Downtube FD Shifter Question
 

The FD shifter on my wife's Centurion Ironman has a full 180 degree range of motion. This is way more than necessary to shift, should there be something limiting the motion (other than the FD itself)? She tends to stop pedaling, and crank it from limit to limit, which may have contributed to the recent cable failure.

Attachment 187291

Ideally the shifter will move a little less than 90 degrees. If you can physically get it to move more than that I would say the cable tension is way off. When you say "caused the cable to fail", do you mean caused the cable to break? That would be unheard of, unless it was severly corroded. Even cheap brake cables are incredibly strong and the stress put on them isn't much to move a derailleur out. Something else could be wrong here. Start with a new cable and adjust the initial tension as per the Park Tools website or any of the other online sources.

Bottom line is that it isn't the shifter, I'd say, but something else. The shifter itself is a fairly "no-brainer" device.

Range of motion required to shift is a function of the FD and the diameter (radius) of the lever and how much slack is in the cable between the forward position and when the FD starts to move after moving the lever "back". Get rid of the slack and see how much the lever needs to move to shift. Typically 90 degrees, maybe a little more.

This reads like you should fit a internal gear hub on her bike rather than a derailleur, they work whether pedaling or not, even at full stop.
derailleurs only work with moving chainrings, so rather than force your marriage
into distress teaching her what must be done to shift a derailleur bike,
get it rebuilt around an internal gear hub.

Easily fixed.

1. Shift the lever to the full forward position. The front derailleur will shift into the small chainring position.
2. Now disconnect the cable from the derailleur, pull it taut and reattach it.
3. The lever will now only move about 90 degrees because the high limit screw on the derailleur will stop it.

This certainly won't help. You should always be pedalling lightly when you shift a derailleur, without yanking on the lever too hard. It may be that repeated stress on the cable from trying to shift a stopped chain has caused it to fail from fatigue.

This is how it looked after breaking:

http://i8.photobucket.com/albums/a39...rokenCable.jpg

Wife mentioned during a ride that shifts weren't happening, and I was able to get a few strands hooked to the FD, to get us through the ride. When I unhooked it at home, it came apart.

I'm embarrassed, because I should have inspected the cables periodically at which point I would have noticed the fraying. Guess I've got a new item for my checklist.

Based on your info, my current theory is that the cable was rubbing against a part of the derailleur, which caused it to fray. It looks like it was coming in contact with something. When I put on the new cable, I'll check that out.

Thanks for the help.

+1. With friction shifters (like your FD downtube shifter) you probably just have the cable too slack when the shift is let all the way out.

You can leave it like that so long as your shifter is still able to move enough to shift your FD to the high limit -- I don't know why your cable broke. That's a separate issue.

The slack should be removed with the shift lever all the way forward and the front derailleur in the small(est) chainring as RG recommended. If you have too much slack when the lever is forward, it's quite possible for the cable to slip sideways off of the shifter's "drum" when you go to upshift to the large chainring.

TromboneAl: I wonder if the slack in the cable was a result of it not being properly seated in, or having fallen out of, the cable guide under the bottom bracket, assuming that is how it is intended to be routed. If it wasn't in the guide it would be rubbing against something else, which might explain the fraying.

Another thing to check is that the shift lever and the parts behind it are installed at the correct angle/orientation. My down-tube shifters are mechanically prevented from moving beyond being parallel to the down tube when pointing forwards. When I was fiddling with it once, I reinstalled it and suddenly the shift lever could keep rotating forwards much furthur, so I took it off the mount again and tried a different orientation for the parts, and this made the lever again stop when parallel with the downtube. If the cable is taught when in the lever is in this fully forward position then, as others have said above, the derailleur should limit the amount that it can move when pulled back to about 90 degrees of rotation.

I thought of that, but the fraying was up near the FD attachment point.

Right. It currently can rotate 270 degrees -- from back parallel to the downtube to forward and 90 degrees from the downtube (pointing torwards the tire). I've never removed the shifters however, so it would have to be something that broke.

Then the lever may have been installed wrongly from the start or by someone else. Try to get it so that the lever can't go past being parallel with the downtube when pushing forwards. Clamp the cable to the FD so that in this fully forwards position, the FD is above the inner chainring, with no cable slack, and set the outer limit screw appropriately. The motion of the lever should then be limited sufficiently.

After looking at the cable I think that you should replace the cable and make sure the cable is taut when it is installed. I don't get my dt shifters to move 90 degrees for a double crank.

I removed the shifter lever, and found it was broken inside. That explains why it didn't stop where it should have.

http://i8.photobucket.com/albums/a39...ShiftLever.jpg

It still works fine, I've instructed wife to not try to force it past 90% from the downtube.

I also figured out why the cable broke (see this thread).