When I was a young Army Officer, I was responsible for a rather complicated tracked vehicle known as an M577 Command Post. It was important to understand the thousands of moving bits and pieces in this contraption and I quickly found there were two major ways that I would be learning about the machine:
1. By performing regular preventive maintenance on it.
2. By asking questions when it inevitably broke.
You can learn a great deal about a vehicle by checking the fluids, looking at the tracks, cleaning everything, etc… but some items just don’t come up in conversation until they break, such as “Sir, we’re pretty much screwed here. The U-Joint is blown.”
“Uh, what’s a U-Joint?” I would sagely ask in the manner of Lieutenants everywhere. My sergeant would patiently suck in his breath in the manner of NCOs everywhere and explain the inner workings of my vehicle’s drive shaft, bringing us back to his original point that we were pretty much screwed.
The same is true with bicycles. I’ve learned a fair amount by regularly cleaning and maintaining my bike, but some things just don’t come up until they break. Things like free wheels, free hubs, and their relationship to dork disks.
I broke yet another spoke on my hybrid a few weeks ago and I finally got around to getting it into the shop this week. I watched the mechanic struggle with installing a new spoke, thereby confirming my intuition that I do not want to take on that sort of project just yet. Eventually he made the repair, which allowed me to ask him a completely unrelated maintenance question.
Lately, my cogs (those small black wheels near the derailleur, pictured above) had taken on the unfortunate habit of moving forward whenever I was coasting at high speed. The forward movement caused the chain to become extremely loose, to the point of actually touching the ground. I think we can all agree that is not a good thing to have happen on your bike as you fly downhill. Liberal – sometimes massive – applications of oil had only a minimal effect at stopping this motion.
Several mechanics listened to my story and a younger one said, “Sounds like the freehub is shot.”
It turns out a freehub is a nifty upgrade from a traditional freewheel. Both devices allow the bike’s wheel to rotate while keeping the pedals still, but the freehub wears out less quickly, is easier to remove, and has fewer axle breaks. It is one of the dozens of small upgrades that make a quality bicycle in a bike shop (even the cheapest ones) cost a few hundred dollars more than the one you will find in Wal-Mart. If the freehub was shot, then it would cause the chain to move even if I wasn’t pedaling. If you want to know more, click the link in this paragraph. For purposes of this story, we can move on.
“Nah,” said one of the older mechanics. “It’s the dork disk. It wasn’t attached properly to the spokes.”
“Ah…” said the other mechanics.
“?” I said to myself.
I’ve written before about my dork disk. It is a clear plastic disk situated between the chain and the spokes of the rear wheel. It’s purpose is to prevent the chain – in the unfortunate event of it coming off the inner portion of the rear cassette – from flying between the spokes of the rear wheel and thus destroying the wheel and possibly the bike and/or its rider. Until this moment, I knew of no negative aspects of the dork disk, other than it being an indication to super cool roadies that I was a dork. But now there was this.
As it turns out, the dork disk is attached to the spokes of my rear wheel by four plastic clamps. The disk had worked itself free of two of these clamps and was therefore placing outward pressure on the rear cassette. This pressure was limiting the ability of the freehub to do its job. The disk, still attached to the spokes by two clamps, was rubbing against the cassette as the wheel quickly spun on descents. Some of this motion was transferred to the cassette, which forced the chain to move forward. Except my pedals weren’t moving (I was coasting, remember?), meaning the crank wasn’t turning the chain in the front. The chain had nowhere to go but dangle, taking the cogs forward as far as they could go in the process.
“Well, I’ll be darned,” I said and I thanked the mechanic for solving the problem for me.
And thus concluded “The Mystery Of The Forward Moving Cogs.” I can happily report that with my dork disk properly attached once more, my chain is behaving properly and the problem is solved.
Stay tuned for our next maintenance problem, “The Curious Case Of The Grinding Noise In My Road Bike’s Bottom Bracket.”