What is 4-post and 7-post Testing?

Nobody should have any qualms about asking this question. Outside of a select handful of industry experts, very few people truly understand what 7-post testing is – even at the top levels of motorsports. Even fewer understand how to harness these capabilities and extract the most from this type of testing, while managing the assumptions that go with it. One of our goals at ZETA has always been to not only improve our customers’ performance but also to teach the ‘how’ and the ‘why’ so they can understand the path that led to the results.

What does the 4-post rig do?

Fundamentally, the shaker rig is used to apply vertical loads to a vehicle and measure the reaction of the suspension system and the body. A hydraulic actuator is located underneath each wheel to push up on the tire, creating the four posts of a 4 post rig.

These four actuators, known as wheel pans, move up and down in a controlled displacement to generate the roughness of a road surface being simulated or move across a frequency band in a controlled sweep to gain a general overview of the vehicle’s behavior across its expected operating range. Because we generate all of these profiles, profiles can also be created for more specific purposes as demanded by customers such as discrete profiles (striking a curb or pothole) or pink noise profiles to study specific energy inputs.

4-post testing is a powerful tuning tool for general suspension characterization and damper tuning, especially for vehicles with symmetric suspensions and low downforce. 4-post testing cannot generate vertical loads as seen from braking, turning, banking or downforce, but it can generate the same frequencies seen by the suspension allowing for effective characterization of the suspension, dampers and body response.

Then what is a 7-post rig?

Expanding upon the 4-post rig, a 7-post adds three additional hydraulic actuators that are attached to the chassis. While the 4 posts push up on the tires, these additional three actuators pull down on the chassis to apply loads seen on-track from downforce and inertial effects such as braking and turning.

Through the use of the 7 actuators, a 7-post can be used to accurately replay the vertical loads seen by a vehicle on track or on the road, allowing for a track to be replayed in a controlled environment while measuring the effects of tuning changes performed on the vehicle.

What does 4-post / 7-post shaker rig testing measure?

The force of the tire on the wheel pan, known as the contact patch load, is measured to study how the tire load varies relative to load inputs – a parameter known as Contact Patch Load Variation. The load seen at the contact patch will increase as the tire is pushed up due to gravity and the reaction force of the suspension, such as when hitting a bump, and will decrease after. Contact patch load variation can loosely be viewed as a measure of tire efficiency or mechanical grip, and it is traditionally accepted that the less the load on the tire changes relative to excitation, the more grip the tire will make. Shaker rig testing is an extremely effective method of analyzing contact patch load variation, which is a valuable parameter for damper tuning.  

Through the use of instrumentation mounted on the wheel pans, the suspension and the body, the response of the sprung mass (the body) is also analyzed. The response of the sprung mass in heave (up and down), pitch (front to back) and roll (left to right) is analyzed. In most cases, roll response is negligible due to a symmetric suspension (oval track cars are an exception) and a priority is placed upon the improvement of pitch response. Reducing pitch response reduces weight transfer between the front and rear improving mechanical grip, and also improves platform of the body, reducing movement of the aerodynamic Center of Pressure improving aerodynamic grip.

What happens with all of the information?

Proprietary software and analysis methods are used to perform analysis on the raw data recorded from the rig, delivering data for the customer both in graphs and tabular metrics in a spreadsheet. Customers are provided information regarding mechanical grip, body response, and aerodynamic parameters.

Zeta engineers are on hand to not only operate the rig but also guide customers through the analysis of results and assist in vehicle tuning. With years of experience in race engineering and vehicle dynamics tuning, Zeta staff is here to ensure customers get the most from their testing efforts.