Question About Air Shocks



patmcf79

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Feb 22, 2007
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I just got a new Specialized FSR and the rear shock tends to loose air over time. A friend of mine that is into off roading recommended filling the shock with nitrogen instead of air because nitrogen molecules are bigger and will leak less because they have a harder time working their way past the seals. He told me that even GM is filling their truck tires with nitrogen now because the tires will retain the proper pressure longer.

My questions are:
1. Is any of what he told me true and if so, if I replaced the air with nitrogen in my rear shock would I see it loose less pressure over time (maybe none at all)?
2. Will nitrogen alter the shocks compression and rebound characteristics?
3. Have any of you out there done this and what do you recommend?

Thanks in advance!

Pat
 

steve_18798

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Feb 15, 2007
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Well Pat. I would have to say that filling with nitrogen will not change much but you might notice the differncce with a harder shock. It may keep air pressure longer but not that long. The only question i have is that do you have a very slow air leak beacause mine has never lost air yet. You may want to check the seals and make sure that everthing is good and tightned down. I've had my shock for 2 years yet with no considerable difference. It also depends on how hard you ride and how much to.
 

patmcf79

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Feb 22, 2007
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steve_18798 said:
The only question i have is that do you have a very slow air leak beacause mine has never lost air yet. QUOTE]


As of right now it's an off the shelf 2007 Specialized FSR that has a Rock Shox Fusion rear shock. I just read in the manual on how to setup the shock properly. The tech at the shop I got the bike from showed me how to set the pressure in the shock and it wasnt by the book. Aparently I guess I have to check the squish a few times after the desired PSI is set inorder to let the chambers inside the shock to eqaulize. Not sure if this is the prob or the seals maybe going bad. Im going to check tonight. Thanks for your response.
 

skootaroo

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Jan 15, 2007
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You'll find the seals are not going bad but coming good!

It took one of mine a month or two for the seals to bed in properly. I was going out on a ride at max pressure and returning an hour later too soft to do a bunny hop.

As for nitrogen the advantage is in the heat expansion properties. Air expands a lot when heated, which will change the pressure in your shock. Nitrogen is more stable, when you are bouncing up and down on your shock, the heat created changes the pressure less.
Will you notice the difference? Most don't!

Have you checked the air in your tyres latley? Are there any strange vibrations in your tyres? It could be the air has gone stale, does it have a low smooth hiss or a high coarse hiss when you let the air out, it was probably stale. There are so many mountainbikers suffering poor handeling problems and noise from not refreshing the air in their tyres.....LOL
 

patmcf79

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Feb 22, 2007
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skootaroo said:
You'll find the seals are not going bad but coming good!

It took one of mine a month or two for the seals to bed in properly. I was going out on a ride at max pressure and returning an hour later too soft to do a bunny hop.

As for nitrogen the advantage is in the heat expansion properties. Air expands a lot when heated, which will change the pressure in your shock. Nitrogen is more stable, when you are bouncing up and down on your shock, the heat created changes the pressure less.
Will you notice the difference? Most don't!

Have you checked the air in your tyres latley? Are there any strange vibrations in your tyres? It could be the air has gone stale, does it have a low smooth hiss or a high coarse hiss when you let the air out, it was probably stale. There are so many mountainbikers suffering poor handeling problems and noise from not refreshing the air in their tyres.....LOL
Yeah the tire pressure is fine. Tires, tubes, rims... everything is new. When I brought the bike home I did a once over and the only thing that caught my attention is that the rear shock seemed to loose pressure. Again I am not sure if its due to leaking seals, tempature, or an incorrect inflation process. Tonight I'm going to adjust the pressure and see how long it holds for.

Thanks for your help
 

artemidorus

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Mar 10, 2004
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patmcf79 said:
I just got a new Specialized FSR and the rear shock tends to loose air over time. A friend of mine that is into off roading recommended filling the shock with nitrogen instead of air because nitrogen molecules are bigger and will leak less because they have a harder time working their way past the seals. He told me that even GM is filling their truck tires with nitrogen now because the tires will retain the proper pressure longer.

My questions are:
1. Is any of what he told me true and if so, if I replaced the air with nitrogen in my rear shock would I see it loose less pressure over time (maybe none at all)?
2. Will nitrogen alter the shocks compression and rebound characteristics?
3. Have any of you out there done this and what do you recommend?

Thanks in advance!

Pat
You are aware that air is already 80% nitrogen? You certainly would be invoking the rule of diminishing returns on that one. I have to say that it sounds like a complete and utter waste of time. It takes me ~60 seconds to pump up both chambers of my rear shock.
 

artemidorus

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Mar 10, 2004
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skootaroo said:
As for nitrogen the advantage is in the heat expansion properties. Air expands a lot when heated, which will change the pressure in your shock. Nitrogen is more stable, when you are bouncing up and down on your shock, the heat created changes the pressure less.
Air and nitrogen, and any other gas for that matter, expand an equal amount when heated equally.


skootaroo said:
Have you checked the air in your tyres latley? Are there any strange vibrations in your tyres? It could be the air has gone stale, does it have a low smooth hiss or a high coarse hiss when you let the air out, it was probably stale. There are so many mountainbikers suffering poor handeling problems and noise from not refreshing the air in their tyres.....LOL
This is complete rubbish. Or were you joking?
 

patmcf79

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Feb 22, 2007
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OK... maybe I should ask a diff question. Could any of you point me in a direction to some good literature about shock maintnance other than the manufactures? You know... things like general rules of thumb, dos and donts, how to's? This is my first decent mnt bike... my old one was a low end Trek rigid with non adjustable RST fork shocks. Im quite interested in proper care of my new ride and any good tech advice is greatly welcome.
 

steve_18798

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Feb 15, 2007
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Well as said earlier in here try to replace the air in the shock every month or so when you start loosing pressure or you will feel a certain squish in the shock area. As for the seals always check them because they to can cause problems but have quite a while in there life span. You shouldn't lose air at all but it does go stall and gets a funny smell to it when released as most people do not know this and think the air in the shock will last forever, so try to replace the air frequently, especially if you ride hard on the bike.
 

skootaroo

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Jan 15, 2007
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Sorry about the stale air in the tyres gag. I guess I may have been a little subtle on that one.

On a more serious note, air does behave as any other gas until you start messing with the water vapor content.
 

artemidorus

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Mar 10, 2004
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skootaroo said:
Sorry about the stale air in the tyres gag. I guess I may have been a little subtle on that one.

On a more serious note, air does behave as any other gas until you start messing with the water vapor content.
Sorry, the whole post was a gag? There are hordes of people who'll believe a lot worse than that if you post it on a public forum. I'm a bit of a misinformationophobe.
 

artemidorus

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Mar 10, 2004
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patmcf79 said:
OK... maybe I should ask a diff question. Could any of you point me in a direction to some good literature about shock maintnance other than the manufactures? You know... things like general rules of thumb, dos and donts, how to's? This is my first decent mnt bike... my old one was a low end Trek rigid with non adjustable RST fork shocks. Im quite interested in proper care of my new ride and any good tech advice is greatly welcome.
All air shocks/forks leak air slowly, or fast, it's a matter of luck. Paying more is no guarantee. As skootaroo has posted, the seals may bed in a bit, or they may not.
All you can do is keep fork stanchions and shock pistons unscratched, and dust them off between rides. Have them serviced at the slightest hint of excessive wear, or even do it when the maker tells you to (I don't know anyone who does that). Keep the air springs adequately pressurised for your weight and style. I'll politely disagree about the stale air thing.
 

patmcf79

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Feb 22, 2007
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artemidorus said:
All air shocks/forks leak air slowly, or fast, it's a matter of luck. Paying more is no guarantee. As skootaroo has posted, the seals may bed in a bit, or they may not.
All you can do is keep fork stanchions and shock pistons unscratched, and dust them off between rides. Have them serviced at the slightest hint of excessive wear, or even do it when the maker tells you to (I don't know anyone who does that). Keep the air springs adequately pressurised for your weight and style. I'll politely disagree about the stale air thing.
Thanks Art.

I pumped the rear shock up on Fri and its holding fine as of now. I was unble to go riding this weekend so Ill see how it does the rest of the week and then go riding maybe next weekend.

Also I have been searching the web high and low for "how to" sites and I found a couple.

http://www.utahmountainbiking.com/fix/
http://www.webmountainbike.com/index.html

If you all know of any other let me know. Im a sponge right now with all of this. Thanks for all the help and advice.

Pat
 

patmcf79

New Member
Feb 22, 2007
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artemidorus said:
All air shocks/forks leak air slowly, or fast, it's a matter of luck. Paying more is no guarantee. As skootaroo has posted, the seals may bed in a bit, or they may not.
All you can do is keep fork stanchions and shock pistons unscratched, and dust them off between rides. Have them serviced at the slightest hint of excessive wear, or even do it when the maker tells you to (I don't know anyone who does that). Keep the air springs adequately pressurised for your weight and style. I'll politely disagree about the stale air thing.


Well... the shock has been holding its air for the most part. I still havent done any real hard riding. I am planning on going for a decent ride tomorrow... If any of you are in Northern Virginia we are meeting up at Wakefield around 4pm