FIRST PRINCIPLES DANNY BOY
exceptions to this, so don’t skimp or cut corners. If you do that, it will come back to bite you.
After we have done our prep
work, we then need to consider whether we require a proper aero test programme. As a rough rule of thumb, if the CLA obtained from either the manual or your hand calculation is greater than, or equal to, 1.5, you have to do an aero test programme. Again, there are no exceptions to this. I would love to say just read the
racecar manufacturer’s manual and you’ll be fine, but I have lost count of the number of times exploiting an oddity in the car’s pitch sensitivity has literally meant the difference between winning and losing. It’s that simple.
Aero programme How in depth you need to go will be determined by your simulation correlations to your data when you shakedown the car. This is an example of why, at least as far as I am concerned, if you are not using lap time simulation, you are committing professional suicide. If your damper displacement
correlations are close throughout the speed range then you don’t need to do pitch sensitivity testing, but often you won’t be that lucky. So during aero testing you should also log the following: • Vehicle speed, rpm, gear and accelerations
• Damper displacements (take this at the damper)
• Strains (if you have them) I know this is obvious but I thought I’d mention it anyway. When doing your aero testing, your goals are as follows: • Start with a nominated front and rear wing and appropriate ride height. Take this either from the racecar manual or experience
• Determine the ride height sensitivity map by sweeping the ride heights
• Once you have done this, return to the nominated ride heights and sweep the wings Bear in mind you might very
well have to do this depending on the wing configuration options you
have available. As a rough rule of thumb, if there are jumps of CLA of 0.3-0.4 between the different configurations then it’s a good idea to do it. That said, if time is at a premium,
formulate the pitch sensitivity map on the configuration you will use the most. This is not ideal but it will be enough to get you by. The most important job you
are going to do in aero testing is constructing the front and rear ride height sensitivity map. You can be forgiven a multitude of sins if you get this right. When we run a racecar and
resolve the forces on the data you obtain a thin sliver of the aeromap. This will look something like figure 1. Our goal is to collect as many of these slivers as possible. When we are done, we are after a plot of front and rear ride height for all our different slivers that look like figure 2. Effectively, the more points you have in figure 2, the better the aeromap is going to be. The way we are going to do this
is to walk in with a test programme then, after each run, look at the data intelligently. The test programme I will run will resemble that shown in table 2. These ride heights can be in any
units the user wants, though my personal preference is in mm. The ride height deltas are going to be car height specific (on an open wheeler or Sportscar, this will generally be about 5mm, while Touring Cars are usually around 10mm). The best way of determining the appropriate delta is to compare damper or strains from run to run. Start small, then increase the delta until you see about 1mm worth of differential damper displacement. That will determine your delta. As with anything in life, the more you do it, the better you will become at it. It should also go without saying that you should not run your car into the ground. It’s important during this
process to review the data in between sessions. In particular, you should be looking at the inferred ride height channels. Ride height can be inferred by the following approximation,
66
www.racecarengineering.com • August 2011
Table 1: sample of a spreadsheet that returns lateral load transfer at the front with estimated tyre loads
Roll centre front (m) Roll centre rear (m)
Front weight distribution (%/100) Front tyre spring rate (N/mm) Rear tyre spring rate (N/mm) Front spring rate (lbf/in) Front bar rate (N/mm) Rear spring rate (lbf/in) Rear bar rate(N/mm) C of g height (m) Front track (m) Rear track (m) Mean track (m)
All motion ratios are damper / wheel Front motion ratio Front roll bar ratio Rear motion ratio
Rear bar motion ratio
Front main spring wheel rate (N/mm) Front roll bar wheel rate (N/mm) Rear main spring wheel rate (N/mm) Rear roll bar wheel rate (N/mm)
rcm hsm
rsf at spring rsr at spring rsf at tyre rsr at tyre Prm
Roll distribution (%)
Weight transfer calculation Car mass total (kg) Car speed (km/h)
Lateral acceleration (g) CLA
Air density (kg/m^3) Aero distribution
Total aero force (kgf) Front aero force (kgf) Rear aero force(kgf) Total lateral force (kgf)
Static load front (kgf) Static load rear (kgf)
Load FL (kg) Load FR (kg) Load RL (kg) Load RR (kg)
-0.0254 0.0445 0.43 260 290
1300 1738 1300 500 0.3
1.63 1.529 1.57243 1.03
0.5869 0.8696 0.621
241.5285 598.6569 172.1603 192.8205
0.014443 0.285557 840.1854 364.9808 198.5558 161.5993 0.567072 48.94622
712 113 2.3 3.2
1.225 0.43
197.0525 84.73256 112.3199 1637.6
195.4463 259.08
348.3707 42.52183 418.5891 99.57079
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