car shock absorber response

That would depend on the use. You are getting into a really technical area here, you will here phrases like 'bound' and 'rebound' a lot. Think of it like this. When your car hits a bump, the spring compresses. the shock 'damps' this compression while the spring resists it. When the spring extends again, the shock then has to work against the spring, slowing the rate of extension to prevent 'bounce', and keep the tyre on the road.

For an extreme example, take a look at a suspension unit on a World Rally Car. These can cost in the region of £50,000 ($100,000) a corner.

I've always referred to "jounce" (bump) & "rebound" (return to road height)

Th difficulty here is that any advanced damper unit offers a differential split in the damping rate.

@ lower speeds the resistance is controlling a combination of surface waviness & vehicle roll-rate (normally the importance is biased toward roll)(to control the behavior during 'turn in')

@ the higher rates, wheel 'patter' is predominate.

YOU WILL NOT BE ABLE TO FEEL ANY OF THE HIGH SPEED CONTROL by any any "hand testing"

Yes, this is very normal. "bump" resistance (as the wheel rises) is typically lower than "rebound" (when the wheel is dropping relative to the body). It provides a much more bearable ride and less obtrusive interior noises than if the bump and rebound damping were equal. An effect not always appreciated is that the car will ride slightly lower (the body will sink relative to the road) on certain types of bumpy road compared to a smooth one.

The other two answers are correct, giving me the opportunity to go somewhat off topic:

Drag racers use up-lock shocks at the front and down-lock shocks in the rear (assuming rear wheel drive). The up lock shocks extend easily (allowing the front to rise) but resist compression (keeping the front up). All this helps transfer most of the weight to the rear wheels.

Ken,

Not sure I see what you mean, encouraging the front to rise and the rear to squat (on pretty short travel/stiff springs, if any) wouldn't make much of a difference to weight distribution would it? On a dragster at launch I'd have thought axle torque did most of that.

You're right. This applies mainly to drag racing of what we called "muscle cars" here. These were cars that, in stock form, could not wheelie, and that had about 45% of their weight on the rear axle, by virtue of having large, heavy (6-7 liter) engines in the front. Up lock shocks would allow the front to lift easily on acceleration, and would prevent the front from dropping on shifts. The weight transfer occurred because of the increase in height of the car's CG, increasing the length of the moment arm between the thrust line and the CG.

When I wrote "drag racers" I was thinking about the people (racing mainly modified or nearly stock street cars) as opposed to purpose-built dragsters. With purpose-built dragsters, where torque alone can lift the entire front end (including unsprung weight) off the ground, the dynamics are different.

Yup, I see that. Brains forms pictures when sitting here so I had a picture of a long pointy thing, front end mainly air, back end mainly rubber with metal in between!

Off-road racers will sometimes use both types at each wheel to tune the bound as well as the rebound separately.

Brad

"Arent they supposed to have about the same damping resistance both on expansion and compression?"

The purpose of 'shocks' is to minimize the effects of an unsmooth or non-flat road surface, not enhance it. The return motion has to be slower to avoid 'bounce'

It is a little more complicated than that. For performance the rate of dampening changes depending on the compression of the suspension. Soft in eXtension, firm in operating norms stiff in compression. But the bound and rebound are not the same for any of these. The orifice and bypass tube systems can take a diagram just to figure out. Had a ex- off road racer friend (Baja 1000) explain it to me and it tooK awhile because of the complexity.

Here is a few links:

http://www.kingshocks.com/

http://www.raceroffroad.com/ricor.shtml

Didn't have time to find more exact info

Brad

If your shocks are longer when a wheel is up and shorter when a wheel is down (that is the opposite to most cars that I have seen but possible), then they are probably OK. But if they are the other way round, your shocks are "shot!"

Normally a shock absorber resists wheels rising from the ground under the influence of a bump, but lets them drop down quickly so that they do not spend too much time in the air!!, if that makes sense?

Did you describe their operation exactly and correctly? Or was it the other way round?

The mechanic is useless if they are as you said!!!! He does not know his job! Go elsewhere!!

On the contrary. When you hit a bump, the spring will resist the wheel's tendency to tuck into the well. The force is proportional - more or less - to the travel.

On the way "out" the shock absorber slows the wheel down with a linear force -independent of travel, but proportional to speed. Without it you would bounce right up like a tennis ball.

I have to admit its a few years since I had a shock absorber in my hands, but the last time I checked, they worked exactly as I wrote earlier.

Of course you don't want the spring shooting the car into the air, but they do not (did not!) work as some others have described.....

I would suggest going to a parts department and checking a new one against your old ones - significant differences, replace them!! That is a simple but good way of working!

You most probably had a gas pressurized shock absorber in your hands. the gas is there to reduce foaming if the hydraulic fluid and hence preventing variations in shock absorbing efficiency. Under unloaded conditions, the gas does force the stem outward but does not contribute in any significant way to absorbing shock. Previous explanations of their operation, that is little force is required to compress and greater force is required to extend a shock when used in normal automotive applications, are correct. The explanation for the drag racing shock is also correct. When purchasing those shocks, commonly referred to as 90/10's you will notice a caution note "NOT FOR HIGHWAY USE". They would cause a pogoing of the front end of the car when used on the highway assuming that is where they are installed.

Gas pressurised shocks (dampers) will normally achieve full extended length in a no-load condition.

This pre-load does add a minute bit to the SPRING COMPONENT.

Does NOT alter the damping. (other than degradation of same, due to cavitation)