My shock failures usually first show signs of leaking oil. Then the end caps have come off, followed by more oil loss and shock failure. I put new springs on my Jeep a few weeks ago and when I was reinstalling the rear shocks, one of them had considerably less force than the other. That is to say, it was much easier to push one shock in than the other. But it did still have resistance. So I wonder if I might be loosing the nitrogen charge first.
I did talk to someone at Bilstein, but he didn't really know what was happening. The short answer was that shock life depends on use, but for a daily driver it should should last much longer than what I have been getting out of them. He did say that winter storage might be an issue if the seals are drying up from sitting and then getting twisted out of place when the shocks are in use again if everything is dry. He didn't really have an answer. He kind of just went along with what I was speculating might be happening. So the answer that Dave has up above is as good of a guess as anything.
I realize shocks are a disposable item, but 5160s are $200+/- each for a new set. I was hoping to get more than 20,000 miles out of an $800 set of shocks. I was considering getting some Fox but they are even more expensive a at $1120 for a set of 4 similar remote resi shocks. And I still have the possible risk of having issues over winter. Not to mention I don't know how they will ride. Although Fox can be custom tuned and people seem to really like them.
Since my 5160s can be rebuilt for $120 each plus shipping, I might just go that route. But even at $500+/- for the set, it adds up quick if I need to do this every few winters. I just cant figure it out. A friend got close to 100,000 on his AEV bilsteins that he drove every day summer and winter. So my thought was storage and lack of lubrication. But I also threw on some old Ranchos as temporary fix when my front shock failed and they worked fine after sitting around for 5+/- years. So why didn't winter storage affect them? Twintube vs Monotube?
Anyway, I'm open to hear any ideas or comments on how Bilstien shocks have lasted for others. Thanks for sharing