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Front derailleur hits chainstay

Old 08-12-22, 04:57 AM
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smasha
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Front derailleur hits chainstay

LBS put together a new build that was factory specced with a 44-32T chainring (3x10), but I need a triple and wound up with a 40-30-22T.

Now the front derailleur (Shimano FD-T6000) sits about 8-10mm above the large ring, because putting it any lower crashes it into the chainstay on the small ring.

Any ideas for making this work properly?

Looking at the specs, I see that the FD-T6000 is made for (up to?) 44-48T rings, and has a capacity of 22T.

Other Shimano FDs (FD-M610, FD-M611, FD-M6000) are made for 40-42T big rings, with a capacity of 18T. And of course Shimano being Shimano, I can't find specs for the FD-M612 or FD-T670, and I'm not sure what else I should be looking at.

I'm wondering: Does a FD with an 18T capacity have a shorter cage than a FD with a 22T capacity? IIUC, an 18T capacity FD should work perfectly with a 40-30-22T crankset, and even if it still can't get as close as it should be, if it has a shorter cage it should be a better/closer fit with the big ring. And what seems like it should be obvious, I'd think that a FD made for 40-42T rings would be a better fit for a 40T ring than a FD that's made to fit 44-48T rings.

Or am I not understanding this properly?

Or is there a better solution?

Or do I need to build a bike with a double-chainring and a Rolly hub to get the range of gears I need?
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Old 08-12-22, 07:12 AM
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A front derailleur spec'ed for 40-42T big chainrings should have a shorter cage and better curvature than one spec'ed for larger chainrings. You can get around the total teeth recommendation by never using small-small to keep the chain from dragging on the fd's tail.
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Old 08-12-22, 07:26 AM
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This could be a chainline issue. When chainline is wrong, you're likely to have a variety of shifting problems, and you're describing one of them. I'd start by measuring your chainline and making sure it's correct for that frame.

https://www.sheldonbrown.com/chainline.html

Last edited by Jeff Neese; 08-12-22 at 07:34 AM.
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Old 08-12-22, 07:29 AM
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Originally Posted by HillRider View Post
A front derailleur spec'ed for 40-42T big chainrings should have a shorter cage and better curvature than one spec'ed for larger chainrings. You can get around the total teeth recommendation by never using small-small to keep the chain from dragging on the fd's tail.
Cross-chaining is a bad practice but all gear combinations should at least work without anything rubbing or dragging.

My money is on the chainline being wrong.

Last edited by Jeff Neese; 08-12-22 at 07:35 AM.
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Old 08-12-22, 08:09 AM
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Originally Posted by Jeff Neese View Post
Cross-chaining is a bad practice but all gear combinations should at least work without anything rubbing or dragging.

My money is on the chainline being wrong.
I'm in the habit of moving the chain in (towards the bike) when I shift down, and moving the chain out (away from the bike) when I shift up. It's just a habit to not have the chain crossed.

That said, a quick test-drive revealed that in the smallest chainring, the two largest cassette cogs caused the chain to rub against the inside of the FD cage. That's definitely not a gear combination that should be rubbing there.

So the problems may be that the current FD is made for larger rings and a larger big/small difference, and/or it may be that the chainline is wrong?

So it may need a FD that's made for smaller rings with smaller capacity, and/or it may need the crankset adjusted farther out, away the centre of the bike?
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Old 08-12-22, 08:12 AM
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Originally Posted by Jeff Neese View Post
Cross-chaining is a bad practice but all gear combinations should at least work without anything rubbing or dragging.

My money is on the chainline being wrong.
Yes all ratios should work but there are times when, for practical reasons, you exceed the derailleurs design parameters and avoiding one or more gears is a worthwhile compromise. Not using small-small is a trivial sacrifice. You may be right about the chainline but having to mount the fd excessively high to keep its tail off of the chainstay is a clear problem.
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Old 08-12-22, 08:20 AM
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If the manufacturer doesn't offer that model bike in a 3x configuration, then the frame might not be able to properly handle a 3x. As you are finding out, you now have to make some compromises or do something else, at more expense to correct those compromises.

Can you return the bike and get something that is built to be 3x or have the low gearing and range you need?
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Old 08-12-22, 08:34 AM
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Originally Posted by smasha View Post
That said, a quick test-drive revealed that in the smallest chainring, the two largest cassette cogs caused the chain to rub against the inside of the FD cage. That's definitely not a gear combination that should be rubbing there.
Try adjusting the low limit screw.
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Old 08-12-22, 08:39 AM
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Originally Posted by smasha View Post
I can't find specs for the FD-M612 or FD-T670, and I'm not sure what else I should be looking at.
Specs for both can be found in Shimano's 2014-2015 Specifications document. It can be downloaded from Shimano's archive in PDF format at this link:

https://productinfo.shimano.com/#/archive

The 670 appears to be rated for 44-48T max large chainwheel if I'm reading the doc correctly. The 612 is rated for 40-42T.
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Old 08-12-22, 09:08 AM
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Originally Posted by smasha View Post

That said, a quick test-drive revealed that in the smallest chainring, the two largest cassette cogs caused the chain to rub against the inside of the FD cage. That's definitely not a gear combination that should be rubbing there.
If I'm understanding you correctly, that can be corrected by simply adjusting the limit screw, unless you're already at the low limit already. The chain rubbing the inside of the cage is different than the cage hitting the chainstays.

As Iride01 suggested, are you trying to get a triple to work on a bike designed for 2X?
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Old 08-12-22, 10:08 AM
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Originally Posted by Iride01 View Post
If the manufacturer doesn't offer that model bike in a 3x configuration, then the frame might not be able to properly handle a 3x. As you are finding out, you now have to make some compromises or do something else, at more expense to correct those compromises.

Can you return the bike and get something that is built to be 3x or have the low gearing and range you need?
Altus, Acera, and Alivio are available in 3x9, but 3x is just out of fashion for higher specced groupsets and new bikes.
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Old 08-12-22, 10:11 AM
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Originally Posted by HillRider View Post
Try adjusting the low limit screw.
No amount of limit screw adjustment will bring it closer the big ring *and* not crash into the chainstay on the small ring.
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Old 08-12-22, 10:14 AM
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Originally Posted by smasha View Post
Altus, Acera, and Alivio are available in 3x9, but 3x is just out of fashion for higher specced groupsets and new bikes.
I wasn't asking about whether the component groups have 3x versions.

It's the frame design that sometimes determines if you can put those 3x components on your bike and have the proper chain line without the chain, chain ring or crank arm rubbing the frame as I thought you indicated it was doing.

If the bike manufacturer never offered that particular model bike in a 3x version then it might be it won't handle a 3x crank. At least not without some more effort and expense on your part and quite possibly further compromises for how well the gear train shifts.
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Old 08-12-22, 10:21 AM
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Originally Posted by Jeff Neese View Post
If I'm understanding you correctly, that can be corrected by simply adjusting the limit screw, unless you're already at the low limit already. The chain rubbing the inside of the cage is different than the cage hitting the chainstays.

As Iride01 suggested, are you trying to get a triple to work on a bike designed for 2X?
As the FD moves in, it also moves down. It can only move in so far, before it hits the chainstay.

Yes, the bike is factory specced for 2x10, but before I ordered it, the shop said it should work fine with a 3x crankset.

I'm starting to suspect (and hope) that the FD is just too big for the crankset. If that's it, it should be an easy fix.
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Old 08-12-22, 10:42 AM
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Originally Posted by Iride01 View Post
I wasn't asking about whether the component groups have 3x versions.

It's the frame design that sometimes determines if you can put those 3x components on your bike and have the proper chain line without the chain, chain ring or crank arm rubbing the frame as I thought you indicated it was doing.

If the bike manufacturer never offered that particular model bike in a 3x version then it might be it won't handle a 3x crank. At least not without some more effort and expense on your part and quite possibly further compromises for how well the gear train shifts.
I'm not sure, but I think it's the same frame that was factory specced with an Acera 3x9 (22T small ring) in 2018.

If that's correct, there should be a 3x FD that fits it well.

Just found another source that shows that model (I think the same frame) in 2018 being available with an FC-M351, 44-34-22T crankset and "Acera" FD. Of course, during the 2017-2018, and 2018-2019 model years, I can't find any 3x10 FDs in the Acera lineup, but I'm getting hopeful that the FD that's now on the bike is just too big for the crankset and/or the cranks are too close to the frame.
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Old 08-12-22, 11:43 AM
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Random thought - the old, narrow Cyclones (stay away from the Mk1s; not stiff/sturdy enough) had shorter cages that would work well and I never had issues with them not having enough reach inboard. I ran them on triples for many years........ The two "issues" - narrow cage; in the inner chainring, you have to re-trim most rear shifts and the chain will rub on the bushing at the cage bottom on the small cassette cogs. I used to remove the bushings and replace with even shorter stacks of small washers. (Narrow cage - better, more secure shifting and few chain drops.) Hardened steel chains hardly care if they are rubbing on milder steel washers and it takes a long time to wear out those washer stacks. Might cost you a quarter to replace them. The rest of the problem is a little noise that can cause real issues between the ears.

Those short cage Cyclones (and other FDs of the era) might work well for the modern small chainring triples where the percent gear ratio changes are big but the number of teeth differences not very different from what I ran. I ran 52 (sometimes 53) -42-28 forever with those Cyclones. 24 tooth difference. You are talking 44-22 = 22 teeth. Now there will be issues that may be show stoppers. Indexing. Total chance that it would work. Ability to trim the FD? The need for old school seattube diameters. (Every Cyclone I've ever seen was clamp-on, not braze-on mount.)

And an old-fart gripe - why can't modern FDs be made to have big reaches inboard? That costs you zero when the FD is narrowed to double width with the 'low" limit screw. I now have a later model Cyclone on a ~1990 steel race bike I just set up and it works like a charm over the sweet 52-42-30 Campy Chorus crankset. Same gaps you are running. Set-up was easy; like that was supposed to happen. Off course, friction DT shifting.

Edit: another thought - look at an old Cyclone. Notice the bolt that runs through the bottom of the cage with a bushing slipped over it. Now buy a small bolt and tube to make the bushing or the stack of washers I described above. And surgery time. Drill through both cages for your bolt as close to the bottom of the cage as you can and still have the cage short enough to not hit the chainstay. Hacksaw the rest of the cage off. Slip the bolt through from the outside cage, through the washers or tubing, the inside cage and secure with the nut lock-tited in place. (I'd do this with a common, easy to replace FD in case this doesn't work.) You now have a short cage triple FD that misses your chainstay and while rub will happen in the small cogs, it won't matter.

Last edited by 79pmooney; 08-12-22 at 11:56 AM.
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Old 08-12-22, 12:37 PM
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To back this up a bit. I haven’t found a problem swapping out double for a triple; the exception is chainstay clearance. But if you can get a decent chainline and inner chainring clearance there should be no issue.

You do have a couple of potential issues. The obvious one is a wide gap between the outer chainring and the FD cage. It is typically caused by an incorrect FD for the chainring you are using.

It may or may not be a problem. If everything shifts smoothly, the large gap may not be a problem. The may part has to do with throwing a chain. I would think there is a greater potential especially when in smaller (outboard) cogs and shifting from middle to outer.

Your SL-T6000 is a Deore trekking FD, so a Deore “M” FD that is spec’d for a 40t would be a good change.

You second issue of chain rub on a triple in the small-small with a non-trim trigger shifter. Part of it is “welcome to 3x10 MB reality” and part of it could be chainline. Ideally, you want to make sure the middle chainring hits all the cogs and is centered.

A shorter more curvature M FD cage “might” give a tiny bit more chain clearance. But if the chainline is good and you get a bit of small-small rubbing that’s life. Years ago it was possible to add a small washer to spread the end of the cage a bit, but rivets ended that option.

John

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Old 08-12-22, 03:13 PM
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Part(most?) of the problem could be related to the bottom bracket drop and the corresponding angle between the chain stay and the seat stay. Shimano makes some FD in spec for 63-66 degree angle and some for 66-69 degree angle. The angle between the cage and the clamp is different allowing smaller rings to have proper vertical clearance in relation to the FD without hitting the chainstay. Some also have a shorter cage. You might look for the T(Trekking) and M(mountain) series that are available in both angle configurations for comparison. A road triple FD might offer you what you need for clearance on the chainstay but adjustment can be a bit challenging as the cage is intended for larger chainrings-it will work but you need patience for the initial set up.

Mainstream manufacturers don't really want cyclists to have gearing that is actually appropriate for the riding that they do.
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