FIRST PRINCIPLES DANNY BOY
a rough rule of thumb. Figure 3: plot of simulated camber
Tyre pressures Our next area of investigation is tyre pressures. This is often overlooked, yet it is critical in determining tyre temperatures and tyre spring rate. This is what I would do: start with the manufacturer’s recommendation and log the data. Advance the front tyre pressures by one psi and log the data. When you’re comparing the data, pay close attention to the lateral acceleration and steering trace and, if you can trust them, what the driver has to say. Then repeat the process for the rear. I would love to claim credit for this, but this is something a former boss of mine came up with and it worked really well. The last step in the process is
to throw as many bars and springs at the car to get as much load variation as you can. This is where table 1 comes in again, because what we want is to collect as much information as possible to fill in the maximum traction circle radius vs load characteristic. This is illustrated in figure 5, below left. When it comes to tyre
Figure 4: lateral sensitivity vs camber
modelling, you can be a forgiven a multitude of sins if you get this right. This is why we need the lateral load transfer analysis in table 1, because the more load variations we throw at the car, the better idea we are going to have of what the car wants. Also remember, this curve isn’t just a simple square function. Due to temperature effects, it can be highly non-linear. With this method of testing we are going to capture this, which is invaluable information.
Once we are at this point, you’ll
Figure 5: traction circle radius vs load characteristic 68
www.racecarengineering.com • August 2011
have the information you need to go chasing lap times, regardless of the situation you find yourself in. You can now use simulation tools to reverse engineer the tyres and aero. Alternatively, you can use the techniques on tyre and aero modelling that I’ve discussed in recent issues and do it manually. Then, you will be in a strong position to know what the car is going to do in a multitude of situations. I also guarantee, once you have this knowledge, your
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