OzVMX Forum
Clubroom => Tech Talk => Topic started by: eric318 on September 12, 2012, 03:43:19 pm
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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
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any cycle should have them.I got one from anaconda.
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http://www.ebay.com.au/itm/TOPEAK-POCKET-SHOCK-DXG-PUMP-GAUGE-/140842591477?pt=AU_Sport_Cycling_Accessories&hash=item20cadf64f5#ht_500wt_1414
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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.
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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.
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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.
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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.
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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.
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Very useful, thank you.
I am on my way to order a PocketShock DXG !
Cheers,
E
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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
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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..... ;)
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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.
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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.
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I am able to professionally nitrogen charge anyone's shock to whatever pressure they wish for $20.
PM me if you need it.
Hardo
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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 :)
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Good 'ol Nathan. Spotted another conspiracy theory.
Ever felt your shock reservoirs after a decent ride or race? Feel that heat in there?? Hmmmm? :o
I'm sure nitrogen won't be such an issue in the new air forks as they certainly won't be charged to the same level as a shock is.
I can't imagine a bike shop that doesn't have a bottle of nitrogen. Most people who are half serious about dirt bikes have one.
This suggestion is right up there with home made air filters, shocks that are the wrong length for your bike and cheap oil in your gear box.
Insane.......
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Don’t know much about shocks but I do know that when welding an enclosed frame tube the air pressure buildup inside the tube is enough to push out through the weakest point which is the molten weld pool.
It doesn’t take a lot of welding to build the air pressure up to the point that it wants to push out through the molten metal.
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Good 'ol Nathan. Spotted another conspiracy theory.
Ever felt your shock reservoirs after a decent ride or race? Feel that heat in there?? Hmmmm? :o
I'm sure nitrogen won't be such an issue in the new air forks as they certainly won't be charged to the same level as a shock is.
I can't imagine a bike shop that doesn't have a bottle of nitrogen. Most people who are half serious about dirt bikes have one.
This suggestion is right up there with home made air filters, shocks that are the wrong length for your bike and cheap oil in your gear box.
Insane.......
Good old Ross. Spotted another opportunity to argue with Nathan.
The bit where you can feel the reservoir with your hand shows that it's not all that hot. ;) so what's the increase in pressure from 20*C to, say, 80*C?
Then remember that the change you could possibly feel is the ratio of the reservoir diameter to shock shaft diameter - sod all, like I said before.
Here's another thought: Lots of MTB fork and shocks run on compressed air, usually from hand pumps. Do you ever hear of them having problems with changes in pressure or oxidisation?
How can this be?!
Nitrogen is the best. My point is that it is nothing like essential, and the arguments against using something else are mostly about mental (and emotional?) conditioning, not physics or engineering or even feel.
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Could work. The squeaky Ross Nimmo voice you get from inhaling it would be a bonus too.
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Don't start talking about yourself in the third person Nathan or you won't be able to use my Nitrogen bottle anymore....
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Nathan doesn't like that idea...
;D
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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.
Rubber contains natural oils (otherwise would it perish?). Oxygen is highly reactive with almost everything. Even to store industrial and electronic equipment such as generators, transformers, cables, motors etc etc etc air is displaced by nitrogen to stop all oxidation caused by oxygen content of the air. If you store almost any items in a sealed container some or all of the oxygen is consumed over time therefore there must be some form of reaction taking place. The heat cycling within a shock would assist any oxidation.
I think I'll still be getting shocks charged with nitrogen regardless of the expert theory outlined here.
Over to you Nathan for the LAST word ::)
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Bugger, I was hoping to have the last...
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Oh lord, what have I started... ??
Hey, I have a MIG welder with a bottle of mix CO2/Helium. May be I can use that to pump my shocks!
:)
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Come on Greg... Tell me how much oxidisation you'll get from the oxygen in a shock reservoir. Be lucky to be a litre in there (at atmospheric pressure...).
That seperator bladder you're worried about: If you left it sitting on a shelf, with no/minimal UV exposure, how many years would you expect it to last before it was oxidisation damaged it noticably? And that's with an essentially infinite supply of O2 doing its best to wreck it...
Nitrogen is the best choice, but the sky will not fall if you use air. :D
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Bugger, I was hoping to have the last...
Impossible if Nathan is involved check every thread better still try for the last word. You will not live long enough ;)
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Apart from the discussion about what damage the moisture from the air is going to do to seals, bladders etc, the preasure from the air building up would have to be placing a lot of pressure on your shaft seal (via the oil that the bladder is compressing) and surely playing havoc with the overall performance of the shock. ???
I'm waiting for the all-time pearler when you tell us that you're wasting your time adding oil to the fuel of a two-stroke.
It can't be far away.....
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Bugger, I was hoping to have the last...
Impossible if Nathan is involved check every thread better still try for the last word. You will not live long enough ;)
Haha ain't that the truth!!
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Apart from the discussion about what damage the moisture from the air is going to do to seals, bladders etc, the preasure from the air building up would have to be placing a lot of pressure on your shaft seal (via the oil that the bladder is compressing) and surely playing havoc with the overall performance of the shock. ???
I'm waiting for the all-time pearler when you tell us that you're wasting your time adding oil to the fuel of a two-stroke.
It can't be far away.....
Your trolling skills are beyond my ability to ignore ignorance.
I assume you finished high school, Mont?
I also assume you did some sort of science back then? Or were you too busy with your interpretative dance electives?
Now, using those high school science skills, explain to me about how "air builds up" in a shock reservoir. I've given you a big fat hint on how to work it out the pressure rise due to a change in temperature - if you try hard, I'll even tell you how much it will alter the effective spring rate (pro tip: sod-all).
You can keep taking digs at me if you want, but I've can back up what I'm saying with science and experience - not just vague 'gut feeling' and fear of the unknown...
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If I remember well the first law of thermodynamics tells us PV=NrT.
So with only T changing say by a factor 4, P will also go up 4x.
Do I get this right?
But this regardless of the gas...
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http://en.m.wikipedia.org/wiki/Ideal_gas_law#section_1
http://science.howstuffworks.com/dictionary/physics-terms/expansion-info.htm
But hey, manufacturers use N2 so it must be the only thing that works, just like we only ever use original grips and tyres and oil to stop our bikes oxidising... :D
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Eric... please... please... please... just try it and report back on how you go...
Enough of the round table discussion... Enough I tells ya, enough... >:(
And no I don't give a tinkers cuss about manufacturers, original grips, tires or f#%king oil... that doesn't mean diddly squat and has nothing, I repeat NOTHING to do with pressurising shocks. UGGGGGGG!!!!!
Good luck Eric. I hope it goes well!!! ::)
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Gotta love when the phrase 'tinkers cuss' is used in anger ;D
Why not R232 refrgeration gas if these shocks get so hot..........worked for Ferrari in their tyres
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i've always used LPG when regassing my shocks, except on really cold days when i've had more success with BBQ gas (i think it's the Butane). i've tried using NG but have problems with mains pressure being too low.
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next time i want to try some helium, might help reduce the weight of the heavy Yamaha shock :o
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Sorry your Honour. Badly phrased. I meant preasure from all that moisture that's being heated up.
I didn't have to get a masters degree like yourself Nath to race motocross. It's not that hard. The people that built these bikes and the associated performance parts knew all that stuff. Not some mad, back-yarding amature that seems hell-bent on bending every law of physics (and common sense) in proving that they can do it better.
Now I understand that you're a wizard at using a gum leaf and Gasket Goo at making a base gasket at a track or using ear wax in a holed piston to keep on racing ( ???) for the day however I'm silly enough to follow the manufacturers recommendations for servicing and not have to rebuild my bikes track-side.
No for me it's pretty simple. I just look at my bikes.
And then I look at yours...... ::)
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Maybe, just maybe, other people have different goals from their involvement in old bikes?
Maybe some of us enjoy increasing their knowledge, rather than meekly opening their wallet whenever something goes wrong?
But hey, you keep wanting to pick a fight because you have an over-inlfated sense of self importance - so I'm happy to keep shooting you down.
So, without resorting to ad hominem attacks again, do you actually want to try to tell me where I'm wrong?
Or have you finally realised that I'm right?
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Of course you're right. :P When have you never been? I just await the next gem (be it bike or outrageous thought).......
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If Nathan was right all the Race teams across the planet would be using air instead of nitrogen in their shocks... But they don't
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So Nathan you don’t consider
“I assume you finished high school, Mont?
I also assume you did some sort of science back then? Or were you too busy with your interpretative dance electives?
Now, using those high school science skills, explain to me about how "air builds up" in a shock reservoir. I've given you a big fat hint on how to work it out the pressure rise due to a change in temperature - if you try hard, I'll even tell you how much it will alter the effective spring rate (pro tip: sod-all).”
To be an ad hominem attack! ::) ::)
Nathan fill your shocks with whatever you want, you own em and have every right to do whatever you want with em.
This is starting to look like one of those 20 page crappy threads that fill up the ‘recent post’ section of the page and hide the truly interesting and informative threads.
Pleasssssssssssse :'( cease
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I'm done. :)
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Look, this one is pretty simple: I made a statement that the three stooges thought was wrong, and they called me on it.
Then I've been able to demonstrate that I was right, and now they're flailing around desperately trying to justify their own incorrect opinion.
The need to misrepresent what I said ("if Nathan was right, then race teams would use air"), ad hominem attacks ('you once bodged a bike to finish a race meeting therefore you must be wrong on this topic' and 'he'll have to have the last word - stubborn bastards can't possibly know about high-school physics') show that you're collectively incapable of refuting the technical points that I put forward.
Maybe you could pause to consider that your inability to counter what I've said is because I'm actually correct - and so while we're all douches who argue on the Internet, I have the advantage of being the one douche who is correct?
:)
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Best we let all the suspension technicians across the globe know that a forum whore from Yass has proven them wrong and they're wasting their money on nitrogen...
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I'll put it to Paul Thede in a few weeks time instead of arguing the toss here. Now I'm done......
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Keep digging your holes deeper fellas. :)
Then go back and read the bit where I said 'Nitrogen is best, but air will work fine'.
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I'll put it to Paul Thede in a few weeks time instead of arguing the toss here. Now I'm done......
Now that will be fun . Good old Mont coming back all Rice tacked :D
its hard to rebuild burnt bridges
ive burnt a couple lol
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Some good info in this thread I think, and some things to ponder as well. But beyond that, well... done to death is the phrase eh? What I'd like to see now is some expert commentary on IT400C's question.
This thread now locked.