uploaded 7/30/2001
Shaker Rig Q & A
I received this email with some questions about the shaker
rig article from Aaron Henningsgaard. Here's his email and my
answers.
Any comments or further questions from anyone?
Paul,
I enjoyed reading the article on shaker rigs and especially appreciated
that
actual data was included. However now that you've shown it,
I
have a couple of questions that I hope you can answer.
The graphs appear to have mapped only a single adjustment
for front and rear dampers, but most dampers used in open wheel
racing have at least two or three adjustments (and the Ohlins
that I'm familiar with, 4). How is data analyzed (or presented)
for these cases? 3-D maps? Overlaying graphs?
In the article it stated that the front and rear grip disturbances
were
decoupled. I gather from the plots that this means that for
a given
adjustment of the front/rear damper, the grip disturbance on
the other end
of the car is unaffected. Is this specific to the car being
tested (Winston
Cup) or is this a trend seen in other series (cars with stiffer
chassis?)?
What is grip disturbance a function of - wheel load, wheel
acceleration, tire properties?
The testing seems to be limited to damper movements caused
by track
irregularities. How can this same information be applied to
a transient
conditions where the dampers also play a large role (turn in/corner
exit)?
Lots of questions...I appreciate any responses.
Thanks,
Aaron Henningsgaard
My reply:
Aaron,
Good questions!
When you have dampers with several adjustments you have to
vary those adjustments independently. That's why the team has
to be prepared and have a plan for what they want to test. They
have to have identified problem areas and have a plan to test
in those areas. The data gets analyzed and presented in the same
way.
As Mats said in the article, NASCAR stock cars tend to be
easy to decouple while open wheel cars, F1, Indy, IRL, etc. tend
to have front/rear coupling. You're right that means that changes
at one end actually effect the other end of the car.
The way Ohlins analyzes data is proprietary but generally
less change in contact-patch forces is better. Remember, tires
are load sensitive so you want minimum change in tire contact-patch
force. I think what Mats calls "grip disturbance" is
more variation in contact-patch forces.
[I added these two paragraphs later. What causes grip disturbance?
The (too) simple answer is the tire isn't following the road
variations. Tire contact patch forces vary more than they would
with a better setup. It could be stiff springs/dampers that cause
the tire to bounce off the road. Or bounce just a little. It
could be the driver turning the steering wheel too fast and too
much low-speed compression in the front dampers. Weight transfer
and inertia forces build up quickly and, if you had force sensors
in the tire, you'd read grip disturbance. Smoother steering input
could give less grip disturbance.
The test I wrote about involved damper changes because that's
what was adjustable on the car that was available that day. The
car owner didn't have enough spare springs to create a usable
matrix.]
Damper effects into and out of a corner have more to do with
driver/car/track preferences. It can be complicated but you don't
need a shaker rig to analyze that. The low-speed adjustments
are used to change the response of the car to suit the car/driver/track
transients.
I hope this helps. Let me know if you have more questions.
I don't know it all but I might be able to help.
Paul
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