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Front derailleur clamp size

Old 08-02-18, 03:01 PM
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Front derailleur clamp size

I've an old (1999) Lemond Zurich frame that I'm restomodding. I'll be putting Ultegra R8000 on the thing. The front derailleur is available for a braze-on, or as a clamp. The frame does not have a braze-on tab, The size of the two clamp sizes listed for Shimano FDs are 21.6-31.8mm, or 34.9mm. But the seat tube in a Lemond is 32-32.1 mm.

0) Did I mis-measure, and is the tube really a 31.8mm?
1) Will the smaller clamp work?
2) If not, are there appropriate after-market clamps for 32mm tube that mimic a braze-on tab?
3) If not, are there shims available that would allow me to bridge the 2.9mm diametral difference so that I could use a 34.9mm clamp? Is it that simple, just using a 1.45mm shim?
4) If not, can an FD tab easily be welded to the frame without damage? It's an 853 frame.

Thanks.
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Old 08-02-18, 03:31 PM
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I suspect that your 31.8mm seat tube has 0.1-0.15mm (measured twice) of paint on it, making the total 32.0-32.1mm.
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Old 08-02-18, 03:36 PM
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0) Probably. Perhaps someone with the same bike can chime in.
1) No.
2) Probably. There's a lot of wild stuff out there. More all the time.
3) Probably, or you can make your own from "generic shim stock" (soda can).
4) No.
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Old 08-02-18, 04:07 PM
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I may give the small band a shot.
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Old 08-02-18, 04:19 PM
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Tubing is commonly imperial but spoken of in metric numbers..


1.25" is 31.75mm
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Old 08-02-18, 10:38 PM
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It will probably fit, but if you are not happy with it, just sand out the diameter .006/.008 of an inch larger. I have a mountain bike with Columbus max tubing that is slightly oval. Sandpaper on soft aluminum does wonders to get it to a particular shape.

The other route is to get 34.9 and use Shimano plastic derailleur clamp shims and sand the shims to fit. I had a tough fit to get a chainline I wanted and have the derailluer shift to the inner chainring. Took a 34.9 to 28.6 shim set and sanded the shims to be slightly offset to bring the derailleur clfrom the seat tube.
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Old 08-02-18, 10:43 PM
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Since I once again accidentally hit submit reply on my phone and BF's Big Lots software won't let me edit, I will continue.

... I basically sanded the shims so the derailleur sat closer to the seat tube and moving it just enough to shift to the inner ring. After all the shims are plastic.

​​​​​​​John
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Old 08-03-18, 09:18 AM
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Well, I ordered the small clamp version. Will see if this works,

Regarding mismeasuring, I used a Starrett caliper (the US made 797, not the Chinese 799) and am pretty sure I got a pretty accurate number. I'm hoping that there's 0.3mm tolerance in the thing. Else, I have to sand or buy the larger one and clamp.

So is it difficult to weld to 853? Is this an allow that requires post weld heat-treat?

PS I had to order the whole groupset, plus the small banded version of the FD. So I'll have a brand-new RD8000 FD for sale....

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Old 08-03-18, 10:39 AM
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If it's like the Shimano FD I recently purchased, it will come with one clamp and two sets of shims. One shim set was too thin, the other too thick. I inserted the thicker set, and I could easily get the clamp bolt to squeeze it down for a good, solid mount.
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Old 08-03-18, 11:11 AM
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Originally Posted by dsbrantjr
I suspect that your 31.8mm seat tube has 0.1-0.15mm (measured twice) of paint on it, making the total 32.0-32.1mm.
I suspect this is the case as well and would get the braze on derailleur and a 31.8 clamp. The clamp has plenty of leeway for exactly this reason.

Shimano Braze On Adaptor | Chain Reaction Cycles


-Tim-
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Old 08-03-18, 11:33 AM
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Powder coating of a handlebar required me to sand some off,
to fit a 22.2 grip shift on it ..

it measured 22.5, originally..
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Old 08-13-18, 09:40 AM
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I ended up touching up the paint as the previous owner somehow thought that cream-colored paint would mask the chips in the brilliant, titanium white paint on the underside of the top tube. Looked horrible, so I wet-sanded down the paint with 600 grit SiCarbide paper, and used white urethane enamel. Overcoated with clear enamel. Looks pretty good. I could use some automotive cutting compound, and some carnauba wax.

The paint was all bunched up under the original derailleur. I did the same wet-sanding and refinishing on that area as well, and the clamp-on derailleur fits perfectly. Will post a pic when I get the thing on the stand next.
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