only accounts for one type of grip known as adhesion grip which is a type of grip that applies to all materials which makes it the most commonly known type of grip. However, when it comes to rubber and tyres there is a second very important type of grip at play, hysteresis grip. Hysteresis grip comes from the distortion energy of the tyre, where if the rubber is pushed one way, it wants to bounce back and reform its original shape. However, with rubber it never recovers all of the energy that was put in to deflect it, this energy has to go somewhere due to Newton’s first law and is called hysteresis. This hysteresis is where the second factor of grip is produced from within a tyre.
For this article we will look into how the tyre generates its grip in both adhesion and hysteresis. We will also look deeper into the chemical compound of the rubber and how it interacts with itself and the ground to generate grip.
Adhesion grip is the type of grip you learnt about in school, where if you were to push a rock along the ground it would not move initially until enough force was applied to begin moving the rock. The force required is enough to overcome the bond between the molecules in the ground surface that have formed a bond with the molecules in the rock. Not all of the molecules present actually form a bond though as only certain ones at high spots form the temporary electrical attraction which holds the two parts in place. A force larger than the magnitude of this electrical bond is required to move the object along the surface. As the rock moves along the surface new bonds will form requiring the force to be continuously applied to keep the rock moving. Should the applied force stop, the rock will stop as the bonds take hold. This reoccurring force is shear force. The diagram below shows how the tyre interacts with the ground through adhesion grip.
If you were to press down on the rock at the same time as trying to move it sideways it would now require more force to move due to the increased downward force causing more bonds to be created. This is where the “N” for normal force applies in our basic grip equation F=uN.
Another factor of adhesion grip is friction co-efficient. This comes down to the road surface, tyre compound, heat and any contaminants on the surface. For example a film of oil on the circuit between the tyre and the ground will act as a barrier between the molecular bonds making the connections weaker causing the tyre to slip.
However, rubber has a second type of grip that is very important to why it has such great properties to provide grip.
One key characteristic of hysteresis grip is that it is not affected by contaminants so gives the same level of grip when there is oil present or not. The diagram below shows how hysteresis grip works with the ground.
Rubber has a very unique make up making it respond to loading in a unique way. It is made of long molecular chains that are coiled like springs. Due to the random shape of the spring shaped molecules they’re able to uncoil easily. However, when rubber is manipulated not all of the energy put in to distort it is recovered when released.
Hysteresis occurs within the rubber compound when the tyre is being loaded in motion. This operation of the surface being loaded and unloaded is called the loading cycle. In order to further understand the hysteresis loading cycle we must look at how the tyre is loaded and how it responds throughout the loading cycle.
To understand where the energy comes from its easiest to first look at a loading cycle of an ideal simple steel coil spring. The diagram below shows how a spring loads and unloads across a distance when a force is applied to it. The force can be paused and there can be different length intervals in the loading and unloading process but the line would remain along the same path.
The key here is observing the energy put into the spring and released from the spring during the process. The area below the blue loading/unloading line represents the energy absorbed and released by the spring. Due to the loading/unloading line perfectly overlapping each other, exactly the same amount of energy put in to the spring is released once returned to zero extension.
Now we can compare this to the loading/unloading cycle diagram of a tyre shown in the diagram below.
As you can see the loading and unloading curves are separate to each other showing that it absorbs more energy in the loading process than it releases when unloaded. Due to the shape of the molecule chains of the rubber, once stretched they do not follow the same path upon their return. This causes the molecules to rub together as they pass against each other due to the different chains returning to their shape at different speeds. This act of rubbing against each other is referred to as internal friction and is the reason for the loss of energy. However, this energy is not totally lost as it is turned into heat. The amount of energy turned into heat is the area between the two blue curves. This act of stretching the coil shaped molecules and the internal friction as they unload is why tyres get hot when used. It is also why the entire depth of rubber gets hot and not just the surface that is in contact with the ground.
This increase in heat within the compound often makes it more pliable and soft, applying more of the contact patch to the surface, therefore increasing the adhesion grip of the tyre allowing it to withstand higher input forces before losing traction.
The Friction Circle
A key way to understand how a tyre generates grip is by looking at the friction circle. This is one of the first models observed and analysed by engineers and drivers alike when embarking into the field of motorsport. The friction circle can be seen below:
As you can see in the diagram above maximum forces in the X direction are generated when the Y forces are at 0. Likewise maximum Y forces are generated when X forces are at zero. Therefore, when accelerating or braking there should be no lateral forces being applied to the tyres in the form of cornering/steering. Similarly, when cornering there should be no longitudinal forces applied to the tyres in the form of braking or acceleration. When forces are applied in two separate directions, such as applying throttle during cornering, the tyre is sheared in two different directions causing the compound to shear easier and therefore reduce the grip levels. That is why it is good practise in racing driving to be off throttle and off brakes through the apex of a corner to maximise speed around the apex. Furthermore, it is good practise to brake in a straight line to generate maximum braking force possible from all tyres making the car stop faster, allowing the brakes to be applied later. Slip angle of the tyre is also a factor which we will go into in more depth in the next article.
Interaction With The Road Surface
First we will take a look under the microscope at the contact patch of the tyre where it makes contact with the track surface. When looking at a track surface it can look very smooth just like the surface of a new tyre. However, when you zoom in, the surface of the track is actually very rough with large undulations between the different pieces of tarmac on a microscopic basis. The surface of the rubber finds these gaps and grooves and applies forces on the leading and trailing edges of the undulation. These undulations cause the loading and unloading cycles to occur at the surface of the tyre and begin the hysteresis process. It is also important to consider the adhesion components at this point which in the dry weather often account for most of the grip generated by the tyre. In fact it makes so much of a presence in the dry that it almost masks the effects of hysteresis upon grip levels. However, if the surface is contaminated with rain or grease to reduce the friction coefficient and greatly reduce adhesive grip, the hysteresis will still apply and be unaffected by the contaminant. Its for these reasons that wet weather compound tyres often have a high level of hysteresis designed into them.
Tyre Compound Choices
Most racers and track day goers alike will be familiar with different tyre compound choices such as hard, medium, soft, intermediate and wet compounds. Within these categories there are also different levels of tyre tread wear ratings. Softer compound tyres are known for generating more grip than harder compound tyres but also for not lasting as long. Therefore there is a fine balance when choosing tyres between maximum grip and the churn rate that you use tyres. When racing this can be a choice between lap times and amount of pit stops, whereas for the track day goer it can be a choice between lap times and the cost of replacing tyres.
The softer the compound, the deeper into these grooves the tyre can go as the compound deforms to mould to the track surface. As the tyre goes deeper into the groove it generates more surface area of contact and therefore allows more force to be applied through the rubber from the car in the form of adhesion grip. Although the softer compound is able to form deeper into the grooves and generate more grip, it is also easier to shear away from the tyre, wearing it down faster due to the softer rubber making the tyre have a shorter lifespan than that of a harder tyre. The shearing is caused by the sharp edges of the asphalt grooves that the tyre is locking into along with the rotation of the tyre applying the linear force to the ground surface directly shearing off the compound where it meets the ground.
The softer compound tyre allows more movement between these layers causing a higher level of tyre distortion, in turn generating a large amount of movement between the layers of the compound, as the molecules rub together rapidly and have their chemical bonds stretched and compressed causing heat to generate much faster and make the compound even softer, generating more grip. However, there is a limit to how soft a tyre can go and how much heat it can generate before causing issues and going in the opposite direction rapidly and losing vast levels of grip.
When a compound gets too hot it begins to degrade and the chemical bonds between the rubber break down rapidly and begin shearing at a fast rate. This can cause symptoms such as delamination of the tyre where strips of rubber come off the surface of the tyre exposing the chords; or blistering of the compound where separation between layers causes the rubber to separate in the forms of lumps and chunks of tyre coming off rendering the tyre useless.
In the next article we will be looking at how to manage and optimise the heat in your tyres in order to optimise grip using different methods. We will also delve into slip angles in depth and the effects of downforce.