6

I am repairing my road bike after a mishap where it swallowed the rear derailleur around the cassette and into the front derailleur. As part of the rebuild, I'm swapping out the badly worn old chainrings and fitting some NOS Biopace chainrings.

Downside is that my Front Derailleur doesn't have clearance when the chainring's highest position is upward.

I am aware that my FD mech suffered and is probably somewhat bent, but it does clear the original worn chainrings.

Here's two shots showing the chainring teeth at both positions.

enter image description here
The lowest area of the chainring.

enter image description here
Highest point on chainrings, showing the overlap highlighted with red.

The front mech is positioned as high as it can go on the braze-on mount point. If I tried shimming the mount, the tail of the cage would lift but this would not significantly change the inner plate's height.

The proper plan would be to buy a new front mech with a smaller inner plate.

However I'd like to not spend too much on this repair. Is it feasible to trim off some of the inner plate?

enter image description here

I'd try to leave enough meat at the top to have something for folding a replacement reinforcement lip, but the lower half is lacking in metal for that, and will likely become quite flexible without additional reinforcement.

Is this a workable solution? I figure if not, I'm up for buying a new mechanism anyway.

Part is a Shimano model FD-2403 so its a triple road, bottom pull, low normal, and the bike was 9 speed.

Update - the cage cut cleanly with a Dremel cutoff wheel. Took a couple of minutes, and made plenty of pretty sparks. I used the dremel to round off the edges.

On installation, I find the mount on the frame is still not high enough to work as required. I've ordered a couple of Band-on to Braze-on adapters to see if the mech can be mounted higher up the seat tube.

I still don't have enough "reach" to the right to get the chain onto the big ring. Hopefully this will also be addressed by a bandon mount with more range.

Improve this question
4
  • 1
    If you cut off that section, then up shifting is probably going to suffer, if it's going to work at all. The reason the 2403 is designed with such a low inner plate is to facilitate upshifting from the smallest chainring. Double front derailleurs don't have all that extra stuff but might work in a triple shift system, although not optimally.
    – Kibbee
    Oct 5, 2019 at 11:06
  • 1
    In the days of biopace the instruction was to set the outer plate of the FD 2-3mm above the highest teeth. This might be problematic with a FD meant for braze-on mount.
    – Carel
    Oct 5, 2019 at 11:57
  • @Kibbee Excellent point - I don't get how this FD cage worked on the old chainrings, but it was always terrible about moving to the big ring. On re-inspection shows that it was never a good fit either. I plan on trying this cut and seeing how it works.
    – Criggie ♦
    Oct 5, 2019 at 13:50
  • @Carel I'm lucky in that the braze-on mount is bolted to the frame, via two small 4mm rivnuts. So the other possibility is a band-on to braze-on adapter to suit the 32mm seat tube. Or a new band-on derailleur to suit. Or go without a FD completely and shift by "hand" though this will very likely derail in big-big gear.
    – Criggie ♦
    Oct 5, 2019 at 13:50

1 Answer 1

Reset to default
3

For completeness, I ended up replacing the bolted-on brazeon mount with a band-on braze-on mount.

This allowed me to raise the derailleur by those critical last few millimetres.

If the bike had a brazed-on braze-on mount then I might have had to cut it off and file the seat tube to a round finish, but fortunately this Cannondale frame had a small bracket held on by two 3mm bolts into rivnuts. The Rivnuts are still in the frame under the band.

(I'm wondering if I should have put two grub screws in the holes to seal them up and reduce rain ingress from the rear tyre.)

Here's the completed fitment:

Own work

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.