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Old 02-24-2014, 02:05 PM
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Default Twin Chamber Forks - Inner Cartridge Damping Rod

     

So, I am rebuilding my Twin Chamber forks and I have cleaned and inspected all the components. I poured the recommended amount of oil into the inner cartridge and stroked the damping rod to purge all of the air. After I was satisfied that all of the air was gone, I installed the compression assembly and compressed the rod all of the way to purge any excess oil, which it did.

The final test is to compress the damping rod completely and then lay the inner cartridge on its side and verify that the rod extends completely by pulling the rod and making sure that it did indeed max out.

My damping rods on both fork legs will extend about 90% in a few seconds and then the last 10% takes several minutes. If you wait a few minutes, you can pull the rod and it is indeed at the end of its travel.

Is this normal or is there an issue?
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Old 02-24-2014, 02:43 PM
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I had this exact problem and question in the past. I'll be darned if I can remember the answer. It seems like I re-bled mine and got better results after that.

I'm going to search and see what I can find. It's been years.
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Old 02-24-2014, 02:51 PM
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That's how mine wound up. I tried bleeding twice to make sure and there was no change, so I figured it was normal.
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Old 02-24-2014, 02:51 PM
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It sounds like the amount of oil put in was too much. In most cases there is a range of oil to be put in them. I almost always gravitate toward using almost the lowest amount. If too much oil is used it has no place to go.

Paw Paw
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Old 02-24-2014, 02:53 PM
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Yes it is an issue. You probably have oil getting past the free piston seal. The rod should extend fully and consistantly. Did you happen to check it before disassembly?

You can't put too much oil in the inner chamber. It needs to be 100% full of oil.
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Old 02-24-2014, 03:03 PM
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The more I think about it, the more I think I re-bled and my technique got better and I was therefore able to get it to fully extend on it's own in a matter of 30 seconds or so.

---------- Post added at 02:03 PM ---------- Previous post was at 02:02 PM ----------

Oh and I agree. The way these work, you can't put too much oil in the inner chamber.
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Old 02-24-2014, 03:38 PM
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I installed all brand new O-Rings on the compression assembly, so I don't understand. I also added the exact amount of oil that was called out for and noticed that as long as the oil exceeds the top of the opening, the excess will bleed out through the blow-by valve.

So, what do I need to do, then? Are you referring to the piston inside the chamber? My manual had no procedure and did not recommend dissassembly to that point.

---------- Post added at 03:38 PM ---------- Previous post was at 03:24 PM ----------

Okay, it looks like I may be able to remove the damping rod by taking off the locknut on the bottom and then pushing the rod back up through the opening. I'm not 100% sure on my model (1995 RM 125 - SHOWA Twin-Tube Forks) but I saw it done to some KYB forks on YouTube.

I did notice that the damping rod was very "notchy" when first bleeding, as in me pulling fairly forcefully and it not moving and then it would "jump" a distance and repeat.

As far as that goes, the notchy-ness is gone, now but that last bit is concerning.

Here's a video with my issue EXAGGERATED big-time. Mine comes out a lot farther than this one (about 90% before slowing WAY down):

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Old 02-24-2014, 04:04 PM
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Quote:
Originally Posted by Woody_393 View Post
The more I think about it, the more I think I re-bled and my technique got better and I was therefore able to get it to fully extend on it's own in a matter of 30 seconds or so.

---------- Post added at 02:03 PM ---------- Previous post was at 02:02 PM ----------

Oh and I agree. The way these work, you can't put too much oil in the inner chamber.
I guess I should have known that this was a "certain bike" without it being stated. Some "Twin Chamber" can have too much oil put in them and works like I said. From now own if the question does not include the bike we are asking the question about, I will ask that before attempting to help.

Paw Paw
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Old 02-24-2014, 04:15 PM
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@PawPaw: Dude, don't sweat it. No harm, no foul.

@Gusco: So, how did your forks perform after you installed them like that?

I would think if the cartridge seal on the bottom was worn out, wouldn't oil be leaking past it down onto the damping rod?
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Old 02-24-2014, 04:21 PM
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acts like there is air in it. one way you get pressure and then when it extends to the point that pressure dies and it is pulled the rest of the way it creates a vacuum and draws the rod back to equilibrium.
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Old 02-24-2014, 04:22 PM
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When you pull the damping rod out the last little bit, does it want to pull back in? If it does your seal 351mustanger is talking about. If not then it could be needed to bleed it again. I try to do that step really slow so I minimize the chance of air getting in too.
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Old 02-24-2014, 04:25 PM
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Okay, I found this thread that shows the issue of the rod sucking back in and need of a free-piston seal:

Left and right fork feel different...

No, mine does not do this. It does not suck back up and it does not leak oil down the rod.

I guess I will try to bleed them again more carefully this time.
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Old 02-24-2014, 04:39 PM
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Definitely try bleeding it again. If it is in fact the free piston seal (in the cap assembly), it will get worse and pull the rod back as nooner mentioned. There will be no visable oozing of oil with this type of leak, as it is internal. The lower seal you mentioned will show oil running down the rod. Regardless of a leaky seal or insufficient bleeding, it should be adressed. Obviously try the cheapest and easiest solution first.
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Old 02-24-2014, 04:46 PM
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Mustang, I have only done the operation mentioned two times. Neither time did I experience the pull back. I did get some "skips" and that was air as I worked the action in and out while filling. I filled mine as full as I could get it working it in a pan full of oil as I went (similar to the way i do shocks). when you mention the cap seal; is that leaking air to make that pull back condition?
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Old 02-24-2014, 09:15 PM
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The leaking free seal lets some of the oil escape up into the cap area. Creating similar to a vacuum inside the chamber. Whenever I have had air in the inner chamber I have experienced the "skip" that you mention. The rod moves slow and steady while the oil is flowing through the valving, and as soon as the air pocket passes through it gets a quick movement, then back to slow and steady when the air pocket has passed. I like to leave mine sit for at least 30 minutes after the bleeding process, to let alot of the micro bubbles surface. If you shine a flashlight in the inner chamber after bleeding, you will see the tiny air bubbles still in the oil. Randog, you could try compressing the rod for a short period of time and see if the condition gets worse, and check for oil leaks. You could also try switching the caps (base valve assemblies) and see if the condition is transfered to the other fork. If the condition follows the cap, it is most likely the free seal.

Just re-read and saw both forks are doing this. Switching caps will not help. If the rod is not getting sucked back in, the free seal is ok. Maybe the springs are getting a little sacked out.

Last edited by 351mustanger; 02-25-2014 at 08:43 AM.
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Old 02-24-2014, 09:52 PM
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Quote:
Originally Posted by Randog Leader View Post
@PawPaw: Dude, don't sweat it. No harm, no foul.

@Gusco: So, how did your forks perform after you installed them like that?

I would think if the cartridge seal on the bottom was worn out, wouldn't oil be leaking past it down onto the damping rod?
Well. They held up with some of my terrible landings due to my "dainty" 200+lb frame and rode smooth. Albeit, I dont have much to compare them to with this being my first dirtbike. Mine had about a 1/2 inch that wouldn't come out.
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Old 02-25-2014, 09:11 AM
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Update: So I re-bled one of the cartridges. I was very diligent this time and took my time. At one point, I thought that my shop light had dropped some dust into the chamber as I saw thousands of tiny particles floating in the oil. To much surprise, I discovered they were bubbles of air! So, I pumped and tapped, pumped and tapped, for an hour and then left it overnight. This morning, I could stroke it and NO bubbles at all. I even left the compression assembly soaking in fork oil. I then capped it and compressed the damping rod to purge the excess oil. The rod shot back out about 99% before slowing to a crawl to that last bit. Success!

Not so fast. After leaving it alone for about thirty minutes, I tested it again and here's what it is doing:

The damping rod extends about 13 inches total. So when I compress the rod, I hear a slight "squish" sound from the compression assembly and then the rod shoots out to about 8 inches and then slows a bit to about 11 inches and then the last 2 inches take about 30 seconds to complete.

It does not suck back in and does indeed extend fully. Am I there yet or is this still an issue?

When compared to the other cartridge, this newly bled one is much better and extends much faster. I am wondering if it will worsen as it sits.
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Old 02-26-2014, 09:07 AM
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After the cartridge has sat overnight. It is "stable" and there is no fluid leaking anywhere. It is still better than the other cartridge and the damping rod extends fully. It's just a bit slow on the last two inches. I would estimate that it takes about thirty seconds for it to extend all of the way. I am in the process of bleeding the other cartridge, now.

I am leaning towards the theory that my spring in the compression assembly may be sacked out. These are from a 95 RM 125 and I don't know where I could get new springs.

How exactly would this affect the performance if left "as is"?
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Old 02-27-2014, 09:35 AM
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Update: The forks are finished! After bleeding the cartridges THOROUGHLY, the damping rods return at the same rate. I did some research and found that it is normal for the damping rod to slow way down for the last inch or two, due to the fact that there is little pressure at the end of travel.

So, to anyone reading this thread that is having similar issues, I say:

BLEED, BLEED, and BLEED some more. Tap the sides of the cartridge. Let it sit overnight. NO BUBBLES!!

I am using Maxima Fork Oil - 5W

There are probably higher quality brands of fork oil that do not require so much bleeding but I chose Maxima. Whatever brand you use, just make sure that you can stroke the rod and see ZERO bubbles come up.
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Old 02-28-2014, 03:51 PM
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One more post for the record:

This is from JB Covington, who has a nice site that sells parts for your suspension:

Randy,
Thanks for the email. My apologies I did not respond sooner.
Likely the seals are not sealing at 100% anymore. However the fact that your cartridge does not leak and does extend all the way says that it is functioning well.
The reason the return of the rod slows down is because the pressure spring force is decreasing as the rod leaves the cartridge. Factor in friction and the return will be even slower.
Nothing to worry about dude. The fork sounds good.
JB Covington
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