The seven-post rig is used in racing work because aerodynamic downforce and track banking add to the wheel loads. The amount of load added can be more than the static initial weight, so it must be included in the test procedure. The seven “posts” are hydraulic cylinders. Four of them have flat pans the tires sit on and support the car. The other three are called the “aeroloaders” and attach to the sprung mass. Normally, two are mounted to the front of the chassis some distance apart while the third one is mounted at the rear on centerline. Loading on these cylinders is done to pull the car down, opposing the four wheel pans. The aeroloaders simulate other forces on the car such as the squashing from inertia loads as the car rolls through a banked turn or deflections due to aerodynamic loading.
By adjusting the load on the three downforce rams we can simulate any combination of roll, heave or pitch displacement to recreate specific conditions seen on the track and repeat that condition. Normally, wheel travels from actual test-session recordings are re-created in the lab. By using the correct deflections indicated by wheel travel with the same springs and bars as those used in the track test, the loads will be correct. Deflections are used because race teams seldom have vertical loads as a measurement.
Now that we all know what a post rig is I would like to discuss here the whats and hows for its operation and understanding. What are the variables used to calculate the suspension optimization and how they calculate them. I guess it may be a vertical load average and its variance. But I also have heard other stuff...
Also it would be interesting to know how its the post rig built. How are the sensors and actuators attached, etc.
I know there are many teams (not F1) that have built their own 1/4 or 1/2 car post rigs with good results.