A Test Using the Shaker Rig
"We ask the teams to bring the setup we use for the test,"
Hedlund continues. "Typically they'll come prepared to look
at spring and shock variables for a specific corner at a specific
track. They know their speed in that corner and they may know
the wheel vertical loads or total suspension movement. If not
we can calculate that. Then we set our downforce actuators to
maintain the loads in that corner at that speed.
"We don't feel using track-data for a whole lap as input
to the car is the right thing to do. For most of a lap the car
is working fine. We'd prefer to work on areas of the track the
teams identify as giving them a problem. We can help them identify
problem areas if they have track data that is accurate enough.
But this type of analysis can be time consuming.
"What's different about the way we test is how we input
energy to the car. There's a random element to it. The rams move
each wheel with randomly distributed displacements creating a
phase difference front-to-rear, left-to-right, and diagonally
to simulate moving over track irregularities. We also prefer
long test runs to allow accurate data analysis. Ours is 80 seconds.
"When a ram moves a tire it moves through some displacement
in some time. The slope of a graph of displacement vs. time is
speed, inches per second for instance. Our input program randomly
changes the slope of that curve in sharp steps during the 80-second
run. We run a low-speed test where the maximum vertical speed
is 4 in/sec and a high-speed run with Vmax at 10 in/sec.
"There are two disadvantages to frequency-sweep inputs.
The hydraulics can't control the rams much over 30 Hz and a sinusoidal
input just isn't a realistic input spectrum. We think our proprietary
random velocity input is more realistic, the energy spectrum
is very close to what you typically see in track data. Every
time the velocity input to the wheel changes it kicks some energy
into the car. And the frequency of the input can go much higher
than with a sinusoidal input.
"A tire is a complex viscoelastic system. We think there
are things to learn about the tires at inputs over 40 Hz. You
want to excite the car over a wide frequency range so any important
dynamics in the tire or dampers will show in the data."
This Late Model Stock Car was tested using a setup and wheel
loadings for a lightly-banked, half-mile asphalt track near Greenville,
So. Carolina. This is a 3,100 pound car mounting bias-ply, spec
tires. The owner told Mats the bank angle and the car speed in
the turn. This allowed him to make some calculations and come
up with static loadings for each of the rams on the rig. The
rig can hold these loadings so the car sees force similar to
those during an actual lap on that track.
Hedlund and Hesling space the wheel pads properly, push the
car on the pads, and go down below floor level to attach the
downforce actuators. Accelerometers are mounted on the wheels
with a rigid adhesive.
This photo shows a wheel/tire sitting on a one of the hydraulic
rams. An accelerometer has been glued on the wheel. The masking
tape on the pad is a reference. The tire can bounce during testing
letting the whole car move around. They periodically used a jack
to reposition the tires on the pads.
Mats asked the car owner some questions about the track on
which he competes. He needed to know the bank angle and the car
speed in the turn. This allowed him to make some calculations
and come up with static loadings for each of the rams on the
rig. The rig can hold these loadings so the car sees force similar
to those during an actual lap on that track.
During data runs the car danced on the pads, lightly during
the low-speed run and more energetically at high speeds. "This
car is fairly quiet," said Hedlund. "A Winston Cup
car can be very noisy. Normally we would go through a spring
matrix first but the owner of this car doesn't have many spring
sets. If a team is properly prepared we can make 50 to 100 runs
a day. The teams learn quickly and the second or third time they
come, the test can be very focused."
In between data runs Hedlund changed settings on both dampers
at the front or rear of the car. Starting with all the dampers
four clicks from full hard, the next run tested the front dampers
two clicks softer. Testing continued making two-click damper
adjustments until all front/rear combinations were covered.
"Frequency Sweep" Explanation
I need to add some explanation about how shaker rigs input
energy to the cars. Mats alludes to "frequency-sweep inputs"
above. Using this method the rams start off moving slow with
large displacement and increase the frequency of the movement
at the same time using a smaller total displacement so that the
maximum velocity of the ram remains the same. Watching a test
using this method the car starts off with the wheels moving up
and down with large movements and as the rams speed up the displacements
get smaller until the car is vibrating on the tires.
There are at least two additional subsets to this method.
Using the observation that, at speed, the front tires encounter
a disturbance before the rear tires you can move the rear tires
a little later but on the same time/displacement curve. It makes
sense to set the time difference to the speed of the corner you've
decided to simulate. This is a frequency sweep with phase lag.
Or you can just excite all the wheels identically.
Another excitation method is using white noise. Each wheel
sees random ram displacement and speed. Again you can use a time
lag between front and rear wheels or input identical signals
to each wheel. Mat's method is a type of white noise. He's using
a random number generator to control the slope of the displacement
and speed of the rams. He can input the same signal to all the
rams or use a phase lag. As he says above, during his 80 second
run he uses time lags front to rear, side to side, diagonally.
He processes the data statistically and comes up with some colorful
graphs which we'll see in the next installment of this topic.
Contact Ohlins in No. Carolina at 828-692-4525.