Automobile shock absorbers

  • #1
I'd like to open a discussion on the ins and outs of shock absorbers on cars. The ability of a device to restrict the extension of the suspension springs works miracles that are not always appreciated.
 

Answers and Replies

  • #3
brewnog
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The damper's purpose isn't to restrict the extension of the springs; its purpose is to prevent multiple oscillations following an impulse.
 
  • #4
The damper's purpose isn't to restrict the extension of the springs; its purpose is to prevent multiple oscillations following an impulse.
I think this is more complicated than mere damping. A shock absorber will generally not impede the compression of the spring to any great degree. On the other hand, once the spring is compressed, the shock absorber resists any extension of the spring. The effect is to isolate the car or truck from the road. If a shock were just a damper operating in both directions equally, then the ride would suffer.
 
  • #5
Integral
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I think this is more complicated than mere damping. A shock absorber will generally not impede the compression of the spring to any great degree. On the other hand, once the spring is compressed, the shock absorber resists any extension of the spring. The effect is to isolate the car or truck from the road. If a shock were just a damper operating in both directions equally, then the ride would suffer.
Seems to me that the effect you describe IS damping. Who said that it has to operate equally in both directions?
 
  • #6
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I think this is more complicated than mere damping. A shock absorber will generally not impede the compression of the spring to any great degree. On the other hand, once the spring is compressed, the shock absorber resists any extension of the spring. The effect is to isolate the car or truck from the road. If a shock were just a damper operating in both directions equally, then the ride would suffer.
It is simply dampening. That's why in the automotive industry they're also referred to as 'dampeners'.
 
  • #7
Mech_Engineer
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I think this is more complicated than mere damping. A shock absorber will generally not impede the compression of the spring to any great degree. On the other hand, once the spring is compressed, the shock absorber resists any extension of the spring. The effect is to isolate the car or truck from the road. If a shock were just a damper operating in both directions equally, then the ride would suffer.
What you're describing is a variable-rate dampener, where the compression side his much less dampening than the extension side. Shocks do not have a fundamental quality that limits their dampening in one direction.
 
  • #8
FredGarvin
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A shock absorber is just that...an absorber. In mathematics it is modeled as a dashpot which is a viscous dampener. The absorber works the same in both directions. It is a hole that a viscous fluid can pass through only so quickly.
 
  • #9
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In a sense, I can understand what the OP implies. Modern high-performance suspension dampers use some pretty intricate speed-sensitive valving.

If you want an example, I can probably find an exploded-view of a motocross bike's fork assembly.
 
  • #10
Seems to me that the effect you describe IS damping. Who said that it has to operate equally in both directions?
I think I said it was more than damping alone.
 
  • #11
What you're describing is a variable-rate dampener, where the compression side his much less dampening than the extension side. Shocks do not have a fundamental quality that limits their dampening in one direction.
Clarify please. What is meant by fundamental quality? Does this mean they do or they don't have symmetrical damping?
 
  • #12
A shock absorber is just that...an absorber. In mathematics it is modeled as a dashpot which is a viscous dampener. The absorber works the same in both directions. It is a hole that a viscous fluid can pass through only so quickly.
Fred, you seem to be saying that the shock exhibits equal resistance to both compression and extension. That is what you are saying?
 
  • #13
In a sense, I can understand what the OP implies. Modern high-performance suspension dampers use some pretty intricate speed-sensitive valving.

If you want an example, I can probably find an exploded-view of a motocross bike's fork assembly.
pantaz, What, please, is OP? Are you speaking of an over the counter shock or something specialized?
 
  • #14
stewartcs
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pantaz, What, please, is OP? Are you speaking of an over the counter shock or something specialized?
OP generally means Original Poster (that's you) or the Original Post.
 
  • #15
Mech_Engineer
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Clarify please. What is meant by fundamental quality? Does this mean they do or they don't have symmetrical damping?
Fred, you seem to be saying that the shock exhibits equal resistance to both compression and extension. That is what you are saying?
Yes, most "standard" automobile shocks have the same damping in both directions. However, there are more advanced shock absorbers that use special valving that changes the damping response based on speed, or direction of travel.
 
  • #16
Yes, most "standard" automobile shocks have the same damping in both directions. However, there are more advanced shock absorbers that use special valving that changes the damping response based on speed, or direction of travel.
This has not been my experience. I know of double action shocks that help with a spring that is not sufficient for the job involved. But I'm thinking that what a regular shock does it is release the wheel over a time interval that is large enough to isolate the body from a continuous series of impacts.
Listen, if this is going to turn into an ugly scene than I suggest that the folks who feel they have a motive bow out know. I know some of my questions have not been phrased in the best manner. But I really don't want to go around the block bickering about semantics.
 
  • #17
Mech_Engineer
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This has not been my experience. I know of double action shocks that help with a spring that is not sufficient for the job involved. But I'm thinking that what a regular shock does it is release the wheel over a time interval that is large enough to isolate the body from a continuous series of impacts.
Shock absorbers are not "timed." In the simplest model, they provide a resistive force proportional to a velocity imparted on them; nothing more. You can basically think of a vehicle's suspension as a mass, spring, and dampener: an undamped system will oscillate indefinitely, so by adding a dampener you can make sure the system response settles. However, settling time is dependent on the input conditions and dynamics involved. Generally, the larger the input (or the closer to the resonant frequency of the system) the longer the system will take to settle.

Listen, if this is going to turn into an ugly scene than I suggest that the folks who feel they have a motive bow out know. I know some of my questions have not been phrased in the best manner. But I really don't want to go around the block bickering about semantics.
We are all answering your questions as best we can; but you apparently have some misguided views as to how shock absorbers actually work, and insist on applying your personal anecdotal evidence as argument against scientifically grounded explanation.
 
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  • #18
Shock absorbers are not "timed." In the simplest model, they provide a resistive force proportional to a velocity imparted on them; nothing more. You can basically think of a vehicle's suspension as a mass, spring, and dampener: an undamped system will oscillate indefinitely, so by adding a dampener you can make sure the system response settles. However, settling time is dependent on the input conditions and dynamics involved. Generally, the larger the input (or the closer to the resonant frequency of the system) the longer the system will take to settle.

PS I'll open another thread and we can all lock that one too.

We are all answering your questions as best we can; but you apparently have some misguided views as to how shock absorbers actually work, and insist on applying your personal anecdotal evidence as argument against scientifically grounded explanation.
I didn't say they where timed. I only asked what you people think. I can see this is going to be difficult. Let's lock the thread and call it a day.
 
  • #19
Integral
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I do not think that the question is difficult. The problem seems to be that you cannot free yourself of incorrect preconceived notions.

Fred says that auto shock absorbers are simple orifice devices. I will take that as a fact. Note that pantaz points out that there are more complex shock absorbers, are they typically used in cars?? I don't know.

At any rate it does not matter. You need to understand that the motion you observe is damped. Uniformly damped motion falls under 3 classifications. Underdamped, Over damped and critically damped. A under damped system will oscillate with decreasing amplitude until the motion stops. Over damped and critically damped systems do not oscillate. The motion is suppressed within a single cycle. IIRC how the system returns to the equilibrium position is the difference.

To really see just what the shock adsorb er does you need to observe the undamped motion, without that you do not know the actual amplitude of motion.

Read the Wiki article on http://en.wikipedia.org/wiki/Damping" [Broken]
 
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  • #20
stewartcs
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I didn't say they where timed. I only asked what you people think. I can see this is going to be difficult. Let's lock the thread and call it a day.
I would suggest that you read http://www.monroe.com/tech_support/tec_default.asp" (it's the main page for the first link I posted). It is from a company that has over 90 years of experience designing shocks.

I think they know what they are talking about and they explain the whole suspension system at a basic level.

Note that there is more than just one type of shock, hence they behave slightly differently, but fundamentally the same.

Hope that helps.

CS
 
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  • #21
I would suggest that you read http://www.monroe.com/tech_support/tec_default.asp" (it's the main page for the first link I posted). It is from a company that has over 90 years of experience designing shocks.

I think they know what they are talking about and they explain the whole suspension system at a basic level.

Note that there is more than just one type of shock, hence they behave slightly differently, but fundamentally the same.

Hope that helps.

CS
Stewartcs,
just read it. thanks, very good the article does mention the higher restriction on extension and my point is that this is a very important aspect of the design. It is so basic that only the more sophisticated "improvements" are discussed in any depth. I agree that damping is what is going on here, but damping on the compression side forms a direct transmission link from the road to the vehicle. The trick is to damp oscillations and keep compressive forces from being transmitted via the springs. The road presents forces into the wheel. It never grabs the wheel and pull it down, if you will. So the isolation has to be on the compression side.
I have a question. And I really am not sure about this. If a spring oscillation were damped exclusively in one direction, would it be effective?
Anyway, please look at some of the prior posts and see if you don't see my point.
thanks again
Alex
 
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  • #22
Mech_Engineer
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Here is a useful article that simply explains some of the intricacies of a vehicle's suspension:
HowStuffWorks.com said:
A typical car or light truck will have more resistance during its extension cycle than its compression cycle. With that in mind, the compression cycle controls the motion of the vehicle's unsprung weight, while extension controls the heavier, sprung weight.
http://auto.howstuffworks.com/car-suspension2.htm
 
  • #23
mech eng,
that's the phrase that I was referring to. Does this article somehow not line up with what I had said?
 
  • #24
Dampers: Shock Absorbers
Unless a dampening structure is present, a car spring will extend and release the energy it absorbs from a bump at an uncontrolled rate. The spring will continue to bounce at its natural frequency until all of the energy originally put into it is used up. A suspension built on springs alone would make for an extremely bouncy ride and, depending on the terrain, an uncontrollable car.
Enter the shock absorber, or snubber, a device that controls unwanted spring motion through a process known as dampening. Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated through hydraulic fluid. To understand how this works, it's best to look inside a shock absorber to see its structure and function.



That's the first paragraph of the link you sent me.. the phrase "uncontrolled rate" is a reference to the time it takes to release energy through the shock. that's what i mean when i talk about time.
I'll look and the rest of the article and I'm sure i can find a ref to asymmetry in the damping.
 
  • #25
Mech_Engineer
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I simply posted the quote to show that they claim shock absorbers generally have higher damping on the extension side of travel (wheel travelleing down). Nothing more.

This seems to make sense from the perspective of trying to make the ride as smooth as possible.
 

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