I recently bought an older Cannondale mountain bike and the rear hub went on it so I started taking it apart and never stopped. Now it's all apart (the entire bike, not just the hub), sanded down to aluminum and I'm thinking that I want it to have a flip flop rear wheel. I primarily ride trails for my health and a little bit off road and a little bit in my hilly little city which brings me to my question(s).
First, I found that when I'm riding distances around 30 miles (or longer), I was very much comfortable riding without stopping using the 42 in the front and the 14 in the back. If one side of my flip flop is fixed, will having a 48 in the front and a 16 in the back be the same as having a 42 in the front and 14 in the back? I'm thinking that it would actually be more efficient if I get into a good cadence. That said, I would like the freewheel side to have 20 or 24 in the back because I'm not all that concerned about going incredibly fast when I'm in the woods unless I'm going downhill. Okay, there's that part of the questioning. Does this make sense? I've never really considered these things until just now.
Second, so that I don't need a chain tensioner, is the Eno Eccentric Hub that keeps popping up online something that I can even do this with? From what I can tell, it seems like it is, but I'd like someone to just say "Yeah, that's totally doable" before I go and spend the money. So what I'm asking is: good idea? bad idea? is there a cheaper idea? any recommendations on whom to go see in person and discuss this with when I'm ready to buy? I hope I'm not coming off as too idiotic. I really am new to this stuff.
The ENO hub is your only choice to do what you're proposing. There are some other esoteric ways to do this, eccentric BB converters, older Bullseye systems... but you want the ENO.
In order to have a tooth difference like you're proposing side to side you'll likely need to use two masterlinks and opo a section of chain in/out for the larger/small size. I'm not sure how much adjustment in chain length the ENO hub allows, but it's not 6 teeth.
What Brad said, and maybe think about running two rings up front. The Eno might give you 2 teeth if you are luck, not really enough for road and off road gear.
Something like 42-14 and 36-20 might work. Both add up to 56 teeth, so your chain length will effectively be the same.
Now when yinz are talking about running two rings up front, you don't mean with a derailleur, do you? Would I physically move the chain when I flip the rear wheel? If that's the case, I assume I wouldn't be able to run the 42-20 or the 36-14 then without adjusting the chain length. I really do appreciate this help. Most of what I have been able to figure out has been from reading product specs and the Sheldon Brown site, of course.
Here's another question: If I run the 42-14, will I wear out the teeth and chains much faster than I would with a 48-16? I'm guessing that having more teeth in contact with the chain at all times will naturally result in less wear and tear on the parts. Is that even a concern given that I'm only riding about 100 miles at about a 9 or 10 MPH pace per week on average?
All this drooling over parts online makes me feel like I'm 8 and the Sears Wishbook just came in the mail with all the new G.I. Joe guys in it.
Nope, not with a derailleur. If you pick the gear ratios carefully you can switch between two very different ratios without breaking the chain. Just use two rings up front with two cogs in back where the difference between each is the same, or within a tooth.
42/14 and 36/20 should work with just about the same length chain. You may have to put a BB space or behind the 14t freewheel to move the chainline out a bit to account for the 42t outter chainring location, but chainline is a whole different topic. Just visualize that you need to keep the chain straight, at that with two freewheels on opposite sides of a symetrical hub only one of the two proposed front rings will be in plane.
Don't worry about the potential chain wear difference between the two. Some people on the internet like to yell about such things, but it doesn't amount to much more than that.
Okay, last question for a while and I should be on the right track. When you mention the BB space, that's a spacer, not a longer spindle (Is that the term?) on the BB, right? I was just looking them up and I'm seeing Shimano BB's in 68 x 107 and up. Is that 107 (for example) indicative of the cranks that I need or will that move my chainline out for each mm longer that it is?
I feel dumb asking all these questions, but the last bike that I owned was a Schwinn that I bought in a bike shop in Johnstown in 1990 or so and I swear that thing never broke or needed any real maintenance for the entire time I had it. I guess I was kinda lucky with that one. I beat the hell out of it.
You need to match the BB spindle length with the one recommended for the crank you have. 107mm refers to the length of the BB spindle. Look up whatever crankset you have with the manufacturer, Sheldon's site or a decent shop and buy the correct length BB for it. You can get really nerdy figuring out chainline measurements for all this stuff, but it may drive you crazy. You might want to stick with trial and error.
Because I am shameless, here's a brief piece from a few months back on chainline: http://www.urbanvelo.org/issue13/urbanvelo13_p84-85.html
I mentioned the BB spacer as a way to move the freewheel outboard, as BB spacers fit on the freewheel threads of a rear hub. This was in the context of... assuming you go the two chainring route, you may need to "force" the freewheel matched for the second, outside ring outboard by a millimeter or two from the hub to keep the chain straight.
Another thought: the ENO hub is expensive enough that you might well be able to find another frame with horizontal dropouts for less money.
have two bikes is not a bad thing either, singlespeed beaters are fun!
Again, Brad's got it covered.
Only thing I'd add is to consider running a geared chain rather than one of the singlespeed variety. Geared chains are designed to have a bit more side to side flex to handle the range of angles between the front sprockets and cassette on a multi-speed drivetrain. Using one on your not-quite-perfect-chainline double single speed might help keep everything happy with a bit of misalignment between your front and rear gears.
Or not, it probably makes little difference, but it always made me feel better when experimenting with things like this.
Thanks guys, this has been extremely helpful to me because honestly, this drivetrain business has been the most confusing part of the whole process.
Alankhg, I looked at that route, but I'm in the market for a new wheelset (or an attempt at building one) anyway, so I figured I'd give this a shot. It's helping me learn a good bit as well.
Once this is over and done with (hopefully by November), I'll have this bike as my rail-trail beater/trainer through next summer while I save to get something new when the 2010's go on sale later next year.
Eric, thanks for the geared versus singlespeed chain advice. That was seriously going to be my next question!
I got more info from this message board in a couple days than just about anywhere else online. You guys just inspired me to become a member next payday so that this can continue. If you're members, give yourselves a pat on the back.
My fixie could use some chainline love (could have used it 5 years ago 
and I've always wondered if BB spindles were symmetrical. From sheldon's site:
Change the Bottom Bracket
Most bottom brackets made since the mid-'90s are symmetrical, they stick out the same amount on each side. If you replace your present bottom bracket with one that is, say, 4 mm shorter, it will move the chainline 2 mm to the left, because it will be 2 mm shorter on each side.
If you go this route to a narrower chainline, make sure that there won't be a clearance problem between the frame and the crank or chainring.
and I can further deduce that the right edge of the spindle will be half the spindle length out from the center of the tube.
