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My chain broke and in a moment of silliness (I was cold wet and slightly annoyed) I threw the broken one away before thinking about measuring the new one.
Is there a guide to getting the chain length/tension correct ?
It's an 8 speed road bike (Shimano Sora back, double ring front)
Put the chain around the big-big gears without threading it through the derailleur. Have the chain overlap by one link. Now with that length thread it through the derailleur.
Starting from a new split chain:
Thread chain onto largest chain ring and largest cassette sprocket. Either thread through front derailleur or move it out of the way. Do not thread chain through rear derailleur.
On the chainring, find the links that will join to form the shortest possible chain that can be connected. Add 1 inch of chain to that (two rivets or 1 inner link and 1 outer plate link).
If using a quick-link, remember that takes the place of one outer plate link.
See video from Park Tool below.
Bonus content: why this is the correct chain length:
Modern derailleurs can accommodate the chain running from the chainring to sprockets in almost a straight line. The shortest chain that fits over the largest chainring and sprocket and that can run through the derailleur is desired.
This is because:
The two extra links added provide enough slack for the chain to run through the rear derailleur.
If one had a system where the derailleurs could be prevented from selecting the largest chainring and n largest cassette sprockets at the same time - which could be programmed into Di2 or eTap systems - then the chain could be sized for the large chainring and n-1th largest sprocket.
