Hi,

I've searched and searched and while maybe I haven't done the best job, I'm at wit's end. I bought an older bike (1992) with a 7 speed RX100 drivetrain. It has indexed downtube shifters and the rear derailleur spring is no longer strong enough to pull the derailleur all the way down to the smallest cog. Rather than tearing the derailleur apart and trying to replace the sprint, I'd like to just replace it. My question is, will a modern derailleur work with the indexing of my downtube shifters? I read that a derailleur really isn't speed specific - it travels the same amount of distance no matter what. So my question is, what creates these indexes? Is it a function solely of the shifter, or is there something else I'm missing? It would seem like I could get any newer derailleur and it should work fine with my 7 speed cassette. Is this true?

If not, then I really don't know what to do. The RX100 ones are hard to find, especially NOS. I'd hate to think I'm out of luck

Thanks so much for any insight anyone has.

-Collin-

Another SIS Shimano derailleur (pretty much all of them with a few old DuraAce exceptions) will work fine, brains are in the shifters (and the amount of cable pulled, so SRAM derailleurs are different).

Awesome, that's great news. Looks like I can pick up a modern 105 derailleur for must less than trying to find a NOS RX100. I don't exactly care to keep this bike original as it's a 1992 Giant Allegre.

Out of curiosity, is it a common problem for Shimano springs to lose their strength and not want to go into the smallest cog? I'd never seen it before until yesterday. Even after loosening the limit screw, I had to push the derailleur by hand to get it into the smallest gear.

-Collin-

Does it behave that way even with the cable/housing disconnected? You probably already ruled that out.

It's not just the cable pull. The rear derailleur's actuation (or throw) ratio is also critical to the distance traveled per click. Fortunately all Shimano SIS or STI RD's have the same throw ratio with the exception of pre-9-speed Dura-Ace
Another consideration is the width of the cage compared to the overall width of your chain. In your case I would try to find a derailleur closer to your 7-speed generation. Not sure but I'm thinking that a 10-speed RD may have a narrower cage. You could run an 8-speed chain with a slightly narrower overall width.
I agree with JanMM. Check to see if the problem is excess friction in the cable housing.

Al

I once read a comment that all derailleurs are unionised, they work for a certain numbers of kilometres per dollar paid. Funny and seemingly very true.

Buy a low end 9 speed derailleur (Sora or Tiagra) and run it with an 8 speed chain. Everything will play nice together. I just did this on my wife's flatbar road bike, works fine.

I already tried without the cable/housing attached. That was my first guess too.

Will a Sora/Tiagra shift as nicely as my RX100? Well, it doesn't shift now but you know what I mean

-Collin-

Tiagra is very close to 105 and is considered good quality and will give you very good service. Sora is decent but a bit heavier and not as well finished. Either is adequate but go for the Tiagra.

Since you've eliminated the cable and housing as the problem, have you thoroughly cleaned and relubed the derailleur's pivots, springs and pulleys? Crud buildup in the derailleur itself could be your problem. I've had several Shimano rd's last 30,000 miles so they don't wear out quickly.

BTW. "8-speed" chain is the same as 7-speed chain. Most 8-speed chains are marked 7/8-speed.

A 7 or 8 speed rear derailleur (Sora or older Tiagra) will work. A 9/10 speed (105, etc) derailleur might be too narrow for your chain. If anything, it would likely just cause some noise. I'd just put a Sora on it.

Yes.

I was making the point that you can safely use the current (9 speed) Sora / Tiagra derailleur with the old (8 speed) chain, as some posters seem to be in doubt about this.

Hi, my memory is a bit sketchy but I think Shimana RDs of that era had a
spring tension adjustment dial on the RD's inner link plate's underside.
The dial looks like a flathead screw and it had 3 or 4 preload settings.
Your RX100 RD might have that. If it has it, increasing the tension may
solve your problem.

Good advice above, and also check that the frame tab for the derailleur hasn't been bent in.
A minor hit to the bike's right side can cause this.
And if it's bent, a trip to a shop may be required to get it properly realigned.

You could try this:
https://cgi.ebay.com/New-Old-Stock-Sh...#ht_2850wt_962

7 speed, nice derailler, put it on one of my touring bikes.

It's been a while since I measured but I don't recall seeing any real cage width difference between 8, 9, and 10 speed rear deraillers. Front deraillers are a different story.

I've never measured. I just remember the propaganda from Shimano in the 90s that said that "everything is narrower" and that you "must" upgrade everything. It wouldn't surprise me if there's no difference at all.

If I remember, I'll measure again this weekend and post the numbers. If nothing else, Shimano has likely adjusted the freeplay of the upper jockey pulley to deal with tighter spaced cassettes. Not sure what effect that would have on an older system though.

Thanks so much for all the help guys, much appreciated. I think I'll take the derailleur off tonight and give it a really good cleaning with degreaser and then re-lube it. The bike is all RX100, so I think keeping the matching derailleur is ideal. Not the end of the world, though. I'll see if I can't find that spring tension setting. Also, I'll see if the derailleur hanger is bent. That would be a bummer. If I can't fix it, I think I'll go the Tiagra route with a new chain.

Thanks again. Really appreciate the help in getting this bike back on the road!

-Collin-

FWIW, the one derailler I came across that would not return full to the high position (small cogs) had a cracked pivot that caused the derailler to bind at that point. It worked fine in all larger cog positions.

How about an Ultegra for less than a Taigra:

https://www.cambriabike.com/shopexd.a...EAR+DERAILLEUR

I am kind of surprised that you don't know this, given your experience, but most of Shimano's 'compatibility rules' are complete BS (bike silliness).

You don't need to match number of speeds of chain with derailleur any more than you need to lather rinse and repeat to make shampoo work.

I thought I replied, but I guess it didn't get posted. Anyway...

It turns out that after a thorough cleaning with carb cleaner and watching all the gunk loosen off of the derailleur, it snaps back to the smallest cog with no issues at all. It actually went too far because I'd been tinkering with the limit screws. Now, everything is back to normal.

I'm a tad embarrassed, actually. The derailleur didn't even look that dirty, but I guess inside is what counts. After cleaning it out, I lubed it with Tri-flow and it shifts like new again. I'm really happy that I don't have to replace it.

Thanks a million for the suggestions. Much appreciated to have that kind of support!

-Collin-

Carb cleaner is great for cleaning chains too if you already have it sitting around.

I used carb cleaner first to clean my Ultegra STI shifter and it took the finish right off the lever, which sucks. It sure cleaned out the inner parts well, though! Just have to be sure to re-lube everything after because it leaves it completely dry.

Anyway, thanks again!

-Collin-

Use "Dry Lubricant" to clean it next time. It's part solvent, part lube, and safe for most finishes. Lube with real lube afterwards, though.

Or, if you need a more powerful cleaner, try Throttle Body Cleaner next time. It's like carb cleaner, but is safe for Teflon-coated throttle plates (most Fords require it rather than a carb cleaner).