Last summer, something funny happened: The bike was in fixed gear mode, and no amount of loosening the screw inside the axle would make it work as a freewheel hub. I was stuck in fixie mode. Puzzled, and willing to sacrifice the wheel to the gods of knowledge, I took it apart.** Upon disassembly, two tiny pieces of metal that had previously been one piece of metal fell out. It was what SRAM describes as the "carriage keeper key."
The Carriage Keeper Key is the Achilles' Heel
The carriage keeper key is a little bit of metal about 8mm long by 1.5mm thick, with a 1mm threaded hole drilled through it. Its job is to keep the metal ring that covers the "fixed" pawls over those pawls until the axle screw is tightened, moving the pin and the ring toward the non-drive side of the hub. I'm not going into too much detail here, but the pin is under pressure from a spring on both the drive and the non-drive sides of the hub. The drive-side spring is substantially bigger and stronger, so in a fight between the two in the absence of the pin to hold things in place, the ring is going to be pushed out of the way and the hub will be kept in its fixed-gear state.
I'm not sure when or how my carriage keeper key got broken. But I know why it broke: it's a tiny piece of metal with a hole almost as wide as it is drilled through the middle. It is, in my opinion, a matter of time before it breaks. While you could theoretically break the pin by over-tightening the screw, I know for a fact that I didn't and that it worked just fine for the first 18 months i rode the bike.
Frustrated, I put the whole thing back together minus the broken bits of the keeper key, resigned to riding fixed for a while. I spoke to a mechanic at my LBS about sourcing a replacement key to see if such a thing was possible. He put in a call to SRAM and found that such a part was not available separately. SRAM could sell me either all of the internals or an entire new hub, I forget which. But not the 50-cent piece of metal I needed to get the hub up and running again. I don't want to waste the shop's time or my own anymore, so I've just continued to ride the thing fixed, secure in the knowledge that it won't break any more than it already has and that it's highly unlikely to just start freewheeling on me.*** That big old spring on the drive side is going to keep that second set of pawls engaged for a long, long time.
As I was riding the other day though, I started thinking about what would have happened if the pin broke while I was riding, particularly while I was freewheeling downhill at speed. The hub would more or less instantly converted from free to fixed. As much as I like racing goldsprints at high RPM's, the idea of going from motionless legs to spinning at 42X17 at 40mph or better is a little scary.
I have scoured the Internet and can find no evidence that this has ever happened to anybody. But I think it could, and would recommend that somebody with one of these hubs learn to crack it open and have a look once in awhile. And if anybody from SRAM is reading this and wants to send me a new carriage keeper pin, have at it. Otherwise, I think I'm going to just rebuild this wheel around a conventional flip-flop hub.
*I have pestered them, however, lamenting that, with a separable rear triangle to accommodate a belt drive, it would be the near-perfect do-everything bike.
**I wasn't that stupid. I consulted the excellent page on the Park Tool site first and watched all the videos there. Park did a great job explaining this hub, better than I can.
***Don't do this. If you do anything like this, it is at your own risk.