Mono-tube spring effect

[BearWRX]

I would like to better understand the difference in the behaviour of a mono-tube shock and a OEM or Koni twin-tube shock on my WRX MY99. Towards this I read the FAQ article on the web site

http://www.whiteline.com.au/faqshocks01.htm#Mono-tube vs Twin-tube - ride characteristics

In particular I wonder about the effect in a mono-tube of the gas above the floating piston which allows for displacement of the shaft. It seems that this will act as a progressive spring with at least several mm of undamped travel. If so, won't this reduce roadholding and maybe increase body vibration?

Any clarification would be helpful.

Thanks,
John.

[Jakub]

Hi John,

You are correct in that the gas pocket seperated by the floating piston from the oil chamber would provide a non-linear spring rate to the damper. This is evident when one tries to compress a damper and the spring rate forces the shaft back out (or the rate is so much you cant compress it by hand).

The gas chamber is seperated from the main oil chamber by the floating piston, so if everything is working correctly, there shouldnt be any contamination of the oil by the gas. So there will always be damping present.
If there was a leak and there is mixing of the two fluids (oil and gas) then there could be moments where air would be passing through the damper piston giving little damping resistance, giving poor roadholding, body vibration.

As always a picture of this would say a 1000 words, i'll see if I can get one up and running soon.

Jakub Zawada B.E. (Mech)
jakub@whiteline.com.au

[Jakub]

Hi John,

Its been a while, although I finally did get one made up. The picture shows a very simple mono-tube damper (sliced down the middle), simple in that it does not show any seals and the main damper piston has small holes which pass oil through them (normally there would be smaller holes with shims which deflect under load). The parts have been sized to show clarity. In the picture the gas is blue, oil is gold, floating and damper piston in white and damper body in red.

I dont think I nailed your question down properly last time, I beleive the "travel" will be damped all the time, even in the first few mm. This is because the floating piston is somewhat directly connected to the main damper piston by the damper oil, which could be viewed as an incompressable fluid (much like your brakes).
My line of reason is that the non-linear spring (from the compressed gas) acts on the floating piston which acts on the damper piston through the damper oil. However the damper piston is "damped" and so the non-linear gas spring would be damped as well.

Regards
Jakub Zawada
jakub@whiteline.com.au