Pumping up rear shocks at home

[eric318]

Hello all,

I am looking for a way to pump up rear shocks (AirFox, Ohlins and all those guys) when I rebuild them without having to leave the garage.
I hear that some modern bike pumps available on the open market for a fistful of dollars  can reach above 200psi. My compressor certainly does  not get there...
Myth or reality?

Thank you,

E

[DR500]

any cycle should have them.I got one from anaconda.

[DR500]

http://www.ebay.com.au/itm/TOPEAK-POCKET-SHOCK-DXG-PUMP-GAUGE-/140842591477?pt=AU_Sport_Cycling_Accessories&hash=item20cadf64f5#ht_500wt_1414

[vmx42]

Fox Airshox can be pressurised with a pump as you describe [although nitrogen is better].

Any shock that uses the gas pressure to reduce oil cavitation should use nitrogen. The moisture in the air will play havoc with your pressures as the temp varies. Plus for Ohlins etc, you will need the special needle to pressurise...

Most suspension companies charge only a small fee to do it for you. Much cheaper than your own nitrogen bottle and tools.

[Nathan S]

Air is actually fine from a performance point of view, providing you have a decent water trap on your compressor.
I did the maths a while back - even if the shock gets to 120*C (NB: the oil life will be very short at this temp), the change in pressure due to humidity in the gas is sod-all.

The bigger (but still not huge) issue is whether the water will cause corrosion - the bladder shocks are pretty bloody safe in this regard.


That said... I used to work in a pushie shop, and the only pumps that went near their advertised maximum pressure were the ones designed for inflating bike shocks/forks. The tyre pumps were all rubbish in comparison.

[JohnnyO]

The problem with air is that it expands a lot more when hot compared to nitrogen so you will get a pressure increase in the shocks. This will be more noticeable in Fox Airshocks, you'd have to start with them a little soft to allow for the increase in pressure when they heat up after a few of laps.

[Nathan S]

Air expands a lot less than most people think - remember that it's 78% nitrogen. May be an issue for a gas-spring shock (although I doubt it), but not one for a coil-spring one.

The thermal expansion coefficient of air is 0.003/degree and 0.002 at 100*C...
This is almost entirely due to the water in it - the other main gases all have even lower CTEs.

[OverTheHill]

in desperation at home i just pressurise any shock of my own shocks with air. Not air/spring type but say Yamaha pressurised cylinder where the gas piston separates the oil from the air 'trick'--or the bladder type like i think Honda have. Probably not the right thing to do but have re-gassed my older 'Gas Girlings' so i could try different oil grades [long ago]. Used Yamaha Gas Valves i had a few of left over from the days of re-building the likes of steel bodied TT250's mono's, just drilled on an angle in the end & tapped to [what was it!!] 1/8 gas thread or something--tapered tap thread, then gassed up [with air] through the needle Yam's use. Like i say desperate measures [home measures] & only get about 120psi but ok for me. Just remembered i carry a 'Soda Stream' bottle in the car set up to pump up the tyre if i get a flatty on the road [& can plug the hole], used it once after letting the tires down to get out of a paddock in the wet but bottle only held enough for say 20 psi in both tires but was thinking--how would it go [co2] pumping up shock damper canisters as bottle i think has 'high' pressure when full, the local guy can re-fill them with air but not sure about nitrogen--maybe. Have a gauge somewhere we used to use at work for the mono's, could run my bottle through that to set 14bar pressure or whatever's required. I Feel A Plan coming on.

[eric318]

Very useful, thank you.
I am on my way to order a PocketShock DXG !
Cheers,
E

[Montynut]

Air expands a lot less than most people think - remember that it's 78% nitrogen. May be an issue for a gas-spring shock (although I doubt it), but not one for a coil-spring one.

The thermal expansion coefficient of air is 0.003/degree and 0.002 at 100*C...
This is almost entirely due to the water in it - the other main gases all have even lower CTEs.

Nitrogen is inert with most components and material inside the shock such as the oil and seals. Oxygen is not inert and will cause corrosion or oxidation of components and the shock oil. If there is moisture present then the oxygen could play havoc with things inside the shock.

It would not seem that manufacturers would go to the trouble of using Nitrogen just because it sounds good. Last time I rebuilt a set of shocks the local bike shop charge $10 to gas them. The actual quantity of gas is very tiny just the pressure is high

[3858]

I am luck enough to have Nitrogen ...... but the conundrum is if I owned a late model KX would I put 33PSI of air in the "Air Forks" or Nitrogen..... 

[PCMAX]

So having read all of the above, and probably not going ahead with the planned $59 spend on the shock pump, can anyone advise which Sydney bike shops provide a nitrogen shock service?
Last time I rang a few of the local bike shops none of them could even suggest where i might find such a service, only one suggested Shock Works about 1.5 hours drive away.

[Nathan S]

Non-emulsion shocks keep the oil and the important innards seperate from the pressurised gas.
In a bladder reservoir shock, the gas is only in contact with a rubber bladder and the inside of the reservoir cap: the only thing to worry about is the CTE, which we've established is a non-issue.

[Hardo]

I am able to professionally nitrogen charge anyone's shock to whatever pressure they wish for $20.
PM me if you need it.
Hardo

[eric318]

For me it is just that I have lots of shocks (none have oil in contact with gas) and my "play" hours are not aligned with any decent shop business hours. So either I do it myself or it does not happen.
Plus a motoX cross champion I am certainly not, just a late night tinkerer...
For me the pump seems to be the way to go... on a warm dry California day