Air piston shaft with negative/top-out spring in left leg, damper
cartridge in right leg.
Upon first inspection, everything looked perfect. The fluids were pristine clear. The fork is barely two years old. It should still be pretty clean inside. It held air impeccably too, adding air maybe once this summer. The damper still seemed to be working correctly. Nothing visibly was wrong with it. There was plenty of shock oil in the damper leg too.
The other leg has much simpler inners. It basically is a plunger for the air spring and a negative spring/top-out spring combo. Being an air fork, air pressure in the left leg suspends the fork. When you hit a bump, the air compresses as the fork collapses. When the wheel clears the bump, the compressed air extends the fork again. The damper in the right leg makes sure it doesn't extend too quickly, and a small top-out coil spring in the left leg prevents the fork from extending all the way out against its mechanical stop with a thud. I won't go into the subtleties of the negative spring. When I flipped the shaft over, something inside the negative spring slid down the shaft. Oh. That don't look right. The top-out spring, which resides inside the negative spring, was broken in two. That explains the harsh behavior.
Look carefully. The inner top-out spring is broken.
I went to the web to find a replacement spring. Nothing. In fact, I could not find any Fox replacement parts from my normal list of online sources. I dreaded having to go through a shop, or worse yet to have to send my whole fork in for service, losing weeks and wads of cash. It was an experience with a LBS over 10 years ago that convinced me to do 100% of my own bicycle maintenance, whether it be changing a drivetrain, a new bike build, building or fixing wheels, or even repairing suspension components. Ten years ago I brought a bike in with a fork problem, still under warranty. After a couple weeks, it was ready. As I rolled it out of the store, I pushed down on the fork to see how it felt. It went down and locked up solid. I was still without a bike. That was back when I basically had only one decent bike.
The Fox website had a toll free service number. I called them on Monday. Turns out Fox directly services all their suspension components, whether sending items in for repair or ordering replacement parts. Also, I learned that the top-out spring is integral to the shaft assembly, so I would have to order the whole thing. I was pleasantly surprised to learn it was only $42.50 and in stock. The MSRP on the fork was around $700. I should have it in a couple days. I keep things like various weights of suspension oils on hand, both for tuning purposes and in case I have to make a repair. Should only take about 30 minutes to put the fork back together again when the part arrives. With the Wicked Ride of the East coming up soon, I need my dualie back in service. Harold Parker State Forest seems to beat me to a pulp every time I ride there. I need all the pampering I can get.
I can only speculate why this spring broke. Seems like an unusual failure to me. I do tend to run my forks fairly firm with quick rebound. Couple this with lots of root garden riding, that top-out spring could take a beating. Perhaps I'll run the fork a little more plush from now on. It is only 100mm travel. Most guys on dualies these days have 150mm travel front and back. Set up plush, it is easy to blow through 100mm of travel and makes the bike dive more when rolling off of things. A plush fork also robs more output when mashing out of the saddle, and this bugs me more than anything else.