This answer could get really into the weeds with trying to describe each adjustable feature of any particular single speed frame. So, I am going to assume that you will have researched the "how you adjust" your particular drop out or eccentric bottom bracket. I will focus on how to know when your chain is adjusted properly instead. This seems to me to be the thing that trips up people the most, not how their adjustments work. In other words, it's all about chain tension and how you determine what is "right".
First, you'll need to figure out how long your chain needs to be. A chain tool to help aid you making the correct adjustment is all you'll need here for tools. I recommend getting something like THIS or THIS. These chain tools will allow you to loosen a stiff link. (See THIS video to learn how to do this)
To determine chain length and to allow for a bit of adjustability, move your rear wheel, (or eccentric bottom bracket) so that it sits in about the mid-point of adjustability. You could move the wheel forward a bit or backward a bit from this point. (Or the bottom bracket if eccentric) Okay, now lace your chain, (I recommend a SRAM PC830 for most single speed applications), and you should find that you have far too much chain. Bring the chain together, overlapping the ends, until you see about where the ends would have to be to meet to make things work out on the bike.
You may not be able to make a perfect fit where your wheel is at, but remember, you can go and adjust for that. Keep in mind the a pair of inner plates has to mate with a pair of outer plates. (I typically stay away from quick links on a single speed, by the way.) Note how much of the chain will have to be removed. Err on the side of "too long", if you are not sure. Then using your chain tool, push out a link's pin, but juuuust enough that it doesn't go all the way through the last outer plate. (See images below) You want to be able to push that pin back through here in a bit. Okay, now with your predetermined amount of chain removed, double check the length. Hopefully you are right in the ball park, but if not you can shorten it up a link or two here. Make sure you end up with one exposed inner set of plates, and one outer set at the other end of the chain. Not two sets of outers, or vice versa!
Then join the two ends as they should mate together. If your pin sticking out of the outer plate is protruding slightly on the inside of that plate, this may help you hold the chain in place a bit as you reconnect the loose ends. (Again, please refer to the images below for reference)
|Once you've determined an approximate chain length, push out the pin which will cut the chain at the spot you've chosen|
|Don't push that pin all the way out. You want it to look like this when you are done. Clean up any extraneous flashing and bits of metal. |
|The opposite, inner pair of plates at the other end of the chain now can be "snapped" into place. It may take a little effort to do this. Also- Make sure the chain is laced through the frame when doing this! |
|Now carefully drive that pin back into the chain. You will feel more resistance when the pin starts to go through the outer plate. See next image.....|
|Ideally, you want to push that pin juuuust a bit too far. See orange arrow. |
|Note the position of the chain in the chain tool. Now gently push that pin that sticks out a hair and this will free up the stiff link you created when putting the chain together. |
Now you've hopefully come pretty close to the ideal chain length, but things are a bit slack, right? This is where you adjust the chain with whatever system you have on your bike. But how tight should you make the chain? There is a trick to doing this.
Not all things that look "round" are actually "round". What I mean by this is that chain rings and cogs often are not perfectly round. They will usually be, but not in every instance, somewhat elliptical in shape, although we cannot see that with the naked eye.
|Checking for chain deflection to help adjust chain tension.|
If you adjust your tensioning system to where the chain has a bit of tension on it, stop there and then rotate the crank about a quarter of a turn. Now, finding a spot in the chain midway between each gear, use your finger and deflect the chain downward. If you cannot push it down a half inch, the chain is too tight. Loosen your adjustment until you get that amount of deflection, but now rotate the crank a 1/4 turn again and check. Maybe now you are seeing 3/4's of an inch deflection. Okay...,.go another 1/4 turn. If you get back to 1/2 inch, you are good. But let's say that you see an inch of deflection at that point after the first adjustment. Go back on your adjustment and then continue. Essentially, what you want to do is to adjust for that 1/2 inch of deflection wherever the chain is "tightest" during the crank's rotation.
A chain that is too tight will potentially cause accelerated wear, be resistant to rotation, robbing you of power, and is more susceptible to breaking under stress. Too loose and the chain is apt to jump off the cogs at an inopportune time, which may cause a serious crash and injury.
|Here you can see various spacers used to align the rear cog with the chain ring on a geared free hub body.|
Chain Line is VERY important with a single speed set up. Chain line is the "line" a chain makes from the front chain ring to the rear cog. If this "line" deflects inward or outward from the chain ring as it goes to the rear cog, you have a BIG problem! Ideally you want this set of components to all be in the same plane. No chain deflection at all. If you are using a geared rear wheel to mount a single speed cog on, you'll need to set up spacers to align the cog with the front chain ring to achieve a straight chain line.
If your chain line is off, meaning that the front chain ring and rear cog are not on the same plane, you will get noises, popping, and you may even have the chain jump off the rear cog. this can result in a serious accident and injury. So, pay attention to chain line!
A single speed specific hub is generally set to a certain chain line, and at that point you'd need to adjust your chain ring. You might be able to do that by mounting the chain ring on either the front of the chain ring mounting tabs or behind them. You may be stuck in that regard depending upon the type of crankset you chose. You may also be able to do some adjusting with the bottom bracket using spacers and shims. See your local bike shop if this gets to be an issue with your single speed set up.
That'll do it for today. Tomorrow we will talk about crank length and how that can affect your single speed success or failure. I will also touch upon gearing and how to choose a good place to start out.