There are a number of rules that need to be followed when changing out cassettes on bicycles and tandems. Most of them are pretty basic:
1. Make sure the cassette you plan to use is compatible with your hub, i.e., you can’t put a Campy cassette on a Shimano freehub or visa versa.
2. Make sure the cassette you plan to use is compatible with the gear specifications of the rear derailleur and shifters, i.e., 9 speed Shimano cassettes typically work best with 9 speed Shimano STI levers and compatible rear derailleurs. Yes, there are some exceptions, but in general it’s best to keep it simple if you don’t like to mess around with fine-tuning rear derailleur adjustments and/or using adapters.
3. Make sure the cassette’s capacity and range are within the operational range of the rear derailleur. There’s a reason that bicycle derailleur manufacturers offer short, medium and long-cage rear derailleurs. Using the right length cage will go a long way towards improving shifting performance.
4. Make sure your chain is the correct width for your cassette AND your chain rings. If you run a chain that’s too wide for the cassette, e.g., a 9 speed Campy chain on a 10 speed Shimano cassette, the chain will skip and jump because it’s too wide to fall into the sprockets. Going the other way, while you can usually get a very narrow 10 speed chain to work on 8 or 9 speed cassettes, you may find that the chain will drop between the front chain rings.
5. Make sure that the brand and model chain really is designed to work with your cassette and chain rings. Years ago, before integrated shifting, all of the rear sprockets on a freewheel and chain rings were straight cut and relied on a cyclist’s shifting technique for smooth, seamless shifts. Nowadays, many cassette sprockets have machined-in shift ramps that mesh with the side plates on chains that are designed to work with those cassettes for super-efficient shifting. If you use chains or cassettes that have these features without the correct corresponding cassette or chain you’ve wasted your money on a carefully engineered component that won’t yield the integrated performance that you paid for when you bought that part.
6. Make sure the chain is the right length for the new cassette. Let me say that again, MAKE SURE THE CHAIN IS THE RIGHT LENGTH FOR THE NEW CASSETTE! I’ve probably done field repairs on a half-dozen bikes or tandems over the years where a chain was installed that was not checked to be the correct length for the bike’s gearing. Now, to be fair, it’s not as much of a big deal to have a chain that’s a couple links too long for a given drivetrain. That is, so long as the rear derailleur’s idler wheel is installed on a cage that’s long enough to take up all the slack when your bike is shifted into the smallest chain ring and smallest sprockets, aka, cross-chained in the small-small gear combination. However, it’s a whole different ball game if you have a chain that’s not long enough to wrap around the largest chain ring and largest rear sprocket without binding. As a rule of thumb, when going from say a 12x27t cassette to a 12x32t cassette those 5 extra sprockets need about 3 extra inches of chain — three full chain links — if the chain was sized correctly for the 12x27t. Three chain links may not sound like a lot, but if you don’t have it and make the mistake of going full cross-chain in the big-big gear combination you’ll be in for a rude awakening as your drive train locks-up.
Why do I bring this up? No, it’s not because I had to rescue yet another fellow cyclist who failed to recognize the need to check the length of their chain after doing a cassette swap, it’s because I had to rescue myself. Yup. As you may recall reading in my TTR Day #2 write-up, I made a bad gearing choice in Tennessee by leaving my 10 speed Shimano 12x27t cassette on our tandem for the hilly Saturday ride. To make amends to my beloved stoker for that oversight that put us into a spot of bother on some steep climbs I promised to switch out the cassette to our 9 speed 12x32t, which I did. In fact, cassette swaps are something I do all the time given that we’re still playing around with three different wheelsets, three different cassettes and two different chains to support those cassettes. And, yes; I routinely bend some of the rules when it comes to gearing on our tandems, but it’s never that far outside the square that defines “acceptable compatibility”. Well, that was until Sunday’s ride from the house. When I changed out the 12x27t for the 12x32t I went and grabbed the longer 9 speed chain I use for the 32t cassette and started to install it when I realized that I’d forgotten where I’d put the SuperLink for that chain. I’m not sure why I didn’t just pull the spare out of our seat pack or get a new one from my tool box, but I made a conscious decision to put the 10 speed chain back on and made a mental note to myself: don’t go big-big!
So, away we go and we’re having a pretty nice ride on yet another hot day with a heat index of about 100°F. Because of road construction, we took a different way back to the house on a road that’s flat enough and long-enough to throw the chain into the big chain ring on our tandem, which I did vs. the typical rollers that are better suited to the middle ring. As we approached a turn that we normally take at a pretty good clip, Cyclo-Math intervened to put a line of cars in the on-coming lane at the exact point where it precluded our left turn. So, seeing this line of cars coming, we slowed to a near track stand and I moved the chain to the lower gearing / taller rear sprocket on our cassette, anticipating the need for an easier starting gear. Unfortunately, I shifted the chain all the way up to the 32t rear sprocket while I still had the chain on the 53t front chain ring: doh!
It’s a horrible sound and feeling that you get from the drive train when you discover your chain is about an inch or so too short for the big-big gear combination. If you’re lucky, the derailleur stays attached to the bike and the rear drop-out’s derailleur hanger doesn’t bend or break. But even then, you can be in some big trouble since as the rear wheel will be locked into the dropouts which precludes a quick fix. Moreover, because the chain is so tight you typically can’t get it to derail from the larger chain ring or sprocket to a smaller one, and pushing a rivet out can also be pretty difficult since all of the rivets are also under a lot of tension. In these situations, and if you’re lucky enough to have cranks fitted with self-extracting crank bolts on the drive-side crank arm + a micro-tool with the right size hex-head wrench end, you can remove the rear crank — chainrings and all — as a way of unbinding the chain and drive train.
Thankfully, I was able to do just that. However, in doing so I buggered up the starter threads on the extractor / crank arm bolt. This made getting the crank bolt threaded back into the Phil Wood titanium square taper axle a bit of a challenge. I was finally able to persuade the bolt threads to engage the threaded axle end and get us back on our way without further incident. As best as I can tell, there was no other damage to the drive train, although I have not pulled the damaged extractor / crank bolt back out of the rear cranks to make sure that it hasn’t been cross threaded, galled or otherwise damaged to the point where the threads need to be re-cut by a die or that the bolt simply needs to be replaced.
Anyway, I thought I’d share this little oversight on my part with y’all in the hopes that you’ll think of this the next time you’re considering a cassette change as part of your own mental checklist and to give you an out with your stoker, i.e., “Honey, it’s not like this is the first time this has ever happened to anyone. Just give me a minute and we’ll be back on our way.” And, with some patience and the right tools, you will be. Thank goodness for self-extractors: never leave home without them.