7 post shaker
Encyclopedia
The 7 post shaker is a piece of test equipment used to perform technical analysis on racecars. A 7 post shaker can apply all vertical forces acting on the car on the racetrack, emulating some of the effects of driving. The shaker has many controls for the hydraulic system and sensors to measure forces and their effects on the car. The forces include banking loads, lateral load transfer, acceleration, braking and ride height sensitive downforce.
, Richard Childress Racing
, Earnhardt Ganassi Racing
and Roush Fenway Racing. The car driven by Jeff Gordon
is shown on a 7 post rig in this video.
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 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.
, downforce
, braking, acceleration, cornering and road irregularity forces. The vehicle suspension and drivetrain components respond to these forces by random motion, chassis and suspension frequency oscillations (under 30 Hz), and tire, engine, transmission and drive axle vibrations at higher frequencies. The forces applied are calculated from a model of the racetrack, the weight of the car and driver, tire pressure, engine RPM, and driveline RPM. The forces that the testing engineers want are placed on the car through the use of four main hydraulic actuators capable of generating 25 kN of force with a maximum velocity in excess of 1 m/s that act on the tires. While the actuators are capable of producing frequencies as high as 500 Hz, this is not necessary as the elasticity of the rubber and air in the tires will absorb most inputs above 50 Hz. The remaining three posts are known as aeroloader actuators, and 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 and unloading in the form of downforce or lift from a wing. These forces are small on road cars where speeds are not normally greater than 90 mph, but are significant on a race car where speeds can exceed 200 mph.
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:
Uses
The 7 post shaker is used for many vehicles in different driving conditions. Earlier versions were the 5 post shaker and the 4 post shaker. The 4 post shaker is commonly used by vehicle manufacturers to investigate squeaks and rattles. Manufacturers do not normally use a 7 post rig for road cars because these vehicles are not normally subject to the same aerodynamic effects as a race car operating at high speeds. This technology was first used in Formula 1 in the late 1990s, and is now also used by other series such as NASCAR and the Indy Racing League. NASCAR teams with 7 post rigs include Hendrick MotorsportsHendrick Motorsports
Hendrick Motorsports , originally named All Star Racing, is a current American auto racing team created in 1984 by Rick Hendrick. The team currently competes in the NASCAR Sprint Cup Series with Chevrolet Impalas...
, Richard Childress Racing
Richard Childress Racing
RCR Enterprises, LLC, doing business as Richard Childress Racing, is a NASCAR team based in Welcome, North Carolina, and is owned and operated by former driver Richard Childress...
, Earnhardt Ganassi Racing
Earnhardt Ganassi Racing
Earnhardt Ganassi Racing with Felix Sabates is a NASCAR team based in Concord, North Carolina, United States, owned by Dale Earnhardt's widow Teresa Earnhardt, Chip Ganassi, and Felix Sabates. This team was formed when Dale Earnhardt, Inc...
and Roush Fenway Racing. The car driven by Jeff Gordon
Jeff Gordon
Jeffery Michael "Jeff" Gordon is a professional NASCAR driver. He is the driver of the #24 Drive to End Hunger/DuPont/Pepsi Chevrolet Impala. He is a four-time Sprint Cup Series champion and a three-time Daytona 500 winner. He is third on the all-time wins list, with 85 career wins, and has the...
is shown on a 7 post rig in this video.
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 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
Line (racing)
In racing sports, the racing line is the route the vehicle must take in order to minimize the time taken to complete the course.When analyzing a single corner, the optimum line is one that minimizes the time spent in the corner and maximizes the overall speed through the corner...
when going around a corner and see how it affects the race car.
Operation
The 7 post shaker places forces on a vehicle and records the forces that the vehicle puts back into the system. The 7 post applies liftLift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
, downforce
Downforce
Downforce is a downwards thrust created by the aerodynamic characteristics of a car. The purpose of downforce is to allow a car to travel faster through a corner by increasing the vertical force on the tires, thus creating more grip....
, braking, acceleration, cornering and road irregularity forces. The vehicle suspension and drivetrain components respond to these forces by random motion, chassis and suspension frequency oscillations (under 30 Hz), and tire, engine, transmission and drive axle vibrations at higher frequencies. The forces applied are calculated from a model of the racetrack, the weight of the car and driver, tire pressure, engine RPM, and driveline RPM. The forces that the testing engineers want are placed on the car through the use of four main hydraulic actuators capable of generating 25 kN of force with a maximum velocity in excess of 1 m/s that act on the tires. While the actuators are capable of producing frequencies as high as 500 Hz, this is not necessary as the elasticity of the rubber and air in the tires will absorb most inputs above 50 Hz. The remaining three posts are known as aeroloader actuators, and 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 and unloading in the form of downforce or lift from a wing. These forces are small on road cars where speeds are not normally greater than 90 mph, but are significant on a race car where speeds can exceed 200 mph.
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
Talladega Superspeedway
Talladega Superspeedway is a motorsports complex located north of Talladega, Alabama, United States. It is located on the former Anniston Air Force Base just outside the small city of Lincoln. It was constructed by International Speedway Corporation, a business controlled by the France Family, in...
where speeds can approach 200 mph than Bristol Motor Speedway
Bristol Motor Speedway
Bristol Motor Speedway, formerly known as Bristol International Raceway and Bristol Raceway is a NASCAR short track venue located in Bristol, Tennessee. Constructed in 1960, it held its first NASCAR race on July 30, 1961...
where the corners are banked
Banked turn
A banked turn is a turn or change of direction in which the vehicle banks or inclines, usually towards the inside of the turn. The bank angle is the angle at which the vehicle is inclined about its longitudinal axis with respect to its path....
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
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
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
Sampling (statistics)
In statistics and survey methodology, sampling is concerned with the selection of a subset of individuals from within a population to estimate characteristics of the whole population....
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