8-Post Shaker Rig
An 8 Post Shaker Rig is a machine which is used to improve a vehicle’s ride and handling dynamics. Shaker rigs have been primarily used in racing to simulate race tracks allowing various vehicle suspension set ups to be tested prior to race events. SoVa Motion’s 8 post rig has four hydraulic “wheel loaders” on which the vehicle is positioned simulating the ground inputs into the suspension, along with four hydraulic “down force actuators” which can be attach to the vehicle’s body and simulate both the inertial and aerodynamic forces that a vehicle experiences. The wheel loaders comprise the first four actuators and can be used by themselves in a 4 post test, or in conjunction with the down force actuators arranged in various configurations for either a 7 Post or an 8 post test. SoVa Motion’s personnel, along with the 8 post rig and controller have the knowledge, experience, and capabilities to provide our customers with the best testing experience through drive file testing and hardware-in-the-loop vehicle simulation.
7 Post testing was developed primarily within the open wheel formula ranks. These cars typically have monocoque structures with a very high torsional stiffness. Accordingly, these stiff vehicles do not experience much torsional, or warp, displacement. Therefore, the three points-defines-a-plane assumption is quite valid. However, when testing production vehicles, or production based race vehicles, such as a Porsche 997, the bodies do flex and with the additional down force actuator we can replicate this warp effect. Secondly, and more importantly, having 4 down force actuators allows us to better replicate the load transfer dynamics that occur in a vehicle. With 3 down force actuators, there is only one way to provide a roll moment. However with 4 actuators, closely coupled with each corner of the vehicle, SoVa Motion’s rig has the capability to better replicate the load transfer dynamics (e.g. roll couple distribution) as they occur in the vehicle. In vehicles that have a higher contribution from inertial forces relative to the aerodynamic forces, the extra control translates to better testing fidelity.
Each of the 4 wheel platens can produce up to 11,000 pounds (5,000 kg) of dynamic force and each of the 4 additional down force actuators can provide up to 4,500 pounds (2,000 kg) of inertial and/or downforce loading. The large load capabilities coupled with 12 inches actuator strokes allow the machine to encompass a wide variety of road simulations. The wheel platens and down force actuators are repositionable, allowing for testing of a wide range of vehicles.
Road Inputs can be created from on-track vehicle data, or custom lab inputs created at SoVa Motion. Both of these input types will help gain a better understanding of the vehicle’s suspension setup and allow teams to make informed decisions when changes are made at the track.
- Road to Lab Vehicle Development
- Collect On Road Data or Track Data from Vehicle
- Generate Representative Drive Files on Multi-Post
- Determine vehicle response and suspension metrics from drive file inputs which correlate to on road subjective evaluations
- Develop Vehicles using metrics and confirmed with subjective evaluations
- High repeatability of vehicle response with minimal test time and part change time
Using the sine sweep in heave, front and rear natural frequencies will be analyzed by comparing both magnitudes and phases. This will determine the balance of the vehicle across the tested frequency range. An unbalanced vehicle will have excessive body motions at either the front or rear of the vehicle over track disturbances, potentially resulting in unpredictable behavior mid-corner or under heavy braking. A balanced vehicle will typically have similar front or rear natural frequencies and magnitudes that can be determined by viewing the transfer function of front-to-rear body accelerations.
Grip (Contact Patch Load Variation)
Using the random road, impulses, sine sweeps, or track drive files, the contact patch load variation (i.e., grip) can be determined. Random excitations will have frequency content similar to most race tracks, but track drive files will be more tailored to a specific track. To accomplish this, load cells in the platens (road) record a range of load values during the tests. Since tires generate the most lateral force with consistent and higher loads, smaller variations should produce better cornering performance from the vehicle. Similar front and rear grip values are also desired in a well-balanced vehicle.
Using the sine sweep in heave, the damper balance can be determined. If excessive damping in either rebound or compression is present, the vehicle will be jacked down or up, respectively. This will potentially reduce damper travels or upset the aero platform. Viewing the time history trace of the wheel displacement will show both of these behaviors.
Using the random road, impulses, sine sweeps, or track drive files, the wheel control can be determined. Similarly to body control, wheel control is calculated by taking the transfer function of the platen (road) accelerations to the four hub accelerations. Ideally, if the hub is following the platen (road), it will have a magnitude of one. Therefore, any deviation from one is undesirable and will either cause a reduction in grip or overwork the tire, causing premature wear.
Using the random road, impulses, sine sweeps, or track drive files, the body control can be determined. Body control is calculated by taking the transfer function of the four platen (road) accelerations to the four body accelerations. Ideally, a well-controlled vehicle will have a gain value closer to zero, meaning the body is isolated from the road inputs. A controlled body will provide the driver and the aerodynamic package with a stable platform. A single metric number is calculated for easy comparison across the relevant frequency ranges.
Optimization is performed using the initial baseline and multi-post rig inputs. Since the test rig measures vertical force only, it does not simulate the entire environment that a vehicle may encounter on the road or track, such as lateral loads. However, trends and directions of vehicle improvement or loss can be identified and used at the track for race-day tuning.
After the vehicle is analyzed in its baseline state, changes to spring rates may be necessary to bring the vehicle back into balance. Once the vehicle is balanced, the dampers are optimized. It is usually necessary to perform a basic damper setting sweep, the purpose of which is to determine the sensitivity of the vehicle to changes in damping levels and to make any necessary adjustments. Once enough runs are made, metrics are plotted that provide a visual representation of trends.