Shock Pots for Downforce Testing

I have shock pots and have used them to attempt to estimate total downforce and aero balance and the associated downforce coefficients (front, rear, total). My experience has been:

1) wind and air density variations introduce potentially large errors, even when testing same day, "same conditions" multiple configurations. Ground speed is a poor analog for dynamic pressure, which can be measured directly with a pitot-static tube, which essentially eliminates air density and (steady) wind speed as sources of error.

2) dive and squat, even at very modest longitudinal acceleration values, significantly affect damper positions. Therefore valid downforce data requires steady speed conditions. A hot lap on track will never provide good downforce or aero balance data via damper positions. For the same reason, the test section needs to be flat, level, straight and smooth.

3) analytical evaluation of wheel rates and wheel positions vs. contact patch loads are very dubious. In contrast, calibrating wheel positions vs contact patch loads by weighing the car with and without known ballast (and known fuel state and driver weight) is relatively easy and it provides a valid correlation between weight/downforce and damper positions for each axle (i.e. average the results across each axle). Calibration data should be conducted at low speed, not stationary, to allow enough suspension movement to break stiction.

4) damper settings can affect results. Too much rebound damping will cause the car to jack down, overestimating downforce. Low damping levels are probably beneficial when collecting downforce data via shock pots.

5) even road sections that appear and feel smooth can result in significant suspension movement. I have found it most useful to test at a constant speed over a significant interval (say 20 seconds), but to sample the data (say 3 seconds) from sections of track where the damper positions are smoothest (i.e. the road is actually smoothest).

6) Even with very smooth data you need to average over an interval to get repeatable results, because the slightest variation in damper position represents a lot of load with the spring/wheel rates typical of track cars.

That getting good data even with shock pots is difficult, reveals how totally inadequate the zip tie method is.

For the last 20 years, I have been successfully using damper data (shock pots), with small purpose built race cars that can produce 400-500lbs of downforce at 100 mph. I have very repeatable results, and can easily measure changes of 2-5%. I combine the suspension position data to get "Front Downforce, Rear Downforce, Downforce Bias.

I have math channels that filter the data so that I can produce tables of average load, aero balance at 100, 120 and 140 mph.

Yeah, zip ties won't do that. But pushrod strain gages are not required to get useful data for the cars I work with.

BTW, have you noticed that you can align data very precisely by using the "shape of the bumps", this allows direct comparison of two setups in a particular spot on the track.

What have you used for sample rate, and what is the suspension position resolution of your data?

I haven't tried to extract data from lapping data, only from straight line tests, as my goal was to determine an actual lift coefficient (using pitot-static data to get dynamic pressure). I can see how a track might still be useful for a direct comparison.

I am logging at 200 Hz at 0.1 mm damper position resolution. 1 mm at the damper corresponds to about 15 lbs at the contact patch (front and rear). At that resolution the data is pretty noisy due to damper position fluctuations, but the average values over a reasonable sampling interval of steady state conditions produce rational data. Since track driving contains very little steady state, I have not been tempted to try use it to extract downforce data.

Hi there James,

You're definitely right, getting accurate aero numbers from potentiometers is extremely difficult, even strain gauges can be difficult to get good, clean numbers if you're looking for small deltas. Like you alluded to, the only real place you can get them is down the straight, and the best way to check them is to do a steady state cruise at constant (high) speed, then average the number. Like David says though, they can definitely get you in the ballpark!

With regards to density changes etc, it is best if you record atmospheric conditions before and after each run so you can know the density, and ideally perform the testing on a day with minimal winds. If you have a pitot static tube as well that's great! However you will still need the air density from the atmospheric conditions.