Re-Building/ valving 2.5" shocks

[BruceTS]

I finally decided it was time to re-build my front shocks. They have been on my rig for over 3 1/2 years without any servicing. The reason why I haven't worked on them any sooner is mainly because I didn't have a nitrogen filling system. I was able to aquire a complete Sway-a-Way shock valve kit and nitrogen bottle off one of the forums at a cheap price so now was the time for me to tear into this project. After a quick stop over at Kartek to pick up rebuild kits, oil and a few other goodies I finally decided to tear apart my shocks today.



The lower heim joint on each shock were worn out. The old one is to the left and the new bearing is to the right of the shock. The upper one's were just fine, but since I bought new one's those were replaced as well.
http://img140.imagevenue.com/loc639/th_49643_D07434_122_639lo.jpg (http://img140.imagevenue.com/img.php?image=49643_D07434_122_639lo.jpg)

Next step is to discharge the nitrogen, then remove the 3 allen bolts. Slide the dust cover up and depress the seal retainer to remove the retainning clip. Afterwards pull the piston assembly out and discard the oil.
http://img175.imagevenue.com/loc944/th_49648_D07435_122_944lo.jpg (http://img175.imagevenue.com/img.php?image=49648_D07435_122_944lo.jpg) http://img172.imagevenue.com/loc1176/th_49652_D07437_122_1176lo.jpg (http://img172.imagevenue.com/img.php?image=49652_D07437_122_1176lo.jpg)

Here's a shot with the parts dissassembled
http://img232.imagevenue.com/loc252/th_49656_D07438_122_252lo.jpg (http://img232.imagevenue.com/img.php?image=49656_D07438_122_252lo.jpg)

To remove the hose from the reservoir, I used a strap wrench. You may need to heat the parts up with a propane torch if loctite was used. This step can be bypassed if you don't plan to replace the hose. I have a new 90 degree hose to install, so off they came....
http://img126.imagevenue.com/loc1159/th_49660_D07440_122_1159lo.jpg (http://img126.imagevenue.com/img.php?image=49660_D07440_122_1159lo.jpg)

I used the same strap wrench around the piston to loosen the nut holding the valving. Afterwards I separated the shims and took measurments since I had no clue what the stock valving was.
http://img181.imagevenue.com/loc546/th_49671_D07445_122_546lo.jpg (http://img181.imagevenue.com/img.php?image=49671_D07445_122_546lo.jpg)
In the picture above, I have the old valving to the right and the new valving to the left. My rebound was ok so I'm only changing 3 of the shims and retainning the 2 largest, just gonna stiffin it a bit.

Now for the compression, that's another story, it bottoms out too quickly on the ruts bigger than 1 1/2 feet, so I need to stiffin it up quite a bit. Problem is you end up losing some of the slow speed stuff and the smaller bumps you end up feeling. So I decided to set up a flutter stack and give it a try. Basically the you stack 2 shims of the same size, but thinner for the first 2, then use thicker one's afterwards. I went up quite a bit from the original valving, but I need to start somewhere. This is pretty much a guessing game, adjust, drive, re-adjust til you finally get the feel you like. There is no set valving that is perfect for all situations, so you need to decide what is the happy medium you can live with.

Now that the valving is done, I can proceed to reassemble everything. Did a good cleaning and replaced all the seals, even though there were no leaks.

http://img148.imagevenue.com/loc808/th_49677_D07446_122_808lo.jpg (http://img148.imagevenue.com/img.php?image=49677_D07446_122_808lo.jpg)

In the picture above I'm setting the floating piston in the reservoir. Note the bolt comming out.... Sway-A-Way has a threaded hole in the middle of the piston that I noticed during dissassembly and wondered what it was for. Well I found a use for it... Attach a bolt and when your filling the shock body, you simply pump the piston back a forth to expell any air pockets in the line. Once this is done, slowly slide the piston assembly back into the body, so you don't produce too many bubbles. Slide the piston all the way down and refill the body about 1" from the top. Check the piston in the reservoir, since this will move during assembly. Using a ruler press the piston into the reservoir body 3 3/4", this should push fluid out of the shock body and expell any remaining air. Press the seal retainer down into the oil and make sure the shock shaft is compress all the way. Now check the depth of the piston in the reservoir body. It needs to measure 3 1/4"(note this is for the 6" long reservoir body) for the outer lip to the piston, if not add or subtract from the 3 3/4" measurment I mentioned earlier and repeat the process. Once this is complete, install the retaining clip and attach dust cover with 3 allen bolts.

http://img187.imagevenue.com/loc414/th_49689_D07447_122_414lo.jpg (http://img187.imagevenue.com/img.php?image=49689_D07447_122_414lo.jpg)
Once complete add a few pounds of nitrogen, then cycle the shock to insure there is no air bubbles trapped, if so take it apart an repeat. Too much oil and you can get fluid lock. Once everything cycles fine, charge the shock to 200 psi. and your done...

[Good Times]

awesome write up bruce!

[BruceTS]

Here's a few quotes I found surfing the net, they explain the function of each component and should give a better understanding how the shocks work.


The function of the gas chamber in the reservoir is to accommodate the volume change that occurs when the shaft enters the shock body and when the oil expands. The reason that the gas(nitrogen) has to be under pressure is to force the fluid through the various orifices the shock If there is not enough pressure then cavitation occurs. Cavitation is when the absolute pressure exerted on the fluid is lower than the critical vapor pressure for that fluid at a given temperature and the fluid begins to boil or vacuum pockets form
Cavitation happens under two circumstances. What I call gross cavitation and localized dynamic cavitation. Gross cavitation occurs when the force on the shock exceeds the force exerted by the reservoir pressure and the whole column of oil above the piston is shoved into the reservoir and a vacuum is pulled on the bottom side of the piston. Localized dynamic cavitation ocures when oil is flowing at very high speeds and is required to change direction through passages or around corner in the geometry of the piston or valve. This causes low pressure zones, if the decrease in pressure is below the critical pressure or zero than vapor or vacuum bubbles form. This can be very damaging the shock. As the little pockets collapse they cause a hammering action on the surface of the piston or valve. You can see this on a shock piston that has cavitated. Usually around the hole that the compression valves cover or on the face of the valve plate. It appears as abrasion or pits in the surface, it often can eat through the hard anodize on the piston.
In both cases it affects the performance of the shock detrimentally and in both cases more reservoir pressure can alleviate the problem.







Dividing piston: used in the reservoir to seperate the oil from the nitrogen. the distance the piston travels can be calculated by the piston dia./shaft dia (pixrxsq.)x stroke; ex. 7/8 shaft 14" stroke=just a little more than 2" of piston movement. With this in mind place ment of the pison can be critcal to nose pressure on the shaft.

Free bleed holes/ bleed holes: these are holes in the shock piston (not dividing piston) that flow oil in both directions. The 4 big shock makers all use a shim stack to control piston movement and use very small bleed holes; like 1 .060 hole in a front shock and 2 or 3 .060 in the rear for a 2.5 shock. They are a "low speed adjustment". These holes affect rebound more than compression but are so small as to have little effect during high speed shock movements. As a side note: some of the big handmade bypass shocks that don't use shims have unusally large holes up to .375" or use many smaller holes.

Shims: These are like thin washers made from spring steel. the comon thicknesses are .008, .010, .012,.015,.020. They are placed in stacks of 5 or 7 of decreasing diameter with the largest against the piston and the smallest farthest away (usally the same thickness) on either side of the piston. As the piston moves through the oil, the oil is forced to go into ports(little tubes) with the shims covering the end, the oil must bend the shim and force its way through. Thicker shims makes it harder for the oil to get through the tube. A "flutter" stack is usally 1 1/2 shim stacks, the first couple shims are usally thinner then a heavier complete stack.