Why does this happen to a Dura-Ace Left Shifter?

[jberenyi]

I am trying to figure out what is wrong with this Dura Ace 9000 left shifter. A woman friend of the family who is quite the athlete dropped her bike off yesterday and asked for me to fix it again in hopes it would be ready for Wednesday’s event and Saturday’s as well. This is the second time this has happened. The lead slug on the end of the cable which fits into a pocket popped out of place. I have no idea why this is happening but it happens once a year. Not sure what could cause this to happen so I thought I would ask the experts. Once put back into place it shifts perfectly and there is no excessive slack at all in the cable. If the cable is always under tension, what could possibly allow the cable end to come out of place?

[Mark McM]

If the cable is always under tension, what could possibly allow the cable end to come out of place?

This could be your problem - in the real world, the cable is not always under tension. When dropping the derailleur from the big ring to the small ring, the shifter releases the tension on the cable momentarily, and then the derailleur is supposed to pull the cable tight again when it retracts and pulls the chain over. But if the shifter cable spool travels faster than the derailleur can pull the cable, the cable head can pop out of place.

I'd check for excessive friction in the cable (or the front derailleur), or anything else that could be preventing the derailleur/cable from moving quickly/smoothly. Or maybe your friend is trying to shift from the big to small chainrings without turning the cranks (which obviously prevents the derailleur from moving)?

[mt2u77]

I would guess there is excessive friction in the housing somewhere (either it's just old, a burr on a housing end, or an overly tight bend) that is making it slow for the derailleur to take up the slack in the cable when you downshift.

[jberenyi]

Thanks guys. Makes sense to me. I will check these issues today and most likely buy her a new cable setup.

[Black Dog]

It can also happen if shifting into the small ring while pedalling very slowly or while not pedalling. This would have the spool moving way faster than the cable and pop the cable head out of the socket.

[dddd]

I would recommend slightly pre-bending the cable at the head, which should allow the dislodged cable head to re-center itself in the spool cavity after losing cable tension (for whatever reason) during a downshift.

Some cables are stiffer than others, so this particular bike might need the cable to be bent where on other bikes it might never surface as an issue.

[dddd]

The head of this bike's front derailer cable also may have a particularly sharp edge along the cable end of the "metal slug", which I would radius with a file.

But I think just replacing the cable with a different brand would very likely prevent this issue from ever recurring, though again I would pre-curve the cable before installation.

[Gummee]

If your person hasn't replaced cables and housing in a while, that's likely your problem.

Another alternative is a kink in the cable and/or housing somewhere.

Regardless, use the green coated Shimano cables and SP41 housing. At the minimum, use Jagwire Pro or Elite polished cables and SP41 housing.

Good luck!

M

edited to add: I HAVE seen a few (very few) Shimano ft derailleurs that freeze up. They just won't move any more for some reason. Then the only real alternative is a new derailleur.

[Dasarbule]

Frayed cable
Scored housing liner
Tight radius in housing
Housing not fully seated in shifter
Sweat/corrosion on cable or shifter
Cables twisted inside frame
Corrosion/dirt/debris at cable guide, liner section under BB
Hindered movement of FD due to dirt/debris

These are a few of my favourite things :)

[dddd]

Frayed cable
Scored housing liner
Tight radius in housing
Housing not fully seated in shifter
Sweat/corrosion on cable or shifter
Cables twisted inside frame
Corrosion/dirt/debris at cable guide, liner section under BB
Hindered movement of FD due to dirt/debris

These are a few of my favourite things :)

Yeah, but...


Thing is that I somewhat regularly downshift a left/front STI lever while the crank is stationary, sometimes it's to create slack so as to fiddle with the cables/housing, or to allow the cable to be more easily tensioned while the anchor bolt is tightened. And sometimes it occurs accidentally.
I never have had the problem described by the OP despite these many tension-loss incidents.

I work on a lot of bikes having various versions of STI and have never seen this problem(!), though I have had a cable head dislodge from a downtube lever and then fail to settle back into the pocket in the lever, which prevented the bike from reaching top gear at the rear derailer. It was that occurrence where I was able to prevent it's recurring by first removing the DT lever so as to slacken the cable, then bending it near the cable head. Also I think that I may have radiused the cable end and/or the entry of the pocket in the lever.

At first glance, I remember thinking "where's that damn corrosion when you need it".

[rccardr]

Could just be a manufacturing tolerance thing where the cable head socket is a tad too large, making it easier to slip out and snag.

Maybe slightly peen or disform the cable head to make it just a smidge tighter? Not enough to make it impossible to remove, mind you, just a smidge. A weensy bit, if you want to get technical.

[HenryA]

I'd go with all brand new Shimano cables outer and inner. I bet it shifts better too after you do this. I used to cheap out on cable replacement and finally gave up because it never fails until you really don't want it to. Its so much easier to fix this at your convenience and in your shop with good tools.