Monotube Damper Without Piston Compression Stack

[jkdmag]

Interested in the opinion of the experts on the necessity of compression shim stack on the piston of a monotube damper with high speed compression shim stack /adjuster fitted to the remote canister. To my untrained mind it would appear that as the flow of oil displaced by the rod is controlled by the canister compression shim stack, a simple check valve is all that is needed on the piston compression side.

 

[gt6racer2]

In theory your are correct. It would work. However, the pressure generated at the remote valve will only act on the rod area, which is very small for a monotube. As a result, the pressure drop needed over the remote valve will be very high, and oil and other compliances and other issues will become significant. The pressure drop across the piston stack works on the piston area, and so generates much more force for a given pressure drop. As long as the pressure above the piston is kept positive to avoid bubble formation, the piston valve is the better one.

 

[jkdmag]

In many application for compression damping, the rod volume as it moves into a cartridge, is the only principal used to establish oil flow through a shim stack valve at the base of same the cartridge. I see the remote canister as an extension of the cartridge with a shim valve.

 

[gt6racer2]

I didn't say it wouldn't work - just that it's not ideal. Yes the model in simplistic terms is the same, but cartridges are struts with rod dia 20-25mm vs montube 11mm - up to 5 times more pressure area. There are also some shocks that used rod displaced valve designs (check valve on top of piston), but those units (if used by a discerning customer) will use 16mm rod Vs 12mm typical. Of course there are down sides to a big rod in a shock - the cost/mass and also that it drives up the overall diameter of the shock, as more rod means less rebound pressure area. What is driving you to pursue this ?

 

[jkdmag]

The background to my initial question was the hi/lo speed compression adjuster option available on some up market remote canister monotubes. If the canister has a hi compression control then what is the piston compression valve actually doing.

Having given it more though, I suspect that I have put too much emphasis on the "hi" and should see it more as a mid speed that eventually will blow off to leave the real hi speed compression control to the piston valve. Am I closer to an understanding of the system.