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I just changed the struts on my car, and was curious how fast a strut piston should return after being depressed. In my case the old ones didn't have any return at all almost ( after a few minutes they had returned maybe 4 inches ). The new ones took maybe a second or two to return to full length. I didn't time it, just seemed quick, but not too quick.

I'm talking about depressing the silver piston in the middle of the MacPherson strut:

enter image description here

EDIT August 4th, 2016

Here is a video comparing my old strut to my new strut.

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  • You compressed what, exactly? Are you talking about suspension shock absorbers? A strut is more than that.
    – I have no idea what I'm doing
    Mar 4, 2016 at 11:40
  • @IhavenoideawhatI'mdoing The silver piston in the middle of the strut.
    – Robert S. Barnes
    Mar 4, 2016 at 11:45
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    I wouldn't worry about how fast it auto extends. How easily you can extend/compress it by pulling and pushing on the rod will give a better indication of its damping strength. The gas in the old ones must have leaked out of its compartment, so stopping them extending fully.
    – HandyHowie
    Mar 4, 2016 at 17:29
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    @HandyHowie could you flesh out an answer so that we can move this question off the "Unanswered" stack? Thanks in advance
    – Zaid
    Jul 30, 2016 at 14:02
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    @Zaid when I change the rear struts actually made a video of how fast the old strut compared to the new strut expands maybe I'll try and link to it.
    – Robert S. Barnes
    Jul 30, 2016 at 21:06

1 Answer 1

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Watch the vocab here.

  • A strut includes the large spring that is attached to the frame at one end and the wheel hub assembly at the other end. It's job is to allow the wheel to move up and down freely over uneven road surfaces.
  • The picture you posted is of a shock absorber. It's job is to quickly slow down the up and down motion in the strut to keep the vehicle stable.

Modern vehicles have both as this provides the best ride comfort and vehicle stability while braking, accelerating, cornering, riding over bumps and potholes, as well as vehicle oscillations induced by the road, the air resistance and driver inputs.

Shock absorbers should return to normal "very soon." It's not the time that is critical, it's the strength that they use.

  • If the shock absorber has lost some of it's "charge" (some are filled with gas, some are filled with liquid), it will loose some of it's strength to return to it's normal length. If it's completely lost it's charge, it won't return at all. These things develop leaks over time.
  • If the valve system inside the shock absorber gets clogged or damaged, the absorber will loose some or all of it's strength as well.

A shock absorber loosing it's strength will take longer to return to normal, since there is deliberate internal friction part of the "tuning" of the absorber for the weight and type of vehicle it is attached to and the ride smootheness it is supposed to provide. As you noticed while shooting your video, the new strut was a lot harder to compress than the old one.

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    You are incorrect the thing I posted a picture of is called a Macpherson strut. It is labeled that in every automotive book I have read.
    – Robert S. Barnes
    Aug 4, 2016 at 18:05
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    @RobertS.Barnes Nope, tlhingan is correct - the MacPherson Strut is the full system, so damper, spring, lower arms etc. What you've pictured is indeed part of a MacPherson strut system, but it's still a shock absorber, and would behave the same as a non-MacPherson one.
    – Nick C ♦
    Aug 5, 2016 at 9:16

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