Tandem Tom

I've got a situation with a XT FD where the cage is out too far in the High position. Looking at the profile of the outer platet "bulges" inward. Wondering about the feasibility of flattening the bulge and making the cages a bit narrower?
Thoughts?
Thanks!

79pmooney

I've been modifying FD cages a long time. In the old days (pre-CPSC) we took crescent wrenches to bent the front tabs in to speed up shifting. For years I,ve been removing the bolt between the cage plates at the bottom and reinstalling with #4 washers in a shorter stack than the original bushing to improve shifting onto the inner rings of triples. I wouldn't have a second thought about placing the outer cage in a vise and flattening it a little to improve shifting or reduce crank contact. (Unless it is a bulged shape where crushing the bulge would reduce stiffness or strength. Old SunTour, no big deal, new highly shaped Shimano? Maybe not.)

Kontact 

Out too far in what way? What's it doing that you don't want, and why is adjusting the angle or hi stop not effective in correcting the problem?

Tandem Tom

The situation is a Surly Ogre with a 73mm BB shell and trying to us a Velo Orange Rando crankset. I can almost make it work but the crank arm is hitting the cage.

Kontact 

Why are you trying to use a triple FD on a double crankset? One of the reasons that style of crank design was low Q is that there is only enough clearance for a relatively old school double FD with a flat outer plate.

The part that is hitting is there to provide some cross chain clearance in the granny gear. You will likely find that if you try to flatten that bottom step, the outer cage plate will effectively get longer front to back than it was, causing it to bow out in the middle, with all the clearance and shifting problems that come with that.

A used double front derailleur would be a cheap and easy solution to your problem.

Tandem Tom

I came in on this project at our shop after it was started. We have tried a few different BB and have a good chain line. It is the clearance between the chain ring and crank arm that is the problem child.

Jeff Neese

Does this imply that you changed the crankset but kept the same BB? It sounds like you may have a chainline problem. Sometimes you get lucky, but oftentimes you also need to change the spindle length when you change the bottom bracket.

I would measure your chainline to see where you're at, before you start bending (or swapping) the derailleur. I'm going to bet that it's off, which could indeed cause the problem you're describing.

maddog34

install a different crankset or a different derailleur..... that crank is wrong for the bike and derailleur combo, period.

you're blaming a well engineered part that is used worldwide, as-is, for a bad decision on another part.

you might be able to find an outdated der. that will kind-of work....something like an LX/DX age der.

once you beat up the der. on the bike now, it will become scrap.
And "Barely enough clearance.... kinda." in the repair stand equals "Not Enough Clearance" when riding the bike.

maddog34

explain how changing the chainline will increase the clearance between the outer ring and the crank arm.

Jeff Neese

Part 2 of my reply. You do need to ensure correct chainline or you're setting yourself up for a variety of problems, but is this the same derailleur that was on the bike before?

If so, changing a crankset almost always involves moving the front derailleur up or down the seat tube, unless of course the large chainring is the exact same size. With a smaller chainring, the derailleur is going to be too high which could contribute to the problem. Use the "nickle test".

70sSanO

Such a lost art. Of course, so many newer FD’s are riveted these days, but your post soothes my soul.

John

maddog34

What's going on is that the outer chainring is too close to the Crank arm to permit using a modern design MTB Derailleur...
the OP already stated that the Chainline is fine, and i noted that changing it would be futile in this situation.
The Velo Orange crankset is almost certainly a ROAD crank... i'd guess that it may also have Arm clearance issues with the chainstays too... any less than 1/4" is asking for trouble.

i'd like to see the angle and/or radius mismatch between the der and the outer ring, too... i bet it's not right, unless someone already bent the cage in that respect.......

funny thing... no one has asked the tooth counts on the crank rings... or the exact MODEL Number of the Crankset......

i guess bending up a nice FD is much more fun to type about.//

Jeff Neese

I missed that it's rubbing on the crank arm. He didn't initially specify what he meant by "too far out". He also didn't initially say that he checked the chainline.

I guess I assumed he would have adjusted the FD for that crankset, but that was admittedly a bad assumption as well. Without pictures it's hard to know what's really going on. I wouldn't be so sure that the crankset and derailleur won't work, without seeing it. I have put road cranks on MTB frames with no issue, and without having to change the FD (or worse, bending things). But again, I always start with a good chainline and correct position of the derailleur.

maddog34

that first post was a bit vague, eh? i read the others before responding... something i've neglected to do in the past.

we still don't know the tooth counts, model number of crank, or the der. part number....
stock randos have a 46/30 combo... and are a "narrow road" configuration.
the OP needs a flat plate double road FD..... or an old school MTB FD, if shifting is friction type... and some place with a wide selection of old Ders to test fit....... or put a different crankset on the bike... kinda sounds like the rando was chosen, and they're doing what they can to make it work.

answer: Find a more appropriate FD...

3alarmer 

...would it be possible to post a photo or three of what you have there ? It is true (as has been mentioned already), that not all front derailleurs work well with all cranks and chainrings. (Or work at all, for that matter). Since it is an "inherited" project, maybe it was not well thought out to start with ? Anyway, my apologies for the current state of how threads in mechanics, asking for help with simple problems, all seem to degenerate into a stream of arguments. At this point, I would be reluctant to make any suggestions at all. But I also date back to the days when it was routine practice to bend the leading edge of the flat plates on front derailleur cages, to make them shift much better.

I'm just not really sure that bending will either work well in your case, or is even a good or bad idea.

I think I understand the issue to be: that the cage you have is pretty wide, so in order for the inner plate to contact and push the chain onto the large ring of your double crank, the whole cage ends up so far out that it interferes with the crank arm. I personally would not use bending in such a case. It might work, or it might not. It's possible that rather than flattening the derailleur cage as you are intending, you might get away with bending just the interior leading plate edge, inward, so it throws the chain sooner. Then your cage won't end up so for outboard as to interfere, and you can just use the limit screw to reduce throw. Hard to predict how well that might work.

Also, make sure you've got good alignment of the cage. Sometimes if the derailleur is a little cockeyed on the seat tube, either the front or rear of the cage will interfere with the crank.

Kontact 

You'll want a very flat outer plate derailleur like this 600EX from 1987.



A braze on version will help because you can use the curved mounting style adapter like below to adjust for your wide chainline. Turn the adapter counter clockwise to the limit of the FD mounting screw to push the FD to the right.

Jeff Neese

You might be right, but I still would like to see some photos. I wouldn't automatically assume the current derailleur won't work without knowing more.

The original post gave me definite "newbie" vibes, but then we learn a few posts later that it's at a shop and nobody else can figure it out either. Just the idea of bending the derailleur cage to fix it seems very questionable, coming from a bike shop or bike mechanic. Regardless, I think we're all guessing and can't really help unless we know more, preferably with pictures.

Kontact 

I'm not guessing. I have dealt with modern FDs on low Q, 70s style cranks before and there isn't room for any sort of curved outer cage plate.

The tooth count is somewhat irrelevant since no one made compact chainring specific derailleurs with low clearance outer plates.

The OP should spend $15 on a vintage FD and try an appropriate solution to their crank compatibility problem.

maddog34

the VO Rando is the same rando that was around decades ago. it's a Vintage 6 bolt design with a nice polished finish.
personally, i think the arms flex too much. Not a good choice for stompers like me.

Jeff Neese

A Velo Orange Rando is hardly a low quality 70's style crank. And we don't know anything about the FD except that it's Deore XT - there are different versions and configurations. And of course tooth count matters - that determines the position of the derailleur.

There's nothing "vintage" about this setup. It's a Surly Ogre with a modern crankset. A vintage FD may or may not work. The OP is not sharing any more information and we still do not have pictures. Until that happens, everything else is just guessing.

Normally I'd advise him to get the bike to a good shop, but he's saying that he works at a bike shop.

Kontact 

What is low quality about a nice crankset made in the '70s? It isn't a quality issue, it is a dimensional issue. And Velo Orange advertises that many of it's components are designed to work with vintage set ups, and that the crank has low Q. That means limited space for complex outer plates. What are you not understanding about that?

A new double FD may work, But old derailleurs definitely work - especially with modern chainrings. A lot of what changed about derailleurs was not to make them work but to make them work with indexing and without trimming.

Jeff Neese

A derailleur designed for a vintage road bike with a 43.5mm chainline may not work on a bicycle with a chainline of 50mm. Can you guess why?

Kontact 

I don't have to guess, and I already addressed how to get some additional right movement with the derailleur, but the 73mm double chainline is only 46. As plenty of tandems use road triple FDs, I think tandem makers have already addressed derailleur location adequately. There are no tandem specific derailleurs, and brifters tandems can't use MTB derailleurs.

The OP is doing something uncommon - using a double crank on a tandem, and an unusual one at that. The derailleur is going to be an experiment as well

Jeff Neese

A Surly Ogre is not a tandem, and has a chainline of 50mm. I'm not sure you should be giving any advice here.

L134 

Regardless, Kontact suggestions, in my experience, are addressing the issue as described which is fairly common when using a low Q factor crank. I don't believe chainlne has anything at all to do with the problem as described. Sometimes I find I get my best shifting with the derailleur set such that, at its most extended position, there will be slight contact with the crank arm but one simply completes the shift and then immediately trims the derailleur back in very slightly. This is very easy with friction shifting and becomes automatic, but maybe not with indexed?