Dangerous Information On Charging Shocks with air. - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 21 (permalink) Old Mar 17th, 2017, 7:03 pm Thread Starter
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Dangerous Information On Charging Shocks with air.

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...
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post #2 of 21 (permalink) Old Mar 17th, 2017, 7:41 pm
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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 todayís rear shocks and not as subject to spontaneous combustion?

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post #3 of 21 (permalink) Old Mar 17th, 2017, 8:07 pm Thread Starter
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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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post #4 of 21 (permalink) Old Mar 17th, 2017, 8:34 pm Thread Starter
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Damn,
Can't believe I spelled "Gas" with two s's in my first post on the Forum. haha
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post #5 of 21 (permalink) Old Mar 17th, 2017, 9:33 pm
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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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post #6 of 21 (permalink) Old Mar 18th, 2017, 7:27 am
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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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post #7 of 21 (permalink) Old Mar 18th, 2017, 9:48 am Thread Starter
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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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post #8 of 21 (permalink) Old Jun 13th, 2018, 8:39 pm
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post #9 of 21 (permalink) Old Jun 13th, 2018, 11:44 pm
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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.

Last edited by xracer; Jun 14th, 2018 at 7:38 pm.
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post #10 of 21 (permalink) Old Jun 14th, 2018, 2:08 am
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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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