That video is very instructional, and better than the one I saw years back! Without benefit of see-through shock body and reservoir, I was only guessing by dyno results that I'd encountered cavitation. I could clearly see a dramatic drop in damping force above a certain velocity, and I concluded it must be cavitation. I remember immediately disassembling the shock, thinking I'd see air in it, but the fluid was crystal-clear. That's about when the Ohlins rep "schooled" me on what cavitation is and later showed me a video.
Fox Performance Series Shock Rebuild Thread
- Thread starter Mike_H
- Start date
So properly charged nitrogen prevents the cavitation? Ostensibly by providing some compression?
How often should one be checking Fox 2.0 nitrogen levels?
-Mac
How often should one be checking Fox 2.0 nitrogen levels?
-Mac
Yes, the N2 prevents cavitation by raising the low-side pressure above a vacuum at all running conditions, keeping it above the vapor pressure of the oil (which is below 1 psi gage pressure, where atmospheric is 14.7 psi gage pressure).
You shouldn't have to worry about it. In many years of doing shock tuning and maintenance, I think I can only remember one leak, and it was human error - an o-ring I nicked while being in too big a hurry. It was a substantial leak I noticed when charging the shock. Other than that - never a leak.
What I did not expect to see was the cavitation start, stay around, and then increase in volume until it migrated into the reservoir. The other videos I've watched show the cavitation start below the trailing side of the piston and immediately dissipate when the piston reverses direction (as it should due to the pressure change). That also helps me to understand the pressure drop you've referred to. The pressure in the oil on the low side of the piston is easy to understand, not so much on the top side for me though.
Yes, I sure have, but you'd probably break the shock or the gage checking the pressure, so I'd recommend not touching it.
There is so little volume of nitrogen in a shock, most conventional methods to check the pressure would actually cause the pressure to drop. I would charge, and not worry about it until it’s time for service.
What they are trying to tell you is you don't really check the pressure. You buy a nitrogen set up for filling shocks and reservoirs and just fill them to the correct pressure as a way to check them. As Mike said, if you try to check them with a pressure gauge, you will bleed off most of what is in the reservoir trying to get the pressure gauge onto the shrader valve.
Mike and Blaine are right on. You can’t just put a gauge on a shock to check the pressure. You have to just re-charge it to the pressure you desire.
If you are going to play with shocks, you need to get a setup that has a zero loss chuck, no loss chuck, or whatever the manufacturer calls it. I still consider it good for one fill. if you connect and "check" it again, you have already lost psi. fill it, and if you think its leaking, fill it again, There are no checks. I'm not sure if that makes since unless you have played with one.
The owner of a rig I built called me not long after he picked it up. One corner is low, the shock is leaking nitrogen. Well, that would be very odd since they have the leak resistant caps on the shrader and they were just filled, plus I check them visually for obvious oil stains and then with soapy water for the nitrogen side purely because I don't want this phone call.
Sorry about that, get the coil over off and get it to me and I'll go get it checked and the leak fixed.
Oh, I think I just need to add some nitrogen.
If it is leaking, that won't last very long plus it isn't good for the seals. They need pressure over the oil or they will weep and eventually leak. Get the shock to me so I can get it fixed.
Pretty sure it just needs some nitrogen.
He never admitted what he did. I know how he does things so there is zero chance he wasn't being cute with his maintenance and tried to check the pressure with a tire gauge. Especially since he never brought the shock to me to get fixed.
Why I asked my question about whether or not that pressure was a maintenance item to check.
I probably would have made the same mistake. Why I read all these posts.
-Mac
For maintenance or pre ride check on non race car things, I suspend the vehicle with all of the shocks fully extended and dump the nitrogen. If it hits with alot of pressure and just a little oil, pressure it back up to the prescribed pressure and go ride. If it just barely hisses or there's is alot of oil, pull it and see what's going on with it, if there's oil on the nitrogen side of the piston, there's nitrogen on the oil side of the piston.
Having a nitrogen kit is the only way to do this but one man can pressure up for ever on a regular size bottle.
LG
Having a nitrogen kit is the only way to do this but one man can pressure up for ever on a regular size bottle.
LG
Another thing about checking the pressure with a tire gage...Most of them go to 100 psi. Shocks are generally at least double that. 150-300 psi nitrogen charge is what I've read as "standard" range, and typical for fox is 200. Its not a tuning thing, its purpose is to prevent cavitation (Speaking in generalities here).
My lossless chuck goes up to 400psi
https://www.summitracing.com/parts/...0GOHndjEgR1wxFju8hFrJ9T4KzqtT6k1tMHxJJEygxTMQ
That’s not a loss-less device. It’s made to screw on the shock valve, open the valve, which draws enough N2 from the reservoir to lower the reading, charge it to the proper pressure, and then close the valve, which is the only loss-less part of the process. You still have to add N2 to stay at the same pressure because there is volume that must be filled to get a pressure reading. A device like that will lower the pressure by about 50psi when taking a reading if you don’t recharge.
Little bit of progress this weekend. My wife had friends In town, so I didn’t get as much garage time as I would have liked.
I did get my res caps drilled and tapped for the new schrader fill valves. I may have to take them back apart though. The Schrader valves are 1/8” NPT. Unfortunately, I ran the tap a bit deep, so the fit isn’t quite as tight as I’d like, even with 6 wraps of tape (standard for NPT is three). Probably bring them to work though to do that, so I can clamp them in a mill vice and counter bore the hold deeper and at a diameter that will allow me to install and remove them with a socket.
I also filled a couple of small pits in the shafts with some JB Weld. Little light filing and some lite sanding, and they will not cut up my new seals. Finally, I filled the pits in the one body that was really bad.
Rustoleum metallics titanium silver is a near perfect match for the clear anodized aluminum finish on these parts. This is the filled body from above. I didn’t paint the whole thing, just the last few inches that were corroded.
I’ve got all the seals installed on all my parts, and I’m putting the shafts back together with a SWAG at valving.
I did get my res caps drilled and tapped for the new schrader fill valves. I may have to take them back apart though. The Schrader valves are 1/8” NPT. Unfortunately, I ran the tap a bit deep, so the fit isn’t quite as tight as I’d like, even with 6 wraps of tape (standard for NPT is three). Probably bring them to work though to do that, so I can clamp them in a mill vice and counter bore the hold deeper and at a diameter that will allow me to install and remove them with a socket.
I also filled a couple of small pits in the shafts with some JB Weld. Little light filing and some lite sanding, and they will not cut up my new seals. Finally, I filled the pits in the one body that was really bad.
Rustoleum metallics titanium silver is a near perfect match for the clear anodized aluminum finish on these parts. This is the filled body from above. I didn’t paint the whole thing, just the last few inches that were corroded.
I’ve got all the seals installed on all my parts, and I’m putting the shafts back together with a SWAG at valving.
Last edited:
It’s not a sickness…or maybe it is. Couple more pictures of how I repaired the pitting from road salt that was trapped in the rubber reservoir mount blocks on these shocks. There was only one shock that was bad enough to repair, strangely. Probably the rear passenger, since the front tires kick the stuff up, and typically there is more corrosive crap on the curb side, due to the crown of the road.
Anyway, first scrub the aluminum oxide with a wire wheel, then wipe it down with acetone. I used some high temp JB weld putty to fill the pits. This worked well, because it stayed put. I struggled with the 5 minute epoxy in tubes because is low enough viscosity to flow. Since the parts are round cylinders, the epoxy doesn’t stay where it should.
I let it set up overnight. Once dry, I used my band file to quickly remove the majority of the now hardened putty. Then I switched to a file to bring the putty down to the surface of the aluminum. Finally a quick couple minutes with some sandpaper to smooth out the file lines.
These will get another quick dip in my ultrasonic cleaner and then I’ll squirt some paint and clear on them. This is the last thing I have to do before reassembly. I have the shafts rebuilt, seals installed, and some valve shims installed.
Anyway, first scrub the aluminum oxide with a wire wheel, then wipe it down with acetone. I used some high temp JB weld putty to fill the pits. This worked well, because it stayed put. I struggled with the 5 minute epoxy in tubes because is low enough viscosity to flow. Since the parts are round cylinders, the epoxy doesn’t stay where it should.
I let it set up overnight. Once dry, I used my band file to quickly remove the majority of the now hardened putty. Then I switched to a file to bring the putty down to the surface of the aluminum. Finally a quick couple minutes with some sandpaper to smooth out the file lines.
These will get another quick dip in my ultrasonic cleaner and then I’ll squirt some paint and clear on them. This is the last thing I have to do before reassembly. I have the shafts rebuilt, seals installed, and some valve shims installed.
Similar threads
