'87 rear shock: nitrogen vs. air, psi?

[sccasey]

have an 87 and the manual says it takes nitrogen (and definitely not air) in the rear shock, at 145lbs/in pressure stock (up to 200lbs/in). three questions -- with everyone saying you should change the oil so frequently on these things, do all of you use nitrogen or if i change the air frequently, would i have problems? if i have to use nitrogen, where do i find it? hot-rod/4x4 shop who does car suspension? also, is the stock pressure good for a 190lb rider? finally, i don't want to have to change the spring... thanks in advance.
steve c.

 

[JasonWho]

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[canyncarvr]

1. The pressure in the canister is to prevent cavitation of the oil in the shock. It doesn't have anything to do with how much you weigh..you don't vary the pressure to match rider weight.

2. There has been considerable discussion on this and other DRN forums. Try a search on the matter.

3. Some use air to what they say are no ill effects.

4. Any bike shop can charge your shock for you. They CAN, but they might not. There's a liability issue....putting 175psi into a shock that was rebuilt by: 1. Someone who knows what they're doing? or.... 2. Not, might become exciting! If you're doing the oil change/rebuild/revalve or whatever and you're going to use nitrogen you may want to check around first for a supplier. Any air/gas welding kinda outfit COULD do it, too.

.....and something a bit more than the oem shock spring rate would be a good idea.

Don't forget to set your sag after you put it back together.

If you're doing the service, you're familiar with the air bleed process?

Bubbles in your shock oil = a shock that won't work.

 

[BadgerMan]

Good advice!

I ran air in my shock for a while with no ill effects. Run at least 140psi and fill it with an ATB shock pump rather than the compressor in your garage. Garden variety compressed air (like that from your compressor) is too dirty and moist.

If you are really serious about maintaining your own shocks, get a nitrogen bottle and regulator. I bought a complete setup a few months ago at a welding shop for less than $150.00.

Check out MX Tech's website for some good instructions on the bleeding process (critical).

Here are some good instructions. You will be primarily interested in the assembly and bleed instructions near the end. I believe you have a bleed bolt equipped shock.

http://www.mx-tech.com/downloads/KYB_shock.pdf

 

[JasonWho]

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[BadgerMan]

The pump I have goes to 300psi but it is real small. It takes about fifty strokes to fill a shock bladder. You would not want to have to do a tire with it! It has a really nice bleeder valve on it too. You can set the pressure very accurately.

 

[JasonWho]

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[Jaybird]

That shock pump will put in just as wet air as the compressor will.
I would never let an air filled shock freeze. It is imperitive that the air be clean and DRY.

 

[sccasey]

good info everyone -- thanks -- esp the bleed procedure. re the wet/dry air discussion, if i fill it during the deep freeze we're currently living through in new england, the air should be pretty dry ;) i think the worry about wet air is the corrosion it could cause on the shock internals. i also think the welding shop would be cheaper on the refill for nitrogen. it couldn't take more than a few seconds to fill that little resevoir. is the stock spring really that weak for my weight? and could someone explain what higher than stock gas pressure is supposed to do -- prevent frothing the oil at more strenuous shock use (eg, mx'ing it)?

 

[KnoxKDX]

Air by nature has a certain relative humidity, but that can be circumvented to a certain extent by (don't laugh) takiing advantage of winter. If you heat your house with natural gas, your relative humidty can drop some 40% from what the same air might be in the summer using air conditioning (IE, think about the dry night-time nose and dry sore throat)...you can use that to your advantage by airing it up inside your house. You would, in theory, only have 16-20% humidity depending on how frequently the gas heat kicks on. This may not be practical, but it's certainly true.

 

[BadgerMan]

Originally posted by Jaybird
That shock pump will put in just as wet air as the compressor will.
I would never let an air filled shock freeze. It is imperitive that the air be clean and DRY.

I was assuming that the compressed air would have a higher moisture content due to the residual moisture in the compressor tank, lines, etc.

It should be of no consequence if frost forms inside the shock's bladder.

i think the worry about wet air is the corrosion it could cause on the shock internals.

The air or nitrogen is contained inside the bladder. The only other surface it comes in contact with is the inside of the resevior cap.

Obviously, nitrogen is the best way to go, but air will work just fine if you have no nitrogen available.


Oh, here is the pump I have, $30.00. Use at your own risk. LOL!

http://www.beyondbikes.com/images/large/pu9000.jpg

It also works great for setting nitrogen pressure since it has a neat and very accurate bleeder button on it. I over fill the shock (with nitrogen) by 10 or 20 psi, connect the pump hose, and then bleed it down to 160 psi with the bleeder on the pump

 

[Jaybird]

Badger, you are probably correct about the residual humidity being higher. But like you also said, it doesn't matter...water is water and none should be allowed in the shock bladder.
The taking the shock in the house won't keep all the moisture out either, even though it would be less if the humidity is different.
Bottom line is that shocks should be charged with a clean, dry, inert gas. If you can't get your air clean and completely dry, use nitrogen.
I won't spam here, but there are filtration systems available that will get the air to a condition it needs to be in, but they aint cheap.

 

[JasonWho]

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[CIER]

The water vapor in the air will cause the pressure to vary greatly with as the shock heats up. That is why race car drivers use nitrogen in their tires. The other problem is that oxygen under pressure, mixed with air is very explosive. Sure the bladder would have to rip to mix the air and oil, and the air is only 20% oxygen, but you never know.
I have a small (1' long) nitrogen tank. When I need to refill a shock I pull the oxygen regulator off of my torch, put on the nitrogen tank, attach a air chuck and crank it up to the max. (150psi). 1 little tank of nitrogen is enough to refill about a billion shocks.

Jim

 

[BadgerMan]

The pressure can vary in theory, but not greatly, and I have not seen any evidence based on my experimenting.

Pressure variation is one reason nitrogen is used in tires, including truck and aircraft tires. Aircraft tires use nitrogen to avoid ice forming at high altitudes. Another reason is to avoid corrosion of aluminum rims. Also, for some reason, proponents claim that nitrogen is not lost through the rubber like other elements that make up air. I don’t understand why, but that’s what I read.

Something to consider. In a manufacturing setting, nitrogen is used on a large scale to achieve higher pressures and shielding mainly because it is relatively cheap as compared to other gases and/or equipment.

As for the explosion part, oxygen is not explosive unless coupled with a spark and a fuel source. You are assuming the fuel source to be shock oil. Where will the spark come from? An aluminum shock body? Even if you could get a fire going with shock oil and a spark from the aluminum body, 20% of a charge that small (the oxygen available) would not be enough to blow your nose.

I ran air for an entire season in my YZF including several long rides in high heat. No explosions, no corrosion, no difference in performance.

 

[Houndog]

I just had my shock recharged after I changed the fluid, it cost me $12 CDN.

 

[Jaybird]

Like Badger stated, oxygen is not an explosive gas.

Yes, it can greatly accelerate the cumbustion process of materials. In fact, pure oxygen can cause things that would not normally burn, to actually combust. But there needs to be a set of conditions existing for this to happen. You need a fuel souce like a hydrocarbon (shock oil) and an ignition source before the combustion can take place.

Compression alone can cause combustion of a hydrocarbon when mixed with an oxidizer such as pure oxygen. But, the levels of oxygen in atmosphere aren't enough to allow the spontaineous combustion to occur. Primarily becasue other items such as nitrogen (78% of atmosphere by volume) exist and these items absorb the heat of the reaction such that it hinders the combustion process.

Using wet air (straight out of the compressor) will hinder this process even further since water is one of the best heat absorbers there is. However, since there are other molecules available to absorb heat (nitrogen, argon, etc) in the atmospheric charge, we need to consider completely ridding the water as it can cause other major problems in a shock bladder.

Look at your forks...they have a fuel source (shock oil) an oxidizer (air chamber on top of the shock) and compression going thorugh stroke, yet they don't explode. Becasue of the conditions I explained above. However, when we mix the oxygen with the hydrocarbon gasses are released as a result. These gasses are what hiss out of the bleed holes on top of the shock when we open them. Even after you bleed these residual gasses off, there has been oxidation and the oxides left on the hydrocarbon chains can be very abrasive to valves, shims, housings, etc... This is why we need to change out our fork oils often.

On tire pressures...when we fill a tire with atmospere, the moisture that remains can cause tire pressure to change as much as 10-12 psi with temperature change. That is why nitrogen or dry air needs to be used in tire fills when we need the pressure to remain stable throughout temp changes.