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Half-step front derailleurs

Old 08-01-21, 08:48 PM
  #1  
ShannonM
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Half-step front derailleurs

So, I built up a Deore FC-MT60 crankset as a 45/42/30 half-step + granny to run with the 14-26 6-speed Suntour freewheel on my Fuji. Got all the parts except the 45T and 42T chainrings out of the bins at the co-op. All was looking rosy, until it came time to put a front derailleur on it.

There wasn't one in the bins that would work. The closest I came to a working one was a Suntour AR double, but it didn't quite have the swing to reliably shift to the 45 with a 68x122.5 bottom bracket. Looks like I'm gonna have to buy a front derailleur.

The obvious choice would be the half-step specific Deore, but I figured I'd ask the Brain Trust if there are other ones that are known to work. I'd prefer a Suntour, since the rear is a mint 1980 Superbe that I love to death, but that's a want, not a need. Also, sanity check: 28.6 seat tube, right?

--Shannon

Last edited by ShannonM; 08-01-21 at 08:54 PM.
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Old 08-01-21, 09:08 PM
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Questions: Does the gap between your 4th and 5th FW cogs line up with the 42 tooth front when viewed from behind the bike? (I put the bike in a stand and raise the rear until I can sight right down the chain/cogs and chainrings.) If the 42 is sitting to the right of the gap, check to see that you have clearance between the 30 and the chainstay, Also the crankset and the bottom bracket face and bike's BB shell. If yes, you could get a shorter bottom bracket and bring the crankset inboard.

My experience is that Shimano likes to place the cranks, chainline, etc. further outbord than SunTour used to. (Makes it easier to simply drop the stuff onto any old bke and have plenty of clearance. Good for production. Less so for those of us who like the old stuff. (And in my case, the old, narrow Q-factors my knees still insist on.) I saw 122mm and my first thought was "wouldn't a 118 work? That would bring the 45 in 2mm.

Edit: There's been a little corrosion in these circles. Here the Brain Trust has been reduced to the Brain rust.
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Old 08-01-21, 09:11 PM
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While not half-step specific, I've been using the stock Compe-V that came on my '75 Fuji S-10S to shift the Sakae 48/38/28 triple I put on it 35 years ago. IIRC, I reused the stock spindle from the original double crank. It may not be the best for a triple, but it works! Triple-capable front derailleurs typically have that 'dropped' inner cage to help with shifting from the small ring.

AND, FWIW, my '84 Univega Gran Tourismo is equipped with its original SunTour Mountech front derailleur for its half-step w/granny triple crank.
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Old 08-01-21, 09:21 PM
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Obligatory picture:



Quick n' dirty measurement (measuring tape and Mk. 1 Eyeball) shows about 1/8" from the 30 to the chainstay, call it 3-4 mm. Looking down the chainstay has the outer ring between the 4th and 5th cog. Looks like the 122.5 is the correct BB for this crank on this frame.

I'll be running it as a 1x6 for another week, it seems. Which is fine, I've been doing that since I bought the bike last year.

--Shannon
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Old 08-01-21, 10:05 PM
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Next trick - when the chain is on the 45 (shifting it up there by hand if you have to) how much clearance to you have between the outer FD plate and the chain in high gear and how much between the inner plate and the chain when running to a middle cog?

If the chain clears the outside plate in high gear, OK it works without rubbing, just the FD won't get it there. If you have a lot of clearance on the inside, second scenario above, well the FD needs a little oomph to push that chain. Ready for a sacrilege? Break out the almighty crescent wrench. (English works but metric is far better. ) Now, notice that the inner FD plate runs forward of the rest of the FD? Tighten the wrench around the forward-most part of it, wrench handle facing forward. Tweak that cage plate front a little to the outside - to reduce the gap to the chain. Try shifting . I'll bet it's better.

Edit: another thought. How much gap between the FD cage and the chainrings. Do the full sweep with the shifter and move the FD down until no more than 2mm is showing at the closest point. (Usually as the outer cage passes over the outer chainring but you may be seeing it as inner over middle chianring or I could be completely out to lunch. I've never done a half-step.)

Also, you can often tweak the FD rotating on the seattube for better shifting, Mark the seattube with tape after the clearance step above so you know if it has dropped as you turn it. We used to call FDs bumbebees. In theory they shouldn't fly but with enough evolution, it seems they do.

Now, if the FD is too new, there may not be any of the cage plate forward of the rest of the FD. Fear of OSHA intervention meant that the FD makers stopped making FDs with that handy tab because it might "qualify as dangerous" in a crash!! Go find an older one. This is a trick mechanics have been doing forever. In the '70s it was routine. (Not all bikes/FDs needed it but if they did, we didn't think twice.)

Last edited by 79pmooney; 08-01-21 at 10:14 PM.
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Old 08-01-21, 11:01 PM
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Originally Posted by 79pmooney View Post
Next trick - when the chain is on the 45 (shifting it up there by hand if you have to) how much clearance to you have between the outer FD plate and the chain in high gear and how much between the inner plate and the chain when running to a middle cog?

Edit: another thought. How much gap between the FD cage and the chainrings.

Also, you can often tweak the FD rotating on the seattube for better shifting.
I don't have a front derailleur on the bike right now, but your in-depth post deserves a response, so:

I tried almost every road FD we had in the Kitchen that looked like it would fit. The best was the first one I tried, a Suntour AR, which would sorta-kinda-maybe make the shift to the 45t, and once it was there it wouldn't rub, but it had zero overshift capacity with the LaPree levers, even with the upper limit screw backed all the way out, and so the front upshift was really, really janky. On the road and under load, it wouldn't have worked at all.

And, yeah, I played around with mounting height and angle. The best way it worked was as described above, which makes me think that I just need more swing.

Really, the more I think about this, the more the Deore half-step FD seems like the right answer. Easy to find, not expensive, triple swing, small inner plate so it won't hang up on the 42, radiused for a 46T outer, and a long tail, which matters because I'm exceeding chainwrap by quite a bit. The 30x14 and 30x16 dragged the chain with the AR. Fortunately, the shift off the granny is 30x20 to 42x26, which worked fine, but the longer-tailed cage on a touring / MTB FD gives some extra Piece of Mind.

--Shannon

PS: Many points for the comparison of front derailleurs to bumblebees... just by looking at them they shouldn't work, but they do.

Last edited by ShannonM; 08-01-21 at 11:05 PM.
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Old 08-01-21, 11:25 PM
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Another thought:

I was looking at mid-80s Fuji catalogs at the Classic Fuji website, and it looks like they used Suntour Mountechs on their half-step touring bikes. Looking on E-Bay, it seems like Mountech FDs came in two versions, one with a deeper, stepped inner cage plate and one that looks more double-ish. I'd assume that, if there were two versions, the double-ish looking one is for half-step triples, and the other is for alpines. Is this correct?

--Shannon
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Old 08-02-21, 04:27 AM
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Originally Posted by ShannonM View Post
I tried almost every road FD we had in the Kitchen that looked like it would fit. The best was the first one I tried, a Suntour AR, which would sorta-kinda-maybe make the shift to the 45t, and once it was there it wouldn't rub, but it had zero overshift capacity with the LaPree levers, even with the upper limit screw backed all the way out, and so the front upshift was really, really janky. On the road and under load, it wouldn't have worked at all.

And, yeah, I played around with mounting height and angle. The best way it worked was as described above, which makes me think that I just need more swing.
.
Check that AR again, I went and had a look at one I pulled.

The outer limit to movement was not - at the extreme - the end of the limit screw. There was clearance between the moving arm and the tip of the screw.

The limit instead was the end of the cable-fixing-screw hitting the casting where it projects to form the ledge the screws go through.

This is a used one, and you see that already there are marks where the screw end hits. If yours is the same, get a swiss file and make a little groove there and you should get that extra mm or two you need.


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Old 08-02-21, 01:03 PM
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Just out of curiosity….Is it a “cable” issue or a “shifter/derailleur” issue?

Will the cage run through your full range?

How did you set up the cable tension?
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Old 08-02-21, 07:09 PM
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If you can still find one anywhere, the Shimano Titlist on my UO-8 works well with 42-45, and I suspect it would also work nicely with a triple with a 30T granny.
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Old 08-04-21, 01:03 AM
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I’ve had great luck with the ubiquitous Campagnolo NR. I’ve used these on all kinds of set ups and I don’t think I’ve ever had one not work. Here’s my half-step/Campy Paramount…works great.
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Old 08-04-21, 07:22 PM
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I'm going to retry the AR this Sunday afternoon. I just realized that I boneheaded the bottom bracket math. I was looking at my very inaccurate measurement of the chainring / chainstay gap, and thinking, "3-4 mm gap. 122.5 - 118 = 4.5. Nope... need a new derailleur."

In my haste, I overlooked two significant facts. First, given that I used an inch / foot tape measure and my eye to take the measurement, the odds are that that measurement is not only wrong, but wrong in the direction of "too small." Second, and more importantly, the difference in gap isn't 4.5 mm, but 2.25, because symmetry.

There's 2 or 3 118mm UN-whatevers in the box-o'-bottom-brackets at the Bike Kitchen, so I'm gonna try that and the AR FD and see if it'll work. I have hopes, since it was oh-so-close before.

One question: Is ring / stay clearance as simple as "if it doesn't rub, it works?"

--Shannon
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Old 08-04-21, 08:47 PM
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Originally Posted by ShannonM View Post
I'm going to retry the AR this Sunday afternoon. I just realized that I boneheaded the bottom bracket math. I was looking at my very inaccurate measurement of the chainring / chainstay gap, and thinking, "3-4 mm gap. 122.5 - 118 = 4.5. Nope... need a new derailleur."

In my haste, I overlooked two significant facts. First, given that I used an inch / foot tape measure and my eye to take the measurement, the odds are that that measurement is not only wrong, but wrong in the direction of "too small." Second, and more importantly, the difference in gap isn't 4.5 mm, but 2.25, because symmetry.

There's 2 or 3 118mm UN-whatevers in the box-o'-bottom-brackets at the Bike Kitchen, so I'm gonna try that and the AR FD and see if it'll work. I have hopes, since it was oh-so-close before.

One question: Is ring / stay clearance as simple as "if it doesn't rub, it works?"

--Shannon
Well if it does rub, it doesn't work.
I have one bike where it's about 2mm, that's kind of close. Most of mine are between 4 & 6 mm, with the doubles closer than the triples.
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Old 08-08-21, 08:37 PM
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Thanks to all who offered suggestions, and especially to 79pmooney for the idea to try a 118 bottom bracket in place of the 122.5 that Shimano specs with the FC-MT60 crank.

Swapped out the bottom bracket for a 118 UN-55, tightened up the crank bolts, and the 30T inner ring cleared the chainstay by a solid couple of mm. Installed the same Suntour AR front derailleur that give me fits last week, set the height and angle, adjusted the limit screws, tightened up the cable, and it just worked.

I will doubtless need to do some tweaking over the next week, and I've gotta get used to the half-step triple, since it's my first one, but fresh off the stand it seems like a winner setup.

Obligatory Pictures:






--Shannon
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