For the technical content series, I would like to start with one of the most important topics in motorsport: the tyres.
As you may know, the tyre is the only part that is in contact with the ground (or at least it should be). Hence, it is a key factor to maximise the performance of it. How you apply power on the tyres, which setup do you use, how do you manage the tyre temperature, the compound. All of these, among many others describe how a tyre performs. As you may know, it is very difficult to know how the tyre will behave.
However, for the sake of brevity and entertainment, today we will not focus the attention on the details but on the basics. In further posts, we will give more details about it.
Tyre Composition
The tyre is made of rubber, which interacts with the tarmac in many different ways depending on the tyre compound, the track roughness and so on.
The tyre engineers difference four different forces that create grip:
Hysteresis
Adhesion
Viscous
Interlocking
It also has great sensitivities with the pressure, the temperature, normal load and the tyre wear which can be controlled by adjusting the setup of the vehicle and the vehicle sliding.
Tyre Forces
How many times we have heard that the Red Bull had a better tyre wear because of the better aerodynamics? The F1 teams must find the balance between end of top speed and cornering speed.
However, assuming dry conditions and a standard setup, some graphics can be extracted from the tyre. The most common one is the lateral force vs. slip angle.
The slip angle is the angle between the wheel truth speed and the direction where the tyre is pointing at. The more slip, the more lateral force is created by the tyre. However, it is worse in terms of tyre wear.
The load that is applied on the wheel; simplifying it, the vehicle weight and the aerodynamic downforce, increases the amount of lateral force for the same slip angle. This is one of the reasons why the teams invest so many resources on the aerodynamics of the race car.
The slip angle characterises the lateral capabilities while the slip ratio defines the longitudinal acceleration performance. This is described as the difference of the rotational speed and the travel speed divided by the latter.
Have you ever seen a wheel spin? This happens when the power applied from the traction system is greater than the power the tyre can handle. In this case, the slip ratio goes to infinity.
However, what is anti-intuitive is that there must always be some tyre slip to create longitudinal acceleration. However, the optimal slip ratio is usually around a 0.1.
Setup influences
The tyres are not only important for the vehicle acceleration, but also for the kinematics of the vehicle. The whole kinematics design of a racecar is focused on maximising the tyre perfomance to enhance tyre's grip. The setup is focused on determining which is the motion of the vehicle to have the tyre always working at its maximum. However, it is always a trade off between performance, handling and the rules. We will talk about it in the upcoming posts.
The tyre also has a huge role in the vehicle stiffiness since it acts as a spring with a certain stiffness which is always taken into account when designing the suspension of a racecar.
As the tyre gets temperature during the stint, the pressure increases and it does not only modifies the tyre grip but also the ride height, which with the new 2022 rules, has a huge effect on the undertrain flow management. This season is extremely important to correctly set the static ride height up to avoid the famous porpoising dynamically.
A clear example can be seen with the reasons behind the fateful accident at Imola'92 that ends with the life of the legend Ayrton Senna.
Tyre Modelling
The easiest way to understand the tyres is to test them in a rig to extract all its features and building a tyre model. There are a great variety of tyre models: the Magic Formula, the Brush model, the Dugoff’s model or even linear or polynomial approaches. The selection of each one will depend on the information available.
The best models such as the Pacejka's ones are composed for more than 100 parameters that depend on:
Tyre Speed
Camber
Slip Angle
Slip Ratio
Tyre Pressure
Vertical Load
However, in some motorsport categories these tests are completely forbidden, so the teams must build their own models using track data.
Tyre temperature
Finally, it is important to mention that every tire has a different operating temperature range. It depends on the tyre composition. Usually, the softer tyres have a temperature range that goes from 80 to 100 deg. However, the harder ones require to apply more energy on the compound to make them work, between 100 and 130 deg. That is why the medium and hard tyres take more time to achieve the operating temperature.
But how the tyres are heated up? There are different strategies:
Tyre blankets and warmers
These are the famous blankets where the tyres are stored before a run. When the tyres are in them, the temperature is electronically monitored to keep them in good conditions to extract the maximum potential as soon as possible.
Zigzaggings and burnouts on track
Who has not seen the cars do zigzaggings on track after the safety car? They need to keep the tyres in the temperature range to get them ready for the re-start. While the zigzaggings are effective for front tyres, the burnouts are used to increase the temperature of the rear tyres by producing wheel spin.
Brake Warmer
The brake warmer is a brake configuration that redistributes the brake balance to heat up the tyres from the brake discs. Therefore, the driver can warm up the axles independently.
Temperature vs. Grip vs. Wear
Every team has to find a compromise between the grip (lap time) and the tyre wear. This, actually, is one of the duties done during free practices. They evaluate the race pace versus the wear to adjust their models.
However, there are a lot of factors that must be taken into account: the vehicle setup, the track temperature, the drive style, the tyre wear, etc. Most of the times any driver has done a full stint with a set of tyres during the weekend so the tyre engineers must predict which will be the tyre wear based on the weekend’s information and previous grand prix.
Despite usually more temperature means more grip due to the viscous forces, it is not always a true. We have already seen that the best lap time is achieved when the drivers donot drive at its 100% during the first sector in order to avoid reaching the last sector with the tyres completely overheated. They are able to control the tyre slip to achive, not the best time at all the sectors but the best lap time overall.
Do you understand now why is so complicated to decide which tyres to use every weekend?
And that's all for today!
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