Fox Performance Series Shock Rebuild Thread

[Mike_H]

Spent a couple more hours tonight taking the rest of the shocks apart. The rear ones were nasty…oil came out black. When it’s fresh, it’s a translucent cherry red!

I have a few more parts to buy too. One hose was linked and is starting to fray…I may just run it, I don't think it was leaking. Depends how much it cost, I guess. Its not particularly hard to tear these apart, but once its installed in the Jeep, I can see it being a couple of hours worth of work to change it post restoration. The bushings I have “in inventory” are the proper length and OD, but they are 3/4” ID instead of 1/2". I’ll probably replace those. I want as much flexibility in the mounts as I can get. The bushings are cheap, and I only NEED two of them. The last thing to figure out is an over-site on my part. The front shock have the correct stem mount on top, but a standard shock eye on the bottom. Need to figure out if I buy a couple bar pins to press in, or go with a bar pin eliminator kit. Depends on my travel biases, I guess.

The valving for the rear was, ah, fairly complex. 20 total shims, with a flutter on the rebound side and a double flutter on the compression side. These came off a JKU, I guess all that complexity is needed to control the big JKU booty.

I circled the three flutter shims (fulcrums). Going to have to do some reading on this. I was really surprised to see a flutter stack on the rebound side. Like, I had to stop and think about how they were mounted and which side of the piston was compression and which was rebound. Also, the double flutter on the compression side is interesting. Accutune brags on their double flutter stack...so it makes me wonder if these were tuned before.

https://accutuneoffroad.com/accutun...VAqKH15etGrBpKu4VSVm2HK36mb_1el4dnW13tr05fQmu

No Accutune stickers on the shocks though, so no way to tell. I mean, I guess I could email them the serial numbers on the shocks, maybe? I don't know...I don't think its that important. I'm still kinda leaning on the Mopar influence. You're a pretty smart engineer to work on that kind of stuff for an OEM. Just to get a job at an OEM, you've generally gotta be an A student, plus have some real world chops. Then, the guys (and gals) who get to work on the fun stuff are a cut above that. So, its entirely possible, in my mind, that Mopar developed this crazy stack. They probably had different part numbers for JK and JKU lifts too. The shock valving might have been the only difference.

As an aside...I absolutely LOVE finding stuff like this. Its the things that make me go, "Hmmmm, I wonder"? I will get HOURS of entertainment out of this, digging into the net and trying to figure something out. Even if I throw these shocks away tomorrow, the hundred bucks and 3 hours or so I spent on them was worth it. I just love this stuff.

 

[macleanflood]

Could throw some epoxy heat shrink on the damaged hose to encourage it to stay together.

My rears are Accutune sourced Fox 2.0s. They're smooth.

-Mac

 

[Mike_H]

General information alert. Found this article from ADS. It’s pretty well written and presents the information in an easy to digest format. I enjoyed it.

https://www.adsshocks.com/support/v...igko6fxi4zolstncqvheqwjbv1m_rk16x-drdxdevdfbi

 

[GoldenGorilla]

Thanks for sharing. having rebuilt my Fox mtb shocks many many times, it's cool to see it all scaled up.

 

[mrblaine]

General information alert. Found this article from ADS. It’s pretty well written and presents the information in an easy to digest format. I enjoyed it.

https://www.adsshocks.com/support/v...igko6fxi4zolstncqvheqwjbv1m_rk16x-drdxdevdfbi

There is a mention in there of high spring pressures. Folks who don't know what that is about may bring that forward in the next tuning ride quality with springs discussion. The high spring pressures are from cantilever style suspension where the coil-over is mounted inboard or away from the axle so they have much higher than our typical 1-1 motion ratio. Similar to how you see the monster coil-overs with very high spring rates on 2000 lb. SXS.

 

[mrblaine]

General information alert. Found this article from ADS. It’s pretty well written and presents the information in an easy to digest format. I enjoyed it.

https://www.adsshocks.com/support/v...igko6fxi4zolstncqvheqwjbv1m_rk16x-drdxdevdfbi

I would have liked to see more about the reservoir mount locations and why that matters so much. I don't think it is intuitive to understand why we need a reservoir. If you had a shock body without a reservoir, filled it with oil with the shaft fully extended, as the shaft moved into the body under compression, it would either lock up and not move or bust a hole in something so it could displace the oil.

The problem is solved by adding a reservoir with a piston in it. The piston has a gas charge over it on the non fluid side so it can allow the oil to be displaced into the reservoir and then the pressure charge pushes the oil back into the shock body when the shaft moves out. (I know you understand, others may not)

Where the problems happen is when we over valve a small shock like the 2.0's so we can work them harder. The high compression valving will blow the oil column apart with a high level of cavitation under the piston and just push the oil above that into the reservoir which causes the piston to plunge. The common answer is to just add more pressure to the reservoir to slow that down. You can read the warning from them about that.

The best answer for us is to use a bottom fed reservoir but the packaging makes that difficult to impossible. It is impossible with a coil over. Fortunately for us, the DSC reservoir makes pretty much all of that go away.

 

[B00mb00m]

I would have liked to see more about the reservoir mount locations and why that matters so much. I don't think it is intuitive to understand why we need a reservoir. If you had a shock body without a reservoir, filled it with oil with the shaft fully extended, as the shaft moved into the body under compression, it would either lock up and not move or bust a hole in something so it could displace the oil.

The problem is solved by adding a reservoir with a piston in it. The piston has a gas charge over it on the non fluid side so it can allow the oil to be displaced into the reservoir and then the pressure charge pushes the oil back into the shock body when the shaft moves out. (I know you understand, others may not)

Where the problems happen is when we over valve a small shock like the 2.0's so we can work them harder. The high compression valving will blow the oil column apart with a high level of cavitation under the piston and just push the oil above that into the reservoir which causes the piston to plunge. The common answer is to just add more pressure to the reservoir to slow that down. You can read the warning from them about that.

The best answer for us is to use a bottom fed reservoir but the packaging makes that difficult to impossible. It is impossible with a coil over. Fortunately for us, the DSC reservoir makes pretty much all of that go away.

Would cavitation in resi lead to a drop in damping or increase?

 

[mrblaine]

Would cavitation in resi lead to a drop in damping or increase?

No idea. Never heard of cavitation in the reservoir.

 

[Mike_H]

Thought I'd try to keep a record of what this little side hustle is going to cost me. Couldn't sleep last night, so I threw some notes down in excel. Not including my time (since my hobby time is "free") nor did I include shipping...which I probably should, since its getting effing ridiculous.

Grand Total so far is $321.12, all in. I think I've managed to consolidate my orders well enough to keep shipping under 50 bucks. Without shipping, I've still spent less than the cost of ONE Remote Res Performance Series shock, so that is a win, to me.

As far as the Time piece of the project, I've got a couple hours into clean-up and teardown. I did find a nick in one of my shafts, so I will fill that (to prevent chewing up the new seals). Then there is the corrosion on one of my bodies...which should be fine, but it will drive me nuts...so I'll fill and paint that one...and if the color match is bad, I'll have to do them all eyeroll.

Note: This project (and many like it) don't always pencil out. I do this stuff because I enjoy the process. This one will probably work out (having remote res, tunable shocks that are pseudo bolt in for less than 400 dollars)...but I wouldn't recommend this to anyone to "save money." If you see a set of shocks on marketplace, be cautious. Its very easy to get over your head. Yes, they are expensive to buy new...but the parts are expensive too. IF you find a really nice set, maybe you just need to do seals and oil. Oil is 80 bucks a gallon, seals from fox are probably 40’ish a shock.

If you find any bad parts…start looking at 20 bucks here, 20 bucks there and pretty soon, you have a bunch of money invested. The first set I rebuilt kinda got away from me like that. Purchase price is uber important here. Paying 50% of retail for something needing a rebuild is probably not a good spend.

All that being said, it’s not a difficult process. Parts and information are pretty readily available. The task itself is pretty easy in the grand scheme of jeep projects. And, from what I hear (and I’ve experienced), shocks make a HUGE impact to the drivability of a Jeep and your enjoyment behind the wheel. So, I say go for it…just try not to overpay!

 

[GoldenGorilla]

Would cavitation in resi lead to a drop in damping or increase?

It's been an issue in mtb suspension from the early days. The early RockShox quadras were pretty notorious for it. They were pretty basic back then. Damping falls apart.

They addressed the design issues around it long ago. The actual damper cartridge in a Fox Factory fork isn't rebuildable at home. You're really just servicing the air spring side.

 

[mrblaine]

It's been an issue in mtb suspension from the early days. The early RockShox quadras were pretty notorious for it. They were pretty basic back then. Damping falls apart.

They addressed the design issues around it long ago. The actual damper cartridge in a Fox Factory fork isn't rebuildable at home. You're really just servicing the air spring side.

How is there enough fluid in the reservoir that any cavitation there can occur?

 

[sab]

How is there enough fluid in the reservoir that any cavitation there can occur?

I don't have any experience with bicycle shocks, or with the DSC Foxes sitting in my shop awaiting installation. However, I re-valved and dyno-tested hundreds of shock setups during my career in Superbike racing. Cavitation can definitely occur in reservoirs. As you're aware, cavitation occurs due to an extreme pressure drop (too much restriction in the valves/orifices). The technical reason is that the vacuum created on the low side of the drop falls below the vapor pressure (a physical property unique to all liquids) of the fluid, so the fluid vaporizes. Vaporized fluid results in dramatic loss of damping force. Most reservoirs aren't simply reservoirs. They have valving in them - either shims, orifices, or both. Just like in the shock body's valving, if that valving is too restrictive (or the gas pressure is too low for the valving), cavitation can occur. In my dyno testing, I was able to see cavitation in both the shock body and the reservoir by using too big a needle or too thick a shim stack.

That said, I have no idea if the parameters that create cavitation exist in either case (bicycle shocks or TJ shocks), as I'm not a bicycle person, and my TJ journey is still in its infancy...

 

[mrblaine]

I don't have any experience with bicycle shocks, or with the DSC Foxes sitting in my shop awaiting installation. However, I re-valved and dyno-tested hundreds of shock setups during my career in Superbike racing. Cavitation can definitely occur in reservoirs. As you're aware, cavitation occurs due to an extreme pressure drop (too much restriction in the valves/orifices). The technical reason is that the vacuum created on the low side of the drop falls below the vapor pressure (a physical property unique to all liquids) of the fluid, so the fluid vaporizes. Vaporized fluid results in dramatic loss of damping force. Most reservoirs aren't simply reservoirs. They have valving in them - either shims, orifices, or both. Just like in the shock body's valving, if that valving is too restrictive (or the gas pressure is too low for the valving), cavitation can occur. In my dyno testing, I was able to see cavitation in both the shock body and the reservoir by using too big a needle or too thick a shim stack.

That said, I have no idea if the parameters that create cavitation exist in either case (bicycle shocks or TJ shocks), as I'm not a bicycle person, and my TJ journey is still in its infancy...

I understand the concept. What I don't understand is how it happens in a reservoir. The pressure drop would be caused by a too high piston speed essentially pulling a vacuum on the trailing side of the piston which is basically splitting the oil column in half.

The only time that can happen in a reservoir is when the shaft is extending on the rebound side. There would need to be something very strange happening to see cavitation in a reservoir. I can't see how the fluid column remains intact in the shock body yet has enough "suction" through the hose to pull the fluid in the reservoir away from the pressurized piston enough to cavitate.

Alternately, is there a better chance that we are confusing an air pocket from an improperly filled shock, fluid movement through the orifice and hose creating an emulsion? (I have issues with the use of term "emulsion" unless we define nitrogen gas as a liquid) I use the term emulsion due to the shock industry calling shocks without reservoirs "emulsion" shocks due to the gas pocket above the oil for shaft displacement. Emulsified oil and the resultant sponginess would certain adversely affect how the shock performs which is why every shock builder out there tells everyone to stay away from emulsion shocks.

 

[sab]

I understand the concept. What I don't understand is how it happens in a reservoir. The pressure drop would be caused by a too high piston speed essentially pulling a vacuum on the trailing side of the piston which is basically splitting the oil column in half.

The only time that can happen in a reservoir is when the shaft is extending on the rebound side. There would need to be something very strange happening to see cavitation in a reservoir. I can't see how the fluid column remains intact in the shock body yet has enough "suction" through the hose to pull the fluid in the reservoir away from the pressurized piston enough to cavitate.

Alternately, is there a better chance that we are confusing an air pocket from an improperly filled shock, fluid movement through the orifice and hose creating an emulsion? (I have issues with the use of term "emulsion" unless we define nitrogen gas as a liquid) I use the term emulsion due to the shock industry calling shocks without reservoirs "emulsion" shocks due to the gas pocket above the oil for shaft displacement. Emulsified oil and the resultant sponginess would certain adversely affect how the shock performs which is why every shock builder out there tells everyone to stay away from emulsion shocks.

The large pressure drop in the reservoir is at the valve assembly, not at the floating piston, and that is where cavitation occurs. During compression, as the rod volume displaces fluid into the reservoir, if the valving in the reservoir is too restrictive, the oil on the low side of the valving (the reservoir side) can cavitate, just like in the shock body.

 

[mrblaine]

The large pressure drop in the reservoir is at the valve assembly, not at the floating piston, and that is where cavitation occurs. During compression, as the rod volume displaces fluid into the reservoir, if the valving in the reservoir is too restrictive, the oil on the low side of the valving (the reservoir side) can cavitate, just like in the shock body.

You're gonna have to get out your crayons and dumb that one down for my simple brain or bring it back into context.

 

[mrblaine]

The large pressure drop in the reservoir is at the valve assembly, not at the floating piston, and that is where cavitation occurs. During compression, as the rod volume displaces fluid into the reservoir, if the valving in the reservoir is too restrictive, the oil on the low side of the valving (the reservoir side) can cavitate, just like in the shock body.

I found a video that shows what happens in that style of shock. The low pressure charge in the reservoir allows cavitation/emulsion on both sides of the piston and due to the very short stroke of the piston in the shock body, after a few strokes, the fluid gets contaminated with the bubbles which then goes through the valving at the entrance to the reservoir which dramatically alters the shock's response to the tune. It was a clear shock and reservoir so you could clearly see the bubbles start and then grow, then completely fill the shock body and migrate into the reservoir.

 

[sab]

Sorry I couldn’t keep up with the thread today. We’re on island time now (South Padre) - drove down all afternoon for a couple nights of R&R. I’m not much of an R&R feller, but this gulf breeze is calming!

I’ve seen those type of videos before. In the early 90s, Ohlins had a video (on VHS tape!) that they shared with us. Cavitation is a bizarre phenomenon. It acts like air in the oil, but as soon as the pressure drop eases up enough that the low side rises above the vapor pressure, the fluid turns instantly back to normal, with no lasting effects. Until I’d seen it, I really couldn’t comprehend it. I’d always thought of it as air in the oil…

Usually, the compression stack on the shock piston and the shims/needle/jet in the reservoir are in series, so it’s possible to have cavitation in one, the other, or both. It just depends on pressure drops across each.

Cavitation on rebound isn’t very common because the forces involved are non-inertial, just stored spring energy. The inertial forces (weight of the vehicle decelerating against the valving) of compression make cavitation more likely in compression.

 

[mrblaine]

Sorry I couldn’t keep up with the thread today. We’re on island time now (South Padre) - drove down all afternoon for a couple nights of R&R. I’m not much of an R&R feller, but this gulf breeze is calming!

I’ve seen those type of videos before. In the early 90s, Ohlins had a video (on VHS tape!) that they shared with us. Cavitation is a bizarre phenomenon. It acts like air in the oil, but as soon as the pressure drop eases up enough that the low side rises above the vapor pressure, the fluid turns instantly back to normal, with no lasting effects. Until I’d seen it, I really couldn’t comprehend it. I’d always thought of it as air in the oil…

Usually, the compression stack on the shock piston and the shims/needle/jet in the reservoir are in series, so it’s possible to have cavitation in one, the other, or both. It just depends on pressure drops across each.

Cavitation on rebound isn’t very common because the forces involved are non-inertial, just stored spring energy. The inertial forces (weight of the vehicle decelerating against the valving) of compression make cavitation more likely in compression.

Non VHS version shows very clearly what I think you may have been dealing with on the bike shocks. They consider cavitation to be the phase change from liquid to gas. This video shows the cavitation start, then fill the shock body with the cavitation, then as the cycling continues, the cavitation migrates through the reservoir connection, then through the valving in the reservoir, and then finally into the reservoir. Start at .29 and watch it several times, pretty clear to see what is going on.

 

[B00mb00m]

Non VHS version shows very clearly what I think you may have been dealing with on the bike shocks. They consider cavitation to be the phase change from liquid to gas. This video shows the cavitation start, then fill the shock body with the cavitation, then as the cycling continues, the cavitation migrates through the reservoir connection, then through the valving in the reservoir, and then finally into the reservoir. Start at .29 and watch it several times, pretty clear to see what is going on.

That’s a great video. Interesting, that cavitates ok both sides of the main piston, the vaporized oil stays vapor and gets distributed to the reservoir. I think it makes sense also if the valve between body and resi was super restrictive you can cavitate there also. They didn’t show the force traces during cavitation unfortunately, it’d be cool to see how much the force dropped off.

 

[hosejockey61]

That's a great video.