Fluid Viscosity to Damping force relationship

Hello, all!

Does anyone know the relationship between an oil viscosity change in a shock (or fork) and the accompanying change in damping force? Specifically in my example, if I were to replace a 12.5cSt shock fluid with a 13.6cSt shock fluid, would that (simply) result in an 8.8% increase in damping force under same-shaft-velocity conditions? Logic being 13.6/12.5 = 1.088.

Perhaps there's some more complicated exponential component to the relationship and/or equation? I hear (and read) of people changing oils to slightly lower or higher cSt ratings to make effective damping changes, but wonder if my math assumption is correct.

Ed

It is more complicated that that, and there are a lot of variables. It would be that simple if damping were a matter of simple check valves routing oil through fixed orifices under every circumstance, but damping mechanisms have become much more sophisticated in their function than that. Modern dampers are a combination of fixed orifices and valving that amounts to pressure regulators.

In a simple form, a contemporary shock valve is a pressure regulator. When the shock moves it he direction the valve is controlling, it opens as the pressure on the advancing side rises beyond the designed level. If it moves faster, or if the oil is thicker due to low temperatures, the initial response of the valve is simply to open farther. In fact, one of the reasons shocks are now built this way is to reduce the effect that oil temperature has on damping rates, so that suspension operation will remain more consistent throughout the course of the day. The truth is that there is more difference in the viscosity of a typical 5wt suspension oil between 70 and 150 degrees (F) than there is between a 5 and a 10wt at the same temperature.

Having pointed that out, however, it also needs to be recognized that, as I said already, modern dampers are a mix of orifices and valves. The orifices in the system will respond in a fairly predictable manner to viscosity changes, as one might expect. The valves will, too, but again, there are variables.

A "perfect" pressure regulator set to maintain a single pressure delta would respond to any change in viscosity or flow rate by simply opening up more and allowing the oil to flow through a larger opening. The first problem with this in the real world is that any such valve can only open so far, and it will have a physical limit as to how much it can flow, and as viscosity is raised, or flow rate increased, that limit will be reached sooner, and the pressure will rise beyond the set limit.

The other thing is that the valve will probably not be designed to have a linear response to flow and speed anyway. It may be designed to start at one rate, then raise the damping resistance in response to faster shaft speed, or lower it, or something even more complex than that.

In general, the only thing that can be safely said as a generality is that a more viscous oil will flow through any system with more resistance than a lighter one. How that actually works out can be highly variable.

Hello, all!

Does anyone know the relationship between an oil viscosity change in a shock (or fork) and the accompanying change in damping force? Specifically in my example, if I were to replace a 12.5cSt shock fluid with a 13.6cSt shock fluid, would that (simply) result in an 8.8% increase in damping force under same-shaft-velocity conditions? Logic being 13.6/12.5 = 1.088.

Perhaps there's some more complicated exponential component to the relationship and/or equation? I hear (and read) of people changing oils to slightly lower or higher cSt ratings to make effective damping changes, but wonder if my math assumption is correct.

Ed

Ah...well, changing the fluid does not replicate the curve into a new position.

In other words, you don't get the same curve in a different spot, as you might with something like a spring change.

The relationship is the molecular count (density) to that of the orifice, which is generated by the size and shape of the port to how the shim is deflecting, (as the shim lifts, the “port” changes).

So… a formula of such would work but only if the size of the orifice and the velocity of the fluid are constant.

Great, Dave. That's what I was looking for!

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