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Old 02-11-04, 04:32 PM   #1
trailhound
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Front derailleur question

I recently converted my rigid Trek 930 into a commuter. The conversion was basically replacing the handlebar with a drop bar, road brake levers, Dura Ace bar cons (mostly roads on the commute). Didn't want to fool with the drive train so left it as it was.... LX rear and STX front derailleurs. 7 speed XT rear hub. Just using friction shift mode on the rear. All works well, except one small issue. No matter how well I adjust the front derailleur (want the use of all 3 chainrings), I get a small amount of chain rub in the smallest 2 cogs when on the large chainring. Any attempt to tune this out screws up the ability to use the small chainring. My guess is the Dura Ace shifter doesn't take up enough cable to pull the derailleur outboard quite enough.

Anyone else try this? Suggestions appreciated!

Thanks,

Karl
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Old 02-11-04, 05:11 PM   #2
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IOW, if you tighten the cable to get the front cage to move far enuf to the right not to rub, it won't then release far enuf left to drop onto the small chainring? Yeah, that sounds like the barcon doesn't "throw" far enough (weird, since it probably rotates on its pivot pretty close to 180 degrees, right?).

One dicey option is to put the cable on the other "side" of the pinch screw on the derailleur. This would put it closer to the fulcrum, and make it throw the cage further for every degree the lever is pulled/pushed.

Possible caveats:
* This increases the effort the lever must endure as it pulls the cage to the right, and may eventually bend/break the lever, or snap the cable
* Since you have more cage movement per degree of lever movement, fine adjustments will be more difficult

I take it you've ruled out things like min/max adjustment on the cage, right?
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Old 02-11-04, 05:26 PM   #3
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hmmm.....I'm using Ultegra bar end shifters on my Mtb. My LX front derailleur shifts perfectly with still enough cable travel to spare.... I'd suggest disconnecting the cable and starting from scratch. Your barcon should have more than enough range to shift your front dr.

George
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Old 02-11-04, 08:47 PM   #4
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Front derailleur question

EeeeeWwwww....

The thought of connecting the cable to the wrong side of the ft. der. just creeps me out... Sorry, just hadta say that!

The head wrench at the LBS said it maybe had something to do with using an LX 8 speed rear derailleur on a 7speed hub, but I find that a bit hard to swallow. Maybe I'm wrong. Adjusted the stops to accomodate.... and I mean all of 'em....

I built this bike as an All 'Rounder, not just as a commuter. That's just how she's been used lately. I really don't need the bottom end, and would be willing to sacrifce the bottom 2-3 cogs for top end performance. But, when I start screwing around with the cable tension for adjustment the small chainring, it messes up the drop to the small from the middle.

Perhaps I'm just destined to run it crippled as a 14 speed, or cop out and go with an 8 speed rear wheel.... hmmmmmm.... that wheel looks a bit used....

Thanks for the input, guys. I'll keep ya' posted.

Karl
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Old 02-11-04, 11:07 PM   #5
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I am assuming that when you say the smallest cogs, then talk about giving up the bottom end, you are actually referring to the ones closest to the spokes, which would actually be the largest cogs, but the easiest ones to pedal. If that is true, then what you have is a drivetrain that is fairly normal for a triple. The chain doesn't want to deflect at the steep angle you want it to between the outer ring in front and the innermost cogs in back. Even on my double, I don't go for the big/big and small/small combos. It is too rough on the chain. When you are in the middle ring, do you have use of all 7 rear cogs? If so then you don't really have a problem.
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Old 02-12-04, 01:00 AM   #6
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Quote:
Originally Posted by trailhound
EeeeeWwwww....

The thought of connecting the cable to the wrong side of the ft. der. just creeps me out... Sorry, just hadta say that!
Well, that's why I included the caveats...
Quote:
The head wrench at the LBS said it maybe had something to do with using an LX 8 speed rear derailleur on a 7speed hub, but I find that a bit hard to swallow. Maybe I'm wrong. Adjusted the stops to accomodate.... and I mean all of 'em....
Yeah, that sounds a little cagey (ouch, bad pun!). AFAIK, the only effect #cogs has on front shifting is where the front cage sits on the index detents, and you originally posted that you're running in friction mode. The total range of position across the cogs is the same. If anything, you'd have more likelihood of chain rub with more cogs.

Seems like it's still a front cage travel issue. Is there anything restricting the throw of the lever? Could there be anything weird about the way the cable routes thru the lever, that affects the distance the cable moves as a a function of lever throw?
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Old 02-12-04, 01:10 AM   #7
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Quote:
Originally Posted by trailhound
The head wrench at the LBS said it maybe had something to do with using an LX 8 speed rear derailleur on a 7speed hub
Irrelevant.......it is strictly a front derailleur issue, period!
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Old 02-12-04, 08:13 AM   #8
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Yeah, I thought the rear derailleur suggestion sounded a little odd. I worked OK when the bike was running with grip shifts.

There is nothing impeding the travel of the lever. It has full motion from past the full down position to just higher than horizontal (maybe 100-110 degrees of motion).... the same as with no cable attached. I've set the low stop screw to a setting that limits the travel of the derailleur to where it is centered over the small chainring, and the high stop to where the inside cage arm will be centered over the large, if forced by hand. The problem is, the shifter won't pull enough cable to move the derailleur far enough outboard.

With the lever pulling as far as it goes, the outside arm of the cage goes maybe 5 mm past the largest chainring. So, it shifts onto all 3 chainrings just fine, but doesn't go far enuff past the large for the chain to clear the cage when the chain engages the 2 smallest cogs.

Pehaps I'll try setting the low stop screw to limit the travel of the der. a bit more, and try taking up the cable more. Again. I tried that before, but it affected the shifting onto the small chainring. Perhaps a little more patience....

FWIW, new cables and housings. I've used it enuff to stretch the cables.

Thanks for the input guys. I'll keep ya posted.

Karl
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Old 02-12-04, 10:32 AM   #9
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Dumb question, perhaps, but could it be the position of the der on the seat tube? Could it be "rotated" so that the cage isn't letting the chain travel past it properly when on the big ring / smallest 2 cogs?
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Old 02-12-04, 11:10 AM   #10
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Your next step (last resort) is to tweek (bend, re-form, de-form, whatever..) the deraileur cage by hand to make clearance. That has always worked for me....
But before you attempt to do that, make sure your derailleur is mounted correctly as Madpogue suggested.

George
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Old 02-12-04, 11:47 AM   #11
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Ya' know, madpogue has a good point. As I recall, I set the derailleur up with a bit of toe-in, but perhaps not enough. Kinda hard to picture it sitting here at my desk (didn't ride it in today), but I'll have a look when I get home. Thanks!
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Old 02-13-04, 07:51 AM   #12
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I fooled around with the front derailleur again last night after work. Rotated the front der. a little more, made some small adustments to the cable tension, and viola! Works like a champ.

Thanks for the input, Fixer, madpogue, and demoncyclist.
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