bark_eater 

I've got a couple bikes with Suntour Mountech Derailleurs, and have been told that the rear derailleur is a ticking time bomb and should be replaced. I was wondering if I could I could swap out the Mountech long cage with a Suntour ARX cage and keep the appearance of a matching groupset?

wrk101

I think ticking time bomb is a gross exaggeration. The one on your bike is likely 35 years old or older. it’s a design I don’t care for, but 35 years life is pretty decent!

Now in the world of friction Suntour derailleurs, I'm a huge fan of the Vx GT. That is my standard go to derailleur.

IMHO, the front Mountech derailleur is not a very good match for the rear. So a Vx GT would look just fine. I like the aesthetics of the Vx line much better than ARX.

steelbikeguy

I wouldn't call the Mountech a ticking time bomb.... I think they are more like a modern electronic time bomb... no ticking at all.

I happen to have one of these in my parts box, purchased only as an item of curiosity. For the sake of the reader, let me offer a couple of photos.





As can be seen, there is an extra piece between the lower derailleur pivot and the upper pulley. There is a pivot and spring built into that upper pulley, which is where things get troublesome, at least that's what I have heard. I'm not in the mood to find out for myself!

It does look like the derailleur body is the same as a Cyclone Mk II, shown below....



I've never owned a Suntour ARX, so can't comment with any authority on how compatible it would be, but it seems like there is a good chance that the ARX cage would work with the Mountech body. I'd like to think that SunTour didn't make tiny changes that prevent using common components on such similar derailleurs.

Steve in Peoria

clubman 

Correct me if I'm wrong but the failure is in the pivot point location so swapping the cage would do nothing to solve the Mountech's issues? Use the Mountech until it fails, or swap it out. No guarantees either way.

steelbikeguy

I think the problem is at the upper pulley, which is also a pivot point and contains a spring in addition to the pulley's bearing. ... but... would like to hear from someone who has run into this problem. My understanding is that the design makes it difficult to lube the upper pulley bearing and the associated pivot.

Steve in Peoria

panzerwagon 

The issue is only present in the earlier models, with the complex upper pulley design. Later models used a conventional upper pulley, and is one of the finest friction-era long-cage RDs. The issue itself is a bit overblown, and with proper maintenance and a good cleaning regimen can remain perfectly functional. Wash off mud early and keep it lubricated. I would not hesitate to run it on a road touring rig.

clubman 

Agreed, it's what I was trying to say, but didn't really.

bark_eater 

ARX mention and a bomb bit:
https://www.disraeligears.co.uk/site...leur_4900.html

jdawginsc 

I’ve never had extensive experience with the Cyclone GT, but the VXGT and the ARX might be the most bomb proof derailleurs I have ever used.

For friction shifters, they have never failed. The ArX is actually good for MTBs...7 speed max though.

Schreck83 

Looks like it will:
ARX exploded view on Disraeli Gears

There are different flavors of Mountech
Here's one from 1984:


Mountech. Lots of friction surface on the upper pulley.

Mountech

ARX

The Golden Boy 

Right!

The Mountech was a re-engineered "improvement" of the Huret Duopar. The Duopar has two parallelograms (hence "duo" and "par"). Apparently, the Duopar always rated "better" on Frank Berto's shifting machine- and Suntour were chasing that title- so they took the MII/AR body and incorporated the 2nd parallelogram and marched off to world domination. Where the Duopar had an external spring in line with the pulley to tension the cage, the Mountech had a coiled spring enclosed in the upper pulley mechanism. And that's where it gets all complicated and funky.


Sachs/Huret Duopar by Dave The Golden Boy, on Flickr
Sachs/Huret Duopar by Dave The Golden Boy, on Flickr



So then Suntour finally gave up on the silly idea and combined the ultra long cage of the LeTech- it also added like a 15 tooth lower pulley- so this version is like a more burly LeTech.

2nd Generation Mountech by Dave The Golden Boy, on Flickr


It shifted "OK," and I imagine it could handle an epically colossal amount of chain wrap. I assumed mine may have had a relatively rough life- or maybe that extra parallelogram and super long arm and monstrous pulley didn't do the best for shifting; but that ultra long cage could not be good for any part of the derailleur or the derailleur hanger- that's a long lever for a little something to do a lot of pulling with a little energy. I think my LeTech also has the same issue. Short story long- if it's got the stylized script it'll be less susceptible to blowing up catastrophically than the block letter script models.

FWIW- I bought a 1984 Stumpjumper from the original owner that would have come with a regular Mountech. When I got the bike, most everything was stock- except it had both an XT derailleur and a replacement rear wheel from around 1992/93. I would lay money that's when the Mountech blew up and thew itself into the spokes.

jonwvara 

I put a first-generation Mountech on my Miyatata 1000 a few years ago, then rode it without incident from San Diego the the east coast of Florida. As I recall, I got it out of the rear derailleur bin at the local bike coop, looked it over, dripped some lube onto it, and called it good. And it was good.

But true, that's only one data point. I would guess the the problems occurred on examples that went on hard-ridden mountain bikes.