How do I know when a derailleur needs replaced?
 

I have this early 90s Trek 1200 that I'm looking to refurbish. It was my former commuter that I inherited from a coworker and it served me pretty well for a few years before it retired when I stopped bike commuting. Now, I'm considering making it a refurb project, since the frame is still in good shape and I'm interested in trying my hand at some mechanics I've not done before. I figure I can spend a few hundred over the course of a couple of months putting it back on the road - not my preferred type of bike but I did not hate riding it.

The chain is definitely worn out and I suspect (but don't really know how to confirm) that the chain rings and cassette are as well - being as a worn chain wears the teeth. This makes me think about the only other part the chain goes through. I don't remember it having any shifting issues - though I used only the friction-shift setting for extra hipster street cred - but if the steel cassette is likely worn, I definitely expect the plastic derailleur jockey wheels to be. This is a Shimano RX100, which is long out of production I think, so I'd have to hunt up a 7-speed unit that actually works with my downtube shifters (which are friction-or-index changable).

When should one replace a derailleur, and is it viable to replace just the jockey wheels if need be?

Replace chain and cassette, and the jockey wheels if they're starting to shark tooth. If there's shifting problems then look at the derailleur.

Derailleurs last eons, typically.

I've found that the word "SRAM" on it is a fairly reliable indicator of imminent failure.

For a rear derailleur, take ahold of the “arm” with the lower jockey wheel and try to jiggle it side to side. It easily moves side to side it could be replaced. If I were you I’d replace chain & cassette, then ride it a bit to see if anything else might need replacing or adjusting.

It is difficult to tell if cogs or chain rings are worn by looking, but if you change the chain and it skips then you will know they are. No skipping, you are good. New cables and housings will help shifting performance a lot as well.

You should determine how far out of spec the chain is by measurement or Park tool (see Sheldon Brown). If it's really toast, consider replacing it, the cassette (or freewheel) and any front rings that have the "scalloped" shape that is customary when they are worn out. Otherwise the old parts will wear out the new ones rapidly IMO. You can procure Shimano Altus level derailleur and cassette for about $20 each on Amazon, chain for $10 or so, keep the shifters (replace the cable and housing if necessary) and locate some chainrings (maybe only one needs to be replaced depending how the bike was used or will be used) on Amazon or ebay.

If you are friction shifting, virtually all rear derailleurs will work as long as 1) the derailleur has enough throw to reach all the way across the freewheel or cassette. (Later SunTour Cyclones handle 9-speed cassettes with ease) and 2) your shift levers can handle the cable pull amounts. This limits you to nearly every derailleur made after 1989 and many before. (Pick up a box of used derailleurs and maybe one or two will be incompatible. Friction shifters are a joy, especially here!)

Shift levers with bigger drums will take up more cable and handle more cable pull at the expense of being harder to shift accurately and requiring more skill.

Very worn pulleys work just fine, if you are not using index shifters.

Ben

Cheers; the chain definitely needs replaced, so that's where I got my casette suspicions from. Should I do chain rings as well if I'm doing casette? It's a two-ring piece, and I already plan to put some kind of chain guard there if I can since it is totally exposed.

It's been some years so I don't remember if there were skipping issues. I'll definitely find out. That said a new cassette doesn't seem all that expensive (I don't know if it's a freehub or free wheel, need to pull it apart to check). The cables will definitely need replaced as some are looking less than good and the one on the derailleur is beginning to fray. Not ever having run cables before, I need to invest in a proper cutter. It'll also be an adventure as the housings run through the frame for some of the cables, which I've never dealt with before.

I do have one of those tools; first thing I invested in when I even floated the idea of a refurb to myself. The result was basically "hilariously stretched", as even the "replace now" tong had significant movement.

Have a strategy before you start pulling things apart. You will need a way to get things threaded back through the frame.

DO NOT remove the both inner wires and the housings from the frame; leave one or the other to act as a guide for the new parts, I'd suggest leaving the wires. If you pull both out you will have to figure a way to route through the frame again, which will really suck. Don't ask how I learned this. Again, have a plan before you start, don't leave yourself "stranded".

You will also need to prepare the housing ends after you cut them. https://www.sheldonbrown.com/cables.html#cutting Use good cutters. I invested in Felco cutters decades ago and have not regretted it.

Yeah, pulling the housing off of the wiring was my wild guess on how to do it - clamp down one end of the wire and thread it through the other. Initial install of those things must be a chore - I could see how you could do it with a strong magnet but still.

I'd considered the Park Tool cutters for the actual cable, but it sounds like I need to pick up some stuff for the housing as well; thanks for the link.

This is a slow project that's just in the planning phase so it'll probably be months before I actually get to work - other things in the Bike Budget Queue must come first. Once I get a new chain breaker in (my old one is trash) I'll start the "pull apart and inspect" phase. Need to get a handle on the hubs - rims look fine from what I can tell, but I remember having to use copious degreaser on the rear hub once due to a sticking issue I never quite figured out.

Checking chain wear is relatively easy, if you have a 12" ruler. Lay the chain flat, and place the ruler on the chain, with the 0" mark at the edge of one of the chain pins. Look at how the 12" mark lines up with the chain pin at the other end. If it's more than about 1/16" off of the edge of the pin at that end, consider replacing the chain, which will also likely mean replacing the cluster. If you're not having any problems with shifting or the chain jumping (especially on the smaller cogs), there's no great hurry, though. Chainrings don't wear anywhere as quickly as chains and clusters, so unless you see that the teeth have an obvious "shark tooth" profile, I wouldn't worry about it. And unless the derailleur pulleys are cracked, broken, or binding, I wouldn't worry about them either.