Can regular compressed AIR be used in shock?

[Eddie8v]

Old post but everyone seems to be missing the REAL issue here.

A hydraulic shock contains oil (aka fuel).

Rapid compression creates heat.

Air contains OXYGEN.

What do we have here kids? Well we have the 3 parts of a fire triangle, Fuel, Heat, Oxygen.

Nitrogen is used because it is INERT not because it behaves a certain way. The bladder on the shock is exactly the same as a Hydraulic Accumulator and EXPLOSION RISK is the exact reason we use Nitrogen to charge accumulators.

Sure air has water which = bad for a shock but it's not the real danger.

Will it explode every time, probably not. Like all explosive risks the conditions need to be just right. Ratio of O2 to fuel and enough heat to cause ignition (Dieseling).

But the reality is it CAN happen so don't do it, even in a pinch unless. Sure guys have done it and got away but you are sitting on a pipe bomb... not my idea of a fun way to end an afternoon!

It's your ass ?

Steve.

25yrs experience, Industrial & mobile hydraulic design engineer.

This is the silliest thing I have heard in quite some time... The are many reasons Nitrogen is used vs ambient air inside a shock absorber, but the fear of flammability inside the shock surely isn't one of them.

Nitrogen is an inert gas so it won't adversely react with the oil and/or shock internals.

The nitrogen molecules are larger than ambient air molecules and this aids in cavitation prevention.

There wouldn't be enough oxygen inside the shock (if ambient air was used inside the bladder vs Nitrogen) to support any large internal fire or explosive event. Also, I believe that ambient air is more than 75% Nitrogen, so there's maybe enough oxygen to support the combustion of a lit kitchen match for several seconds, at best. There's really not a lot of volume, there.

I don't recall off the top of my head but Nitrogen likely expands less than ambient air given the same temperature change (moisture-related, I assume, and perhaps inherently) so that would mean less impact on damping changes throughout a long moto due to system pressure changes due to heating.

Edited April 15, 2014 by Eddie8v

[ramjetV8]

isn't the gas on one side of the bladder and oil on the other? last one I had open back in 86 LOL sure didn't seem like the oil and gas were mixed. I am sure the gas was in a bladder that compresses as the oil flows in and expands as it flows out. Or have shocks change a whole lot since then?

[Eddie8v]

You're right, not mixed.

[EnglertRacing]

Dude,

This will not work...Don't do it or you will end up on your head and possibly in the back of an ambulance....Most decent motorcyle shops have a nitrogen tank and will charge your shock for you for 15-20 bucks...Air is simply not stable enough. Not only does the exhasut have some effect, but the oil gets very warm while riding and has the greatest effect on the air temp..

 

massive exaggeration.

 

HEY welcome to 2014.

we use air in our forks now!

[EnglertRacing]

massive exaggeration.

 

HEY welcome to 2014.

we use air in our forks now!

 

hey has anyone heard of an air shock?

maybe they used to use air in air shocks?

even air in emulsion shocks with no bladder and they never exploded.

 

but nitrogen is used because its dry,

and can hold a charge slightly better than air because the molecules are a larger size.

but air is already 78.08% nitrogen 20.94% oxygen 0.93% argon and 0.03% other (CO2 mostly)

another benefit is that it prevents deterioration of the bladder and oil through oxidation since there will be no oxygen present.

I know a thing or two about the contents of the atmosphere and properties of gasses I operate a plant(s) that Produces 600 tons per day of liquid nitrogen / liquid oxygen by cryogenic distillation

 

 

so nitrogen is better mainly because it is dry the pressure will change less per degree of temperature increase

I have personally run Air instead of nitrogen and noticed no difference,

but personally i dont want moisture in my shock.

I wonder if anyone has tried nitrogen in their air forks?

I bet since they are entirely pneumatically sprung the effects of nitrogen's consistency may be felt, unlike in a shock.

[Eddie8v]

The bladder is porous, so whatever gas you put in the reservoir will end up in your oil. Nitrogen molecules are bigger than air's so less will leak through over time, and because it is inert it will do little to no damage.

[KVSteve]

 The are many reasons Nitrogen is used vs ambient air inside a shock absorber, but the fear of flammability inside the shock surely isn't one of them.

 

True that N2 properties given it's almost pure form may well be superior to plain air. 

I will also admit that I have been unable to locate any info pertaining to explosion risk of air in a shock application so I will give you that.

 

The nitrogen molecules are larger than ambient air molecules and this aids in cavitation prevention.

 

 

How exactly will molecule size prevent cavitation? 

Can you please define cavitation?

 

There wouldn't be enough oxygen inside the shock (if ambient air was used inside the bladder vs Nitrogen) to support any large internal fire or explosive event. 

 

 

this is simply not true. Enough entrained air can be pulled from a slug of oil to cause ignition under the right circumstances. Adding a volume of compressed air will definitely provide enough O2 for ignition "under the proper circumstances".

 

I could actually show you a video of a hydraulic cylinder with 1 pint of oil inside (ZERO air space) being physically pulled open and the entrained air will allow the cylinder to stroke over 1 foot.

Upon rapid compression a phenomenon called 'dieseling' occurs acutally igniting this air oil mixture. There is enough entrained air in a pint of oil to ignite over 200 times. So to say adding air doesn't present enough O2 for combustion is simply wrong.

 

The idea that there is a bladder there to isolate oil and air ignores the fact that bladders fail/leak eventually. Actually, some are likely to add air after finding out their shock is low in pressure, likely due to exactly that, a bladder failure so IF a hazard exists by using air/oil then relying on a bladder where there is a safety issue is not acceptable.

 

I will say this again, I am conceding to you that my initial post relates shocks to accumulators but after some googling there does not seem to be any published evidence to support the notion that the same risk applies to shock absorbers as it does to hydraulic accumulators.

 

The notion that it COULD BE POSSIBLE is however sound. Using air in an accumulator (that is what the little bladder reservoir on the shock is) is a VERY real issue in hydraulic applications and a SERIOUS no no due to explosion hazard.

 

There are reasons that this may NOT pose a threat in a shock application, the most logical that I can imagine is that working pressures inside a shock are not high enough to cause dieseling or auto-ignition of any combustible air/oil mixture. This situation DOES existing in hydraulic equipment for sure.

 

For me I know what can happen in the industrial world and I'm not willing to sit on one, YMMV. 

Add to that the other numerous valid reasons N2 is superior to plain air and the bottom line is IMO why risk it.

[Eddie8v]

True that N2 properties given it's almost pure form may well be superior to plain air.

I will also admit that I have been unable to locate any info pertaining to explosion risk of air in a shock application so I will give you that.

How exactly will molecule size prevent cavitation?

Can you please define cavitation?

this is simply not true. Enough entrained air can be pulled from a slug of oil to cause ignition under the right circumstances. Adding a volume of compressed air will definitely provide enough O2 for ignition "under the proper circumstances".

I could actually show you a video of a hydraulic cylinder with 1 pint of oil inside (ZERO air space) being physically pulled open and the entrained air will allow the cylinder to stroke over 1 foot.

Upon rapid compression a phenomenon called 'dieseling' occurs acutally igniting this air oil mixture. There is enough entrained air in a pint of oil to ignite over 200 times. So to say adding air doesn't present enough O2 for combustion is simply wrong.

The idea that there is a bladder there to isolate oil and air ignores the fact that bladders fail/leak eventually. Actually, some are likely to add air after finding out their shock is low in pressure, likely due to exactly that, a bladder failure so IF a hazard exists by using air/oil then relying on a bladder where there is a safety issue is not acceptable.

I will say this again, I am conceding to you that my initial post relates shocks to accumulators but after some googling there does not seem to be any published evidence to support the notion that the same risk applies to shock absorbers as it does to hydraulic accumulators.

The notion that it COULD BE POSSIBLE is however sound. Using air in an accumulator (that is what the little bladder reservoir on the shock is) is a VERY real issue in hydraulic applications and a SERIOUS no no due to explosion hazard.

There are reasons that this may NOT pose a threat in a shock application, the most logical that I can imagine is that working pressures inside a shock are not high enough to cause dieseling or auto-ignition of any combustible air/oil mixture. This situation DOES existing in hydraulic equipment for sure.

For me I know what can happen in the industrial world and I'm not willing to sit on one, YMMV.

Add to that the other numerous valid reasons N2 is superior to plain air and the bottom line is IMO why risk it.

Molecule size vs cavitation: rubber is porous and the air or nitrogen will leak through it over time. Because the nitrogen molecules are larger, it will leak into the oil-side much more slowly. Once a gas has mixed with the oil, cavitation can be more readily prominent.

There is NOT enough entrained air in the oil of our shocks to do squat. Absolutely not. In a mixed air-oil industrial application there is, for sure, but absolutely not our shocks.

In an industrial application, I have seen air enter via poor sealing issues, that's a big problem.

Also, pulling up on your non-air cylinder isn't the expected mechanics of that hydraulic cylinder: air can be introduced when the vacuum is created in a design where the seals are only built to handle the internal hydraulic pressure, and not any introduced vacuum. I.e the seals only seal against a delta-p in one direction. Totally normal for air to get by the seals that way, creating a reverse-pressure situation. You simply need dual-mode sealing in applications that could incur this mode of operation. I.e, the seals will seal under a higher delta-p from either side of the seal(s) vs only 1 side. Like most of the seals in our bike, if you install them backwards, they'll leak asap as the seal design allows passage under only moderate delta-p if that delta-p occurs in the wrong direction. Guys put the older water pump seals in backwards all the time, as soon as the system sees pressure, the coolant slips by the seal. The pressure delta actually increases the sealing power when proper operating conditions are present. If you reverse the delta-p (install the seal backwards, or pull up on a hydraulic cylinder) a 1-way seal setup will leak.

I have plenty of experience on applications where fuel is used to power hydraulics vs oil, it's called fueldraulics. I assume that's a big "no-no" to most, but not if properly designed...

Edited April 16, 2014 by Eddie8v

[gary gary]

Funny cos my Kawasaki warning sticker says  use  only air or nitrogen gas ...gpz1100  ..official Kawasaki sticker