Like vegetables, gears are a good thing but sometimes you just want to throw your gears away and eat chocolate instead. And just as it is difficult to explain to someone who has never eaten chocolate what an excellent idea it is, so it is hard to make a convincing case for riding singlespeed (one sprocket, one chainring, no shifters, equals one gear and therefore just one speed). You have to take our word for it – or, better still, try it yourself.
The first amusing thing about riding singlespeed is you instantly lighten your bike without spending a fortune on Space Age materials handcarved by astronomically expensive robots. The second thing is you realize how much ride time you spend faffing around changing gear. Having no distracting gears, you just ride along. Sometimes you cannot speed up because you don’t have a high enough gear, and sometimes you have to get off and push because you don’t have a low enough gear. These things happen to me on my geared bike as well. The final thing you notice is that riding is much quieter than normal. Still not convinced? That’s fine – someone has to read the ’adjusting gears’ page to make writing it worthwhile.
There are two big issues. First, what gear to choose. In the end, your personal preference wins, but as a guideline, choose something around 65 inches. Gear size is equal to chainring size over sprocket size multiplied by wheel size in inches – so a 42-tooth chainring with a 17-tooth sprocket on a 26-inch wheel gives you 42/17 x 26 = 64 inch gear – which is as good a place as any to start.
The second big issue is chain tension. The chain mustn’t slap or slip, nor can it be so tight that the pedals don’t turn freely. Normally, the tension jockey on the rear derailleur does this job. On a dedicated singlespeed bike, the dropouts (where the wheel bolts to the frame) are horizontal, so you can adjust the chain tension by moving the wheel back in the frame. Almost all geared mountain bikes have vertical dropouts, so that the wheel cannot slip forward in the frame even if it isn’t done up correctly. If you are converting your existing bike, you probably have to find a way to tension the chain. For initial experimentation, try using your old derailleur.
Fit the derailleur and run the chain through it as normal. If you can, wind in the high end-stop screw so that the top jockey wheel sits directly under the single sprocket. Often the end-stop screw won’t go in far enough because it’s not designed for this kind of use. Employ a little cunning, and use a piece of cable to hold the derailleur in place. Take a short section of gear cable, still with the nipple attached (it doesn’t have to be in particularly good condition, since it’s not going to move). Set the barrel-adjuster on the derailleur at the halfway position. Thread the cable through the barrel-adjuster so that the nipple sits in the barrel. Push the derailleur across so that the upper jockey wheel sits just under the single sprocket. Clamp the cable in place under the pinch bolt. Use the barrel-adjuster to set the position of the jockey wheel directly under the sprocket. Remove your chainset and take off the smallest and biggest rings. Refit the middle chainring and the chainset. Dedicated chain tensioners, which look like half a rear derailleur, are available to buy as well. These will do the same job a bit more neatly.
Singlespeed – simple and silent
Refit the chain
If you have vertical dropouts and are reusing the old derailleur to tension the chain, set the chain length quite short, so that the jockey wheels are at 45 degrees to the ground. If you’re using a singlespeed chain-tensioner, the instructions in the packet explain how short to make the chain. If you have horizontal dropouts, shorten the chain to take up all the slack, then adjust the position of the back wheel for at least 10mm (3⁄8 inch) of vertical movement in the middle of the top stretch of chain. The chain tension will change as you turn the pedals, so find the tightest spot and measure from there.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Chainsuck: causes and remedies”