[su_list icon=”icon: angle-double-right” icon_color=”#3498db”]
- Part 1 – Tyre as a vertical spring
- Part 2 – Road Interaction & Contact Patch Grip
- Part 3 – Longitudinal Forces
- Part 4 – Lateral Forces
- Part 5 – Load Sensitivity
- Part 6 – Combined Tyre Forces
To close this series of articles about tyres, we will look to some of the settings and metrics that have the biggest effect on tyre performance:[su_list icon=”icon: angle-double-right” icon_color=”#3498db”]
- Camber angle – The inclination of the wheel in front view. Normally race cars use a certain amount of negative camber, with the top of the tyre leaning in towards the inside of the car.
- Tyre temperature
- Tyre inflation pressure[/su_list]
Negative camber normally produces an increase of maximum lateral forces (at least on the outside tire) and a change in slip curve’s shape, up to a certain point at which this effect saturates and there is an inversion. In parallel to this, it also normally reduces the available longitudinal grip. The increase in a force is associated to a contribution that is often referred as camber thrust.
Physically, the reason of camber trust is a distortion of the contact patch, with the road applying a force on the tread trying to rotate the tyre back in a straight up position.
How big camber thrust is depends on the design of the tyre itself, but both effects are normally related to vertical load too. Higher loaded tyres normally accept larger camber angles without experiencing any saturation.
This is well shown in the following picture, where loads increases moving from “Load 1” to “Load 4” (tyre friction coefficient is higher for lower vertical loads):
We can identify how, for smaller vertical loads (see for example load 1), the friction coefficient (that here refers to the maximum lateral force (or peak force) that the tyre can produce at the given vertical load) goes up reaching a maximum at a certain camber angle and then start to decrease. On the other side, if we look at the plot relative to load 4, we cannot identify any maximum. The friction coefficient simply increases without reaching any inversion point, at least up to the maximum shown camber angle.
Camber has significant impact on tyre performance and a lot of attention is paid when designing suspension to trying to set the correct camber angle. It also has a direct impact on tyre heating and wear. A compromise must often be found to have both acceptable performance and durability.
Pressure and Temperature
A Tyre’s pressure and temperature (with their effects on grip, stiffness, etc) are probably the most complex parameters to analyze because they are so closely related to each other.
An increase in temperature is normally associated also to an increase in pressure. As we have seen, pressure also has an impact on tyre stiffness and contact patch shape. This again has an influence on tyre heating and final temperatures.
Everything connected to tyres pressure and temperature is extremely difficult to measure and study. There is a strong interconnection between each parameter, and it is extremely difficult to isolate each phenomenon during testing. This makes it very difficult to properly understand how each parameter influences performance and behaviour.
Nonetheless, it is universally accepted that tyres performance depends a lot on temperature and pressure management. A lot of energy is invested in setting the car up (on the engineers side) and on driving it (on the drivers side) in a way that allows the best compromise between performance and durability.
Tyre surface temperature effects on performance are normally represented with a quadratic curve.
There are endless discussions about how sensitive tyres are to temperatures, and very little documentation publically available that is relative to race tyres. Every tyre is different to every other, plus what happens at the contact patch (where temperatures cannot be measured) is not completely known.
Tyre manufacturers always try to find a compromise between the overall maximum performance and temperature sensitivity. It is of course preferable to have a higher friction coefficient. To make a tyre easier to use (which means here to make it effective in a wider range of temperatures), the curve shown in previous picture should be as flat and wide as possible in its top part and this could go at the cost of maximum performance.
To further complicate things, tyre surface temperatures changes very dynamically, not only during a lap but also during a single corner. How this happens can be measured using infrared sensors, but the physics of these phenomena are extremely complex.
Luckily, tyre surface temperature measurements are more common today than they were some years ago. According to the publicly available data, a difference of about 35-40 degree Celsius between the braking point and the moment where the tyre reaches its maximum temperature in a corner is not uncommon.
With such a large and quick variation, if tyre’s grip would be too sensitive with regards to surface temperatures (as some of the few available sources suggest) and would change as quickly as surface temperature changes, we would probably see the behavior of the car changing dramatically during a single corner, making it very difficult to manage.
This could indicate that tyre surface temperatures are not the only factor influencing grip and that other parameters (thermal inertia, carcass temperature, tyre pressure variations, structural behavior, etc) also play an important role, maybe even more important than what happens (so quickly) on the outer side of the tyre.
Also a factor to consider is that, although teams always try to measure tyre temperatures as close as possible to the contact patch, it is of course impossible to measure what exactly happens there and we can expect very quick temperature variations as soon as the rubber rolls out and loses contact with the road.
Nowadays, top motorsport teams often use sensors that can measure tyre carcass temperature, air temperature (inside the tyre) and pressure in up to five points, dynamically. This allows a better understanding of the relationships between tyre inflation pressure and inflation gas and carcass temperature, but also opens the door to investigation how dynamically these metrics change and how they affect grip.
Tyre surface temperatures are also among the best indications to understand if camber settings are correct, too conservative or too excessive. A larger camber angle yields to higher temperature differences between the inside, middle and outside of tyre tread. Reading tyre temperatures on three points of his tread is the best way engineers have to find a balance between performance and durability and try to set camber correctly.