7 post shaker

The 7 post shaker is a piece of test equipment used to perform highly technical analysis for racecars. A 7 post shaker can apply all vertical forces seen by the car on the race track. This is akin to driving the car on the racetrack without actually driving it on the racetrack. There are an exceptional amount of controls for the hydraulic system and also many thousands of sensors to detect what is happening to the car as it is exposed to the virtual racetrack and the associated forces. These forces include banking loads, lateral load transfer, acceleration, braking and ride height sensitive downforce. This analysis allows the chassis engineer to replicate the actual corner loads seen at the track and to analyze the interaction between suspension setups and the resulting corner loads. This allows the engineer to study the understeer or oversteer characteristics at either one specific section of the racetrack or the entire racetrack. This detailed analysis is required at high levels of racing. [ [http://www.autoresearchcenter.com/index.php?main=services&subMain=7-post%20rig ARC - Auto Research Center ] ]

Uses

The 7 post shaker is used by a multitude of vehicles in different driving conditions. This type of testing equipment is also known as the 7 poster, the 7 post, a shaker rig and a shake table. There are also earlier versions of the 7 post shaker, such as the 5 post shaker and a 4 post shaker. The 4 post shaker is commonly used by vehicle manufacturers to determine if their vehicles will handle specific road inputs. Manufacturers don't use a 7 post because most production cars will not feel the kinds of aerodynamic effects that a race car will feel at 200 MPH. This technology is used by the top NASCAR team Hendrick Motorsports and others. You can see the Dupont Chevrolet driven by Jeff Gordon on this [http://www.youtube.com/watch?v=M1CalYk5VWA&feature=related video] Other top NASCAR teams using the 7 poster include Richard Childress Racing and Roush Fenway Racing. Aside from NASCAR other racing entities that use this advanced technology include Champ Car and the Indy Racing League.

Operation

The basic premise of all engineering systems is to place forces on another part of the system through force manipulation or to have forces put on the system and have this energy accumulate within the system to a predetermined level or catastrophic failure. The 7 post shaker is an engineering system unto itself as it places forces on a vehicle and records the forces that the vehicle puts back into the system. The forces that the 7 post puts into the system are forces that are commonly felt by a vehicle such as: lift, downforce, braking, acceleration, cornering and road irregularities. The vehicle suspension and drivetrain components feel these forces in the form of random motion, chassis and suspension frequency oscillations (under 30 Hz), and tire, engine, transmission and drive axle vibrations up to 500 Hz. These forces can be derived from a model of the racetrack, the weight of the car and driver, tire pressure, engine RPM, driveline RPM and acceleration of gravity. The forces that the testing engineers want are placed on the car through the use of four main hydraulic actuators that act on the tires that are capable of generating 25 KN of force with a maximum velocity in excess of 1 m/s. The actuators are capable of producing frequencies as high as 500 Hz, however this is not normally necessary as the rubber and air in the tires will absorb most inputs above 50 Hz. The remaining three posts of the 7 post are known as aeroloader actuators. These actuators are responsible for the sprung mass of the vehicle. The forces that these three actuators represent are inertial loads that come from entering a curve or aerodynamic loading/unloading in the form of downforce or lift from a wing. These forces are small on street driven cars where the speeds are normally 90 MPH and below, but on a race car where speeds can be in excess of 200 MPH these forces are significant. [ [http://www.grandprix.com/ft/ftpw006.html F1 - Grandprix.com > Features > Technical > Movers and shakers ] ]
Now that we have some of the basics out of the way, we can look at how an actual test is set up. The very basics that need to be initialized are the input forces to the vehicle from the road surface. The drivers and engineers want to look at how the car reacts to specific tracks, as the car will respond differently at the tri oval at Talladega Superspeedway where speeds can approach 200 MPH than Bristol Motor Speedway where the corners are banked 24 to 30 degrees. This data is extremely hard to collect and assemble as the road surface is highly irregular. The gathering of this data is far beyond the scope of this article. Once a racetrack is loaded into the testing computer the vehicle can be loaded on to the 7 post. Almost all variables are eliminated by using ballast for the weight of the driver and the weight of the fuel in the tank. The test lab temperature is highly regulated to standard temperature of 70 degrees Fahrenheit. Once the unit is started transducers in the form of accelerometers and strain gauges, convert the mechanical movement of the vehicle into an electrical signal. This signal is sent to a processor which converts and amplifies the signal, and sends it on to the computer.
Also of particular interest to the engineer is the force between the tire and the road. This is of interest to the car designer because it reflects the grip that the tire has on the road surface. This is more difficult to test because the sampling frequency has to be at least five times as high as the highest frequency. In this case the incoming frequency is 100 Hertz so the sampling frequency must be at least 500 Hz.
In vibration analysis, as in all engineering problems, the output data must be looked at in a methodical way. When testing on the 7 post shaker, all variables are inter-related and can be analyzed while the effects of the actual installation can be quantified. The damping force curve can be extracted from the data to understand how installation stiffness and other variables affect the damping force. Some seemingly unimportant trends need to be verified so the engineers can be sure that the trend will not continue or that the trend is expected.
The analysis path in this case is:

* Input - The road or race track
* Unsprung Mass - Weight not felt by the springs
** Tires - Act as dampers to the input forces
** Wheels - Add weight
** Brakes - Add weight
** Springs - Respond directly to the input forces
* Sprung mass - The rest of the vehicle, in particular:
** Shocks - Dampen input forces appreciably
** Frame/Rollcage - Distributes input forces over the entire vehicle
** Driver - Directly fatigued by vibration, body roll, and steering wheel feedback

What do the vehicle designers use this for?

Vehicle designers use the results of the testing on the 7 post shaker to adjust specific aspects of the vehicle. They can adjust spring rates, shock valving, steering ratio and many other factors. Also, the driver will use the results to vary how they drive on that particular track. It is like being able to drive a racetrack ahead of time and instantly be able to change variables instantly without having to wait until the next pit stop or after qualifying. When testing on a particular track the testers can choose a different line when going around a corner and see how it affects the race car.

[ [http://www.circletrack.com/techarticles/seven_post_shaker_rig_suspension_dynamics/index.html Seven-Post Shaker Rig - Suspension Dynamics Machine - Circle Track ] ]

References


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