Chain is rubbing the front derailleur even after trying to align using indexing screws

Question

I have a Trek Marlin 5 (2022) MTB with Shimano Altus M315 as the front derailleur. The problem is the chain (KMC Z8.3) is rubbing the derailleur on higher gears (for example, when using 2x7 or 2x8) with the highest being 2x8. I understand that on higher gears it is expected to have some rubbing as the chain is a bit crossed but the noise is too much at this point.

Given this is not an uncommon problem, I tried to move the "H" indexing screw counter-clock-wise to move it outward following some suggestions but the problem is still there. I have tried to look for other causes/solutions but my limited knowledge in this area is failing me.

Here's a picture:

Please let me know what can I do to fix this or make it bearable at least. Also, please let me know if you need any more info.

Answer 1

This kind of noise can happen if the cage is not positioned properly. It's difficult to judge from the picture, but if by 2x8 or 2x7 (the gear indicators on my bike are not marked, so I don't know in which direction they go on yours), you mean big chainrings, small sprockets, the derailleur is not aligned properly. The picture would match a "big chainring, big sprocket" situation, where you may have rubbing because of chain crossing.

Normally, the cage should be parallel to the big chairing. If it's not the case, you can address that by moving the whole derailleur (by loosening the clamp that attaches the derailleur to seat tube). In theory, it is straightforward, in practice it may be a bit of trial and error as sometimes the derailleur rotates a bit when tightening the clamp.

Parktool has published a video on this topic, the section related to the positioning the cage is 2 min long - it's the section after the introduction (and shows the different mounting systems for front derailleurs):

Answer 2

Does this picture show the derailleur in the position where it rubs? If so, it looks like the inboard plate is what's rubbing, not the outboard plate. Moving the derailleur further outboard wouldn't help, if that's the case.

If you really do need to move the derailleur further outboard, it may be that there's not enough tension in the cable to pull it all the way out. The fix for that obviously would be to add tension to the cable. If you've got a barrel adjuster on the cable housing, try screwing that out to add a little tension. If not, or if that's not enough, put the derailleur on the small ring, loosen the cable-fixing bolt, and increase tension. A 4th-hand tool can be helpful. May take some trial and error.

As Renaud suggests, you may need to rotate the whole derailleur around the seat tube. You also want to make sure that the derailleur is riding just a few millimeters above the chainrings, so you may need to lower it a tiny bit.

Finally, it is possible but unlikely that there is contouring in the sideplates that rub in the extreme gears, even when you've got everything perfectly adjusted.

Answer 3

It definitely looks off..Derailleur nose pointing outwards. Make sure it's parallel to chain. Loosen the collar bolt, and move it a few mm's towards the tire.

When the derailleur is adjusted correctly, there should be no rub. I even don't have rub on Tourney groupsets. Proper adjustment is quite tricky. Most of the internet sources I've come across propose wrong methods to adjust it. It's no dark art; it's just a 3 step process.

Just follow Shimano's own user manual for the process. It works perfectly. Do not bother with internet mechanics. Follow the instructions religiously, and every Shimano front derailleur works perfect unless they're bent, or not properly installed.

Answer 4

Assuming this can't be solved by getting it perfectly aligned (and this one does look skewed clockwise, looking top-down in the photo...)

And noting that this usually only happens in "cross chaining" gears that can usually be avoided by selecting the correct front chainring...

An alternate answer that won't be popular but will be useful in some cases.

For some bikes, it's nearly impossible to get a front DR position that won't rub on one edge of the cog, especially with 3-position indexed shifting. This kind of thing drives me nuts. Ways I've solved it periodically across a couple dozen bikes (including home-builds and rebuilds with mismatched parts, etc)...

1. After ensuring big and small ring limit screws are fine, adjust barrel / tension in the offending cog combo. You can sometimes move it just enough so it clears but won't hinder the top/bottom shifts. Since the front DR doesn't work the same as the back, a change in cable tension here won't affect shifting much or at all.

2. Try shimming the drive side BB outboard by 0.5, 1, or 2mm with spacers.

3. Use a "micro indexed" front derailleur. Sram makes twisters that do this. Microshifts high-quality thumb shifters do a great job of this, allowing you to trim the front DR a tiny bit.

4. This doesn't apply to your current derailleur, which fits the post, but for those that require a round shim because the post clamp is bigger than the post: Shim the derailleur position in/out by using an eccentric derailleur mount shim, one that has offset center points. This allows small adjustment to the derailleur cage position that might make a difference. These are easy to knock up on a 3D printer.

5. Widen the derailleur cage. You can usually unscrew the short screw at the bottom of the derailleur cage (that holds it together) and slip in a single washer that will open up the cage 1.5mm or so. Again, a 3d printer can be your friend as you can print a thin PETG one, or a variety of thicknesses. Afterward, the cage will be a bit of a trapezoid, but that doesn't usually matter for shifting, and the increased space can be just enough to eliminate rubbing. You may need to find a longer screw to make this work, however. (I have needed to do this most often when pairing 10 speed front derailleurs with 9 speed systems.)

6. Mod a shifter. This is the most extreme option, and necessary only rarely, but very cheap microshift twist shifters are easy to take apart and the indexing is just a notch in a plastic ring that is engaged by a leaf spring. It's trivial to grind an alternate indexing spot or 2 or 3 into this plastic ring.