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Real World Measurement of Drag and Lift

Down to earth aero testing techniques

by Julian Edgar

Click on pics to view larger images


This article was first published in 2008.

In previous AutoSpeed stories we have covered in detail the direct measurement of aerodynamic pressures (see the series starting at Undertrays, Spoilers & Bonnet Vents, Part 1 and we’ve also covered airflow visualisation with wool tufting (see the series starting at Aero Testing - Part 1).

But there are also other down to earth aero testing techniques that can be used.

Airflow Meter Output

At higher speeds, the power developed by the engine is used almost entirely to overcome aerodynamic drag. If engine power is then measured in different aerodynamic configurations, changes to drag can be seen.

An indication of the amount of engine power required to overcome drag can be found by measuring the engine’s air consumption at a constant speed on a level surface. This can most easily be carried out by measuring the airflow meter output voltage or frequency by means of a multimeter mounted in the cabin. (Note that while it may at first appear that a more accurate result would be gained from measuring injector duty cycle, in practice this figure jumps around a lot, even at a constant speed!)

Click for larger image

A test was carried out on a Volkswagen Golf cabriolet, measuring the airflow meter voltage output. Soft top cars have much higher drag when the top is down, due to the separation (and so turbulence) that then occurs. This graph shows the results of the tests, which were all done in fifth gear at a constant 100 km/h.

As can be seen from the graph, the drag is lowest with the roof and windows closed. The next lowest drag occurs with the roof open but the windows up. Having the windows down but the roof up is next poorest and the worse results are gained (as expected) with the roof open and the windows down.

This testing must obviously be done on a flat road on a windless day and with the speed held absolutely constant. Higher speeds will show aerodynamic drag variations more clearly.

Measuring Lift and Downforce

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The following technique can be used to measure the lift or downforce acting on the car.

A linear potentiometer (“pot”) is attached to the car such that the wiper arm of the pot is moved with the suspension travel. This can be done using a lever pivoting from a front or rear sway bar, or with the pot mounted parallel to a strut.

A multimeter is placed inside the cabin of the car, reading off the pot’s resistance. As the car passes over bumps, the value being read on the multimeter will constantly change but on a smooth road driven at speed, a change in average ride height can be seen. For example, at high speed, the average resistance of the pot might change from 500 to 600 ohms. To calculate an accurate average reading, an averaging data logger is required. (See Real World Spoiler Development for an example of this technique being employed.)

With an average figure gained, and the car again stationary, an assistant can then be used to press down the front or rear of the car’s body (or lift it) until a similar reading is measured inside the car. The force applied by the assistant will then show the magnitude of the lift (or downforce) that is occurring on that axle. This type of test will be most effective on a car with soft suspension.

If a wing is placed at the rear end of a boot or fastback car’s hatch, the downforce developed by it can be approximated by the following test. For the sake of clarity, the test procedure is described only for a three-box sedan with a wing mounted on the trailing edge of the boot.

Click for larger image

Firstly, a spring is temporarily attached to the boot hinge assembly so that the boot is held slightly open. This means that closing the boot the last few centimetres extends the stiff spring. An assistant is placed inside the car. He or she accurately describes the height of the wing by lining it up with a mark made on the rear window. Obviously, the passenger’s eye level must be held constant throughout the testing. Before testing commences, the boot lid is closed against the spring tension, being shut as far as it will go before it starts to compress the boot’s rubber seal. The assistant notes this position, and a piece of masking tape is applied to the rear window to show this ‘closed’ level.

The car is then driven with the boot latched shut. At about 50 km/h, the boot is opened using the remote release, allowing it to spring up a little. As the car is then driven faster, the wing pushes down on the boot lid, closing it against the spring tension. The minimum speed that holds the wing in its previously marked ‘boot closed’ position is then noted.

For example, with a certain spring pre-load, the car might need to travel at 110 km/h before the boot lid is pushed down to its ‘closed’ position. With the car then stopped, the wing is loaded with weights until the boot lid reaches this same closed position. The minimum weight placed on the wing that closes the boot lid to this position is the downforce being exerted by the wing at that speed.

A test was carried out in this way on a R32 Nissan Skyline GT-R. At 100 km/h the standard wing exerted a measured downwards force of just under 6kg.

Top Speed and Drag

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The calculation of a car’s approximate drag co-efficient can also be carried out. If you know the amount of power that the car has available at the wheels in watts (kilowatts x 1000), the car’s frontal area in square metres (~ height x width), and the car’s top speed in metres per second (km/h x 0.278),

Power at wheels
the equation used is: Cd = --------------------------------------------------------
0.6 x Area x Speed x Speed x Speed

For example, for an R32 Skyline GT-R with a top speed of 255 km/h that becomes:

200,000
Cd = ------------------------------------------------ = 0.40
0.6 x 2.35 x (70.9 x 70.9 x 70.9)

The drag coefficient of the R32 GTR at the time of its release was quoted at about 0.38.

Alternatively, if you know the power it takes to maintain a certain speed, you can do the calculation without having to drive very fast.

Click for larger image

For a slightly modified EF Falcon, the power at the wheels at 105 km/h is 13kW. This was ascertained by noting the instantaneous fuel consumption shown on the trip computer and then dyno testing the car at the same speed, measuring the power at the wheels when the instantaneous fuel consumption read out was as occurred on the road at that speed. With a frontal area of 2.7 square metres, the calculated Cd is 0.32. At the time of release, the claimed Cd of the EF Falcon was 0.31.

While this equation is good fun to play with, it has several limitations. It does not take into account rolling resistance (only a small proportion of the total resistance at full speed) and you also soon discover that altering a variable like engine power or speed makes a dramatic difference to the calculated Cd value. So you cannot use this equation to give a definitive figure - but it can give some interesting results.

Conclusion

It’s easy to assume that directly measuring aero data is impossible without a wind tunnel and sophisticated gear. However, that is simply not the case.

Interested in do-it-yourself car aerodynamics? You’re sure then to be interested in the Amateur Car Aerodynamics Sourcebook, available now.

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