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Discussion Starter #1
Please read I joined this forum because I needed information on the Nitrogen charge (Pressure) in the 748 rear Shock.
When I found members were talking about charging the shock with Air I became very Alarmed..
.
I am a 30 year Aviation Mechanic with 90% of my experience in Hydraulics. Both, Self contained (suspension Systems) and Dynamic (pump charged constant supply and pressure)
The common Reservoir Shocks found on modern rear suspension contain either a piston type or a Bladder type accumulator.
This Accumulator is the Gold cylinder attached to your shock.
The Accumulators job is to keep your shock oil under constant pressure While keeping the oil and Nitrogen separated.
As use wears the parts, i.e. O-rings on the piston or deteriorates the rubber bladder in the Accumulator
oil enters the Nitrogen side of the bladder..
Not a big problem, just means things a wearing out..
NOW THE IMPORTANT STUFF



Nitrogen is inert.. I contains no Oxygen. Oxygen being one of the three things you need to make FIRE . Take away Oxygen, fuel or heat and you get no FIRE
DO NOT PRESSURIZE YOUR REAR SHOCK WITH AIR.

Air contains 20% Oxygen. the oil in the shock is usually petroleum based ( a little bit heavier than #2
Diesel) Diesels ignite because of Compression.

If Air ( the stuff we breath) is used in the accumulator of the rear shock, You have a potential Pipe bomb under your seat.

Consider this... You're out for some spirited riding. The suspension is working hard, compressing and expanding all morning.
The day is warming up, your shock gets hotter with every bump.
The oil that found its way into the accumulator in getting atomized, a fine mist.
Your and you fabulous Italian engineered beast hit a pot hole, the sudden compression drives the air temperature into the realm of lower compression diesel engines
You have know clue what happened. all you heard was a loud bang. You and you machine have parted company.

This phenomenon is called Hydraulic Dieseling OSHA has incidents on file, can be found on line.

PLEASE DONT USE AIR TO CHARGE YOUR REAR SHOCKS
NITROGEN IS INERT. THEREFOR NO FIRE OR BIG BANG UNDER YOUR SEAT
NITROGEN ISN'T USED BECAUSE ITS LESS PRONE TO TEMP CHANGES... IT'S A GASS, SQUEEZ IT AND IT GETS HOT, EXPAND IT AND IT GETS COLD.
Hope this helps all here.
Now I'm going to search for the info I need.
Have a great day...:smile2:
 

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@Supersidecar
Thanks for the crash course. That was very interesting and makes tremendous sense. One troubling detail however:
I once owned a Suzuki with air adjustable forks. The engineers at that time would have known this yet offered it to the public. Were there no danger because the fork pressure is lower than today’s rear shocks and not as subject to spontaneous combustion?
 

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Discussion Starter #3
Thanks for responding.. You have made a valid point.
I have the '78 GS1000 once owned by my father. Yes the forks are air adjustable. But the air pressure
max Is, I believe somewhere around 17 psi.. Fork travel should be insufficient to heat the huge volume of oil. Owners manual warned of to high a pressure causing damage.
The most probable event would most likely be blown seals and loss of fluid..
With the volume of the oil and the low air pressure temps and pressures should not get high enough for hydraulic
Dieseling.
Ever use a piston type camp fire starter? A small chip of dry wood or a small ball of cotton.... smack the piston, and tip out the glowing smoking amber... Makers warn buyer not to use oil of fuel.. Yes people have tried the oil...
Good exchange....thanks
 

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My old CBR had 5-8 psi recommended in the forks, again, too low to develop heat and there was a bladder seal separating it from the oil. It was more like a rubber marshmallow I believe.
 

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My shock instructions stated Nitrogen was preferable but air could be used. What you wrote makes perfect sense, so I must assume the shock manufacturer didn't know this would happen, or didn't care.
 

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Discussion Starter #7
Don't have a picture of my shock to post but the caution label on the reservoir warns of "explosion risk".
Perfect weather for a morning ride.... I'm outa here. :)
 

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Not to take anything from the OP but to add. As you compress air what you are effectively doing is INCREASING THE NUMBER OF MOLECULES OF OXYGEN in the system. Air is roughly 80 nitrogen 20 oxygen. So at around 5 bar (73.5lb) you effectively have pure oxygen. More pressure, the more oxygen is in the system. And as everyone knows oxygen is a great oxidiser and high pressures will lead to degradation of anything in this pressurised air. Tyres, air forks, footballs etc. won’t have problems with the small pressures. As an aside, breathing compressed air at deeper then 50mt is not recommended because of the number of molecules of oxygen you are breathing in. 100% oxygen funnily enough will kill you in a short amount of time.
 

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I'm guessing that the compression explosion issue does not apply to air assist forks simply because the fork seals would give out long before the pressure required for combustion is reached. With the smaller volume, sealed canister reservoir of a rear shock, much higher pressure is easily reached.
However, I'm no engineer, although I did once build half a Thunderbird 2 out of Lego.
 

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I'm guessing that the compression explosion issue does not apply to air assist forks simply because the fork seals would give out long before the pressure required for combustion is reached. With the smaller volume, sealed canister reservoir of a rear shock, much higher pressure is easily reached.
However, I'm no engineer, although I did once build half a Thunderbird 2 out of Lego.
No explosions because there’s just nowhere near the amount of compression to heat the air fuel/oil mix that is highly unlikely to exist in a fork tube. And on top of that the ratio of air fuel/oil has to be correct and the fuel/oil atomised to a small enough partical to begin burning.
Thunderbird 2 was my fave as well. Sadly my model burnt to a crisp when I tried (in vain) to attach a couple of sky rockets to see if it would fly.
 

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Whew ! What a relief. The thought of rear shock shrapnel piercing my privates was keeping me awake at night.
 

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Not my first choice, but air has been used as a backup by others from what I've read. The guys on the DRZ forum are reporting using oxygen for their frequent valving/shim changes. I have not heard about any explosions with motorcycle shock bladders.

Is not air already 78% Nitrogen and 21% oxygen? Makes sense that performance wise, it will do backup but not ideal.

Also, if you squeeze air into a tighter space, the number of oxygen molecules remains the same so the concentration is same relative to the other elements in the confined bladder. I don't think this would increase the risk of explosion either.

I'm curious about the temperature of the shock oil under working load. I would think it would need to vaporize to become explosive.

If the bladder leaks, nitrogen or air, then you will have air bubbles in your shock oil. Now, that would be dangerous while riding?
 

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I'm one of the guys that frequently use air in my shock after making a reshim. If succesfull I take the shock to the shop and replace the air (from a mountainbike shock pump) with nitrogen. After three times inflating/deflating I hope most of the the moisture is out.....

Since my shock is mounted between the exhausts it will heat up anyways. Last weekend I charged the shock with air up to 145 psi and after a test drive (while stinking hot) it reached close to 200 psi.

While this is a huge pressure increase, it's not in a dangerous territory.

Anyways, for long term usage I would suggest just using nitrogen, but no immediate dangers exists from using air.

Michel
 

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Hi,
Just had the ohlins rear shock absorber oil seal fail on my 750s. Am more than happy to strip and replace but need to know if the gas can be left in the reservior while the unit is apart with no oil in it. Will the bladder fail, and if it does can it be replaced? Am struggling to find a definitive answer. If the shock might be irrepairably damaged i will have to go for a service repair but £120 seems steep for what appears to be a straight forward strip down and seal replace.
Should I risk it??
Dave
 

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Call me whatever you wish to, but I'm not keen on using an accelerant in a pressure vessel on my motorcycle. Oxygen is an accelerant. A very efficient one at that.

Nitrogen is inert. It can be used to extinguish a fire, that is how efficient it is. Oxygen is a highly active accelerant ... it is used to excite a fire. Which is why it is used with acetylene because O2 increases the BTU ("heat") of the fire by leaps and bounds.

If you biff, and the fuel in your fuel system ignites while your bike is on it's side burning away on the street, when that reservoir with high pressure O2 ruptures there's going to be some serious fireworks. If you're unconscious and first responders approach the bike without being provided with that crucial bit of information (pressurized O2 in a sealed vessel) and the reservoir ruptures ... which is not uncommon .. you may have someone's death on your conscience for the rest of your life ... "Maybe not today, maybe not tomorrow, but soon, and forever".

I clearly remember my mother often telling me when I was young ... "if everyone were jumping off of a cliff, would you as well?". I say that in response to the idea that ~big shot teams use O2 to pressurize their suspension systems, so I'm going to use it too~

I mean, y'all do what you want.

I'm not tellin', I'm just sayin'.

:grin2:
 
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