imobilinpedalus

My Shimano XT front derailleur is messing with my head, and so this is where you come in (Trek 26" full.susp)

So i`ve had this issue for quite some time now and i can`t seem to wrap my head around it. Maybe i`m just not trying hard enough

The bottom side of the chain (when inside the cage of the front der. and at the inner chainring) is floating on top of it like a boat in the ocean bobbing in the waves making these metallic rubbing sounds. My cassette has 10 cogs and this problem does not go away before shifting to the 2nd bigggest cog. Easy fix right, just reposition the front der. on the frame until it sits low enough to clear the chain on the smallest cog.

So far so good...well not so fast.

If i do this, the outside of the derailleur cage (when moving outwards) will actually collide with the inside of the biggest chainring when attempting to shift to the biggest chain ring. Shifting to the biggest chainring basically becomes impossible.

This is basically how i`ve been riding my lightning rod lately. Works great in the mid and big chainring position but miserable on the smallest chainring.

WHAT on gods green earth is going on ???

masi61

As you might expect: somebody is going to ask for photos. So yeah, let's see a photo of both the big chainring rub as well as the bottom of the front derailleur cage rub you so entertainedly described.
It makes me wonder if your Deore XT front derailleur is a mismatch to your chainrings - could you let us know what chainring teeth you're using?

Andrew R Stewart 

Rear suspension plays havoc with front der cage/chain/ring relationships. The greater the travel the worse the likelihood of chain rub on the cage end. This is a big reason why the, nearly complete, industry is going single ring for suspension bikes.


I wonder if this has been a problem for the OP since the bike was new. If so then he's against the bike designer and the product manager's choices. Andy

HillRider

Thanks Andy, this is very interesting information. I don't ride an MTB or off-road so I've been a bit puzzled by the mad rush to 1xXX drivetrains. Can't these people design a decent front derailleur and what's so hard about front shifting? Now I know what the incentive is.

rosefarts

It's not a new problem. I had a mid 90s proflex. Great bike for the era. I eventually got a Sachs 3x7 hub for it and ditched the chainrings. The fd never worked well on it.

HillRider

I agree there were a lot of poor shifting front derailleur designs in the past, completely unrelated to the requirements for rear suspension, but that's not true anymore. For road bike use I have both Shimano 105 and Campy Veloce 10-speed triple front derailleurs on four different bikes and they shift extremely well.

Jon T

Need a longer cage that hangs down farther on the fd.
Jon

FBinNY 

Odds are that the underlying issue is chain overall width vs chainring spacing.

I can't sketch here so I'll try to draw a word picture.

Imagine the tips of the teeth of the inner ring next to the inside face of the outer. If they're close together, the chain will slide along the wall and drop onto the ring. But if they're farther apart the chain can lean on the face, with the plates just inside the teeth of the inner. It can stay that way until something moves the chain off the wall and it drops in.

This is a common problem when people, for instance, use a 10s chain with 9s or 8s chainrings, which are spaced a bit too wide.

So, get the chain riding the tops as you describe, then get off the bike and look from the back and see if that's it. If so the solution is either a wider chain, or bringing the chainrings close together (which isn't that easy in most cases, but not overly hard either if you're good with a file).

well biked

An even bigger motivator than full suspension for 1x drivetrains is "plus" sized tires (roughly 3" width in both 29+ and 27.5+), which are more and more common on some new mountain bikes. With "normal" spacing/standards, the chain clearance around the big tires isn't conducive to running more than one chainring without special offset cranks, etc. When I say "normal" spacing/standards, I'm talking about not using the extremely wide dropout spacings and bottom bracket widths that true fat bikes use (4" and 5" tire widths). And these days, might was well count "Boost" spacing as normal (148mm rear), because it's very common on new mountain bikes.

maddog34

umm.. pretty sure the OP is talking about the chain dragging on the TAIL of the cage... you know, the bridge between the two halves of the cage, Francis.
he needs to quit cross-chaining on small small, or find a longer droop cage der.

FBinNY 

Yes, I misread his description and thought he was saying the chain was dancing on the ring.

So, if I'm now reading him right, the bottom of the shifter cage is too high for the inner ring, and lowering the FD is out of the question because it's already too low for the outer.

If that's right, the key here is the angle of approach for the chain with respect to the FD.

Either the FD is rotated too far forward (ie a steep seat tube angle) OR the BB is too high for the rear axle so the chain comes in too low. Normally the BB is below the axle by an inch or two, but possibly he has the suspension jacked up so the BB is too high.

He also might look the FD position especially if the frame has a curved seat tube, or only a stub, which may be at an angle steeper than 73° or so. This used to be a problem with some TT bikes with curved seat tubes, and some builders compensated by pushing the braze-on back to the correct place with respect to the BB center.

In any case a photo, showing the entire rear triangle, including the BB rear axle and FD, along with a horizontal frame of reference, ie the floor, so we could see the relative positions of the parts. best would be 2 photos, one with the chain on the small/small, and once shifted to the outer ring, or as close as possible.

READ WHAT'S BELOW FIRST----

IF the OP is having this problem on the stand, and isn't talking about when riding, then he should raise the FD to the normal 2mm clearance over the outer ring, and test tide the bike. On the stand suspension rear wheels drop a few inches, and this entire issue may simply be the result of that droop.

imobilinpedalus

Sorry to keep the guys in suspense. I wrote the stuff the other day but all of a sudden it was devoured by the omnipotent onlines.

Thankyou FBI in NY for clearing that up for me. I didn`t realize the effect would lessen when riding the bike. It didn`t go away completely but it did help putting the weight on the bike. While riding, the chain touches the bottom side of the cage on the 2-3 smallest cogs. There is a chance i can work my way through this but no fun learning this is a big issue for manufacturers and why the trend is moving away from 3x drive if i read the other guy correct.