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I decided to have the shock reconditioned after more than 2 years of my riding and god knows how many years before that has it not been serviced, if at all.

wr 99

mech said, there was no pressure, but oil was not that bad, so the parts did get its necessary lubrication.

Well, riding the bike before the fix was quite nasty, especially over rocks, but I thought its an old WR, the suspension wasnt yet figured out good, since the rebound actually was working and also had the sag done per the manual.

Got the shock back, for the price of 150$, since rubber membrane had to be replaced...signs of wear was visible, he said because of no pressure.

To the question..what is the purpose of gas, because now, bike is just incredible in the trail (for my knowledge)

I can feel the bike is less powerfull, since rear is so glued to the ground.

Other than that, fantastic, soft, maybe too soft for the track, but trail is what I love and thats where it performs good.

So, oil influences damping, what does gas do?

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Gas has nothing to do with damping itself - it is confined to a small space inside a collapsible bladder inside your shock.

Oil, when flowed at high rates through small orifaces to create turbulence(aka Damping), tends to cavitate or form small bubbles in it. This is actually a form of boiling, and is aggravated by heat. However...if pressure above atmospheric is put on the oil, this is prevented from happening, even at high temperatures. This keeps damping more consistent over a broad range of shock/oil temperatures, and prevents the major inconsistencies in behavior that come when the oil boils/cavitates and proceeds to emulsify.

In your case, the bladder probably ruptured and allowed alll the gas to become emulsified in the oil, and as you rode you only worsened the problem. This has a stark effect especially on rebound damping, so much that clickers are rendered almost useless.

Have fun!

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What thillsam said and a few other things.

First of all, the "gas" is an industrial grade nitrogen, meaning that it's about 99.99% pure nitrogen, assuming that the re-builder purged the atmosphere air from the bladder before doing a final fill, (by the way, you can get two other more pure versions of nitrogen).

The purpose of having a bladder within the shock has been covered and quite well might I add, but there are a few other reasons why nitrogen is used.

The main reason is that it keeps the shock significantly cooler then just about anything else, which is a key factor for the amount of heat that is captured by the shock.

Other benefits include matters of longevity and stability. Details.

That aside, it's important to understand that the complete shock is a damper. I mean, on this board we often speak about damping this-and-that via the pistons and valve shims, but the bottom line is that any assembled shock or fork is effectively a damping device. Springs included.

Why this concept is important to understand is that nitrogen pressures do affect the "damping" performance of the shock. The more pressure in the bladder, the more force it takes to compress, and the faster it will tend to rebound. Actually, the difference in even seemingly small pressures can be significant.

Setting nitrogen pressures is a lot like setting fork oil levels. The settings can be used to control bottoming, and in some cases, affect ride height. And of course, if you were to add or remove the nitrogen from the bladder, you may have to make some changes to damping clickers that control the flow of oil about the piston much like you would if you changed the spring rate of the shock.

In my little world, I've learned that having the equipment to adjust nitrogen pressures in the field is just as important as the benefits of being able to set ride height or sag. As you may have noticed, your shock feels perfect for the trails, but seems a bit low or soft for the track. If your spring pre-load is where it should be, you can just about assume that you need to dial in the nitrogen pressure. So, ideally, each track may require a unique pressure setting as one with do with spring and fluid damping settings. If you were that particular…I guess.

That aside, there is one other thing.

First of all, the more nitrogen you add, the greater the force required to compress the shock initially, say the first millimeter, (call this A) and the greater the force to fully compress the shock, (call this :thumbsup:. And as more pressure is added in the bladder, the greater the difference between A and B.

For example, if you set the pressure to 150lb. it would take about 35lb. of force to compress the shock the first millimeter and about 54lb. of force to fully compress the shock. The difference between these two numbers is 19lb.

When you pump the pressure up to 250PSI, the values are about 60lb. and 90lb., a difference of 30lb. Also note that 150PSI at full extension will put the pressure in the bladder at about 175PSI when fully compressed. At a 250PSI start, compressed pressure goes to about 305lb. I’m sure it’s even worse when it get’s toasty.

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Setting nitrogen pressures is a lot like setting fork oil levels. The settings can be used to control bottoming, and in some cases, affect ride height. And of course, if you were to add or remove the nitrogen from the bladder, you may have to make some changes to damping clickers that control the flow of oil about the piston much like you would if you changed the spring rate of the shock.

Dave, I don't agree with you. take a shock a try to compress it - no chance. now remove the spring - you can compress it completely because it's so soft, though it's filled with nitrogen. maybe it does influence the spring rate by 0.1%, don't you think I am right? :thumbsup:

if you want to have an influence you have to put in maybe 1000psi or much more, but then you will hear a bang :ride:

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I agree with Dave. Try compressing that shock at 15m/sec and tell us the nitrogen pressure has no effect.

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I agree with Dave. Try compressing that shock at 15m/sec and tell us the nitrogen pressure has no effect.

I have to kinda agree with Dave also even tho for many years I've had the belief otherwise.

I expect it does make some difference kinda like how lighter or stiffer ICS springs will make a difference on how a TC fork feels. Even tho I don't especially like, or try to tune with (more pressure or different ICS springs) these variables, since there are other ways to make things work.

doc

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First of all, the "gas" is an industrial grade nitrogen, meaning that it's about 99.99% pure nitrogen, assuming that the re-builder purged the atmosphere air from the bladder before doing a final fill, (by the way, you can get two other more pure versions of nitrogen).

The purpose of having a bladder within the shock has been covered and quite well might I add, but there are a few other reasons why nitrogen is used.

The main reason is that it keeps the shock significantly cooler then just about anything else, which is a key factor for the amount of heat that is captured by the shock.

Other benefits include matters of longevity and stability. Details.

My understanding is that purified Nitrogen is used mostly because it is garaunteed to be free of humidity, and as a result of this the pressure as a result of temperature change follows the Ideal Gas law much closer. The same could be said of most purified, inert gasses...but Nitrogen is the cheapest to isolate from atmospheric air. Also, it's an inert gas - meaning it wont react with anything or corrode anyof the materials commonly used in a shock reservior.

How does Nitrogen aid in heat evacuation(over any other industrial gas)?

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Dave, I don't agree with you. take a shock a try to compress it - no chance. now remove the spring - you can compress it completely because it's so soft, though it's filled with nitrogen. maybe it does influence the spring rate by 0.1%, don't you think I am right? :thumbsup:

if you want to have an influence you have to put in maybe 1000psi or much more, but then you will hear a bang :ride:

I have a collection of shocks that I used for testing various matters such as this. Shocks with and without springs, shock with and without shims...etc.

You may find that a shock will glide in and out relatively easy even with a nitrogen charge in place, however, remove that nitrogen and the dynamics of the shock will change dramatically, such as 5VALVE discovered. Additionally, what feels like an easy movement is actually not when you begin to take actual measurements on it.

Lastly, when you bump the PSI on the charge up, everything charges and rather quickly.

The last series of tests that I performed on this I used an 03 KYB shock, with fresh fluid, shims removed. I took a measurement in ft. lb. that it took to compress the shock past the initial movement, and the amount of force that it took to fully compress the shock to the top of the bumper. I started off at 100PSI and worked my way up at 10PSI increments until I got to 250PSI.

At 100PSI, it only took 33lb. of force to compress and hold the shock to the bumper. At 250PSI, it took over 90lb. of force. That, to me, is a considerable difference.

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I have to kinda agree with Dave also even tho for many years I've had the belief otherwise.

I expect it does make some difference kinda like how lighter or stiffer ICS springs will make a difference on how a TC fork feels. Even tho I don't especially like, or try to tune with (more pressure or different ICS springs) these variables, since there are other ways to make things work.

doc

I guess my application is primarily when you need to reduce bottoming on a shock that has ideal damping and spring loads on it. So far, I think that’s about it.

My understanding is that purified Nitrogen is used mostly because it is garaunteed to be free of humidity, and as a result of this the pressure as a result of temperature change follows the Ideal Gas law much closer. The same could be said of most purified, inert gasses...but Nitrogen is the cheapest to isolate from atmospheric air. Also, it's an inert gas - meaning it wont react with anything or corrode anyof the materials commonly used in a shock reservior.

How does Nitrogen aid in heat evacuation(over any other industrial gas)?

I have to admit that I’m certainly no expert on gases including nitrogen. I just know what I know based on testing and some reading.

I’m not even sure what inert is since I have read some very detailed publications that indicated that, contrary to popular belief, nitrogen was not inert. I still don’t know what to believe.

And I always assumed that the number one reason that nitrogen was used in shocks was because it compressed at a linear rate (or closer to it than outside air). This of course is not the case, but lately some of my testing has been showing that it clearly does not compress at the same rate as outside air, but it still not linear.

So I have to assume that this “gas law” is applicable when comparing gases such as pure argon and helium to nitrogen.

That aside, I also ran a series of tests on a few shocks in which I tested various "gases" at various temperatures, measuring general performance of the shock as well as changes in the pressures within the bladder along the way.

The biggest lessons learned from this testing was 1. I was not able to explode the shock filled with outside air, (yes, I tried) and 2. that it took a significantly greater amount of BTU’s to heat the nitrogen filled shock than those of other gases, and that this shock cooled down significantly faster than the others. This is when it became apparent what the greatest advantage to running nitrogen was.

This led me to the belief that the more I learn the more I realize how much I don’t know. :ride:

Now, one last thing,

When you run nitrogen in your tires you can really feel a big difference and I have no scientific explanation as to why this is. The tire just feels more “cloud” like, (how’s that for technical?). Still firm and planted, but doesn’t seem to hit and vibrate as it does with outside air.

I have a super stiff X5 that I switch over to nitrogen and was really impressed with what it did to the ride quality. I mean, it’s a small improvement that seems to make a huge difference, if that makes any sense. :thumbsup:

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I have to admit that I’m certainly no expert on gases including nitrogen. I just know what I know based on testing and some reading.

I have a little more for you, if you have a moment...

I’m not even sure what inert is since I have read some very detailed publications that indicated that, contrary to popular belief, nitrogen was not inert. I still don’t know what to believe.

http://en.wikipedia.org/wiki/Inert_gas

From the article:

"An inert gas is any gas that is not reactive under normal circumstances."

and

"Some gases which are not usually considered inert but which behave like inert gases in all the circumstances likely to be encountered in some use can often be used as a substitute for an inert gas. This is useful when an appropriate pseudo-inert gas can be found which is extrememly cheap and easy to come by. For example carbon dioxide is sometimes used in gas metal arc welding because it is not reactive in the circumstances encountered in arc welding even though it is often reactive in other circumstances."

So I have to assume that this “gas law” is applicable when comparing gases such as pure argon and helium to nitrogen.

http://en.wikipedia.org/wiki/Ideal_gas

It applies to approximate a given pressure at a given volume and temperature for a given number of gas molecules. From the article:

"Real gases do not exhibit these exact properties, although the approximation is often good enough to describe real gases."

daveJ, your habit of testing makes you more of a scientist than many scientists I know! The temperature-saturation tests are itneresting to me, I have never has the opportunity to observe this. And the car tire phenomenon is something I'll be trying! I dont have any sort of scientific explanation for that one, not from books anyway. I'm sure it can be explained, though...

You'll laugh at this, but I've been using air to charge my shocks for alooong time...BUT its AZ desert air, maybe 10% humidity. I'm curious now to charg a shock with the "right" gas and see how wrong my valving is!

thanks for offering your experience and insight!

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Dave, thanks for your explanation, though I cannot imagine I would feel the difference. I remember a friend of mine who tried different bladder pressures and ended up with a burst shock - so I decided not to do that. but again, I appreciate your knowledge and will to test such things :thumbsup:

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Somehting else to add; I have read that nitrogen molecules are bigger than outside air (even though it's about 70% nitrogen) and have a harder time leaking through the rubber bladder into the oil over time.

I have also read that nitrogen is more stable than air, meaning over the course of a moto the heat that builds up in the shock will raise the bladder pressure less with nitrogen than with outside air. More pressure during a moto will change the handling usually resulting in a harsher ride.

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This led me to the belief that the more I learn the more I realize how much I don’t know. :ride:

Man, I have that feeling all the time...... :thumbsup:

doc

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Uhhh, guys, some heavy reading.

Probably have to agree on gas influencing the ride.

Although bike is now kicking up faster when manually pressed, it eats up bums, rocks, series of bumps much softer...it almost took the soul out of human controlling the bike, but than again, I rose to another level of riding in just 5 minutes.

Still have to test it in slippery mud mixed with unstable rocks, of course quite steep uphill.

On previous shock (even in dryer conditions) rear would just float, throwing me left to right, not giving satisfactory traction and only manpower and reasonable speed pulled me through.

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At 100PSI, it only took 33lb. of force to compress and hold the shock to the bumper. At 250PSI, it took over 90lb. of force. That, to me, is a considerable difference.

Dave,

You might want to take the application into context, not just you pushing on a shock and feeling a difference of 60lbs.

That 60 pounds of force is negligable when taking into account the force to bottom that shock with a normal shock spring on it.

To do a test that really makes sense, you would probably want to test the shock mounted on the bike, with it's spring at proper preload, and then measure the force required to move the rear wheel 1 inch and then bottomed, with both your low pressure and high pressure settings.

Testing without a proper context is kind of like peening in the wind...

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Dave,

You might want to take the application into context, not just you pushing on a shock and feeling a difference of 60lbs.

That 60 pounds of force is negligable when taking into account the force to bottom that shock with a normal shock spring on it.

To do a test that really makes sense, you would probably want to test the shock mounted on the bike, with it's spring at proper preload, and then measure the force required to move the rear wheel 1 inch and then bottomed, with both your low pressure and high pressure settings.

Testing without a proper context is kind of like peening in the wind...

I think you need to also take into consideration when these test are taken. ambient or operating temps or the differences between the two.

I think it was last year JC and I did some shock dyno testing on the effects heat played on bladder pressure's. To be honest I was quite shocked at the numbers.

At a 155psi fill ,hand bled, the bladder pressure rise was increased to 283 psi at a operating temps of 240F. This ended up increasing the shock compression from 325 lbs of force (at 140F) to over 430lbs of compression force at 240F. It also reduces the rebound force by over 100lbs of force. Its makes for an harsh shock with a fast rebound. Most notable was the effect various PSI fills has on nose pressures.

The nose pressures can be felt by way that Dave has mentioned within testing various PSI fills and by bench testing. Basically your feeling the true low speed but never the less it is noticeable by doing it this way if you want to experiment.

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I think it was last year JC and I did some shock dyno testing on the effects heat played on bladder pressure's.

Russ....I was thinking that I had read about your tests awhile back too. Why or how was such a high operating temp (240F) used? I know a hard charging pro will get that shock temp up but with most amatures and local races being so short I don't think most people get their shocks that hot.

I have been using a heat gun lately while testing and at the races and seems like temps run more in the 150F-180F range. Can you remember approx how much the pressures rise at these temps?

doc

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Doc

These test were on the dyno in which produce a stroke of 22in/per sec. I don't remember the lapse time involve off the top of my head, but it did not take long to achieve this temp. Yes real fast guy's do get even higher tempsthen 240F. I will look around and see if I can find the read out.

Not sure what it would be Doc at 180F. Man this is why I wish I had a dyno just to play with, to have actual numbers to support what we think is going on and was is actually going on. :thumbsup:

Dave I do have to agree that different N2 pressures will effect ride height, but would think this would through off consistancies in your sag measurements. I do understand that you said Quote" the right spring preload" but I need to think about this more.

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I would like to here some comments on the numbers that were produced in the above paragraph dealing with the rebound. As the shock heated the rebound was more than 100psi lighter then at 140F. So my question here is. how many here feel they would think this as being shock fade do to the faster rebound that was produced, even though the compression increased ? Alway's another side of things in suspension in the way they are precieved.

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I would like to here some comments on the numbers that were produced in the above paragraph dealing with the rebound. As the shock heated the rebound was more than 100psi lighter then at 140F. So my question here is. how many here feel they would think this as being shock fade do to the faster rebound that was produced, even though the compression increased ? Alway's another side of things in suspension in the way they are precieved.

I FOR SURE think thats what riders feel as "fade" Russ. This has also had me thinking about why I (my customers) seem to like like a little more "free" sag. Instead of the somewhat normal 1" or 25mm's free sag I most times set bikes up with 28-30mm's free sag and seem to have less rider complaints about "kicking", especially as the tracks get rougher. I've also been leaning towards 10-25lbs. less pressure in the shock with some of my test riders with no real problem.

Makes ya wonder if maybe the shock sag should be set right after the moto :thumbsup:

doc

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