Figure 4: a comparison with an on-camber corner that hasn’t been simulated
perfect it should get you within 10 per cent, and I can tell you from practical experience that this is good enough. Lastly, I want to go through
a general check list of what you need to tick off when putting your base set up for the sim:
• Springs, bar rates • Dampers – peak force vs peak velocity curves
• All bump rubber information and gaps
The next step in the process
is to get race data that matches your set up. Again, incredibly obvious, but so important. The things you want to focus on
Equations 1
iR Z = iR ⋅ t φan( )rc y where,
iRz = normal curvature iRy = inverse corner radius Ørc = road camber angle
2
V GF V
= ⎛
⎜ ⎝
⎜
SIM ACT
⎞ ⎟
⎟ ⎠
where, GF = grip factor to apply
Vact = actual simulation speed
VSIM = simulated speeds 2
when getting the data ready are as follows:
• Ensure the speed trace is smooth. When it comes to circuit creation, brake lock ups can ruin your day
• Ensure the longitudinal acceleration is related to the speed, don’t just take it on face value
• Ensure the lateral acceleration and throttle sensor are behaving themselves
• Dampers – zero them on the ground, typically leaving / entering the pit lane
• Steering – this is the driver input calibrated at the tyre
I call this creating the monster
file. This contains the bulk of the information you need to validate your model. Once we have the data, hand
calculate the downforce at the fastest point in the circuit. This gives you a great picture of what the aero is doing. Also, if you don’t have pitch sensitive data, set the aero map at these hand calc values for CLA, CDA and aero balance. It’s not perfect but it gives you a very good start. When you have the data and
the set up, you are now ready to create a vehicle model. I strongly suggest you start from a model that most closely resembles your car and, slowly but surely, make the changes one at a time and test with a pre-loaded circuit.
Doing it this way will ensure you don’t make any mistakes. As I have mentioned before, you don’t get extra points from starting from scratch, you get credit for a model that produces results.
THE CIRCUIT MODEL When you have entered your model, the next step is to create the circuit model. A typical circuit model consists of the following:
• Curvature file – this is the circuit trajectory. It is a plot of inverse corner radius vs distance
• Bump profile – this is the road surface trajectory
• Altitude / road camber file – this plots road camber and altitude
• Bump scale factors – local scaling factors for the bump profile
• Grip scale factors – again, local scaling factors
dictate the grip on the circuit. We’ll discuss how the other elements come into play shortly. Now, log the curvature and
bump profile to data and do an initial comparison. What you are interested in is the general speed trace. If the speeds are down in every corner, or greatly increased, this is your cue to adjust global grip factors. Remember, for a non-calibrated model, correlation of 3-4km/h is good enough. The next step is to look at the comparisons where the corner speeds have a differential of 10km/h or more. At this point you should be asking yourself the following questions:
• What’s the road camber / normal curvature here?
• What are the bumps inputs at this point?
Only when you have asked these questions are you ready to play with local grip variation.
"make the changes one at a
time and test with a pre-loaded circuit"
I have deliberately listed
the components in order of importance. The curvature and the bump profile are what you do first. This is critical because they are the two most important elements of what is going to
Let’s now explore this in further detail. The reason road camber is so important is because of the effects the normal curvature will have on tyre load. Effectively, the normal curvature it causes can be described by equation 1.
February 2012 •
www.racecar-engineering.com 71
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