Cheap and easy measurement of car downforce and lift
Julian Edgar
Find my automotive aerodynamics, suspension and electronics books on Amazon.
Text: Julian Edgar
If you don't have a wind tunnel, aerodynamic lift/downforce is best measured by monitoring suspension height. If lift is occurring, the car’s body will be lifted on its springs – that is, ride height will increase. If downforce is occurring, the car’s body will be forced downwards on its springs, and so ride height will decrease.
By mounting sensors on the car’s suspension, we can therefore measure whether lift or downforce is occurring, and by doing some further simple testing, measure how much lift or downforce (e.g. in pounds or kilograms) is occurring.
In this test my 16-year-old son just wanted to find out if the available rear spoiler/wing made any measurable difference to lift/downforce on his 2002 Subaru Impreza. The car was tested in three configurations: with no wing or spoiler, with the standard spoiler, and with an STi-replica wing.
The standard rear spoiler.
The STi wing.
A ride height sensor was installed on the rear suspension. It is a modified Range Rover air suspension height sensor. Its output was fed into a simple analog smoothing circuit that gave an average ride height.
All the graphs are to the same scale, with sensor voltage on the vertical axis (higher voltage = higher ride height) and speed in km/h on the bottom axis.
The car without the wing or spoiler clearly develops rear lift – ride height increases. When the standard spoiler is refitted, all the lift is cancelled – ride height changes little with speed. The STi wing clearly provides downforce, causing the suspension to be compressed as the car was driven faster.
Note that in each case a slow speed run (20 km/h) was done to establish the baseline for that configuration. (At 20 km/h, the aero forces are near zero.)
In addition to giving measurable downforce, the large endplates on the rear wing move the lateral centre of pressure rearwards, giving better straight line stability.
This article is based on an extract from my book Car Aerodynamic Testing for Road and Track.
MS in ME Research '26 at CMU | BTech in ME '22 at IIT Bombay | Ex Jaguar Land Rover Vehicle Dynamicist
3 年This is awesome Julian! Could you please tell me how you filtered out the noise from the potentiometers?
President/CEO AERA, Currently Retired
3 年What ECU is receiving sensor data?
Motorcycle Suspension specialist
3 年That’s really interesting thanks
Senior Consultant at RINA UK Ltd
3 年Excellent ingenuity, I'm definitely going to check out the book for more ideas :-)
Unlock your vehicles top speed ? Director ? ChassisSim Technologies
3 年This is the definitive way of measuring downforce. I ram this home with all the bootcamp attendee's and I've lost count on the number of times I have had to do this. The other point I ram home is your correlation between actual and simulated data shows you what you have got right and have missed.