Is my indexed shifter compatible with my long cage derailleur?
 

Here's the situation: I have a folding bike that I'm cannibalizing for parts. It had an indexed shifter (six speed) and a short cage shimano derailleur. I put the indexed shifter on an 18 speed mountain bike that has a long cage derailleur.

The rear cassette on the mountain bike also has a bigger range than the cassette on the folding bike did.

I have it tuned now so it is working somewhat reasonably but not quite perfect. It lines up perfectly for gear 6, but as I move up it skips a gear somewhere so that when the shifter is at 2, the chain is all the way on one.

What I'm wondering is whether it's just a matter of more adjustment, or if this shifter only works properly with a short cage derailleur. If that's the case I can swap out the derailleur but then will I need to shorten the chain?

Not nearly enough information.

In order to index all of the related component parts have to match. Some do, some don't. If you're mixing Shimano and Sun Tour parts, for example, I wouldn't expect them to work together very well.

It's all shimano.

I will try to provide a little more info...

Bike 1 was a six-speed folding bike. It had:
- A Shimano indexed shifter. I couldn't see a model name or number other than "SIS Indexed". It has the grey button for down shifting and the black lever for up shifting.
- A standard width six gear rear cassette. I measured about 1 1/8" width from top to bottom gear. Smallest gear has 14 teeth, biggest has 28.
- A short-cage shimano rear derailleur. All I sse on it is "Shimano SIS".

Bike 2 is an 18 speed Miele mountain bike. It had:
- Old-school handle mounted shifters. Not indexed, but they do "click" as you turn them, if that means anything.
- same width (1 1/8") rear six gear cassette. smallest gear has 15 teeth, largest has 36.
- A long-cage Shimano rear derailleur. Again, I couldn't see any other model name or number on it.

So far what I've done is to take the indexed shifter from bike 1 and put it on bike 2.

Unfortunately, as you haven't given us models or model #'s it's hard to be certain, but as you say bike 2 is not indexed, it a good bet that it's derailleur is not compatible with the indexed shifter.
The shifter & the derailleur have to be designed together so that the amount of cable pulled by shifter the produces an exact movement by the derailleur.

The length of the cage has nothing to do with the compatibility for indexing (a given derailleur model is frequently offered with multiple cage lengths and they work ok with the same shifters). But it sounds like the long-cage derailleur you currently have was probably not designed to index at all and may well have slightly different cable-pull requirements than the 'Shimano SIS' derailleur you have on the folder.

IF the RDER from bike2 doesn't sya SIS or have a Shimano model name/number on it, it's probably pre index era.
They didn't worry about it before indexing because they didn't need to. Or if they did, they would force you to buy an index compatible RDER.

Nothing wrong with using the shifter from bike1. Friction is pretty forgiving on a 6 speed.

Okay so it sounds like if I also move the derailleur from bike 1 over, then I should have a matching set and the indexing would work, but I worry that would create other issues:
- I might need to shorten the chain as the short cage will not take up as much slack. Not a big deal except I don't have a chain breaking tool right now.
- I understand there might be issues with using a short cage derailleur if there are three front gears?

Sound about right?

Thanks for all the replies so far.

Right now it works reasonably well so I might just leave it as is, I only use it for a 10 minute commute each way over pretty level ground.

Sounds to me like you should leave well enough alone if it's working ok at the moment.

The reason for having longer cages on rear derailleurs is to take up more chain slack as you shift between gears and have differing numbers of teeth in use on the chain rings and cogs. RDRs with longer cages have a greater "capacity" which is defined as the [largest chain ring teeth - smallest chain ring teeth] + [largest cog teeth - smallest cog teeth]. Your mountain bike with 3 chain rings will need a RDR with a larger capacity than the folder which only had a single ring. It's not just a question of shortening the chain since if you shorten it enough to work in the (small chain ring and small cog) combination then it'll be too short to handle the (large chain ring and large cog) combination.

Yeah, it's working well enough for what I need right now. If I want to improve things down the road maybe I'll buy a new set of shifter and RDR. The other RDR is pretty dirty and rusty anyway.

Thanks for all the advice, you saved me a potentially frustrating experiment at a time when I don't exactly have a ton of spare time. Will leave well enough alone.