Gears & Gadgets: You Know It’s Time to Service Your Bike When…
By David Cohen, Maryland National Capital Park Police Reserve Unit
Like many of you, the pandemic turned my world upside in terms of time management. One of the items that got pushed to the back burner was the routine maintenance on my personal bicycles. So, when I finally had the opportunity to start turning wrenches again, the wear and tear was far greater than I had expected. However, it gave me the perfect opportunity to both demonstrate and correct this kind of wear and tear on our department bikes, and provided plenty of fodder for the next few G&G columns. Let’s start by looking at what a very worn drivetrain, piece by piece.
Chain
Regardless of whether you are riding a human powered or an electric powered bike, the chain is the critical component that enables the bike to get from one point to another. As chains age, they stretch. As they stretch, they start wearing other components down. If a chain stretches far enough, it will break, usually at the most inopportune time.
The best way to check the stretch of a chain is with a chain checking tool. Park Tool manufactures a range of chain checkers ranging in price from $10.95 to $27.95. The Park Tool CC-3.2 for $10.95 is good enough for department bike fleet applications. It works quite simply: you put one end on a chain link and lay the tool down across the chain. If the end of the tool is resting on one of the chain links, the chain still has life in it. If the end of the tool fits into the gap between the chain links, the chain has stretched. Time to replace the chain!
If you don’t want to spend the money on a chain checker, there are two less precise methods of checking chain wear. The first method is pretty straightforward: shift to the smallest rear cog and the largest front cog. Try to pull the chain from the chainring. If you can see daylight between the chain and the chainring, it’s time to replace the chain.
The other method is to measure the chain with a ruler. Use the pin at the middle of a chain link as your starting point, or “0”, on the ruler. Count 24 links on the chain. Number 24 should line up with the 12-inch mark on the ruler. If there is more than a 1/16th of an inch difference, it is time to replace the chain.
Cassette
Wear on the cassette is always a little harder to spot. As we ride, the chain starts wearing down the rear sprockets. If you find yourself using certain gears more than others, those will likely be the ones with the worst wear. I tend to use the middle chainring in combination with 2nd, 3rd and 4th gear most often. Those are the ones that have the most wear. It is easy to spot as sprockets can be worn to the point at which they are pointed. Unfortunately, most manufacturers no longer produce cassettes that let you replace the individual sprockets. Replacing the cassette is an all-or-nothing proposition. Even if replacement sprockets are available, it is best to replace the entire cassette. When replacing the chain, you should always replace the rear cassette. If you do not, the new chain will not ride smoothly over rear sprockets and will cause the chain to wear prematurely.
Chainring
Typically, replacing a chain and cassette does not always necessitate replacing the front chainrings. Chainrings tend to have a longer life than both the chain and cassette. However, wear does happen. In my case, it was clear that the middle chainring was well beyond the point of replacement. The chainring looked more like a throwing star than a bicycle component. While the large and small chainrings do not have nearly as much wear, if you are going to replace a chainring, you might as well replace the others. Unlike the rear cassette, though, you can purchase individual chainrings. This not only allows you to replace ones that are the most worn, it also provides you with the opportunity to change the gear ratios up front.
With many bike manufacturers going to a single front chainring, the amount of wear on the front chainring will increase, as there are no smaller or larger chainrings to help “spread the wear”.
Jockey Wheels
The jockey wheels on the derailleur are probably one of the most overlooked components. However, for the entire drivetrain to work properly, these two little wheels need to do their job smoothly. Like the front chainrings, it is not necessary to replace the jockey wheels when replacing the chain. In fact, they will typically even outlast front chainrings. That said, heavy wear is certainly possible, especially if you are operating in an environment (such as natural surface trails) that introduce dirt, mud and other abrasives into the picture.
While from the side, jockey wheels may appear to still have life in them, but when you look at them from the top, you can tell if they have been worn thin by the chain. If so, they too will need replacement.
Disc Brake Pads
Besides the entire drivetrain being shot on my mountain bike, the disc brake pads were badly worn and contaminated. Unlike rim brakes, where the wear is relatively easy to spot, the only way to assess the condition of the disc brake pads is to remove them. Check the thickness of the pads and look for uneven wear patterns and signs of contaminants, such as oil or brake fluid, on the pads themselves. If they have any signs of wear or contamination, it is time to replace them.
Over the next few G&Gs, I will detail the replacement of all these components so that the bike will ride, and stop, like new again.
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.
Photos courtesy David Cohen.