rigid body modes

[zerodof]

I am looking at the Engine/transmission rigid modes of the transversly mounted engine of passenger vehicle from the point of view of locating there positions.

In the mode development exercise, what mode should be priortised to be the lowest mode ie bounce or pitch and what should be the criteria for the lowest frequency ie should it be based on avoiding human sensitive frequency range or suspension modes. What should be the optimum lowest rigid mode of p/t for idle vibration/suspension/human sensitivity inputs.

The other question on how do we calculate the hop and tramp modes.

 

[GregLocock]

I'd be most surprised if you have great deal of choice in the order of bounce and pitch

You need to develop your modal alignment strategy based on human response, in a whole vehicle model. This will govern the interactions between engine modes and suspension modes.

Hop and tramp modes are calculated (badly) much the same way - for a hand calculation just use the unsprung mass on the tyre vertical stiffness.

That won't agree particularly well with an experiment, because both of those numbers are flaky. Best bet is to fit your target tyre to a similar vehicle and measure it on the road.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[zerodof]

Does that mean the lowest mode of P/T should avoid 4-8 hz human sensitive area. Then you get very small 8-12ish hz band to play with all modes.

When I said bounce and pitch, which of these two are most sensitive to suspension modes . Surely if the dominant bounce is most likely to get excited in transverse layout then bounce mode should be kept lowest in order or vise versa. This is what I am not sure.

 

[GregLocock]

Work out the order of the modes for any practical arrangement of engine mounts. Then you'll see what I mean.

You can't use 12 Hz, that is too close to wheelhop

I agree it would be nice to get the bounce mode out of the 4-8 Hz region - good luck

What configuration is your engine, what is the idle speed?

I strongly suggest that you benchmark your image car. Then think about their modal strategy, what areas would you like to improve?

Sorry to be so cagey, this subject is difficult, but it is a core competency for NVH



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[zerodof]

It is a transverse engine and it idles around 750 rpm approx.

 

[GregLocock]

How many cyliders, how are they arranged?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[zerodof]

4 cylinders, 3 pt mt system

 

[GregLocock]

OK, so now you know nearly enough to set the maximum bounce frequency - you need to attenuate the second order from the engine at idle (25 hz). If you know the noise transfer function from each mount, and the desired level of idle noise, and the vibration on the engine at idle. You'll also need to know the damping characteristic of your engine mounts. Are you using hydromounts?







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[zerodof]

yes hydro mounts