How to determine an optimal single speed chain length? Why is it important? When explaining this, it is important to note that it goes for all the bicycles with only one chainring at the front and at the back, including bicycles with gear hubs (that have multiple gears built inside the rear hub mechanism). For bicycles with derailleurs, see: Chain length sizing for bicycles with derailleurs.
A new bicycle chain usually comes with 116 links. This is long enough for the biggest chainrings and for most distances of rear wheel from front chainrings. So for optimal length a new chain is usually shortened from the 116 links that come in the box. In rare occasions – touring frames for example, with extremely long chainstays, 116 links might not be long enough, so a few links off another new chain need to be added to reach an optimal chain length.
Too long chain will increase risk of chain dropping, while too short chain puts too much stress on the drivetrain, or can damage it, if it can be put and closed over the chainrings in the first place.
For bicycles without derailleurs and without a chain tensioner, the chain length should be such that the chain has some slack, but not too much. Such bikes usually have only one speed, sometimes even “fixed gear”. Some might have rear hub that allows for different gear ratios, with speeds built inside the hub. However, with or without hub speeds, chain measuring method is the same, like this picture explains:
So the chain should be tight, but not too tight. Half way between chainrings the chain should move about 2 cm (1 cm up-down). Frames for single speed, if no chain tensioner is used, usually have rear dropouts that allow moving rear wheel back and forth – so that correct chain tension can be acheived once the chain is cut to size. One chain link is exactly one inch long. So the minimal step for shortening a chain is 2.54 cm.
Chain tension should be checked throughout the pedal turn, because chanrings are not always completely round. If chain gets too tight at some point, it should be loosened a bit (rear wheel moved slightly forward). At the tightest point there should be at least 5 mm of play up and down.
In case the frame has vertical dropouts that don’t allow for setting chain tension by moving the rear wheel, special chain tensioners can be used, or improvised from an old rear derailleur. Also there are rear hubs with oval shape that allow for setting chain tension by rotating hubs, but such parts are exotic.
Some people have problems with setting an optimal chain tension on bicycles with horizontal rear dropout. They find it hard to move the wheel just little enough to make it right. This is because they don’t use jockeying technique to move the wheel back and forth in the dropouts. Use the following procedure:
- Insert the wheel in the dropouts with loose wheel bolts. Put the chain over the chainrings, it should be loose as well.
- Wheel is then moved to the left – away from the chain. Non drive side bolt is therefore put slightly to the back. Then the non drive side bolt is tightened.
- Now the drive side bolt is moved back and forth to achieve desired chain tightness. Then it is fastened.
- Non drive side bolt is loosened. The wheel should now align itself if proper chain tension was set up. If it doesn’t align 100%, align it by hand. Then tighten the non drive side bolt and you’re good to go. 🙂