DIY rear shock re-gassing

[Ewan67]

Hi all.
Thought there may be some interest in this. When rebuilding my MK1 (soon for sale), I realised how poor the damping was on my MK2 in comparison. As I had a spare shock that was in good nick I thought I would have a bit of a look into re-gassing. I know you can pay to have it done but being inquisitive (and a tight git) I thought I'd have a go. Procedure as follows:

Obviously remove rear shock!
Mark / take close note of banjo position on the shock as you don't want to be moving this once you have tightened it.
Depress the valve on the reservoir (the thing that looks like a tin can). Mine had barely any pressure, hence poor damping.
Undo the locking ring and relieve any pressure on spring. I fully undid mine.
Disconnect banjo on the shock and place the reservoir and pipe to one side. It will leak oil so don't put it on dining table.
Now slowly push the shock piston in and drain the oil into a suitable container. I drained mine then flushed with fresh oil.
To refill the shock, I placed it in a vice with the banjo hole slightly higher than the rest of the shock.
Pull the shock piston out to full extension and slowly fill with fork oil of your choice. I left this overnight to allow any bubbles to escape.
Now to the reservoir.
The disc with the Schraeder valve in is held in with a circlip. You need to push this disc down and hook out circlip with a small screwdriver. Mine pushed down easily by hand but I have read you may need to give it a gentle tap with a socket over the valve.
Once the circlip is out you need to put a nut or cap on the valve stem to protect the thread. Grip the nut and gently pull the cap off.
On the back of the cap is a thick rubber bladder. This is what holds the pressure.
I cleaned the reservoir and damping adjuster.
The next part was quite messy when I did it but others may find a better way.
I part filled the reservoir with oil and when it began to come out of the pipe I screwed the banjo onto the shock. I left the banjo loose for a little while to allow oil through.
Tighten the banjo back in the same position it was when you started.
Now I clamped the reservoir in the vice with the shock dangling down and half filled with oil I left mine overnight.
Now gently push the bladder into the reservoir. You need to ensure there is enough oil in the reservoir so be generous. As you push the bladder in oil will seep out.
Now push down on the cap and replace the circlip.
Replace the spring nut and locking ring to the desired position.
Now to re- gas.
I had read a lot about using nitrogen but as there are no metal parts in contact with the gas used to pressurise the system I reckon air will be OK. Nitrogen is used as it is reasonably inert and dry. Air is 80% nitrogen, so close enough for me.
I used a high pressure pump used to pump up mountain bike suspension bought off Ebay for £23. Mine is a Fox one.
Pressure you put in a a personal decision but I used 150 PSI and seems good so far.
The usual disclaimers apply. If your seals are about to give out then re-gassing even to the right pressure could force oil past and mean a complete rebuild is required. Maybe I have been lucky, but I have done the shock on my bike and my spare with no dramas.
Be careful out there!

[fallenmikethebike]

On the face of it that sounds like you've been resourceful and fortunate.
Generally higher pressures than that are used. The piston, and shaft seals were all in good condition as well then?
Mike

[Ewan67]

Mike
From the little information I found online, CRs of the same era were charged to 10 Bar (around 145 PSI) Didn't check piston or seals; that can wait 'til another day. Really just wanted to see if I could recharge gas. No oil leaks yet but that's not to say the seals won't fail under a hard landing.
Ewan

[fallenmikethebike]

my mistake, hope it all works out OK.
Mike

[knackeredMk1]

The biggest issue you have is with the moisture in the air that was pumped into the shock. This will corrode the piston and seals faster than having 'dry' nitrogen only. Whether that is your problem or the person you sell it to in a couple of years time ................?

[Ewan67]

Knackered

The high pressure gas doesn't get anywhere near the piston or seals. It is held within the bladder as described in the post. The bladder applies pressure to the oil. I'll post photos when I have time.

Ewan

[knackeredMk1]

I see. So why do the manufacturer's use nitrogen in bladders and not compressed air. Does the oxygen and water react differently to temperature? Do they cause breakdown of the bladder membrane?

[Ewan67]

My only guess is that nitrogen is used ,cos it is inert. Formula 1 use it to inflate tyres. No idea why an 80% nitrogen oxygen mix isn't good enough, but I suppose they are at the cutting edge. From memories of physics and Boyles Law (?) or something, air and pure nitrogen will behave almost identically for the purpose used here. As for damaging the " membrane" this is a rubber bladder about half the thickness of a tennis ball. I don't think there is anything in the air round here that will affect it, however Dungeness is just down the road!

[biofotis]

I used compressed air on my rgv as well.
I think the reason they use nitrogen is because normal air'a density fluctuates when under heavy load (ie. tracks) therefore changes the characteristics of the shock.

[Ewan67]

Quick update. 100+ miles both on and off road (much the same as the roads round here are shot) both on my own and two up. No leaking or blown seals and damping is working fine. It's like a different bike!

[fallenmikethebike]

That's good, and an option for some. Personally, if the shock was a bit on it's way out I might explore this option, but for a rebuild would prefer some nice clean inert nitrogen, IMO.
Mike