Starting a thread which hopefully will be useful to some people. My hope is to provide some good theoretical data about shocks, in particular to start off with air spring curves. Then move on to providing some real world numbers from a home build spring measurement system.

Phase 2 would be a similar idea/concept but for compression damping and rebound. That is to say if Phase 1 seems interesting and successful I'll move on to building a shock dyno. I'd love to have a means to get empirical data on many of the various shocks out there on the market.

So to start this off.. One night i was on a really boring conference call for work doing some random IT stuff which more or less involved me sitting around waiting for other people to do things for 4 hours. I got to thinking, as I have mused all summer, about how there is very little data released about bike suspension systems. Shock curves, damping data, data sheets in general appear to be totally missing. Perhaps these are available to bike builders/designers (which I am not), but in general it seems like there is an information black hole. So I went out to garage grabbed a pair of calipers and a Fox 7.875x2 RP2 and went to town measuring it and reproducing a digital copy of it.

It looked more or less like:

I left out most of the complexities, especially of the top cap, because measuring that would have been super painful. But all told I think I did ok.

So What do we really need to know about an air spring to be able to calculate it's curve?

Well I figure its:

A known starting point with known:

-positive chamber volume

-positive chamber pressure

-negative chamber volume

-negative chamber pressure

So fortunately I could bisect the model, get a profile of the empty volume of both the negative and positive chambers at full compression and zero compression states. Those models looked something like:

The top being a fully compressed shock and the bottom the shock at rest. So I measured out some volumes, and I didn't really keep good notes and it's been a few months since I did all this, but I believe my numbers were something like:

Shock in uncompressed state - positive volume 4.3 cubic inches.

-negative volume .07 cubic inches.

Shock in compressed state - positive volume 1.4 cubic inches.

-negative volume 1.82 cubic inches.

So... assuming the the amount of air actually being compressed into the negative chamber when you install the air sleeve we can assume that at full shock compression the volume of the negative chamber is at 0psi for the full 1.82 cubic inches. Which means when that volume is compressed to .13 cubic inches and we apply Boyle's law PV=T we can come up with something like

1.82*athmosphere pressure (15psi at sea level) (this part I'm a bit flakey on to be honest as pressure at sea level vs absolute pressure screws me up)=.13X

restated: 1.82*15=.13X

solve for X gives us X=210.

Remember to take away atmospheric pressure at sea level as we don't include it in any of our future calcs so X=195.

Here's another part where my notes get messy (I didn't take any... just have an excel workbook full of ugly calculations...). I apparently threw that number out at some point in the future and started using a value of 240 for the pressure in the negative chamber when the shock is at rest. The difference in volume of the negative chamber is admittedly a large percent, but as a value its very small so in terms of measuring off the model this is a total crap shoot, an initial measure/guess is .07 cubic inches, using X=240 would imply that X=.107 cubic inches. Other numbers I ran went as high as .13 cubic inches. Ultimately it doesn't matter much because we're going to throw the negative chamber talk into the closet for a little while.

So now I've got pressures and volumes for both chambers in both states and I can measure out the piston area from the model. I got 1.76 square inches of surface area for the positive piston and .87 square inches of surface area for the negative piston. Thus we can put together a little spreadsheet that looks like this:

I left out some magic that is in another table and needs to be its own discussion about IFP forces. For those who know how to calculate it (not hard) the assumptions I'm making are:

Shaft diameter .375"

internal floating piston diameter: .8" (guess work from some cut-away views of rp23's)

inital pressure: 400psi (more guessing/internet heresay)

gives a shaft ratio of 4.555 and an ifp stroke of .439 inches.

Assuming an ifp set height of .65 inches (again from cut-away views and guess work)

that means likely minimum force from IFP is 44lbs and max would be 136lbs.

So given all that, we now have a model of a shock that is somewhere in the ballpark or atleast with in a few kilometers of the ballpark.

Next step I took was to think about "shimming" my shock, aka reducing the positive air volume by inserting a non-compressible solid into the shock body. I was looking at this Changing FLOAT Air Spring Compression which lead me to this table: 2012 FLOAT Air Spring Summary

So fox is talking about compression ratios, well looking at my shock a 7.785x2 with high volume can. I had done all my modelling to date as if it were a standard volume can and not included the HV sleeve. But I realized that If I knew what the delta between a standard air can and the XV1 can which I have was that I could use the charge Fox has provided to backwards engineer the value for what the positive chamber must be. I mucked around with it for a long time and did a lot of spreadsheet work and the best fit came out to be:

Now the chart is based on me fiddling with a bunch of stuff and finding the best fit. In reality fox likely rounds up and down those numbers, it's not likely that compression ratios are nice neat numbers to 1 decimal place, I'm sure some rounding or possibly even creative rounding has been taken into consideration in order to get pretty data to release to the public.So take it with a grain of salt just like everything here.

So a 7.875x2 shock with a standard air can comes out to have a positive air chamber volume of about 4.9-5 cubic inches via approximate measuring or reverse engineering some fox specs. So we're definately in the ball park now. Same goes for the compressed volume of the positive chamber at 1.4 cubic inches.

Using the numbers above plus some undiscussed IFP calculations we can create a reasonable model of an air shock and spit out a graph and IMO it looks reasonable. We can also use the formula to determine what effect putting a spacer into the positive chamber would have. Though I'm still waiting on my official FOX float tuning kit 803-00-612 I made up a spacer of my own with a volume of approximately .6 cubic inches and threw it into my shock. My butt informed me after the first ride that it did make a very big difference to bike feel, and the data suggests it should. But saying I feel a difference isn't really very scientific.

So in the next installment of my ramblings I'll document how I built a spring rate tester and I'll start to post some pretty graphs/data of actual shocks with various settings. We can explore things like:

What impact does the fox float tuning kit 803-00-612 have on a shock?

How does a DHX air boost valve and bottom out control affect the spring rate?

How linear is a coil shock?

What impact does IFP pressure have on a coil shock?

How many graphs can I make before I lose my sanity?