Constructing a Simple Chain Keeper [From BA 43-300]

Deconstructing A Chain Keeper Best of Boneshaker

"With a little effort and even less money, the rider of a retrofit singlespeed can create a simple component to keep the chain in place."

Over the last century, the bicycle has morphed into a state of ever-increasing complexity: the latest cassettes come with as many as eleven gears; suspension forks have been developed for cycling applications that do not really require suspension; and even the frame itself has become a complex array of linear fibers, epoxy resins, and nano particles. The necessity of these latest developments, and whether or not they truly enhance the enjoyment of bicycling, however, is of course debatable.

Bucking the trend for the latest consumer product is a subculture within the cycling community that strives to return to a state of simplicity. Just as the ancestors to modern whales shed weight-bearing limbs to return to life in an aquatic environment, many urban riders have been subconsciously trying to recreate the minimalism of the Safety Bicycle of the 1880s.

Recently, the bicycling industry has been more than willing to accommodate the urban rider, shipping from Asian factory to local showroom ready-made singlespeeds and fixies. Either through lack of financial resources or a desire to conserve the planet’s natural resources, some urban riders have chosen to avoid the factory-made urban velo, opting instead to repurpose an older bicycle.

A frequent problem with these budget-minded singlespeed retrofits, however, is that the chain may not be as willing as the rider to return to a life with only one gear. As the rider climbs a hill and torques the pedals, the chain wants nothing more than to help, leaping onto the next smallest chain ring to ease the rider’s efforts. Of course, in the absence of a smaller chainring, the bicycle’s momentum comes to a grinding halt.

Fortunately, with a little effort and even less money, the rider of a retrofit singlespeed (or other type of single chainring bicycle) can create a simple component called a “chain keeper” to keep the chain in place. The chain keeper is particularly useful in the following circumstances:

1. The chainring is ramped and pinned to assist gear changes (Figure 1).

2. The bicycle frequents rough conditions such as singletrack, dirt roads, or potholes that cause the chain to slap violently upon the chainstay.

3. The chain line is not perfectly straight.

4. Despite the absence of the conditions 1-3 above, the chain refuses to stay put.

Figure 1: The chainring is ramped and pinned to assist gear changes.

Figure 1: The chainring is ramped and pinned to assist gear changes.

Paul Components Engineering fabricates a very nice chain keeper in Chino, California. However, for the cost of this artfully designed device, the reader can buy three and a half copies of The Best of Boneshaker and still have money left over for a burrito and a beer. As such, we can create our own chain keeper by simply modifying a front derailleur.

The front derailleur is basically a cage that attaches to a clamp via four pivots that comprise a type of three-dimensional parallelogram (Figure 2).

We can think of the cage as consisting of three plates: an inner plate that lies between the chain and the frame, an upper plate above the chain, and an outside plate. Protruding from one of the sides of our parallelogram pivot system is a lever arm that attaches to the shifter cable (Figure 2). Within the parallelogram is a spring that pulls the cage inward toward the frame. At the top of the parallelogram are two limit screws that provide adjustable constraints to the cage’s lateral movements (Figure 2). Our goal is to cut down the front derailleur, removing those parts that aid shifting and preserving those parts that keep the chain in place.

Figure 2: View of a front derailleur from behind. The cage is to the right, the clamp is to the left, and the parallelogram linkage system is in the middle. The lever arm is colored black. The adjustm  ent screws are up top.

Figure 2: View of a front derailleur from behind. The cage is to the right, the clamp is to the left, and the parallelogram linkage system is in the middle. The lever arm is colored black. The adjustment screws are up top.

The only tools necessary to transform a front derailleur into a chain keeper are a bench vise, a half round file, and a hacksaw. Bench grinders, angle grinders with cut off wheels, and/or rotary tools with grinding and cutting bits can certainly speed up the de-fabrication process.

The key to getting a front derailleur on the cheap is to go to a bicycle shop and talk to the wrench who often keeps a pile of used components leftover from upgrades. He or she might sell you an old front derailleur for just a few dollars or even just give you one to reduce clutter in their work area.

Now, we want our chain keeper to behave like an aristocratic child: barely seen and never heard. We want to avoid chain rub completely, and we want the appearance to blend into the contours of our drive train. Any type of front derailleur can function quite well as a chain keeper, but only those with a clamp lower than the top of the cage yield the desirable, subtle aesthetic with the clamp and parallelogram pivots hidden behind the curve of the chainring (Figure 3). Derailleurs built for thicker 5, 6, 7, or 8 speed chains have wider cages than those built for 9, 10, or 11 speeds. The wider the cage, the lower the chances of noisy chain rub.

Figure 3: Two front derailleurs. The derailleur on the left has a clamp lower than the top of the cage.

Figure 3: Two front derailleurs. The derailleur on the left has a clamp lower than the top of the cage.

Common derailleur clamp sizes are 28.6, 31.8, and 35mm in inner diameter. Older steel framesets from the 70s, 80s, and 90s usually have a seat tube with an outer diameter of 28.6mm. The clamp cannot be smaller than the seat tube; however, if the clamp is too big, we can make a simple shim using rubber from an inner tube or plastic from a shampoo or soda bottle.

We do not want to create a chain keeper plagued by spontaneous movement, which would require frequent burdensome adjustment, so our first task is to remove the spring. The spring usually rests within the parallelogram, usually to the front of the lever arm. Often, the lever arm is held in place by two C-clips. We can release the C-clips by placing the point of a nail inside the contour and pulling opposite the gap of the “C.” With the lever arm out of the way, we can use the tip of the nail to push one of the ends of the spring off its mount in the parallelogram. Once loose, we can easily remove the spring, and return the lever arm and the C-clips to the pivot system.

We want our chain keeper to have a low profile, so we should cut off the portion of the lever arm that extends beyond the pivot of the parallelogram system. We should use a file to smooth the jagged remnant of the lever arm.

We can dramatically cut down the outer plate of our cage. For many derailleurs, the bottom of the outer plate bends or curves inward to assist down shifts (Figure 3, left). If left in place, this curve will narrow the dimensions of our chain keeper, raising the chances of chain rub. We should remove this curve, and cut off the tail end of our front plate, so that the front plate does not extend more than 1cm beyond the rear of the top plate (Figure 4). We can smooth the jagged cut ends of our front plate with a file, creating artful curves or points. The top plate should not be modified.

Figure 4: A front derailleur after cutting down the outer plate of the cage. The inward curving lip visible in Figure 3 (left) has been removed. The dark line represents a good location to cut the rear plate, right below the lower pivot.

Figure 4: A front derailleur after cutting down the outer plate of the cage. The inward curving lip visible in Figure 3 (left) has been removed. The dark line represents a good location to cut the rear plate, right below the lower pivot.

From our inner plate, we should cut off all metal that extends below the lowest pivot of our parallelogram (Figure 4). The front of the inner plate usually curves outward toward the chain to assist with up-shifts. We can file down this curve so that the outer surface of the front of our inner plate is flush with the metal behind it. We can cut down the tail of our inner plate more-or-less even with the new tail of our outer plate. File the cut surfaces smooth (Figure 5).

Many derailleurs have a roof on the parallelogram pivot system made of flimsy plastic. If we remove our adjustment screws, the plastic roof usually slips off. Before mounting the chain keeper, reinsert our adjustment screws and play around with them to figure out which one pushes the modified cage inward toward the frame and which one pushes outward.

Figure 5: The cut edges of the front and rear plates of the cage have been filed smooth. The holes in the front plate are optional but might appease one’s inner weight weenie.

Figure 5: The cut edges of the front and rear plates of the cage have been filed smooth. The holes in the front plate are optional but might appease one’s inner weight weenie.

We should mount our chain keeper, tightening the clamp around the seat tube (Figure 6). We should apply just enough torque to resist gravity but not so much that we cannot rotate the keeper around the seat tube or move it vertically with our fingers. For singlespeeds, adjust the rotation of the clamp around the seat tube and tweak the limit screws of our pivot system so that the inner and outer plates of the cage run parallel to the length of the chain. The chain should be centered within our modified cage.

While turning the cranks, lower the clamp along our seat tube until we hear the top of the chain rubbing against the bottom of the top plate, then raise the keeper a few millimeters until the rubbing noise disappears. For singlespeeds, a gap between the bottom of the top plate and the top of the chain must be much smaller than the height of a chain link or the keeper will be ineffective. Once we've found the “sweet spot,” fully tighten down our clamp and set our adjustment screws so that our cage can't wobble.

Figure 6: Finished chain keeper mounted on a frame and drive train.

Figure 6: Finished chain keeper mounted on a frame and drive train.

For bicycles with multiple rear sprockets and a rear derailleur (but a single chain ring), mounting the chain keeper becomes more complicated. With the chain on the biggest sprocket (low gear), we want 0.5 to 1mm margins between the chain and the bottom of the top plate and the outside of the inner plate. With the chain on the smallest sprocket (high gear), we want a 0.5 to 1mm gap between the chain and the inside of the outer plate. Cycle through the gears, making adjustments to eliminate noisy chain rub. When we find the right settings, fully tighten down our clamp and set our adjustment screws.

To test the new chain keeper, simply head out for a long ride. If we constructed and mounted the chain keeper properly, it should dramatically decrease the amount of time spend returning a fallen chain (and keep those fingers clean).

References:

Sheldon Brown, Harris Cyclery, “Single Speed Conversions.”

Evan P. Schneider