I want to set up a singlespeed mountain bike. How do I know what chain
length to use? I know I want it tight, but how tight?
You didn’t specify whether you are starting with a singlespeed mountain bike frame or a standard mountain bike frame. Bikes made for singlespeed or Rohloff and other internal-gear hubs have a system to vary the distance from the crank to the rear hub. Bikes not made for these hubs (i.e. standard mountain bikes) can be fitted with adaptors to tension the chain.
The simplest made-for-singlespeed system is rear dropouts with long, horizontal slots in which you can pull the wheel back and forth and clamp it in place where you choose. Some of these have adjuster screws with locknuts on them; you push the wheel forward or back by turning the adjuster screw(s) and lock it into place with the locknut(s).
Sliding rear dropouts are a more elegant system for varying drivetrain length on singlespeed/internal gear frames. The axle ends sit in normal, near-vertical dropout slots, but to tension the chain, you loosen the bolts securing the dropouts to the ends of the chainstays. Slide them back and forth while the wheel is already clamped in, and lock it in place by tightening the bolts securing the sliding dropouts to the frame.
A third method is an eccentric bottom bracket; the bottom bracket shell is oversized and the bottom bracket bearing and axle assembly mounts into a tunnel bored off-center in an aluminum cylinder (I’ll call this the eccentric cylinder). The eccentric cylinder clamps into the oversized bottom bracket shell, either by means of a slot in the shell closed by pinch bolts, setscrews on the shell driven into the eccentric cylinder, or a sliding wedge piece in the eccentric cylinder. You tension the chain by loosening the pinch bolts, setscrews or wedge bolts and rotate the eccentric cylinder (usually with a pin tool in a pair of holes in the face of the eccentric cylinder) to move the crank toward or away from the rear hub.
With any of these types, first set the distance from the wheel to the crank at its minimum by sliding the wheel or the sliding dropouts all of the way forward or by rotating the eccentric bottom bracket so that the bottom bracket spindle is as close to the back of the oversized bottom bracket shell as possible. Wrap the chain around the cog and chainring and make it the minimum length that still allows you to connect the ends. Tighten the chain by sliding the wheel back or by moving the bottom bracket forward, depending on the system you have. You want the chain not to be taut like a drumhead, as this would create excess pedaling resistance as well as prematurely wear out the hub and bottom bracket bearings. You also don’t want it loose enough that it can fall off. A good test is to push the chain over gently with your thumb just behind the chainring while turning the crank. If you can derail the chain this way, tighten it more.
To adapt a standard frame to use a singlespeed or internal-gear hub, you can install a chain tensioner to pull a slack chain taut. Often called a “singleator,” after the Surly model of that name, it is usually a spring-loaded arm with a single jockey wheel or roller on it that bolts into the rear derailleur hanger on the dropout. The jockey wheel or roller pushes up or down on the chain coming to the bottom of the rear cog to keep it tight (two springs are often supplied so that you can choose if you want the roller to push up or down on the chain). The jockey wheel shaft is often free to move laterally in order to center the roller or jockey wheel on the chain.
Another type is a U-shaped clamp with a roller bolted through the ends of the “U.” The unit clamps around the chainstay and can be slid back and forth to vary the pressure of the roller on the chain.
For single-speed adaptors with a single roller, make the chain just long enough to give the minimum amount of slack when wrapped around the chainring and cog without the roller in place. The chain tensioner will do the rest.
A twin-jockey-wheel type (the Paul Melvin, Rohloff Twin Pulley, or Shimano Alfine), which cannot be used with a fixed gear or a coaster brake, also bolts to the rear derailleur hanger and has a pair of jockey wheels in a spring-loaded cage that force a Z-bend in the chain to keep it tight. It is set up like a fixed-position rear derailleur and consequently requires more chain length than a single-roller tensioner. You want enough chain length that the jockey wheels are not stretched out in a straight line but not so much that they cannot take up all of the slack. There is a lot of latitude here.
Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
Follow Lennard on Twitter.