Loaded questions

OptimumG’s series on tuning springs and dampers continues with an explanation of the importance of tyre load variation

- CLAUDE ROUELLE

In the previous edition of Racecar (V30 N3), we tried to answer the question: how do we determine suspension spring rates and damping for a vehicle?

To answer this, it was necessary for us to first define the purpose of the suspension. We looked at how the suspension plays an important role in improving the driver and passenger comfort, handling, aerodynamics, mechanical grip, vibration and transient response.

With the purpose of a suspension defined, the next step was to select our spring and dampers. It was explained that to choose these it is necessary to first define the goals for the suspension system: driver comfort, tyre comfort, body control.

We then closed the loop by discussing how you should define your goal based on the track scenario: bumpy or smooth circuit, off-road, passenger vehicle or high/ low downforce vehicle. This month we will be looking at one of these goals: tyre comfort.

As previously mentioned, tyre comfort is important for mechanical grip. Mechanical grip refers to the efficiency (friction factor) of the tyre. Figure 1 shows a typical lateral force (Fy) versus slip angle (SA) curve, for five different vertical loads at zero degrees of camber, and a constant pressure. When testing a tyre, a typical chart used to understand the tyre’s lateral performance is an Fy-SA, like this. In this article we won’t delve into analysing the tyre’s lateral performance based on this chart. Instead, we will use this to help illustrate mechanical grip.

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