My road bike has an Ultegra groupset, but some parts need replacement (chainrings, chain, and cassette). The chain is worn out, the cassette is sliding with a new chain, and the chainset is also worn out. Can I keep my 3-9 derailleurs and mount new Ultegra chainrings with a Sora cassette (9) and a Sora chain to save some money, or should I keep riding and mount ultegra chainrings, chain, and cassette when they are fully worn out?
- Most groupset parts don't wear out on a predictable replacement schedule. Derailleurs, brakes, cranks, and brifters should last for a long time, barring unusual wear or crashes. And I hate to say it, but if you do get to the point where you want to replace those parts, it might be cheaper to buy a new bike.
- Modern Sora and Ultegra chainrings are 4-bolt, and are completely incompatible with your 5-bolt Ultegra cranks. You could replace the entire chainset (aka crankset), but a modern Sora chainset will be incompatible with your bottom bracket (your Ultegra chainset uses a separate BB; the Sora and other higher-end Shimano chainsets today integrate the spindle into the left crankarm), so that would need to be replaced too. You might be better off looking for NOS Ultegra rings.
- Sora-grade chains are lower quality (less precise spacing between rivets), which will wear out the cassette and chainrings more quickly.
- The Sora cassette will work.