Resonance frequency semi trailer

[321GO]

Im trying to calculate the resonance freq. for a semi trailer

The different spring rates of the airspring itself and the tyre should be multiplied by the number on the trailer right?
So, basically 8x spring rate tyre and 4X spring rate airspring?

From my scriblings im somewhere around 1.2Hz for the trailer (loaded) - as attached

Somewhat close?

Peace

 

[Denial]

I haven't thought about your raw values or your units conversions, but your treatment of springs in parallel and springs in series is correct.[ ] Your formula is correct, you are using a valid set of consistent units, and your final answer is in accord with the converted input values.[ ] (However you did seem to misread a 260 as a 280 at one place, and you left a square root sign off one of your expressions even though you pushed the appropriate button on your calculator.)

 

[321GO]

Hi Denial, yes im sloppy
Thanks for your input, highly appreciated

p.s. what will actually happen when the trailer is experiencing a long duration of this frequency eventhough the vehicle is dampened?

Since it is dampened it will be limited to a degree but how far will this actually go?

 

[GregLocock]

That frequency is known as the corner frequency on cars where it is also 1-2 Hz.

At least with cars it doesn't cause any particular problems.

Cheers

Greg Locock


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[tr1ntx]

As for what will happen:

Air-ride suspension on any trailer I've seen is not a closed system. There is a height control valve that will bleed or add air as required, so if it is tripped, the spring rate of the air-spring changes. I think there is a pressure relief valve as well to prevent overpressure.

 

[321GO]

tr1ntx, I think the feed lines have resitrictions in them, probably to prevent activation in situations like this (during normal suspension travel)

 

[321GO]

The sage continues...now I want to dampen this trailer let's say 10% (just a random value)

I have calculated C critical and divided this by 10.
Ok, can I now use Formula Fdamper = c x vdamper to determine the actual required shock forces (of course dividing them by the number of actual shocks)?

I'm not sure what to use for the velocity since the bounce is a sinus motion with non-uniform velocity.

I have attached some more scribblings

Damper specs are force - speed, but since the are also tested with a sinus, what speed is this actually, max speed when passing true zero? But the body motion speed is not uniform, how to do this?

Gracias

 

[GregLocock]

"max speed when passing true zero"

yes

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[321GO]

Thanks Greg

Can anybody perhaps explain why the stiffness value k will eventually become
keq=kspring*ktyre for the unsprung system and keq=kspring*ktyre/kspring+ktyre for the sprung system.

I thought k+k was two spring parallel and k*k was two springs in series, but how does this relate to the actual system

 

[321GO]

For instance when calculating the critical damping Ccr. = 2sqrroot(k*m)

Would k then be the "equivalent k value", so kspring+ktyre(+ not *, mistake above) for the unsprung part of the system and kspring*ktyre/kspring+ktyre for the sprung part of the system?

It's been a long time...

 

[GregLocock]

Re your springs, for springs in series apply a unit load and work out the displacements, which will add.

For springs in parallel, apply a unit displacement and work out the forces, which will add.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[321GO]

Guys, i''m also trying to get some damper forces, please see the attachment.

In the formula i'm not sure how to handle the speed term, since the damper speed is probably more or less a sinewave(cosinus function). How accurate is this formula, or nothing more than a rough ball park value?

Also i'm not sure if the damper ratio "i" is to the power or simply a ratio (like i assumed)

Is a given damper ratio also contant regardless of the actual amplitude of the suspension travel (assuming a contant frequency)?
I guess not since the damper force-speed function is not progressive enough in most cases?

Again, thanks a million

 

[GregLocock]

Sorry, you are asking too many similar questions in different forums.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[321GO]

 

[321GO]

For someone reading this in the future.
I basically made numerical calculations based on a spring mass damper system.
That way i could take in account the non-linear and asymmetrical behavior of the dashpot.

for instance m*a+d*v+k+x=0

peace