Chains are generally the cheapest and the weakest part of the drivetrain, yet can do a lot of damage to your cassettes, chainrings, and derailleur pulleys if not maintained properly. They are exposed to all weather and terrain conditions, like water ingress, dust or mud, etc. You’re not driving a car: if you’re traveling long and remote distances, especially alone, you have to take care of your bike, and learn to maintain and repair it. Chains are fragile. At least, you have to know how to clean, lube and remove / install them.
The most obvious sign the chain is giving up is elongation. Contrary to popular belief, chains doesn’t elongate because plates get stretched out: they get a bit longer, because rollers and bushings get worn.
Personally, I lost few good chains to corrosion,too! I was shocked to see those hardened steel rollers flaked off. That was a phenomenon I happened to see in marine engines a lot. Here is a weird sounding advice to you: if you wont ride for a long time, remove your chain! If you cannot, or you are lazy like me, shift the front (if you have!) to a smaller chainring. Why? Even on Deore XT cranksets, and as I recall, on XTR’s too, smaller chainrings are steel. This stops galvanic corrosion. If all your chainrings are steel, fine. As a rule of thumb, keep steel and aluminum as separate as you can. Top-notch, or so-called, cassettes can have eccentric materials like aluminum and titanium. Titanium will cause corrosion, too. So, know your cassette, and leave your chain on a good, old, steel cog! I’m talking about galvanic corrosion, #1 worst enemy of your bike. Steel and aluminium don’t get along well. Those who have stuck seat posts because of galvanic corrosion know this very well.
So, if you look after your bike, usually won’t have complex issues. 99% of the time, chains worn; only problem. You need to have a good quality Chain Wear indicator tool to determine if chain is bad. I say “good quality”, because you’re looking for elongation that is under 1%!
Use your tool to take measurement from a few places on the chain, as they may partially stretch out.
Once you know your chain is elongated, you’ll need to buy a new one, obviously.
You may want to replace a chain because it’s bent. I never bent a chain, but if I do, I probably don’t replace a nice, healthy chain. Instead, remove bent link, and add another master link. If it’s bent in multiple links, well, don’t bother and replace the chain then.
Shopping advice for a new chain
I do not suggest a brand or a model; but I generally buy “midrange”, which is generally Shimano Deore for me. I used Sram chains and frankly cannot see any difference between the two. Buy a known brand that is compatible with your drivetrain, and you’ll be fine. But be vigilant if you have 11 or more cogs; because I know 12-speed chains are not compatible: Shimano and Sram parted their ways after long decades. If you have Campagnolo…well, it’s Campy. It doesn’t play well with others, never did, never will…
At least for me, spending too much money on a chain that is 80-100 gr lighter doesn’t make sense. Chains are consumables, and the earlier you change it, the better. I generally use 10 speed on my bikes. 10 speed is cheap, quality parts are still available and gear spacing is more than enough to me.
With 12 or 13 speed, things get weirder. Shimano 12 speed comes with Hyperglide+ system. Chains have different inner plates. I’m not talking about inner width. They have a different shape internally. The same goes with 12 speed SRAM. Tough they will fit each other, they won’t shift without zero problems, or nicely as with their original chain.
Chains are generally sold with 116 links. This is more than adequate if you don’t own a tandem bike. Before fitting your chain to your bike, you need to size it correctly.
A worn chain elongates. So, if you cut your new chain to the size of the old one, it may be a bit long. In some cases, you may not have an old chain at all, like building a bike from scratch, or changing the drivetrain, even replacing chainrings with smaller / bigger ones.
I even don’t bother using the old chain as a guide. I do not change chains very frequently, but do a lot of upgrades, downgrades, changes to drivetrain. I always have few chains, in good condition, left somewhere, waiting to be used for a “different” drivetrain. Sometimes, I change the chain, because I opted for bigger cogs or chainrings, and old one won’t fit. One day, I’ll use that short chain in somewhere else. Dealing with chains quite a lot, I just put the new one, size and cut it, without using the old one as a template. I think this is a good habit to make, especially if you have more then one bike, and you replace things often.
The art of sizing chains
Think twice, cut once. You may use two quick links if you cut it short, but lets be honest, its stupid. And makes the chain much more fragile.