Setting up a Duopar Eco
 

Is there a specific chain length or secret process to setting up an Duopar eco? I have higher quality derailleurs that I am using but I have the Duopar in the shed and I would like to play around with it. I had it on my Super Course briefly while it was in the stand. It would not even handle the 33 on the cassette cog. The pulley rode against the cog because the second parallelogram was not dropping the pulley enough. I could push the parallelogram down, but it would pull back up as soon as I pedaled. I wondered if the chain needed to be shorter but I didn't want to break the chain since I was planning to put the Suntour one back on.

For the record, I had the correct washer to hold it in the right position on the dropout and I used the wedge screw to hold the derailleur in place while allowing it to move on the dropout.

I would first check to see if both pulleys turn freely.

I don't really know if the Duopar does or doesn't respond to changes in chain length in the same way that any other particular derailer does.

However, there are two ways that you might be able to test the effect of a longer vs. shorter chain, without actually messing with the chain itself.

You can of course shift the front derailer back and forth.
And (if your bike has horizontal dropouts), you can move the rear axle in both directions.

Hopefully, yours will work as well as the Duopar on my Grand Prix that moves the chain over a 6s 14-34t freewheel (quietest and most responsive friction shifting I've yet experienced).

I also have a DuoPar set up to manage a 34 tooth freewheel cog. I did not have any particular issue with it.

https://cimg3.ibsrv.net/gimg/bikefor...71842caf8.jpeg
1973-4 Grand Jubile along the canal outside Reims

Just in case you are not aware, don't run the chain backwards with the mech misaligned.
The chain can fall off the top pulley and then Bad Things happen.
There is a modification (on the foum somewhere) that prevents this.

The main problem as I see it is the inner cage plate doesn't come up high enough. Well that and the weakness of the second parallelogram.

Here's how I fixed the inner cage plate problem, by replacing it with a piece off a Suntour (a Cyclone I think).

https://cimg3.ibsrv.net/gimg/bikefor...badaf9df0a.jpg

Secondary advantage, it lets you take the chain out of the pulley cage without breaking the chain or removing one pulley. Like old Suntours of the V-GT style, only the opening is in back.

To head off the inevitable questions, no I don't remember how I did it exactly, so I can't give instructions. I did it about 45 years ago.

I think I can say it works though, because the chain never derailed off the pulley after the mod. Though maybe it wouldn't have anyway — they can go a long time without trouble even as-delivered. So I can't prove it's my mod that's responsible for the long streak of no derailing incidents.

Here's Mark Stonich's version:I think he brazed the extension on, which limits the usefulness to only those people who can braze, and only works on the Eco not the titanium one. The Ti one could be TIG welded, but it needs a back-purge, so even most people with TIG welders would not be able to do it without making or buying extra purging equipment. Unless they're already making Ti frames or parts. Hey that's an idea, bring it to someone who makes custom Ti frames. Maybe they'd take it on if they have time and if they would find this quirky little project interesting. (I know, I'm dreaming.)

Found a pic of Mark Stonich's mod to the cage:My mod, with the Suntour piece grafted in, brings the inner cage up higher than his, possibly making mine even better at preventing the
"DuoPar Blues". But it's possible to make it too high. You don't want the inner cage rubbing on the largest sprocket when the chain is on the 2nd-to-largest. Mine didn't rub, but I don't remember what the sizes were, or more importantly the jump in sizes from 2nd to 1st. I think the jump in sizes is what limits how high the cage can come. That mech isn't on any bike currently, so I can't really test that. (I sold the bike it was on, but the buyer wanted Campy, so I still have the one I modded.)

One nice thing about the Stonich method is you could start with it taller than the one in the pic, and then saw or grind some off, if it turns out to be too high.

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