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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?

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.

Aaron Henningsgaard

My reply:


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.




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