Gears & Gadgets: Two Little Sprockets
By David Cohen, Maryland National Capital Park Police Reserve Unit
A smooth-running bicycle drivetrain is a joy to behold. There’s nothing quite like a bicycle chain smoothly and effortlessly passing through the front chainring, rear cassette, and then those two little doohickeys attached to the derailleur.
Those two little doohickeys attached to the derailleur are jockey wheels. Their purpose is to maintain the proper chain tension throughout the entire gear range. However, if those two wheels start getting gummed up, they can throw the rest of a magnificently tuned drivetrain completely out of whack.
Maintenance for the jockey wheel is a pretty simple affair. Every few rides, take a rag and pinch the jockey wheel while turning the crank to remove grime and gunk. Pay close attention: there’s not much room to operate and it’s easy to slip and get the rag or your finger caught up in the mechanism. If the jockey wheels are heavily gunked-up, as tends to happen with bikes ridden on natural surfaces, a flathead screwdriver can be used to remove the built-up gunk. A couple of drops of chain lube on the friction part a couple times of year keeps the pulleys spinning freely.
Jockey wheels do wear, though. What makes jockey wheel wear patterns difficult to detect is that it is a very gradual process. A rider could potentially go through two or three rear cassettes – or more – before the jockey wheels need replacement. The best way to see how much they have worn is to compare them to relatively new ones.
Replacement is not a difficult process. First, make sure you order the correct replacements for the type of derailleur that is on the bicycle. The diameter and number of teeth will vary from derailleur to derailleur. If in doubt, contact the derailleur manufacturer. Then take a picture of how the chain runs through the rear derailleur. This is very helpful during the reassembly process.
Replacement of the jockey wheels can be done with the chain still on the bike, but I recommend replacement at the same time the cassette and chain are being replaced; this way, everything is off the bike. Removal is a straightforward affair – a single bolt goes through the derailleur cage which acts as the axle for the jockey wheel. Remove the bolt and the jockey wheel comes out. Start with the lower jockey wheel, and then remove the upper one. Mark the jockey wheels “top” and “bottom” so you can match them to the new set. Some derailleurs have different jockey wheels for the top and bottom.
While the derailleur cage is apart, this is also a good time to clean and degrease the insides of it. If you spot unusual wear on the cage – perhaps a divot due to a chain rub – and the cage’s structure is compromised, it would be a good time to replace the entire rear derailleur.
Assembly is the reverse. I will sometimes put a very light coating of grease on the bolt. When reassembling the jockey wheel and the cages, you only want to tighten up the top wheel just enough to hold the derailleur cage on. It is much easier to attach the bottom jockey wheel if there is a little bit of play in the cage to line things up properly.
With the new jockey wheels inserted into the derailleur, and presumably a new cassette on the rear wheel, it is time to run the new chain through the jockey wheels. Remember that picture I told you to take? This is when it comes into play. Routing a chain incorrectly through the jockey wheels is a recipe for irritation at a minimum, and could be downright dangerous at worst. I find it easier to route the chain through the jockey wheels from top to bottom, since gravity is on your side. Some like to run the chain from bottom to top to “pull” it through, and you don’t have to worry about the chain folding up on itself when you “drop” it from the top.
Connect the two sides of the chain, make your derailleur adjustments, and the bike will be good to go with a smooth drivetrain for many hours of riding. How long will the replacements last? It depends on the riding style. I’ve found that in most cases, by the time a derailleur is onto its third set of jockey wheels, the derailleur springs have worn out and the entire derailleur needs replacement. However, I do have one 8-speed Shimano Deore LX derailleur from 1998 on its third set of jockey wheels and still shifting crisply. YMMV.
Let’s go riding. Be safe out there!
Gears & Gadgets is a blog written by David Cohen #1116. It features various repair tips and tricks that David has picked up in nearly 25 years of cycling, including 12 years of public safety cycling. Gears & Gadgets will also periodically review some of the latest “gadget” items of potential interest to public safety cyclists. The advice and opinions in this column are solely the author’s and should not be interpreted as IPMBA mandates or recommendations unless explicitly stated.
About the author: David Cohen is a 12-year volunteer with the Maryland-National Capital Park Police. When he isn’t riding or tinkering with bicycles, David can be found tinkering with vintage cars or World War II airplanes. An avid historian, David enjoys researching and writing as well. He can be reached at onyxsax@aol.com.
(c) 2022 IPMBA. This article also appeared in the 2022 Conference Highlights issue of IPMBA News.