TECHNOLOGY – SIMULATION
Figure 5: an example of when bump scale factors need to be applied
A graphic illustration of this is the Indianapolis Motor Speedway. I was always aware the track banking was approximately 9.2 degrees, yet the first time I saw it for real, I thought 'is that it?' Yet it is this relatively slight banking that allows high-downforce open wheelers to go flat out, and this is why it’s so important. A tell tale sign that a corner is on camber is when you have a simulated vs real corner that looks like that shown in figure 4. The first trace is speed,
the second trace is the front dampers and the third trace is the rear dampers. The actual car is coloured, the simulation traces are black. What is immediately clear here is the actual car is squatting significantly more than the simulated car. This is an indication that more road camber has to be added to this corner. However, most on / off camber
corners can be very difficult to detect, yet their effects are still significant, so what do we do? The first method is to look at circuit surveying data. This provides a conclusive answer one way or the other. The other is if you have GPS data. Your fall back is either track walks or watching in-car camera footage using tools such as YouTube. While it’s very subjective, there is nothing like a picture (or even better, a video) to illustrate what you are looking for. It saves an awful lot of second guessing and indicates what to aim for. Once the road camber has
been established, the next thing to look for is what the bumps are doing so that bump scale factors may be applied. What you are looking for is something like that shown in figure 5. As we can see here, the bump magnitudes
(as illustrated in the second and third traces) are the same, yet the corner speed differential is in the order of 20km/h. When you see something like this, you apply bump scale factor. What you apply will depend on a case-by- case basis but, for something like
These will vary from case
to case (particularly if you are investigating fine changes on ovals) but again, this will get you in the ballpark. One of the biggest mistakes I see people make using racecar simulation is they waste an inordinate amount
"The goal in aero modelling
is to establish both a ride height sensitivity map and general downforce levels for your wing changes"
that shown in figure 1, as a rough rule of thumb I’d be applying 70 per cent front and rear. Once this is ready and there
is still cornering differences, you then apply grip scale factor. Again, as a rough guide, equation 2 applies for grip scale factors, If you need to get to this
point, then what you are applying are very small corrections (in the order of 10–20 per cent, if that). However, it’s important to work through the other steps first because you don’t want to miss any important information along the way. This is why there is no auto grip matching in ChassisSim. It may make you look like a hero, but in reality it can skip over many significant effects that can catch you out. Also, in correlating your
circuit model, you don’t need to be accurate to the nearest 0.01km/h. You just need to be in the ballpark. At this point let me give you some further rough rules of thumb: • Un-calibrated model: 3–5km/h • Calibrated model: 1–3km/h
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www.racecar-engineering.com • February 2012
of time achieving the perfect correlation when, in reality, they should be focussing on ensuring the model is capturing what is going on with the racecar.
CAR MODELLING Now that we have discussed how to model the circuit, let us now talk about what’s involved with car modelling. This should be done in this order:
1. Aero modelling – the aero loads play such a significant effect on tyre loads that this has to be done first
2. Tyre modelling – once you have done the aero loads, this will fall into place
The goal in aero modelling is
to establish both a ride height sensitivity map and general downforce levels for your wing changes. To begin, a unity aero map using the fastest point of a long straight forms a good start point. That being said, as a rule of thumb, if you have a CLA value greater than 1.5 and car masses
less than, say, 1200kg then you need a pitch sensitivity map. In terms of tyre modelling, the necessary ingredients are the car data, the curvature file, the bump profile and any bump scaling and the circuit altitude and road camber data. This information, plus the monster file, forms the basis of the ChassisSim tyre force modelling toolbox. Once you have these elements you are ready to do tyre modelling. To cover this in detail is beyond the scope of this article, but it will be an iterative process and the rewards are more than worth it. So, in summary, our racecar
simulation procedure is just a matter of going through a simple series of steps. These are:
• Measuring the car • Entering the set up • Creating a model from an existing model that closely resembles your car
• Creating a circuit model • Refining the circuit model • Aero modelling • Tyre modelling
It’s not rocket science, it’s just
a matter of connecting the dots. What’s more, a lot of these steps are just extensions of what most racing organisations do, so all of this is well within reach. In closing then, as we can
see racecar simulation isn’t hard, it’s simply a matter of attention to detail and working through things sequentially. Once you learn to do this, racecar simulation will become a valuable tool because, as shown in figure 1, it gives you so much valuable information. And the great news is that all of this is well within reach of every motorsport professional / enthusiast.
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