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kawagumby
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Post by kawagumby »

About the nitrogen...I have to admit that I'm a skeptic also. If it weren't for the bottled nitrogen being virtually moisture-free I'd have to assume that there wouldn't be much difference in performance either way. What really puts the kink in the nitrogen vs air debate for me, is that professional tuners do fill the bladders with AIR to hold the shape during assembly.
I read an industry report on the use of nitrogen in truck and auto tires, which pointed to moisture having the greatest impact on volume/pressure changes as tires heated up. Soooo, for those of you who might be interested, I lifted a scientist's response to a question regarding the kinds of musings we have indulged ourselves of within this thread:

"Isai-
I am not up on the known effects of inflating with nitrogen
only, if any.
It is not obvious to me that there are any significant effects,
but there could be some minor ones.

This pressure-loss rate claim is the first thing I would ignore,
for exactly the reason you stated.
There is not a big mass difference between N2 And O2 molecules, as
relates to diffusion.
I can say that oxygen is roughly a couple times more soluble in
rubber than nitrogen,
just as it is in water or virtually any liquid solvent.
I have not yet estimated whether dissolved gasses diffusing through
rubber are the main cause of pressure loss.
With me it is probably an old dirty valve.
But if diffusion does occur, then oxygen from the outside air will
be diffusing inwards,
a.la "osmosis", a litlle bit faster than the equivalent excess
nitrogen diffuses outwards.
Possibly this helps keep the pressure up a little longer.
Making sort of a pressure plateau compensating for the first ~2psi
of nitrogen loss.

I think one physical effect might be a change in the heating
generated by pressure-changes
in the gas when the tire bounces, and by sound inside the tires.
Each kind of gas molecule has a particular set of spins and vibrations
which have low enough quantization energy to be active at room temperature.
(This is the consideration that gives each gas its characteristic
value of specific heat.)
These rotational modes in turn have some particular rate of
energy-equilibration with the translational kinetic energy in the gas.
This rate leads to broad peaks in the absorbtion_rate-vs-frequency
spectrum of sound.
I think the peaks tend to be in the mid-audio range.
With mixed gasses, extra loss-effects occur, related to the
difference in equilibration rates.
It is a question of: when the gas is briefly squeezed and released,
do you get all the work back and gas at the original temperature,
or do you end with less energy and warmer gas?

An allegorical explanation:
If you somehow suspended powder particles in a pure gas,
when the gas was squeezed it would get hot, and the powder would
absorb some of the heat.
Later if the squeeze was released slowly the powder would give
almost all its heat back to the gas,
helping the gas expand and give back the mechanical energy used to squeeze it,
and there would be little energy loss.
But if the squeezing was removed faster than the powder can cool off,
that heat would be stuck in the powder until the gas was already
expanded and cold,
and that would be mechanical energy wasted, turned into net heat generation.
The second gas, oxygen, is somewhat analogous to the powder.

So perhaps one pure gas would have less conversion of pressure
changes to heat.
Less heating is generally good for tire durability.
But I do not know if this business is strong enough to help, or too
weak to matter.

Chemically perhaps it is good for the tire's rubber, the inner part at least,
to have less oxygen and water permeating it
while it is working hard and getting hot due to flex while driving.
Industrial nitrogen not only has less oxygen than air, it also has
less water vapor.
Maybe less of other things, too, like oil used to lubricate
compressor pistons.

When a compressor pressurizes air, 50% humidity becomes >100%, and
liquid water forms,
which hopefully the machine will be good enough to remove.
But it requires an effort, and it is difficult for a "blind"
customer to check this....
On the other hand, if the shop is using bottled or liquid nitrogen
as they claim,
the gas will be bone-dry, more than likely,
and the interior of your tire will stay that way too.
Water condensing and evaporating during pressure fluctuations
is a very likely mechanism of mechanical inefficiency and heat generation,
probably more so than N2/O2 rate differential.
Any tire wet with water inside should have measurable loss due to it.
Even without liquid, the vapor H2O molecule has yet a third set of
mode-equilibration rates,
much more different from N2 than is O2.
Nitrogen inflation fixes all that with assurance and cleanliness, in
a simply deliverable way.
So I think this "water" business is the best reason I can think of.

Wish I knew how big these effect really are,
and why nitrogen inflation is becoming a common offering."

Jim Swenson
====================================================================
1994 KDX200, Beta 200rr, yz125, yz250, kx100 modded for adult, gasgas contact 250.
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canyncarvr
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Post by canyncarvr »

Good reading! I've been long curious how the rate of
energy-equilibration with the translational kinetic energy in the gas effected pressures inside tires!

:hmm:

I'm SURE I have been! Even if I didn't realize it at the time!!

The thread I was reading awhile back had to do with tires. The guy measured pressure changes in the different 'fills' at temps up to hundreds of ºs. It was the tire that had too much water in it, the beads having been liberally soaked during the tire mount process, that showed a significant change in pressure as the heat went up.

As noted above:
When a compressor pressurizes air, 50% humidity becomes >100%, and liquid water forms, which hopefully the machine will be good enough to remove.
That's what the drain valve is for on the bottom of your compressor tank, 'eh? So....all the moisture that WAS in the air before it was squeezed is now sitting in the bottom of the tank..under pressure..so it's not going back INTO the compressed air.

This is has been most informative and interesting. Thanks for all the input..much appreciated. Tidbits like pressurizing the bladder to show the shop that it wasn't going to blow up when THEY did it was great. Which end of the shock to put in first was very helpful. Good stuff, Maynard!

I do have that 0-300psi shock pump on the way. That's particularly 'neat' 'cuz I just asked the shop down the road if they would pressurize my shock after a refresh and they said, 'Yep. $15.'

Here's something to take note of: I grabbed a bottle of RaceTech oil off the shelf (happened to say US2 on it) and asked the counter-guy if that's the fluid he had mentioned on the phone. He grabbed a mech walking by, asked him. The mech asked, 'What kind of bike?' I told him. He said, 'Use US0...ONLY..in the KYB shock.'

The US2 I had in hand did say 10W..and I thought that was too thick, but it was the only stack of bottles ON the rack. Then I noticed that beHIND the US2 was the US0! (Which, btw, is 2-3.5W) At the stellar bargain basement price of ONLY $29.95 a quart...I should'a bought a gallon of it!!

:shock:

Yeah...I'm sure there are those that use Quaker State 30W and, '..doan haf no probwum wif it..' :wink: I figger the weight savings (between 10 and 2-3.5) will make me so much faster up Smoke-a-Clove (a climb at Quartz Ck.)!!!

...and maybe I won't even have to give it 2-3 tries to GET to the top, neither!

I've taken note that compression of the shock is absolutely soundless now. I'm hoping there won't be popping/squishy bubble sounds when I'm done.

One more thing...CM noted the bleed at the top of shock. Yep..I know it's there. The shock is going to be right side up to get any air 'up' to that point. What happens to the pressure if you loosen that screw? Obviously it goes down, as the volume of fluid will be reduced. But, how do you make that up? Charge the bladder to less than the end value, bleed the shock with the screw, then charge the bladder to the full amount?

Thanks all!

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Post by scheckaet »

I was told (a shop :roll: ) you could use the same oil for the fork and the shock. I've used 5w in both and it worked for me but heck, I'm no race demon or engeneer so I really dunno :?
Charge the bladder to less than the end value, bleed the shock with the screw, then charge the bladder to the full amount?
I dunno but it seems to me that the bubble should be out when you assemble the shock (i.e when you add just enough pressure to the bladder so that it holds its shape)
If you charge the shock at 150 psi (or even 1 for that matter) and open that screw, you will lose oil very quickly (at 150 psi) and also pressure in the bladder. That screw doesn't act the same as it does in the forks.
If I was convinced I had air in the oil, I would use just enough psi to push it out slowly then go to 150 (or whatever pressure it should be) and be done
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Post by canyncarvr »

Re: 'I dunno but it seems to me that the bubble should be out when you assemble the shock (i.e when you add just enough pressure to the bladder so that it holds its shape)'

Yes. It should. Done correctly, there should be no need to touch the bleed screw. But...maybe I wasn't clear that I was the one doing this?? :wink:


Re: 'If I was convinced I had air in the oil, I would use just enough psi to push it out slowly then go to 150 (or whatever pressure it should be) and be done'

Which was my intended point.

It's all conjecture at this point. After I'm done, the rod may well go in/out of the body without a single 'pop' or 'spit'. If not, then I would try the bleed screw before I'd take it all apart and do it over.

Especially at $30 a qt. for the magic junk I just bought.

I have read several places that fork and shock oil are NOT the same....and DO NOT confuse/mix-up the two.

Maybe it's great strides in technology...but the RaceTech bottle does say it's good for forks AND shocks.

I doubt (a whole lot) I could tell the difference between 5W and 3W in final result. But...someone whose job it is to know telling me emphatically to use ONLY US0 in a KYB...I'll give it a shot.

Re: 'If you charge the shock at 150 psi (or even 1 for that matter) and open that screw, you will lose oil very quickly (at 150 psi)..'

I'm missing something? I do not understand that sentence.

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Jeb
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Post by Jeb »

So Carvr . . . what are you "servicing" anyway? Are you gonna' replace the seal/bushing or just the oil & N2 repressurization?

A Racetech rebuild "kit" is less than $20 if memory serves me correctly. Includes seal, wiper, bushing. Fairly straightforward; I think I still have some snaps of the seal "head" that I'll load when I return from my voyage to MO
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Post by Jeb »

I suppose moisture could be a problem (not for the reasons Mr. Swenson states), but that could be largely remedied through filtration. Other forms of contamination, i.e. dessicant, dirt, etc would likely have a larger impact than the moisture in my opinion. I know this is true of pneumatic field devices, particularly in the older diaphragm-type controls but also the i-to-p (current to pressure) controls.

I would think that clean, dry air - "instrument air" - would probably work OK if available. But the N2 is probably as easy if not easier to get!!

A BTW - just about every bit of what this Swenson guys describes is gobbly-gook; I started to dissect it all and realized that it's not worth the time. But don't go thinking he knows what he's talkin' about just because of the technical wording and such. I personally suspect that some form of intoxication was at play in his offerings . . .
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Post by StickyTeflon »

I'm a respiratory therapist at my day job... and deal with different gases daily.. My "guess" on the nitrogen would be 1. it doesn't bond with hydrogen as well as oxygen dose (moisture) 2. and may be more importantly it's a much larger molecule. we use a few instruments calibrated / charged with nitrogen and its because O2 / hydrogen / helium / and the other gases are not as dense and find there way out through small imperfections, Nitrogens fat @ss is stuck in there like a 400 lb. lady with a life preserver around her waist. :mrgreen:
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Post by Colorado Mike »

I never bled my shock with 150 psi in the bladder. I would be concerned that the bladder would fill the cavity as soon as the bleeder screw was cracked, and very possibly fill my cavities with expensive shock oil.

:blink:
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Post by scheckaet »

it doesn't bond with hydrogen as well as oxygen dose
wouldn't that make N2O or Nitrous Oxide? :supz: that'd make a hell of a fast KDX :mrgreen:
Last edited by scheckaet on 11:31 pm Apr 01 2008, edited 1 time in total.
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Post by scheckaet »

Re: 'If you charge the shock at 150 psi (or even 1 for that matter) and open that screw, you will lose oil very quickly (at 150 psi)..'

I'm missing something? I do not understand that sentence
I never bled my shock with 150 psi in the bladder. I would be concerned that the bladder would fill the cavity as soon as the bleeder screw was cracked, and very possibly fill my cavities with expensive shock oil.
I know it wasn't too clear, that's what I was trying (rather poorly) to explain :?
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Post by canyncarvr »

Re: N2O

Hey...take a hit off my shock bladder!!! :partyman:


Re: What I'm 'servicing'.

I'm simply changing fluid. The guts were replaced 1 1/2-2 years ago..I'm not worried about them.

OK...this has been a lot of fun..and likely a lot of making it 'too hard' too.

I've not messed with a shock at all 'ceptin' clicking some clickers and setting sag levels.

I know the bladder volume is small..still, the puny 'ppffft' I got out of the thing last night was..well...puny! At that rate, a 5lb nitrogen bottle would last you for darn near forever if you used it only for shock charges.

The bladder compressed easily. Wasn't stuck. I did use my vise. The seal assy could be pushed in using just fingers.

I was a bit dismayed to see the clip on both sides is simply a round wire clip. Why not use a snap ring that you can get to with a pair of pliers? But, no. The only way to get the thing out is to lever a scribe between the shock body and the clip..and that certainly wouldn't be my first choice of where to apply force..against the aluminum body of a $600 shock.

Both clips came out simply enough: Lever the clip end from the groove just enough to slide it UP the scribe with another tool. Didn't seem wise to lever the thing completely out. Too much pressure on the shock body and too much clip compression.

I was surprised to hear/see what came out of the cannister when I eased the bladder out. It made some hissing, spitting noises, and the oil that came out was almost frothy. Hhhmmm. The oil would make that noise and have that appearance if there was still pressure in the shock, but there wasn't. Maybe what's released from the bladder doesn't completely neutralize the shock pressure. Didn't have any trouble pushing the rod in after the bladder charge was released...and it stayed in. Maybe there is still enough 'push' inside to spit the oil out past the bladder.

The old fluid still has color to it! That's a sign of not being WAY overdue! :wink:

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Post by scheckaet »

I seem to remember one of the ring was smaller, you might want to check that out before you put everything back together. Wouldn't be too good to hear the shock blew up in your hands :?
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Post by canyncarvr »

Ha! I thought of that..have them separated for that express purpose. What was odd was looking at the two of them, they appeared to be the same part!

But I kept them separate anyway.

Thanks for the reminder!

P-O-W...BANG...KAWBLOOOYEE

ooops

:wink:

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Post by scheckaet »

they appeared to be the same part!
Yeah, I know, I wondered the same thing, on the microfiche they do have different # but I wonder if they really ARE different. Anybody care to try swapping them on their next re-build and see what happen with 150 psi??? :busted:
Please share the results with our curious mind
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Post by canyncarvr »

They are not the same. Obviously different if you look at them even askance....

OK..done.

BUT...this is where my question about which end first fits. I did put the bladder in first. Putting the rod assy in afterwards, though, means you have to push the assy in with your fingers far enough to expose the ring groove....and that means you need to compress the bladder enough (through the top clicker assy) to move the oil volume required to get TO that ring groove.

And it's further 'in' than the bladder groove is, too.

That takes considerable pushing!

I ended up with the necessary amounts of oil gushing out all over the place. I suppose that is a good thing.

Following CM's idea, I pressurized the bladder to 100psi (air), snapped the bladder out to the ring, and everything seemed to work fine. The rod did come back out of the body as it should after compression.

I put the thing up to my ear and thought I could hear a bit of a 'squirt' at the outer limit of the rod.

Likely a procedural error.

I let the pressure off the bladder, repressurized to about 15psi, right side up'ed the shock, worked the piston in and out a few times, tapped the body of the shock, then loosened the bleed screw to just past the o-ring. A bit of oil came out, tightened it back up again.

I didn't hear the 'squirt' sound anymore.

I don't know that all shock rebuilds result in zero, none, nada air in the assy. If done right, yeah...but how often does that happen?

Maybe a speck of air won't result in an emulsified mess after ten minutes of riding? I dunno.

Guess I'll find out! It's off to the Yamaha shop tomorrow for a nitrogen fill. I don't know that the shock pump I ordered will get here in time to resolve the issue if it doesn't make it. The shop said, 'In by 9:00, out by close of business.'

Taking it Thursday gives me some leeway on THAT, too.

Thanks for the ahsistance, folks!

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Post by Mr. Wibbens »

you ordered a shock pump?

I'll try that route next time I'm do for a service...

which was prolly about a year or two ago lol
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Post by TWMOODY »

All this talk bout the shock oil change has me wondering if the
OE shock oil has the same qualities as the OE fork oil ??
I think I remember someone on here calling it pond scum.
Before I put on the USD's I changed out the OE fork springs
and changed the oil. I was quite amazed that pond scum
would have been better than what came out of those 6 month
old forks.
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Post by canyncarvr »

Whatever is in the shock off the showroom floor, I don't think it cost anyone $30/qt.


I've been thinking..consider that the head/valving assy is quite full of holes (oil passages and such)..you clean it by maybe dousing it in spirits, blowing off the excess with compressed air. When you place it back into the shock body, those holes are not going to instantly fill with oil.

Seems reasonable to me to fill the tube with enough oil to overflow, place the rod/piston assy in the tube enough to seal the oil in, pump the rod a few times (forcing oil into those cavities), move the seal out enough to REfill with oil (leave the valving assy in oil), THEN seat/clip the assy.

..with a good number of taps with a dead-blow along the way to loosen up the bubbles.

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Post by Colorado Mike »

The pond scum thing was me, and I would have to say the shock oil must be higher quality. I got into the home-rebuild game when shortly after my KX fork swap, I noticed that my rear shock performance completely sucked compared to the front. I figured, genious that I am, the KDX shock must not be up to the task , and maybe I should get me a KX shock and shorten it up to make it work. Then I went all out and actually looked at the thing. It was totally blown out, pumping oil out around the shaft. When I drained it, my oil was dirty and black, but at least it was consistant in viscosity. Not a gloppy congealed mess of pond scum goo like my 6 month old OE forks had been filled with.

Glad your refill went well CC, If you end up having to repair a leak in the future, you might want to buy the complete seal head assy. from Race Tech or similar. getting the old seal out is a pain in the ass. Plus I think you are supposed to replace the bushing (which I didn't do), and that's all in the new seal head.
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Post by Jeb »

You can replace the bushing separately - I replaced the seal, wiper, and the bushing. Actually, you SHOULD replace the bushing because as it wears travel gets sloppy resulting in premature seal wear.

It is a little bit of a pain as opposed to replacing the whole seal head, but nothing like trying to separate the base valve from the compression cylinder on the forks . . .
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