Spring Rates + Valving

[Moto Pro472]

This is probably a stupid question but is there any way to find out the spring rates of the springs by a number or something on the springs. I have a set of Elka quad rate long travel shocks and was thinking of using front quad suspension for a system modeling project. I would also need to know the valving to find the dampening coefficient.

What I need to find out is the 4 spring rates of each spring and the dampening coefficient of the dampener so I can model it in simulink.

I don't expect anyone to know this but figured it can't hurt to ask

[dalejiw25]

I'd give Works Shocks or GT thunder a call. Thats a pretty high tech ?

[tri5ron]

is this something you are trying to do on a set of stock 400ex shocks? or some other type of shock fitted to the 400ex?

[dustin_j]

Yes, a simulink model would be sweet to see how damping coefficients and spring rates would react without the need to have a shock dyno or revalve your shock each time. I thought of this also, but there are several obstacles I will list below to begin discussion.

There are supposedly markings on the springs, but mine were worn off so I used a spring rate calculator (GT Thunder's website) using wire thickness, diameter, and number of coils. There is also a spring rate caused by the nitrogen that needs to be considered (which varies based on shaft position and displaced oil).

I wonder how constant the damping coefficient is? Most shock builders use high and low speed damping (shims seperated by a small crossover shim), and my elkas even had a third stage for some reason. This will change your damping coefficient depending on shaft velocity. I would be interested as well if anyone has data from a shock dyno relating shaft speeds and force on a known shim stack. Another issue is that different shock companies use different pistons, so the data would have to be from an Elka.

Hopefully someone is able to shed more light on this topic, it would be a great modeling project. Are you an ME student?

Dustin

[Moto Pro472]

Actually im an EE student taking an ME class lol... What exactly is a shock dyno? Some sort of hydraulic press which compresses the shock at a certain velocity with a certain applied force?

Looks like this project might be more complicated then I thought without knowing how the shock was made up (which Elka probably wouldn't want to give away their secrets).

Since the quad rates that I have have no adjustments besides ride height I was assuming that each of the 4 springs would have a specific spring rate and since the compression and rebound are non-adjustable I figured it might have a specific dampening coefficient but maybe I am wrong?

[Moto Pro472]

Originally posted by tri5ron
is this something you are trying to do on a set of stock 400ex shocks? or some other type of shock fitted to the 400ex?


Not stock shocks... Quad rate elkas.. Stock shocks would be easier though...

These are the shocks i'm trying to simulate...
http://img20.imageshack.us/img20/7044/dsc00092ao.jpg

[MtnEX]

Nice shocks!

[dustin_j]

Originally posted by Moto Pro472
Actually im an EE student taking an ME class lol... What exactly is a shock dyno? Some sort of hydraulic press which compresses the shock at a certain velocity with a certain applied force?

Looks like this project might be more complicated then I thought without knowing how the shock was made up (which Elka probably wouldn't want to give away their secrets).

Since the quad rates that I have have no adjustments besides ride height I was assuming that each of the 4 springs would have a specific spring rate and since the compression and rebound are non-adjustable I figured it might have a specific dampening coefficient but maybe I am wrong?

Good to see you branching out from your major, it'll benefit you for sure. I'm an ME and I should have taken more EE, haha.

A shock dyno is a device (generally powered by an electric motor) that compresses a shock at a known speed and a defined stroke, and measures force. Since F=c*v (Force = damping coefficient * velocity), you could back calculate across different speeds to see if the damping coefficient was constant, if you had a dyno.

You are right that there is a specific damping coefficient that corresponds to the internal shim stack for your shocks; however, finding that coefficient is the problem. Also, as I mentioned previously, the valving setup has different damping coefficients based on shaft velocity; high and low speed damping. I'm not sure what your school has, but if you could rig something up to input a velocity and measure force you'd be set (remove the springs of course).

The spring rates are constant for the four springs. I tried to model my springs to determine proper spring rates, but I was informed that I forgot the spring rate provided by the nitrogen. I believe you could estimate this based on volume of your reservoir (subtract roughly .5" in length to accomodate for the floating piston and small amount of oil) and the diameter of your shaft.

Hopefully this helps. I'd like to rig up a low tech shock dyno someday, but this doesn't help you now :p

Dustin