shock absorber-vs-dampener/damper 4

[metman]

Ref: thread1010-89899

Is not absorption of shock a dampening effect?

Please explain the distinction between shock absorber and damper.

Jesus is THE life,
Leonard

 

[CanEngJohn]

How can the initial force to the vehicle be larger than the input force? What in the system is adding to the input force?

Everything in the system acts against the input force so in the worst case - no energy loss - the force the occupants see is the force acting onto the system. In the case where there is an energy loss - which is the primary purpose of a damper - then the forces transmitted through the system have to be less than the input force.

 

[MintJulep]

The existence of a spring and damper in the system introduces a phase-shift between the excitation at one end that the response at the other, so the term "initial force" seems somewhat ambiguous.

An "ideal damper" has no effect on the displacement. The displacement is controlled by the spring. The damper affects the velocity. In reality ,"shock absorbers" have a non-zero spring constant.

A stiffer damper provides a smaller phase shift.

An effective automotive suspension also tends to "spread out" the response over a longer time then the excitation. I think that you are not considering the time-variant nature of the event.

 

[ivymike]

How can the initial force to the vehicle be larger than the input force?
It's not - for a given "bump," the "input force" depends on the characteristics of the suspension. The force "seen" by the vehicle will be only slightly less than the force "seen" by the bottom of the spring (acceleration of spring&damper components making the difference). The "input force" is larger if the suspension is "stiffer" because the acceleration of the vehicle mass will be greater. ("stiffer" suspension deflects less, "stiffer" meaning greater spring stiffness and/or higher damping coefficient)

What in the system is adding to the input force?
The lack of deflection in the suspension means that a given bump will result in greater suspension force.

 

[ivymike]

An "ideal damper" has no effect on the displacement. The displacement is controlled by the spring. The damper affects the velocity. In reality ,"shock absorbers" have a non-zero spring constant.

For a given bump and vehicle speed, how can you affect the suspension compression velocity without changing the peak suspension deflection? At rest, you're right, the damper doesn't influence suspension deflection. When you're talking about a transient "bump" event, with or without elasticity in the damper, the statement is incorrect.

If I can't persuade you, perhaps trying some calculations on your own will help. Here is an example that I found online, which you might build from:

http://claymore.engineer.gvsu.edu/~kerne/345paper.htm

 

[GregLocock]

I had a play around with CarSim, to pull the forces out for a reasonably sophisticated model of a car hitting a step. The results are on my increasingly disorganised website at

http://www.geocities.com/greglocock

, on the gallery page (sorry, can't direct link to the pictures). The two cases are examined are of one with adequate damping, and the other has only 10% of that.

The peak total instantaneous force is about 10-20% less with the weaker dampers. There's also a plot of the way the two components (spring + shock) add up.


Cheers

Greg Locock

 

[Speedy]

So with better smoother roads does it make sense to increase the level of damping. That is, keep resonance down at the expense of the lesser occurring higher shock loads and therefore higher peak forces.