FIRST PRINCIPLES
the longitudinal properties, my first port of call is multiplying the function to dial in my desired characteristics. If we want more acceleration, I increase this globally, and vice versa for reducing acceleration. To illustrate this, I’m going to increase the front function by 10 per cent and reduce the rear by the same amount. The effect of doing this is shown in figure 5, above. For clarity, I’ve zoomed in
Table 2: three rough rules of thumb for camber sensitivity
Car
F3/GP2/LMP 3-4 deg FIA GT/GT3 4-5 deg
Opt camb Sf_cam_y Sf_cam_x Ux Init 2-4
2-4 Touring Car 5-7 deg 0.4 – 1
2-4 2-4
0.4 – 1
1 1
0.7 - 1
on just one corner, but the speed and throttle trace are very revealing. The overall peak longitudinal g hasn’t increased because that is being dictated by the maximum braking capacity of the car. That said, there is a slight increase in speed as we approach the final turn in section, which makes its presence felt in acceleration. Looking at the throttle trace, the black is our change and the green is the baseline. In some spots it’s nearly seven per cent done and the speeds out of the
corner drop by about a 1km/h. In summary then, when dialling in our tyre model the following list is our order of business here:
• Dial in the grip by adjusting global grip factors and, if necessary, the tyre force curve
• Dial in the base understeer / oversteer by taking deltas of grip factors and increasing / decreasing peak slip angles and slip ratios
• Dial in the traction circle and ellipse properties
As you can see, this isn’t terribly difficult to do. It comes as a consequence of the physics we outlined at the start in equations 1 and 2. It does take time, but anything worthwhile usually does. However, this is the knowledge you can use to win races. In part two next month, I’m going to show you how to extend this to temperature modelling.
Figure 5: modified µ′ TC δcamb ,Fz ( €
)
function compared to baseline 80
www.racecar-engineering.com • September 2011
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