
Gathering some shock data
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 cutaway 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 cutaway 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 noncompressible 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 4.95 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 80300612 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 80300612 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?

I've been through this exercise for the dual air forks. Was wanting to do exactly this for my RP23. I'd been having thoughts of building my own dyno just yesterday.

Originally Posted by dberndt
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
This assumption isn't valid (even though the numbers in your table seem to balance out). There is a bleed port and this balances the chambers and assures relatively low preload on the spring. The bleed port might or might not be up at the end (full extension) part of the stroke, so you either have to assume that the pressure in the two chambers is equal at this point or measure where it really is.
I find it easier to not get lost with these calcs to explicitly mark values as either psia (absolute) or psig (gauge). If you are converting a pressure to a force, always make sure you are using psia. If you are looking at a balance of forces make sure you include all forces. e.g. atmospheric pressure acts as a constant force on the cross section of the damper body.
... but then again I am some crazy Brit engineer who thinks the SI system of units makes sense... even though I grew up using Imperial units and therefore live in a world of mixed units. Most of my spreadsheets have input in Imperial and calculation in SI.
Your starting point values on the volumes don't agree with Fox's published compression ratios data. I've just done an exercise (because I'm away from home and needed a diversion) to back calculate the positive air chamber volumes based on Fox's compression ratio chart: 2012 FLOAT Air Spring Summary
Believing the Fox numbers implicitly, I end up calculating a positive chamber cross section area of 1048mm^2 (1.625in^2). Your numbers give a CSA of 1.45in^2. Not sure who is right. My calculation depends heavily on the Fox numbers meaning what I think they mean which is a long stretch for the imagination.
My approach would be to work out the effective crosssectional areas and the volumes at rest for all chambers (pos/neg/ifp) and net to atmosphere.
The crosssections of interest can all be described relative to the following diameters:
body inner diameter csa (bi)
body outer diameter csa (bo)
can inner diameter csa (ci)
shaft outer diameter csa (so)
The relevant pressures are n,p,ifp and atm. In the following I use the above bid/bod/cid/sod symbols to reference the crosssection areas, rather than the diameters themselves.
The force balance on the whole assembly is:
F=ifp*so + p*(biso) + p*(cibo)  atm*bo  n*(cibo)
For the variance of each of the pressures with shaft position, you need the at rest volume and the relevant c/s areas to derive the effective length. The pressure in each chamber can then be calculated using Boyle, just considering the length ratio.
i.e.
ifplen=ifpvol/so
plen=pvol/(ciso)
nlen=nvol/(cibo)
ifp=ifp_stat*ifplen/(ifplenX)
p=p_stat*plen/(plenX)
n=p_stat*nlen/(nlen+X)
N.B. negative chamber at full extension has same pressure as positive chamber because of the assumption that the bleed between the chambers is at full extent.
That should be it. If you give me the volumes and crosssection areas/diameters (assumptions where applicable) I can adapt my spreadsheet to do the rest, with graphs all ready to go.

I've got to get to work so will digest and respond more to your post later petercam, but for now to keep you occupied i'd ask. If there is a bleed valve between pos and neg how do you explain people getting "stuck down" shocks. A bleed port would not allow that to happen would be my understanding.
My start points are based on rough guesses and poor spreadsheet math, I probably can't hope to defend them from a full on engineer who seems to understand this a lot more than I do, but I would say that I'm not sure I understand how you went about using the chart on 2012 FLOAT Air Spring Summary to back calculate any sort of chamber cross sectional area? There seems to be no data or linkage between the data their putting up about compression ratios and cross sectional area. Further to that drawing the shock and then determining CSA with a CAD/CAM package seems like a fairly easy way to go about this that should land me within a few % accuracy, its not a difficult part of the assembly to get right... That being said In the bottom of the bottom middle of my first chart I explicitly list the CSA as 1.76 pos and .87 neg.
Gotta head to work, more later.

Having read your entire post... looks like we've looking at many of the same sources. Here is where I back calculated from the the (rounded) Fox CRs to suggest a consistent result for the crosssection area:
What I did was say that the vols for endspace and stroke were x and y respectively.
The compression ration (no volume reducer) is by definition: CR0 = (x+y)/x
With a volume reducer in place, this gives: CR1 = (xvr+y)/(xvr)
From known CR0 and CR1 with a known volume reducer, you can solve for x and hence y. Given the stroke you can solve for the crosssection area.
I'm away from home so not able to measure anything, so using the Fox chart let me do some thoughtmaths. I'm not trying to negate or disrespect any of your work. I'm amazed at the timing that I literally looked at this yesterday.
I think we are in great shape to fully describe the air spring characteristics.
As for stuck down...
Normal behaviour is for the positive pressure (at any part of the stroke except at full extent to exceed the negative pressure. The seals will wear according to this pattern of pressure loading. Eventually the seal system separating the negative and positive will fail causing a bleed of high pressure over to the negative chamber. The shock now reaches equilibrium at a point that means it doesn't reach the equalisation port (full extent). It is stuck down. Any attempts to force the shock to full extent are fighting against... the negative now having higher pressure than it was ever intended to have and the bleed port sitting right up at the most progressive phase of the negative chamber's compression. Additionally, the seal system is now facing a reversal in pressure differential (the negative is at higher pressure), so the seals will shift in their glands and rest on a nonworn part, so there isn't likely to be any bleed back in the other direction.

As I actually have an apparatus for measuring shocks throughout their stroke can we devise a test to establish the existence of this bleed port and it's function?
If the pressure equalizes at top of travel (full shock compression) then I would assume that if I installed the air sleeve and did two full compression cycles I'd see a delta between the compression stroke on the first pass and that of the second, as the second would have benefited from the equalization of pressure via the bleed port.
Sound fair? What are your thoughts?
As to how you came up with CSA that's good work. It certainly makes sense now. I failed to realize that we could go from volume to surface area using stroke length, it should have been obvious. Anyways on CSA I think we more or less agree, my drawn/measured number is within ~10% of your calculated number, I think it'd be fair to assume that this comes from the margin of error in the FOX data sheet and any measurement/drawing errors.

The bleed port It's really easy to see in Fox shocks, it's just a dimple in the air can at the beginning of the travel.

Pic? I dont see it...
as a reference for example.

Originally Posted by dberndt
I've got to get to work so will digest and respond more to your post later petercam, but for now to keep you occupied i'd ask. If there is a bleed valve between pos and neg how do you explain people getting "stuck down" shocks. A bleed port would not allow that to happen would be my understanding.
As mentioned by others, the bleed port is a notch pressed into the aircan from the insideout. You can see the lump from the outside.
A stuck down shock is when this bleed port stops working, it can be caused by many things including the seal getting soft enough that it deforms into the port (notch) and stops any air escaping on the way past. Combined of course with positive air pressure leaking past the seal deeper in the stroke.
My main question is "what model are you using to predict the pressure rise?"
*edit*
I just found in your first post that you used boyles law "aka ideal gas law" to predic the pressure rise. Unfortunately that doesn't work unless you compress the shock slowly enough that the temperature doesn't rise.
You need to use adiabatic compression instead.
*/edit*

I'm sorry, I have a DHX air and an rp2 here in front of me and I don't see it. Show me the money...errr bleed port. I've looked inside, the air can, outside the air can, there is no obvious depression that I can find.
Also, for now all I have is an a device to slowly compress a shock against a load cell, giving me an output of force vs displacement, as it takes 30+ seconds to do one compression stroke I'd consider that temperature increase is likely extremely minimal, though i have no data to back that up. So I'm just going to continue to use Bowle's law and do some experiments to compare theoretical data vs real world.
Eventually I'd like to work at higher speeds and lower forces by isolating the damping circuit (removing the air or coil spring). But the combined/holistic view of the entire shock at once is likely never go be realistic, the forces required to generate the shaft speeds in question with a pressurized shock are going to be a lot more than I'll likely want to deal with or be able to afford to deal with.

Originally Posted by dberndt
I'm sorry, I have a DHX air and an rp2 here in front of me and I don't see it. Show me the money...errr bleed port. I've looked inside, the air can, outside the air can, there is no obvious depression that I can find.
I don't have a DHX air to compare, my DHX is coil. I have a few FLOAT's lying around, the bump is just like this one. On the left edge of the can, halfway between the decal lower edge and the last reduction in diameter.
Same place on this one.
Originally Posted by dberndt
Also, for now all I have is an a device to slowly compress a shock against a load cell, giving me an output of force vs displacement, as it takes 30+ seconds to do one compression stroke I'd consider that temperature increase is likely extremely minimal, though i have no data to back that up. So I'm just going to continue to use Bowle's law and do some experiments to compare theoretical data vs real world.
Your 30 second test is a long way from real world. I've never encountered a 30 second bump.
Adiabatic compression is what you need. The exponent will likely have to be varied from the 1.4 used for air to get a good fit with accurate experimental results.

Thanks, I found the bump finally... no external mark on my cans but if you look far enough inside its there. Interesting. Will have to change my calculations.
I don't disagree that it's not a real world test, but it can be used at low speed to prove the model out and generally goof around. For making real world changes and testing on the bike the numbers can be recrunched for Adiabatic compression.

Originally Posted by petercarm
The crosssections of interest can all be described relative to the following diameters:
body inner diameter csa (bi)
body outer diameter csa (bo)
can inner diameter csa (ci)
shaft outer diameter csa (so)
Can you define the values you are looking for a bit more? I don't think I follow exactly in particular what bi vs bo is.
I believe bi would be a reference to the diameter of the IF piston? I haven't disassembled and rp2/float/etc enough yet to know that, but I have a good guess. bo is 27mm, account for some wall thickness, say 1 to 1.5mm and you get a BI D of 24 or 25mm, surface area .76square inches.
BO diameter 27mm, giving an area of .887 square inches.
ci or sealhead OD 1.5" dia, or 1.76sq in
small inner shaft so has Diameter .375" or surface area .110 sq in.
Hopefully those numbers are what you are looking for, or make sense. Let me know if you're looking for something else. I'd post my excel spreadsheet but to be honest it's a pretty big mess. I can clean it up and post if need be but it sounds like you already have a handle on the maths and could easily share?

Last remaining measurement of interest is how far into the stroke the pressures are equalised. i.e. where the bump centre is relative to the centre of the seal gland.

bump centre is approximately 1.02" from end of can, in that one inch you'd have .57" of bottom of can seals. plus whatever the negative chamber space is, plus i''d assume half the seal head on the which is ~.352/2.
1.02.57(,352/2)=.254. It's a bit of a crap shoot to measure and figure out exactly what that means for the negative chamber, also the start of the negative chamber in the can is not at 90 degrees, there is a chamfer which eats some volume. setting my model at ~.192" nches stroke on the neg chamber accounts for the chamfered area in which I imagine the piston never travels into or bump stops against. Resulting in a negative chamber .20 cubic inches.

Originally Posted by dberndt
bump centre is approximately 1.02" from end of can, in that one inch you'd have .57" of bottom of can seals. plus whatever the negative chamber space is, plus i''d assume half the seal head on the which is ~.352/2.
1.02.57(,352/2)=.254. It's a bit of a crap shoot to measure and figure out exactly what that means for the negative chamber, also the start of the negative chamber in the can is not at 90 degrees, there is a chamfer which eats some volume. setting my model at ~.192" nches stroke on the neg chamber accounts for the chamfered area in which I imagine the piston never travels into or bump stops against. Resulting in a negative chamber .20 cubic inches.
1.02.57(,352/2)=.274
I manage to get this to match your 0.20 cubic inches volume.
The negative air chamber acts as the top out spring, but it is ramping up pretty fast at that stage. I'm going to assume the centre of the bleed port as the datum for the stroke. It just makes things a bunch easier and I think it makes next to no difference.
I think I am set with all the numbers I need.

As I said the chamfer at the end of the travel means that not all of the air can is available as stroke length.
That is why 1.02.57(,352/2)=.274 doesn't match volume.
I think you're making reasonable assumptions/choices with starting at 0 stroke being equal pressure.

I've been looking at the cutaway diagrams myself. Just sense checked a calculated 200psig figure for me on a 2.4 leverage ratio bike with an 8.5 x 2.5 shock.
seeing as that's the bike I ride, I was happy to see it predict 28% sag.
I've got a bit of work to do but it is looking hopeful.
Sent from my GTI9100 using Tapatalk

This a a great thread, I have skimmed it, but it will take me a while to really understand a lot of it.
I see there is some info Fox floats of various volumes (not sure how to interpret the data). I would like to see some of this spring curve data graphed (pardon me if you've already done this and I don't understand what I am looking at), but more importantly, I would like to know how it compares to other models.
The practical application of this for me is that I am considering getting a 7.5x2.0 Monarch high volume shock, but I cannot find any info that compared that spring curve of this to the RP2 with an xv1 or xv2 can. I've been told that the Monarch is very "linear" feeling, but I'm not sure if this is due to a more linear spring rate, or better mid stroke damping support. I was very surprised when I asked Push and they could not give me an answer about the actual spring curve.
15mm is a secondbest solution to a problem that was already solved.

As a purely practical matter, how do I know what volume reducer (if any) is in my RP2? Is it something I can easily remove or change?
Thanks
15mm is a secondbest solution to a problem that was already solved.

Originally Posted by kapusta
As a purely practical matter, how do I know what volume reducer (if any) is in my RP2? Is it something I can easily remove or change?
Thanks
remove from bike, release pressure, place can end hardware in soft jaw vice. Unwind can (as you would for an air sleeve service). Look under the top out washer. If there is a translucent plastic piece there, you have a volume reducer.
Fox instructions say to pull away the steel washer down the shaft and use a4mm allen key to pry the volume reducer out of position.
On this thread, I should have a model for the fox spring system by tomorrow (with curves)
Sent from my GTI9100 using Tapatalk

Originally Posted by petercarm
remove from bike, release pressure, place can end hardware in soft jaw vice. Unwind can (as you would for an air sleeve service). Look under the top out washer. If there is a translucent plastic piece there, you have a volume reducer.
Fox instructions say to pull away the steel washer down the shaft and use a4mm allen key to pry the volume reducer out of position.
On this thread, I should have a model for the fox spring system by tomorrow (with curves)
Sent from my GTI9100 using Tapatalk
Cool, thanks. I've had my old floats and AVA apart, never really though about if there were reducers in there.
15mm is a secondbest solution to a problem that was already solved.

I'm still waiting for my set of fox float spacers to come in. I have a few I made myself but I would prefer to only post graphs/charts with data for spacers other people can actually get. Hopefully by mid next week I'll have some real world charts posted up.
I don't have anything other than a fox rp2, dhx air 5.0 and a van coil rc. Eventually i'd love it if I could get my hands on or temporarily borrow some RS rocks and also some of the newer fox's with boost valve.

Originally Posted by dberndt
I'm still waiting for my set of fox float spacers to come in. I have a few I made myself but I would prefer to only post graphs/charts with data for spacers other people can actually get. Hopefully by mid next week I'll have some real world charts posted up.
I don't have anything other than a fox rp2, dhx air 5.0 and a van coil rc. Eventually i'd love it if I could get my hands on or temporarily borrow some RS rocks and also some of the newer fox's with boost valve.
I would like to see some spring curve data with the boost valve. I have read a few people claiming that the boost valve increases the spring rate at the end of stroke, but I would think that it would be so small as to be negligible. It would be good to see actual spring curve data.
15mm is a secondbest solution to a problem that was already solved.

Boost valve on an rp23 doesn't do anything to spring rate, as far as I can see. It looks like the sense of boost valve between the rp23 and the dhx are the complete opposite.
Boost on rp23 gives more platform around sag. Boost on dhx gives more bottom out control.
I'm basing this on some blurry cutaway photos rather than having dismantled the shocks to see the innards.
I have my graphs ready. Will post as soon as I get off this plane.
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