Why did my chain come off?
Go to this link to learn all about chain issues: https://www.sheldonbrown.com/chain-drop.html
Why did the chain come off the sprocket?
It is fairly common for a bicycle chain to come off . Contributing to this problem can be:
- A poorly-adjusted derailer or one which is mismatched to the chainwheels or rear sprockets;
- Riding over a big bump, or crashing;
- Laying the bicycle down on its side for storage or transport;
- A chain which is worn or has a bent or stiff link, or is clogged with dirt;
- A chainwheel which is bent, or with a bent tooth or burrs on the teeth;
- Chainwheel tooth width or spacing between chainwheels mismatched to chain (narrow chain and wide chainwheels/spacing, or the opposite);
- A stiff link in a chain;
- Derailer chain hanging slack due to insufficient chain takeup in the rear derailer, or too much slack in a non-derailer chain;
- Excessive chain angle (outside chainwheel to inside rear sprocket, or vice versa);
- Incorrect shifting or backpedaling;
There may be other factors too, I'm sure I haven't named them all. Occasionally a chain will come off even on a well-maintained bicycle.
Preventing chain incidents and crashes
Your first line of defense against chain incidents on a derailer-equipped bicycle is to shift with care. It's become somewhat of a lost art with the advent of Hyperglide, but if you stay seated and pedal lightly when shifting, your feet will warn you that a shift isn't going well. Taking this care also will prevent damage, and will let you maintain control of the bicycle if the chain falls off. Spinning is more efficient too: low gears for climbing are preferable to macho "honking" up hills. Save standing for short accelerations and sprints when you don't have time to shift.
Shift down before starting, and once stopped, backpedal lightly to raise a pedal into the 10 o'clock position for starting. You should feel no resistance. If you do, a derailer is misaligned with the sprocket that holds the chain. Reduce force on the pedal and adjust the shifter(s) until the chain runs smoothly backward.
Well-adjusted derailers don't overshift beyond the limits of the cassette or chainwheels. Keeping the chain in good condition and replacing worn sprockets along with the chain, avoids skipping.