Why nitrogen in shock? Why not oxygen?

Nitrogen is an inert gas?

You wouldn't want a bomb going off between your legs. High heat, oil, and oxygen would create such an item.
dan

Nitrogen will not heat up and expand like oxygen will, meaning that your suspension will stay much more consistant. It also will be less likely to leak down, as the nitrogen molecules are larger than oxygen molecules. Nitrogen gas works the same way in your tires as well.


Mike

Nitrogen is used because it is the cheapest and most readily available DRY bottled gas. The key word being DRY, this means that it has no water vapor in suspension in the gas. If water vapor is present in a gas then its pressure is far more sensitive to temperature. The gas does not obey the third gas law, that pressure is proportional to temperature or P/T is constant. As the temperature rises the water vapor saturated pressure increases.

Simply put this means that if you heat a dry gas up in a confined space its pressure will rise at a constant rate proportional to the temperature, but if you heat a "wet" gas up its pressure rises faster the hotter you get, up to a point.

This means that your shock will be more sensitive to pumping up at temperature with "wet" gas than dry which is a bad thing.

Any dry gas will give the same effect as dry nitrogen. Nitrogen is used because gasses are separated from the air using fractional distillation and 80ish % of the air is nitrogen so most of the product from air distillation is dry nitrogen so its cheap.

The other point is that oxygen is reactive and will corrode the inside of the resevior, where as nitrogen is not and will not. You could use any other dry inert gas Argon, Helium etc. You can not use dry air because dry air is not truly dry, the methods used to extract the moisture do not give a truly dry gas unlike distillation.

Ben
JBSracing

Plus it's friggin' cheap! Who's charging you so much for nitrogen that you'd even consider something else???

HusaDave

I wrote this just as you submitted the above post. So I may as well post it now I've typed it....................

Have you ever pumped a tyre up with a hand pump? You will notice that the valve adaptor is hot when you have finished. This is due to the friction of the air rushing through the small orifice.

In a shock oil, which is much thicker than air is rushing through small orifices several times a second. All of the energy than is absorbed by the shock is converted to heat by friction that is simply how a shock works. So yes they do get quite hot in use.

Pure gaseous oxygen and oil are hypogolic when mixed, meaning they spontaneously combust, hence it says on your oxygen regulator "use no grease or oil". Oxygen and fuel in a sealed chamber is a bomb. Whilst I think it is unlikely that your shock/bomb will go bang if it does it probably won't blow you to kingdom come in a blue flash but it may knacker your shock bladder, seals and shim stack.

A Detroit diesel will run on and on even with the fuel shut off if a blower seal goes out allowing motor oil into the combustion chamber at combustion temps in the 300+ degree range. Never had a temp gauge on a shock, but have been told by people that know a lot more about it than me that a Pro on a rough MX track can get them THAT hot.
dan

You probably won't notice any difference in the shock while all is OK.

I don't agree at all with claims that oxygen will heat quicker or that it is less likely to leak out than nitrogen, or that its cost makes a difference. The big problem is when some of the gas gets past the bladder or piston in the gas cylinder & lives as vapour bubbles in the oil, which will inevitably happen. The machanics of the shock produces very high localised compression rates, especially as the oil whisles through the valves & galleries. High compression rates in the gas bubbles produce high temperatures in the gas, well above the auto-ignition temperature of oil vapour which by default must also be present - exactly how a diesel engine works.

The only thing that stops the oil vapour catching fire in a normal shock is the lack of oxygen. If the shock has been charged with this then internal ignitions (ie fires) are virtually unstoppable. If the fires are large enough - ie a lot of oil reacts with much of the oxygen at once - then the pressure produced could rupture the shock housing. This is unlikely but possible.

What is more likely is that the oxygen will gradually get consumed by the fires, gradually reducing gas charge pressure. This will increase the voids & gas bubbles in the oil, making the she shock soggy or not work at all.

I would also expect that, from a safety point, race organisers would not let a bike compete if they knew that it had oxygen in its shock/s.

I have run with welding argon in shocks & it seems to work OK, but I would not recommend it.

I don't agree at all with claims that oxygen will heat quicker or that it is less likely to leak out than nitrogen, or that its cost makes a difference.

Click to expand...

Like BundyBear says oxygen won't heat up (read than expand) quicker if it is DRY which welding oxygen in a bottle will be as it is produced by distillation as I said. I agree that it is not more likely to leak than nitrogen.

But cost is a real factor - Oxygen is 3 times more expensive (from a gas refiner, not necessarily your local supplier) than nitrogen, and Argon and Helium and other noble gasses are 20 times as expensive. Nitrogen IS the cheapest bottled gas in the world.

If you were trying to decide what gas to use you'd go for in order a dry gas, a non-reactive gas, then the cheapest. Many gasses fill the first two buy Nitrogen is the cheapest. That is why it is picked.

The point I am trying to make is that Nitrogen has no magical properties that make it suitable for shocks or other gasses not suitable. It is just that it fulfils all the technical requirements i.e. dry - inert - available - Cheap.

Has any other gas been tried? And no, don't go that way.

Well we know HusaDave has tryed oxygen and BundyBear has tryed argon.

But more seriously - I have read articles suggesting that some F1 teams have run Helium.

I don't think it would really have any advantage over Nitrogen for us lesser mortals though.

All the nitrogen is really doing, is holding the shock shaft seal in place. It provides a pressure load on the shaft seal as the Damping Pistons move up and down. The Nitrogen Piston only moves a very short distance. The more critical is how you set your Suspension, Spring, Sag, Shim stack...

The only reason I used argon is that I always have a bottle of it for TIG welding. A $50 bottle of welding argon should charge a few hundred rear shock absorbers = less than 25 cents worth of gas. I didn't worry about the cost.

I did not use oxygen due to the fire/explosion risk. I came close to injuring myself once dicking around with a mixtre of oxygen & alcohol vapour in a drink bottle. 8O Very spectaculiar but!!

Apart from charging accumulators there aren't many reasons why the average punter would need to keep compressed nitrogen on hand but if you are in the business of doing lots of shocks you'd be crazy not to use it.

I have heard of racers pumping up tyres with helium, I believe to help save weight.

The pressurised gas is there to keep the oil in the shock under a higher-than-atmosperic pressure at all times. This allows the working parts of the shock to "pull a vaccuum" (a really crappy term) on the oil up to the charge pressure of the gas without the oil cavitating. Cavitation is bad. The extra pressure against the shaft seal is a bad side-effect.

I agree with BundyBear on the above.

It seems to me that most people on this site understand their shocks pretty well, and there are excellent topics on how to set up shocks on this site. I can't offer anything about setup of shocks I'm no real expert there, but as an engineer and having been involved heavily in the design of shock absorbers when I was at college I will try to explain briefly how modern shock absorbers work for anyone that is interested.

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There is no great mystery to how shock absorbers work they are in fact fairly simple in principal. Even the very latest High/Low adjustable compression and rebound shocks are easily understandable. I do not profess to be a suspension expert, but I do know how they work and have seen the internals of most brands of high-performance shocks on the bench.

Sometimes you have to be inducted into the terminology of a subject before you can begin. Lets start from the absolute basics and go from there:

Looking at the rear of your bike you will see the shock unit. It has a spring coiled around the outside, in the middle of the spring is the body and hanging out to the side is the reservoir.

The Spring - The spring STORES energy, it does not ABSORB energy. You get all (almost all) of the energy you put into it back out. When you hit a bump the energy of the bump compresses the spring against the inertia of the bike and you, as you go over the bump the force on the rear wheel relaxes and the spring pushes the wheel back down against the inertia.

The Body - The body of the shock is filled with oil, a piston with small holes in is attached to the end of the rod which sticks out the bottom of the body, as the shock compresses the piston is forced through the oil. Image a coffee plunger for those of us who use them. This is the damper, the damper ABSORBS energy it does not STORE it, all energy in put into the damper is lost as heat you do not get it back out.

The Reservoir - The reservoir is there to take up the oil displaced by the rod as it moves into the body and as BundyBear says keep it above atmospheric pressure at all times.

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Now probably almost every body knew all that, so to the damping circuits. Inside a modern shock there are usually 4 damping circuits, they are Low Speed Compression (or Bump)LSC, High Speed Compression (or Bump)HSC, Low Speed Rebound LSR and High Speed Rebound HSR.

10 years ago - LSC and LSR were adjustable externally, and HSC and HSR were non-externally adjustable (2-way adjustable)

Here is an example of a 2-way shock http://www.ohlins.com/46prc.shtml

Today - LSB, LSR and HSB are all externally adjustable on most shocks (3-way adjustable)

Here is an example of a 3-way shock http://www.penskeshocks.com/Adjustable% ... Manual.pdf

Today - The best shocks have adjustable LSC, HSC, LSR and HSR

Here is an example of a 4-way shock http://www.ohlins.com/pdf/productnews/t ... racing.pdf

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So how do they all work -

Well this is what is happening in the shock, lets say the shock is coming up to a medium sized bump - as the back wheel beings to ride the bump the acceleration is small and the shock is operating in the LSC circuit only, as the wheel gets on to the steeper part of the bump the acceleration is higher the swinging arm is trying to push the rod into the shock faster and the force of the oil pushes the HSC circuit open oil is now flowing through the LSC and HSC circuits. As the wheel goes over the crest the force is suddenly relieved from the wheel, the spring pushes the wheel back down with its stored energy fast causing oil to flow through the HSR and LSR circuits, as the force decreases the HSR circuit closes and oil flows through the LSR only.

So how each works -

LSC - Both low speed adjusters work by varying HOLE SIZE, they are in effect needle valves, much like the needle and jet in your carb. The low speed compression is usually regulated by the adjuster at the top of the shock connecting the body to the reservoir. It does not regulate the flow of all the fluid just the small percentage of it displaced by the rod as it moves into the body, obviously as it moves in it displaces its own volume in oil. If you turn the LSC in the needle moves into the hole and the size of hole that the oil can bleed through is reduced, turning it out has the opposite effect. Note - Turning the LSC fully in will make the shock feel very stiff and completely out very soft to the touch but this is not the whole story because pushing on the bike with you hand you can't generate enough force to open the HSC circuit. That's why most top racers bikes feel very stiff if you bounce them up and down in the pits they set the LSC very high but the HSC soft so the bike hands well in corners but when the shock takes a hit like off a jump the soft HSC opens easily and the shock softly soaks up the hit. Note 1 - Sometimes instead of a needle valve they use a disk with a series of holes in it each bigger than the last, and you select a hole rather than a needle position.

LSR - Is a needle valve the same as the LSC, but is usually located at the bottom of the rod, the LSR needle valve needle is very long and runs all the way up the hollow middle of the rod. Unlike the LSC it works on all the oil in the shock, unless the HSR is open.

HSC - This a fixed size hole (or more often series of holes) with a shim stack covering it, the shim stack is made of about a dozen or so thin metal disks, like thin washers, stacked on top of each other. The oil pushes against the under face of these shims and if the oil pressure is higher than the spring force that the shims offer pushes them open. You can adjust either the thickness of the shims, the number of shims, the preload, the diameter, number of slits etc to get different opening and closing forces and characteristics. This is what is meant by re-shimming a shock, or "We do factory shimming" type adverts you see. For adjustable HSC the shims are replaced by (or more often supplemented/accompanied by) a coil or shim type spring which the pre-load of which can be adjusted with a screw, like the preload for the main coil spring on the outside of your shock. So the higher you adjust the HSC the harder it is for the oil to open the HSC damping and it will take a bigger bump/hit to open it, and vice versa.

HSR - This works on the same principal as the HSC.

These are the principals as I understand them, each brand does it slightly differently but the principals are the same.

So endeth the sermon for today. I hope that was useful to any one who managed to read all of it, I know most of you knew all that but if not it is hard to find that kind of info' and I hope it has increased your understanding of your shock.

Ben
JBSracing