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

 

[MintJulep]

The term "shock absorber" is generally understood to be a damper as applied as part of an automotive suspension system. however, "shock absorber" is technically imprecise.

As shock pulse has a magnitude and a duration. This defines an amount of energy.

The word "absorber" implies that the shock goes in, but doesn't go out. Like a sponge absorbs water. In reality a damper converts kinetic energy to heat energy.

 

[GregLocock]

As you increase the force/velocity coefficient in a shock absorber it increases the reaction force in the body.

What it does do is damp the subsequent resonance.

That is why they are dampers, not shock absorbers.

Cheers

Greg Locock

 

[metman]

On thread1010-89886

Greglocock said "Dampening has a definite USAn ring to my ear.

Incidentally shock absorber is a terrible name. When you apply an impact to a typical isolation system a shock absorber TRANSMITS the shock to the isolated body. Damper is a much better descriptor, technically."

I will come back later and read the responses in between. Gotta get bakc to work.

Jesus is THE life,
Leonard

 

[CoryPad]

Leonard,

You should use the word damping (to diminish the activity or intensity of) rather than dampening (to make damp/wet).

Regards,

Cory

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

 

[CanEngJohn]

However the general public looks at the automotive shock absorber as ...
a device which reduces the shock effects on driver and passengers resulting from vehicle operation over a defined surface
(it reduces the shocking forces resulting from bumps in the road - as quoted from a non-engineer)

In their eyes the forces are absorbed by the device so that they do not get transmitted. Since the forces are "shocks" from the road the name Shock Absorber is quite appropriate for the general public. We could call them fluid based force-energy conversion dampers (as opposed to gas based-air shock or solid based-coil spring) but that doesn't roll off the tongue easily.

This is one of those scenarios where the iten did not get its name from an engineer and that is probably a good thing.

 

[ivymike]

doesn't it tend to INCREASE the shock experienced by the occupants, and reduce subsequent vibrations?

 

[CanEngJohn]

Not that I am aware of. You have resistances in both directions. Compression is lower than the Rebound however.

 

[ivymike]

isn't it the resistance to upward movement that increases the shock experienced by the passengers?

 

[MintJulep]

Assuming a "bump" rather than a "dip", on the initial upstroke the energy generated by the shock pulse travels in parallel into the spring and the damper. Some of the energy is converted into heat and dissipated by the damper, but most is converted into potential energy by compressing the spring.

As the spring lengthens more of the energy is dissipated as heat by the damper.

This cycle continues for however many oscillations the suspension designer has allowed.

In addition to dissipating the shock pulse energy, the shape of the pulse reaching the vehicle structure is also modified. Typically the magnitude is decreased, while the duration is increased.

 

[ivymike]

Isn't the force at the bottom of the spring/damper assy virtually the same as the force at the top of the spring/damper assy, at any given point in time?

 

[GregLocock]

Good to see polite persistent questioning being used to iron out the details.

At low frequency it is entirely safe to ignore the mass of the shock absorber, so Mr Newton says Ftop=Fbottom.

If you take the shock absorber off the car it MUST reduce the force into the body for the intital pulse, since both the velocity and the displacement are increasing together.

Therefore, a car without shock absorbers will see lower forces up to the first peak (roughly).

Thereafter all bets are off, phasing becomes important.

Cheers

Greg Locock

 

[CanEngJohn]

Here we are having a nice discussion of the English language and suddenly a discussion about physics breaks out.

Bloody Engineers. Can't keep them interested in anything but practical science.

 

[MintJulep]

Greg,

Your statements:

"At low frequency it is entirely safe to ignore the mass of the shock absorber, so Mr Newton says Ftop=Fbottom."

and

"Therefore, a car without shock absorbers will see lower forces up to the first peak (roughly)."

appear to be contradictory.

I'm not convinced that Mr. Newton's laws of motion for rigid bodies apply. In the automotive application neither end is constrained. We need to consider work and power I think.

Consider a fully rigid system. Force applied vertically up-wards to an unconstrained mass. The mass will accelerate up-wards (assuming the force is of sufficient magnitude to overcome gravity).

Now consider a a system with a very soft damper of infinite stroke installed between the force and the mass. You can conceive of a system such that the force will cause the damper to stroke at a constant velocity, thus doing work and resulting in no acceleration of the mass. The work and the speed of compression determine the power the must be dissipated as heat.

Somewhere in between these two extreme cases lies the real world.

 

[ivymike]

Aren't suspension dampers constrained at both ends (to the vehicle at one end, to the suspension at the other)?

hmmm... maybe this simplified example will help:

Consider a body (vehicle) supported at a distance above the ground, and held at constant height. Below the vehicle there is a (massless, rigid) wheel, which is free to move vertically with respect to the vehicle, but is not allowed to move laterally (in any direction). The wheel encounters a "bump" as the vehicle moves down the road. The bump forces a displacement of the wheel in the vertical direction, much as a cam would displace a follower.

* with no spring or damper connecting the spring to the wheel, there is no force between them, and the wheel moves up freely. No force is applied to the vehicle.

* add a simple spring - now the force between the vehicle and the wheel depends on the displacement of the wheel, and increases as the wheel moves upward

* add a simple damper - now the force between the vehicle in the wheel depends on the displacement of the wheel (spring) and also on the vertical velocity of the wheel (damper). As the wheel moves upward faster, the force on the vehicle is increased.

For all of the above cases, the force on the body is the same magnitude as the force on the wheel, because the wheel, spring, and damper are all represented by "ideal" massless entities.

It's easy enough to see that (f.spring + f.damper) > f.spring > 0 when the wheel is moving upward on the flank of the bump.

 

[MintJulep]

InHiding:

The damper is constrained at the top only WRT the car body, and at the bottom only WRT the suspension compnent it is attached to.

Both of thoese items are free to move. The body is most definately not held at a constant height.

Your third bullet correctly notes that force transmission through a damer is a function of velocity. This contradicts your conclusion that the force on the wheel is the same as the force on the body.

 

[ivymike]

The body is most definately not held at a constant height. In my example it was. If you think there was something fundamentally wrong with the example, please elaborate.

Your third bullet correctly notes that force transmission through a damer is a function of velocity. This contradicts your conclusion that the force on the wheel is the same as the force on the body

It might be worth it for you to draw a free body diagram of that - if the velocity of the damper piston is constant, what is the relationship between the force at the bottom connection (to suspension) and the force at the top connection (to the car)? I maintain that they're equal. If they're not equal, where does the extra force come from?

 

[mloew]

Please see the following article:


Viscoelastic Damping 101
by Paul Macioce, Roush Industries, Inc., Livonia, Michigan

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

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

 

[MintJulep]

InHiding,

Holding the body at a constant height is an over simplification. It results in artificially constraining the mass to have zero acceleration.

On a more fundamental level, you are attempting to apply a rigid body analysis to a non-rigid system.

I agree with you that if the velocity of the damper piston is constant then the forces are equal, however your example stated a change in velocity of the piston, ie an acceleration.

Once acceleration comes into play you can no longer assume massless bodies.

Furthermore, this is an energy problem. The wheel traveling over a bump may be represented by a shock pulse having a time-variant magnitude and a duration. This defines a fixed amount of energy available.

As the event progresses the total energy must remain constant. A portion of the energy is converted into potential energy by increasing the height of the unsprung mass. A portion of the energy is converted into potential energy stored in the compression of the spring. Because the spring and the damper compress, work is done (force x distance). For this example we can safely ignore internal damping within the spring. The energy used to compress the damper is converted to heat, and dissipated. The energy left over is available to accelerate the sprung mass up-wards.

The The position-time curve of the sprung mass is dependent upon the characteristics of the spring and damper, and the amount of mass.

Differentiate the position-time curve twice to obtain the acceleration-time curve for the sprung mass. Dividing by the mass results in the force-time curve.

It seems highly improbably that this force-time curve will be identical to the initial shock pulse.

 

[ivymike]

The point I have been trying to make is that the initial force transmitted to the vehicle body is greater when the damper is present than it would be if the damper was not present. Do you agree?

I question the supposition that the mass of the damper is significant for the purposes of this discussion - they seem pretty light compared to most of the other parts in consideration, and the force to accelerate the damper piston is certainly much smaller than the damping force "seen" by the suspension.

You are right that if you put a given amount of energy into the system, it must be split between the various storage / dissipation methods on the vehicle. I'm not so sure that the bump (forced displacement profile) represents a fixed amount of energy, but that's another discussion.

Let's take another simplified system, where we have a spring, a mass (vehicle), and a damper, where the spring and damper are connected in parallel and support the mass, and we apply a pulse to the bottom of the spring/damper.
* The energy stored in the spring increases as one end displaces relative to the other
* The energy dissipated by the damper is related to the velocity of one end of the damper relative to the other, the damping coefficient, and the distance that the damper displaces
* The energy stored in the vehicle is related to its velocity and its mass

A stiffer spring will displace less when the pulse is applied, resulting in greater energy transfer to the mass (and greater acceleration, velocity of the mass)

A larger damping coefficient will result in less displacement of the damper, resulting in greater energy transfer to the mass (and greater acceleration, velocity of the mass)

If you disagree, can you give me any real value of a damping coefficient for the damper that will result in more suspension deflection (less body deflection), on the first half of the pulse, than you would have with zero damping?