Shock Piston Failure - 2010 FE390

JohnnyQuest · Jan 27, 2011

I dropped my shock off at the shop to have the oil changed. I’m told that when he opened the valve to release the nitrogen, a geyser of oil shot out. He said that the oil and nitrogen were completely mixed and that he had never seen that before. The piston was plastic and a bit deformed. The O-ring was ok. He replaced the piston with an aluminum one. Has anyone else had that experience?

JohnnyQuest · Jan 29, 2011

Ok, here's more:
I reinstalled the shock and forks today. I figured I'd check the rear sag just to make sure the new piston didn't change anything. I was shocked (pun!) to see that my static sag was 73mm and my race sag was 133mm!

And no, I haven't put on weight. The bike has 42 hours on it. I set the sag originally when it had 2 hours on it and never checked it since. Could things have changed that dramatically in those 40 hours or is this huge difference due to that piston failure (or something else)? Maybe the piston let the oil and gas mix from the very beginning, but thought that sag should be dependant on the spring alone.

What else is strange is that I needed to tighten up the preload (compress the spring) to the limit in order to get the proper sag. I was able to get the static sag to 35mm and the race sag to 108, but I tightened it up almost to the bottom of the threads.

I only weigh 150lb with my gear on, and that's what I weighed 40 ride hours ago. I have the stock 72 spring on it as I always have. It seems very strange that I needed to tighten it up that much for that spring when I'm at the very low end of what the manual says is the range for the lighter spring. When I originally set the sag, I barely had to put any preload on the spring.

What do you suppose is going on? I can't test ride it since we have about 12" of snow on the ground. Just bouncing it up and down everything seems fine.

jcolleymx · Jan 29, 2011

Are you saying it only sank 60mm when you sat on it? Your sag figures now are good, but how many mm of preload did you add? It is going to be off when you get it back from being worked on and will need reset.

JohnnyQuest · Jan 29, 2011

jcolleymx said:
Are you saying it only sank 60mm when you sat on it? Your sag figures now are good, but how many mm of preload did you add? It is going to be off when you get it back from being worked on and will need reset.

I'm saying that from the unloaded (wheel-off-the-ground) position it sank 133mm (5 1/4") when I sat on it (race sag). Under just its own weight (static sag), it sank 73mm (2 7/8"). In order to get those numbers smaller, I needed to add a LOT of preload. I had to turn the adjuster ring down to near the bottom of the threads in order to get the static sag to 25 mm (1 3/8") and the race sag to 108mm (4 1/4"). I find that very strange since I only weigh 150lbs (68 kg) and have the stock 72 spring.

jcolleymx · Feb 12, 2011

You don't want 25mm static sag, you want 35 - 40. The stock spring should be right for your weight. Where did I read 60mm - did you edit your post? The WP shocks show alot of thread and your mechanic may have just slightly snugged the nut knowing you would have to reset it. The clickers do make a little difference, so make sure they are at stock settings the first time you set the sag after your rebuild.

popup · Feb 13, 2011

if it's sagging more now than before, & the spring & preload is the same .it must have been regassed at a lower pressure.
what is the pressure now that it's regased?

DaleEO · Feb 13, 2011

My following verbiage is based on what you had to say in your post Johnny, so forgive me if I have made some incorrect assumptions. And, respectfully submitted, the title of your post should be something like, shock reservoir piston failure.

1. If the shop you dropped it off at to get the oil changed does not have a spring compressor, then they had to back off the pre load rings to get the spring off to service the shock. And, if they didn't measure the spring length before they took the spring off then there is a high degree of probability the shop tech did not re set the sag correctly. When you get your shock serviced there should be no change in the pre load setting, or in the sag numbers when the shock is serviced correctly, all things being equal, but, in this case they were not according to the shop.

2. Keeping in mind that you weigh 150 and you have the 72 spring, you should be good. I suggest taking the shock off and measure the spring where it's at now. I'm guessing it to be around 240mm's, or at least it should be.

3. Then remove all the pre-load on the spring and measure the spring length to see how many mm's you are actually pre-loading the spring. The spring should be around 250mm's in length with no pre load on it. It is not unheard of for springs to collapse, although it is rare these days.

4. To get the static number you are talking about, there should be about 10mm of spring pre load with that spring.

Please let us know what you find.

Hope this helps,

Dale

JohnnyQuest · Feb 19, 2011

I talked to the shop. They agreed that it was strange so they took it apart again and checked everything. It lost no nitrogen pressure and everything checked out. I put it back on the bike and again I had to tighten the ring nearly to the lower limit. After I got the sag right (35 static and 108 race) the spring measures 238mm without load.

Therefore, I should be ok. The only thing I can conclude is that the problem with the shock piston and the mixed nitrogen and oil occurred at initial assembly. When I bought the bike (new) I set the sag after a couple of hours and the preload was very different than where it is now. Back then, I barely had to put any preload into the spring to get the sag correct.

Since there is still snow on the ground, I can't properly test it, but I bet it will feel/handle a lot different now.

Thanks for your help.

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