Yaw Rate

[Mark23]

Hi all,

I have an question about the yaw rate of an vehicle. I have simulated the fmvss 126 test with an van with standard suspension and with air suspension on the rear axle. I want to explain the difference in the graphs but I cannot tell what this is.

Can anyone tell me what is an big influance on the yaw rate of the suspension?

By the way the things that are replaced on the vehicle are the lear springs for an half leaf spring with air bellows, shock aborbers and bumpstops. Practicly everything.

 

[Mark23]

By the way I simulated empty vehicle and a vehicle with maximum load.

 

[patprimmer]

It is not my area so maybe I missed something, but the answer seems so simple it should be obvious to a student. Are you a student.

Hint. Reaction rate of compression to load applied of leaf spring vs air. Hint hint, spring leaves have friction between the leaves.

Deflection causes bump steer. Bump steer is yaw.

Regards
Pat
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[GregLocock]

To withn experimental accuracy the first graphs show no difference purple vs blue.

There is a difference in behaviour in the second set.

Almost anything can affect the on limit behaviour, and 30 to 40 deg/s is basically limit behaviour.


Cheers

Greg Locock


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[GregLocock]

Incidentally modelling limit handling of vehicles requires specially calibrated tire models and vehicle models that are robust and properly validated.



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Greg Locock


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[Mark23]

The validation of the model I want to do with an fysical test, but this cost a lot of money so this is postponed.

I forgot to say, the vehicle has only one leaf spring so there cannot be friction between the leaves that causes the difference. The reaction rate of the compression of a lear spring vs air is causing the main difference. That is true, but I want to know why the graphs are different. Bump steer I havent thought of that yet. In the attachment I placed the characteristic of the leaf spring, and air suspension in loaded and unloaded condition.

By the way, thanks for the answers so far!






patprimmer

 

[GregLocock]

I have a feeling I'm pretty much wasting my time, but what I'd do in this case is to substitute, one by one, the parts from one model into the other.

You'll probably find most components have little effect. Then run K&C on the remaining parts.

You'll probably find load transfer is vitally important.

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Greg Locock


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[GregLocock]

Incidentally if you want to understand the differences you probably want to (a) concentrate on one particualr test and (b) look at far more graphs. My standard deck for limit handling consists of 12 different graphs.

Cheers

Greg Locock


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[Mark23]

Sorry I wasn’t so active lately. I was a couple of days ill so I lay all day in bed. What you said Greg, change all the parameters that I have already done. I noticed that the biggest influence on the yaw rate is the spring characteristic. I changed the characteristic of the dampers and the lateral distance between them.

Because there are no changes done in the geometry of the suspension, I didn’t change that. I only can conclude that the suspension is the biggest factor in the difference in yaw rate. Also the load transfer has an big influence, but I am comparing the results of the yaw rate in unloaded and loaded condition.

 

[GregLocock]

So now you need to build a conceptual hypothesis of why the spring rate affects yaw behaviour, and then think of a test to evaluate that.

Cheers

Greg Locock


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[cibachrome]

Make a plot of roll angle or roll velocity or roll angle vs yaw rate. My money is on that.

Is the air suspension model complete with gas dynamics or is it just another spring?