Seven-Post Shaker Rig - Shaken, Not Stirred

What is a seven-post shaker rig, and why would anyone in racing need to use one? These are questions being asked in the higher levels of professional racing all over the world. Some know the answer, while others are just learning how much they have been missing.

Dynamics and vibrations are much harder to understand than static forces. Ever since man has been building and driving cars, the complex systems of springs and dampers have created a complicated symphony of noises and vibrations. The passenger car industry has been mounting cars on four-post shaker rigs for years, since it allows for more precise evaluations of body and suspension dynamics than running on a road. The inputs can be simple repetitive vibrations (sine waves), or they can be representations of real roads. While undergoing input from the road, sophisticated dynamic measurement devices provide insight to how the system is working. Generally, unwanted noises and vibrations entering the passenger compartment are the focus of these investigations.

In racing, the only objective is to go fast. One of the main limiting factors is how well the tires stay in contact with the track surface. The basic use of the shaker rig is to optimize the springs and shocks to minimize tire load variations while maintaining reasonable body motion control.

We have all seen a car running down the highway with a bad or missing shock absorber. The body is bouncing up and down like a boat on big waves, and the tire may also be hopping up and down showing daylight on each up cycle. This is a representation of what happens when the system of springs and masses is very under-damped. The shock absorbers are the key element here. They have to do the job of controlling the body motions as well as the wheel motions. One device (normally a shock absorber) mounted between the chassis and the suspension is asked to control a spring connecting two different masses, each with its own natural frequency. To further complicate the issue, the four corners of the car work independent of each other but are tied together by the body structure. The wheels basically move straight up and down relative to the chassis while the body has several motions relative to the ground. Engineers call these body motions “heave (movement up and down), pitch (forward and back) and roll (side to side).” Each of these motions is resisted by the springs at the four corners of the car. Resistance to these motions causes force variations between the tire and the road. The trick is to find the balance point. Tie the car down too tight and the force variation goes up, but freeing it up too much can do the same thing in the opposite direction. There has to be a compromise for the correct amount of damping that gives the best load control. Finally, there is one last, but very important, variable to throw into the mix. Driver preferences come into play here in a big way; some drivers like very little body motion, while others don’t mind a car that moves around a little more.

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.

To perform a useful test with a seven-post shaker rig, you must decide what input forces to apply to the car, and you must have transducers to measure the response, or output. You then need to know what to look for in the response plots so you can judge if improvements have been made.

The input side is normally a sine wave with all wheel pans in phase (operating in unison). You can, however, run them in any manner: one at a time; the two fronts out of phase with the two rears, which is pitch input; the right sides out of phase with the lefts, which is roll input; or any other combination. You can also run inputs from recordings from actual on-track test sessions.

The instrumentation typically used measures acceleration, travel and force. The vertical accelerations of the sprung and unsprung parts of the car are of particular interest because they show the natural frequencies of the system.

The force between the tire and the road is also of particular interest because it is the most direct measure for assessing the effects on grip. Digital recording is the current state-of-the-art method and allows engineers to process the data efficiently. Because the test is for studying vibrations over a range of zero to 100 cycles per second (hertz), the digital sampling rate needs to be over 500 samples per second.

All vibration problems must be studied in a methodical way. There is an input source for the vibration, at least one spring, mass and damper system that responds to the input and a transmission path from the input disturbance to the responding mass. Each of these factors must be understood in order to control or change the overall response. In racing, we cannot change the input; it comes to the car from running around the racetrack. The transmission path is through the tires, which are usually specified by rules and unchangeable except for pressure. The responding systems are the car’s sprung and unsprung masses. These all relate back to how the driver perceives how much grip he has.

Remember, grip is the ability of the tires to stay in contact with the road. The seven-post shaker rig allows us to simulate conditions as a car rolls through a turn and repeat this over and over while changing the shocks, springs, sway bars, tire pressures, nose weight, etc. until we have found the combination that provides the least load variation in the tires. One thing the shaker rig does not do is simulate lateral forces. In the tests, tires only see vertical loads. There are some differences in the tire’s vertical spring rate when operating at a slip angle (around a turn) versus no slip angle (rolling straight ahead). Teams can measure the effect of lateral forces on grip by performing a second test with the best shock proposals learned from sessions on the shaker rig.

Chassis response to input from the racing surface can have varying degrees of what engineers call “coupling.” Coupling is the amount of energy transference from one tire to another. For example, you go into a turn and hit a bump with the right front wheel. You would like for just the right front to react to that bump and return to normal quickly. If the tuning is not correct, the bump at the right front will cause reactions at the other three tires. When all four tires are involved, the overall grip is diminished just from hitting a bump with the right front. If you can take this into account and tune for it, the car can maintain the fastest average speed through the turn while hitting that bump.

Any race team can benefit from a session on a seven-post shaker rig. It will give the crew chief a clearer picture as to what sort of shock changes he should be making for conditions at the track. Like almost everything else in racing, current customers are almost exclusively Winston Cup teams, but the technology will eventually trickle down to lower level teams. The going rate is around $5,000 per day. There are only a few of these facilities available for commercial use. Most car manufacturers have four-post rigs and can build up seven posts from available laboratory test equipment. ARC in Indianapolis, Ind., and Ohlins USA in Hendersonville, N.C., are the two main facilities available to any race team. They both have engineering staffs who are very helpful in directing the test procedures as well as interpreting the results.

Yes, this is elite technology, but you just might be scheduling time on a seven-post shaker for your Late Model in the not-too-distant future.

Terry Satchell is the chassis specialist for Ford Racing.