# Thread: Coil shock vs air shock, what do their force displacement curves look like?

1. ## Coil shock vs air shock, what do their force displacement curves look like?

What do the force displacement plots actually look like in comparison to one another for your typical rear shocks?

I've been researching, even been doing a bit of preliminary calculations, i understand a coil will be linear and a air spring is progressive.

However i've seen some images where air shocks have a greater stiffness for ~ first 5%, then is less tiff than a coil in the middle region, then ramps up at the end of travel.

It's the initial stiffness region and saggy mid that i don't get, where do they come from? Is this what it's actually like in real world?

Below is an example of what i'm unclear of-

2. I fixed your graph to try to make it clear to you. Its far from exact as there are many variables when talking air spring curves, but its the general idea.

3. Originally Posted by DeanFBM
What do the force displacement plots actually look like in comparison to one another for your typical rear shocks?

I've been researching, even been doing a bit of preliminary calculations, i understand a coil will be linear and a air spring is progressive.

However i've seen some images where air shocks have a greater stiffness for ~ first 5%, then is less tiff than a coil in the middle region, then ramps up at the end of travel.

It's the initial stiffness region and saggy mid that i don't get, where do they come from? Is this what it's actually like in real world?...
Without a well balance negative spring, an air spring has preloaded air pressure to make usable weighted sag and would be stiffer in initial travel than a linear coil spring with no negative spring.

A negative spring counteracts the preload of the positive compression resisting spring. Also a negative spring resists inertia that could top out harshly from rebound momentum. The negative spring prevents harsh top out, and can prevent full travel to the full extension of mechanical travel.

4. Originally Posted by derby
Without a well balance negative spring, an air spring has preloaded air pressure to make usable weighted sag and would be stiffer in initial travel than a linear coil spring with no negative spring.

A negative spring counteracts the preload of the positive compression resisting spring. Also a negative spring resists inertia that could top out harshly from rebound momentum. The negative spring prevents harsh top out, and can prevent full travel to the full extension of mechanical travel.
This is a minor nit pick, but a negative air spring does make the initial travel "stiffer", based on the technical definition of spring rate. It takes the preloaded initial travel section of the force-displacement curve, and pulls it down slightly. There is less preload, but the curve is steeper, hence stiffer.

I agree that the negative air does make the initial travel seem less "harsh", maybe because you aren't topping out all the time, or because you aren't overdamped.

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