4

Back story: I bought a Giant Defy Advanced Pro 2 second-hand, with Shimano Ultegra components. The previous owner had used it with different components and had the original ones re-assembled before selling.

In lower gears (up to ~4th one in the back), the chain rubs on the front derailleur cage. In the lowest one, the rubbing is quite extreme.

chain rub on lowest gear front derailleur mount

After I failed to configure the system in a way that would avoid rubbing, I took the bike to a shop. The mechanic there told me that the cage appears to be too far from the frame, so it's not possible to avoid chain rub, but couldn't explain why this could happen (especially given components are original). Allegedly there's not too much one could do differently with the given parts.

What could be the issue here - is it possible that in fact something was mounted incorrectly, or that some custom parts were used? Any ideas how to mitigate the situation?

Update: Here's a different view on the front derailleur cage; I'll post more once I get home.

diagonal view on front derailleur cage

Update 2: After some hints, I started testing around by pushing the cage by hand and felt it hitting a "limit" that was not caused by the L screw. Turns out that there were two reasons:

  1. The tension screw gets in the way if it's not fully in, so I put it all the way in.

tension screw getting in the way

  1. The H screw somehow limits the movement on the lower side if it's in too deep. Screwing it back a bit allowed me to go low enough to avoid chain rub on the low side. However, now there's significant rub on the high side, because I had to move the H screw!

H screw getting in the way

I'm starting to wonder whether tension on the wire is too low, although tension adjustment itself is difficult because of the screw touching other parts as above. Trying to tighten it a bit yesterday had no effect, but I think I failed to actually put enough tension - putting the wire into the holding with tension felt very fiddly, and fine-adjustment only goes so far.

As asked by Criggie, here's a side view as well:

side view

Still happy to hear suggestions what could be wrong with my setup!

7
  • 1
    Can you give us the side view too? straight parallel with the bottom bracket axle? Front Derailleurs are always faffy, and this style also risks stripping the thread. My gut feeling is the cage has been squashed in somewhere.
    – Criggie
    Apr 9 at 4:28
  • 1
    I only have a diagonal top view for now; added this one. I'll take another picture and post it once I get home. Apr 9 at 6:21
  • If there is really no adjustment left on the bottom-out screw try Claudio's point about ensuring the cables are clean and free moving (and no kinks or crimps are jamming things up), then you could try to remove the cable, wind the adjuster right out and just push the parallelogram by hand whilst looking down the chain line to see the total rang of movement the front mech offers and if this clears. -- Criggie's suspicions may be right!
    – Bucky
    Apr 9 at 16:43
  • (ran out of space) -- After that try loosening the mech mounting bolt and see what if any movement you can get twisting the furthest end of the mech out to clear the chain. Effectively you're trying to move the front end of the mech in so the back end moves out to clear. If this looks likely to work but you loose the slightest bit of movement you got when you tighten the bolt again then it's possible that very cautious use of a file to take a fine shaving off the inside edge of the vertical slot on the braze-on mount will give you the room you need (at your own risk!)
    – Bucky
    Apr 9 at 16:48
  • Turns out the tension and H screws are getting in the way (see update on question), but I still don't quite understand why and how that's possible. Also, added a side view photo. Apr 10 at 8:43

3 Answers 3

3

GRX FD

I have the impression that it is a low limit screw issue, but am surprised that the bike shop didn't think about it if it is the origin of the problem (it is one of the first adjustments to make when installing a front derailleur).

The picture shows the front derailleur on my bike (a GRX, but same design as the Ultegra): you can see that your cage doesn't as far inwards compared to the one on the picture. I circled in red the limit screw of the derailleur, that seems much further on your pictures (second one).

The limit screws are located here - the one you need to adjust is the L one: enter image description here

Source

3
  • In the shop they said they went all the way to the limit but it's still rubbing (and that was my impression as well). Do you think something about the screw or thread might be broken, so it doesn't go all the way? In any case, I'll take a closer look once I get home. Apr 9 at 11:55
  • @CedricReichenbach Indeed, looking at mine, the screw should be "fully" out to be closer to the frame, which matches your pictures.
    – Rеnаud
    Apr 9 at 12:18
  • 1
    Pushing around the cage by hand revealed that actually the tension and the H (yes) screw get in the way on the lower side (see update on the question for details). Moving H out of the way causes chain rub on the high side, so I'm not sure what to do about it... :/ Apr 10 at 8:37
2

After some back and forth, the issue turned out to be related to lack of tension on the cable.

To increase tension, I put the derailleur into a position near the "trim position" (one click below top, see Shimano's manual). To keep it in place while I was attaching the wire, I simply clamped some object of appropriate size between the frame and the cage.

object stuck between frame and cage to keep derailleur in place

Before re-attaching the cable, I partially released the tension screw, so I could fine-tune and add more tension later.

After the cable was in place with more tension, I was finally able to set up positions with the L and H screws in a satisfactory-enough way. There is a tiny bit of chain rub left in lowest and highest configurations, but that's probably not a big deal.

I noticed that moving the limits further out would cause more chain rub on the other side, i.e. high front with lower back gears and vice versa. This is of course expected to some degree, but it seems more of a problem than what I've seen on other bikes. But that's a question for another day.

1

Unfortunately I don't have enough reputation to make a comment so I should probably not be proposing an answer :D

There might spacers between the chain ring and the chain guard (although this is an unlikely from the comments below). Perhaps those spacers were misplaced, displacing your chainring.

Update: this might be a dumb idea, but have you checked that cables are not rusted or blocked, thus preventing the derailer from coming all the way down?

Hope this helps!

6
  • Good one, and it would have been relevant for MTB cranksets - that require a 2.5mm spacer on the drive side. But that's one of the subtle differences between road and MTB groupsets - road bottom brackets do not need spacers.
    – Rеnаud
    Apr 9 at 10:52
  • Not sure I fully understand (not a native speaker and not a bike mechanic); do you mean there's a spacer on the crank axis between chain ring and frame? I'll take a look once I get home. Apr 9 at 11:58
  • 1
    @CedricReichenbach on MTBs, you need a spacer between the frame and the bottom bracket shell, which pushes the crank further (that would have helped in your case, but that's not the solution).
    – Rеnаud
    Apr 9 at 12:20
  • @Rеnаud Not explicitly true, there are many different BB designs for various types of bikes, some utilise spacers and some do not (consider BB30, PF30, DuB, etc.) Some standards from 'yesteryear' indeed did include spacers for road bikes too! -- However in the case of the OP: with a modern road bike, "braze-on" style front mech hanger, and one piece crank arm... suggests 'spacers' are unlikely to be the issue
    – Bucky
    Apr 9 at 16:30
  • @Bucky Good point, I should have added "Shimano" in my comments.
    – Rеnаud
    Apr 9 at 16:31

Your Answer

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

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