Snowmobile Springs: How to Determine Pre-Load Force and Dampening Rate

Damping is expressed as F=Cv, so it is similar to spring rate which is F=Kx. Your spring approach is correct, apply a known force and measure the deflection, and you can easily back out K.

Damping is a bit trickier because now instead of applying a constant force, you need to apply a constant velocity to the compression/extension of the shock and measure the force. This is probably easier said than done.

Measuring the force is pretty easy with a load cell (try PCB Piezotronics in New York, not too expensive). But you will have to come up with some sort of rig that compresses the shock at a constant velocity.

Do you have only the spring or the full damper?

If you have only he spring you can only measure its stiffness as you intend to do. If you will use down described procedure for the damping the same approach can be used for the stiffness putting the spring in a press in series with it the force sensor and measuring force and displacement when the spring is compressed. The graph' slope is the spring stiffness.

In the second case all what you need is a good recording of force and displacement over time with a quite fast acquisition capability.

The easiest way is to mai tain the damper vertical and let fall a mass on its piston. The force transducer will measure the force and the displacement sensor the stroke versus time; processing the 2 records you can determine the damping ratio of the damper.

I ,presume that you can even rent only the equipment if your normal work does not requires such sensors and conditioners, and acording to your approach it is not the case.

The ones I have seen the spring and damper are all one unit. But that does bring up the complexity of sorting the spring force from the damper force.... Again the spring can be determined with a static weight and measure the deflection.

If you take nicknames approach, then you would have to subtract the spring force to get the damper force, and then calculate C.

Not necessary since the damping factor is a dimensionless value obtained from the logarithmic decrement of the following oscillations.

And also when the damper as a whole is deformed at -as suggested- constant speed if the spring is in, the damper stroke changes the spring force and according to what is said it also has to be deducted. A damper with a spring cannot be compressed without changing the spring force. Or am I wrong?

An interesting test can be to let the load fall on the damper without oil and with oil, the 2 displacements curves versus time will give a way to compute the damping effect. Of course if the damper is oil filled.