Damping and shocks

[jjustin]

Hi Guys,

I'm hoping somebody more clued up than me about such things could possibly answer this question for me please.

I've just this morning taken delivery of a pair of rebuilt rear shocks for my car. If I compress them at the same time on a hard surface (kitchen floor), they decompress at entirely different rates, with one of them fully decompressing about three seconds before the other and is noticably easier to compress.

Is this normal or should a pair of shocks be matched?

Having a pair of shocks that decompress at a different rate, does that mean that they each have different damping characteristics which would cause handling problems? If they decompress at different rates, this must mean that one is stiffer than the other, right?

The shocks were rebuilt by somebody very reputable in the Ferrari suspension industry so it's hard to accept that there could be a problem.

Any advice about this would be much appreciated.

Thanks in advance


Edit: When switched to 'hard', theres a good 30 seconds difference in the time they take to decompress. This cant be right.



>>> Edited by jjustin on Wednesday 12th January 17:28

[joospeed]

Did you bleed them fully before carrying out the test?

[jjustin]

Thanks for replying.

No I didnt bleed them. Is it the norm to have to bleed dampers before fitting them to a car?

[joospeed]

depends what design they are, would make sense to do it anyway.

[jjustin]

I sent him an email casually asking if they need bleeding or anything before fitting and he said that they shouldnt be touched because they're very high pressure and they require special equipment and nitrogen to ensure proper operation.

Can anyone confirm that they're not right if they're decompressing at entirely different rates? I need to try and get a little more clued up before contacting him again to explain the problem that I think I have.

[joospeed]

you don't need to open the dampers up to bleed them .. what you're trying to do is get any gas bubbles out of the oil .. so with the damper in it's installed attitude (ie whichever way up it's mounted) repeatedly push the piston rod in and out of the damper, this displaces any air bubbles out of the oil to the top of the oil. If there's air bubbles under the damper vale assembly it won't resist movement as well .. you need only oil and no bubbles under the vale assembly for it to damp properly.

If you do that and they still behave differently conact him again .. possible reasons are :
different valve specs, oils, gas pressures, friction on seals, poor quality control during build etc.

Don't forget that at slow speeds you're possibly not even pushing oil through the valve assembly, it could all be going through the bypass valve on the adjuster (if they're adjustable) so this might be the fault in the bypass.

Other than that it might just be that that's how they are .. and of course this static test isn't truly prepresentative of real-world damper conditions .. if you want me to test them independantly on a proper dyno let me know, i can possibly arrange to do that and get a print out and evaluation of their performance if my damper expert is willing..

[jjustin]

Thats excellent advice, thank you.
I'll try pushing the rods in and out a few times to see if the characteristics change at all and if not, I'll send the guy an email to see what he says.

[steve_D]

I would look to at least 10 full strokes to clear the air.
Most shocks only work in compression so the rate at which they return is of little importance.
If on the other hand you have bought shocks which are supposed to have a rebound rate then they are duff.

Steve

[joospeed]

steve_D said:

I would look to at least 10 full strokes to clear the air. Most shocks only work in compression so the rate at which they return is of little importance. If on the other hand you have bought shocks which are supposed to have a rebound rate then they are duff. Steve

other way around steve, a typical adjustable road damper will have 3-7 times the rebound rate as it has compression.. in other words easier to push in than it is to pull out. If you have something like a single adjust early koni for example it'll only be rebound adjustable, compression resistance would be fixed and unadjustable. If you were to specify one adjustment it would be rebound, not compression.

[GreenV8S]

joospeed said:

a typical adjustable road damper will have 3-7 times the rebound rate as it has compression.

2:1 is more typical in my experience, I would have thought that much more than that would cause jacking problems (this was one of the problems I had with the single adjustable Konis).

[joospeed]

Next time I see you Peter remind me to bring some dyno graphs from Nitron's dyno .. shows pretty well the massive difference between rebound and compression through the range of adjustment.