"Force Measurement in the ARC Wind Tunnel"
In the ARC Wind Tunnel, which can test up to 50% scale models, the model is mounted in the tunnel with a sting. The sting comes through the top of the model. Mounted inside the model is a 6 component balance that measures: Drag, Lift, Sideforce, Rolling Moment, Pitching Moment, Yawing Moment. This balance attaches to the ‘spine’ of the model. The main balance is held within a model motion system that allows the tunnel operator to drive the m
odel to different yaw angles, ride heights and pitch and roll, all whilst the moving belt is spinning at 110mph. Lasers mounted in the scale models ensure the model is at the correct position for testing.

When starting a test run, 'tares’ are completed. These tares are done with the road on only (i.e. no wind) and the weight of the model in all 6 components is removed. Therefore only the measured aerodynamic forces are given in results. Tares are done for every run, to remove the effect of different test parts on the model and ensure no creep occurs.
Model motion with balance inside

In addition to the main balance, in commercial truck testing a secondary six component balance measures all forces in the tractor unit. This helps the aerodynamicists when they are developing new aerodynamic parts to understand where the drag is being generated.
Dual balance system in commercial truck scale models

Wing balances are often used for high performance and race cars. These attached to the spine of the scale model and have an internal balance to measure the load of the wings separate from the rest of the body.

For open wheel race cars, wheel arms are sometimes used to separate the wheels from the scale model. This allows for the wheel forces to be separated. In order for this to be accomplished, a complex motorized suspension movement system must be installed in the model. At no times can the wheel and suspension touch, otherwise additional false forces would appear in results, yet the suspension and wheel distant relation must remain realistic.

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