Hi, can anybody expalin me what

[sasank]

Hi,

can anybody expalin me what is a flex mode in modal analysis? What is the significance of low frequency modes of an automotive frame/cradle?

Thanks
sasank

 

[MikeyP]

I've not come across a "flex mode" before (unless it is intended to be some non-technical qualitative description of a mode shape.

A "flexible mode" is one where there is strain energy stored in the structure. ie. it is bending, twisting or stretching in some way. It is the opposite of a "rigid body mode" where the whole structure moves as one. RB modes only occur where the structure under test is suspended freely or nearly freely. In priciple there will be 6 RB modes in 3D space: displacement in the x, y and z directions and rotation about the centre of gravity around the x y and z axes. For genuinely free supports these 6 RB modes will occur at 0 Hz. For supports which are slightly stiff they will occur at very low frequencies which increase as the stiffness of the supports increases. In modal analysis you tend to design the supporting structure such that the highest frequency RB mode occurs at 10% or less of the lowest frequency flexible mode.

In terms of a vehicle, you want to know how it will behave on the road. So rather than suspending it on elastic cords or some such, you use the suspension system of the vehicle as the supports. Examples of RB modes in this case would be: "bounce", the vehicle bouncing up and down on its wheels (displacement); "pitch", the front of the vehicle goes down while the back goes up and vice versa (rotation); "roll", the left side goes up while the right side goes down (rotation).

In each case there is no deformation of the chassis or body shell.

In summary, flexible modes are those modes which are not rigid body modes!

M

 

[GregLocock]

We often say flex or flexural modes, with the same meaning as Mike.

What we actually mean is that the steel is bending, rather than the component bouncing or whatever as a rigid body on its isolators.

Subframes, cradles and cross members need to be stiff because they are terminating isolators, if they have resonances in the range of excitation seen then they will have a reduced impedance, and so more noise and/or vibration will be felt by the car's occupants.

Japanese car companies often seem to specify minimum modal frequencies for subframes and so on in free-free conditions. I don't think this is very helpful, and would much rather look at the mobility of each isolator mounting point, as installed in the vehicle. The sad truth is that many subframes are stiffened by the body, even though they are supposed to be isolated! This doesn't happen so much these days, fortunately.




Cheers

Greg Locock