This is actually very limited advice on bike repair. But first, the background.
Three weeks ago, I bought a fixed gear bike on Craigslist. I had been eying the offerings for a week already, and then I saw three postings on Saturday, and went out to see them. Given my eagerness to get into the sport, I didn’t want to have a delay and not get the bike that week, so the first one I saw seemed pretty good and I got it. I didn’t have too much qualms about it–probably a mistake, since I knew little about bikes and nothing but what I had read about fixed gears–because the owner seemed to be really into the sport, had the whole thing nicely styled, and seemed very interested in showing it to me. I figured if I didn’t like it, I could turn around and sell it right away, since it was at the bottom of the fixed gear bike market.
It was great. A conversion from and old bike dating to the 1970s it seemed, the parts were mostly original (a strike against, since they wear out). I didn’t really notice, since my bike at home is also mostly original, and works just fine; I’m not really one to pay for quality I can’t detect. It was fun riding around town at all hours in all places.
However, I started riding the bike to work, and then the problems started. Literally 500 feet out of the train station, I turned a corner after slowing down, and I heard a clink and then half a second later, the chain fell off. In case you don’t know anything about fixed gears, the chain should never, ever fall off. It should have no slack in it, and the only way for it to physically leave the cogs between which it is tensioned is for the mechanism to suffer serious mechanical failure. I pulled over and noticed a bolt had fallen out of the chain ring, which is the front gear. That was the clank I had heard.
No problem, I used the wrench in my flat-fix kit to reattach it. However, I noticed that the ring itself was bent, and it kicked the chain right off when I turned the crank. What happened was in the course of falling out, the bolt and chain applied extreme torque to the chain ring and put a lateral bump in it. I went back to the bike store by the train station, but it was closed. I asked a passerby for some help to get it back into ride-able condition; he lived a couple blocks away and I used his hammer to straighten out the kink as much as I could. I was back on the road, and proud of my improvisation.
No dice. Later, I noticed the bolts were coming loose from only a few miles of riding. Literally, they stuck up several millimeters having started out flush and tight. I took the bike into a shop, and when the bolts were lock-tite’d in, the bend reappeared. I should have replaced the chain ring almost immediately; instead, by waiting I got the bolts refastened and the misshape bent the entire crank, broke the chain on the bike, and unwound the rear cog (that might have been sloppiness on the part of the mechanic I went to as well.)
In the course of having this problem checked out, I discovered from the mechanic that the wheels were totally trash. The hubs were so decrepit that they wobbled and wasted enormous energy; going down the street became a very vibrating affair. I resolved to buy a new wheel, not yet realizing that the chain ring was a serious problem.
The first mechanic I went to picked up on most of the problems, but definitely was not knowledgeable about fixed gear bikes; I blame him for removing and then incorrectly reattaching the (converted) cog on the back, which then flew right off the wheel the following day.
The second mechanic I went to got my repair work. Although he was more knowledgeable, he wasn’t really an expert in the subject of fixed gears, or he didn’t care enough about his job to do it properly (I think it’s the former given our later conversations). We agreed he would replace every moving part except the pedals and the rear cog: the wheels, the chain, the crank and chain ring, and the bottom bracket. When I picked it up a week later, I found that he had changed the mechanical gain from 5.2 to 5.8, which may not sound like much, but man it was killer, and I detected it immediately (on sight, and then from riding it). He gave me a longer crank (with which I had a pedal strike almost immediately) and a much larger chain ring. I started with 40 teeth in front and 16 in back, 27″ wheels and 165 mm crank, and ended with 46 teeth in front, 16 in back, with 700c (680 mm diameter) wheels and 170 mm.
Not wanting to wait for a new order of mystery at the shop, nor to miss another weekend of riding, I went myself down to Harris Cyclery. I made up a spreadsheet with all the gains on it, and found that I would be pretty much in the same spot I started in if I went with a 42×16 and 165 mm. I found the mechanic who helped me extremely helpful and knowledgeable; he dropped what he was doing (forcefully!) and was willing to listen to me, offer his opinion and guidance, give me the choices, and not make me feel clueless (which I mostly was).
Finally, I dropped the new parts off at the shop that still had my bike, and they changed out the parts. I went for a ride on it, and damn it is smooth. The gain feels the same now with the smaller chain ring and cranks, no anxiety in turns as with the 170’s, and smooth as a whistle (?). I am extremely excited to ride this puppy around, and I doubt if you’ll really see me tomorrow as a result except by coincidence as I ride up and down the street. 
Anyway, the moral of the story is twofold. First, whenever you hire someone to do work, you must be proactive. Don’t be afraid to ask questions, sound stupid, and deliberate. Don’t put yourself in the situation of being in such a time crunch and with so little preparation that you’re at their mercy. This goes from buying the bike — I should have been more patient, and more knowledgeable — to getting the work done. Don’t feel like you owe them anything; it’s strictly a business arrangement.
Second, only have fixed gear work done at a shop known for fixed gears. A regular bike shop won’t do it justice.
Third, and this isn’t much of a moral, but bikes can really add up quickly in cost.
