More Chain Suck and Derailleu Questions...

oldguy00 · #1 08-11-2020, 11:30 AM

Hey all, think I posted a question about chain suck on this bike before....we are still trying to work through some issues...
Bike is a stock (2019 or 2020) Trek Marlin 7.

Issues we have are some shifting issues in some cross chained gears, and bad scratching on the chainstay presumably from chain suck.

Pics attached. Issues:

1. We know that technically you shouldn't be cross chaining, but when he has the chain in the big ring, and the two bigger gears in the back, the derailleur cage actually looks like it is twisted outward (towards the chain line I guess) and it tends to jump the chain back down to the next gear down on its own. Should that really be happening?

2. When in the small chainring, and using some of the smaller gears in the back - but not even a bad cross chain, like maybe the third or fourth gear in, the chain is touching the bottom of the front derailleur cage. One option I guess is to lower the front derailleur, but not sure there is a lot of room to do this before it will be too low to clear the large chainring.

3. The scratches from chain suck. Now I admit to not having experience in this....the shop claims the chain suck is somehow from the small chainring being worn out, and they are even going to replace the chainring for free...but I don't get how a bike that has probably been ridden for a total of 10 hours before this happened, could have a worn chainring, or how a worn chainring could cause chain suck. And the chainring looks new to me (pic attached).

Any advice for this stuff?? Thx!!

Note - not all pics are in the same gear, so no really good pics of the chain resting on the bottom of the FD when in the smaller gears in back, but it does in fact do that!

yinzerniner · #2 08-11-2020, 11:35 AM

That FD looks like it's set up WAAAAAYYY too high off the big ring. 1-3mm is usually the rule of thumb, and your pics look like close to 7-8mm.

Also, the bottom of the FD cage hitting the chain could be the cause of all your noise and chainsuck issues.

I'm not sure who set up the FD but it looks like a pretty poor job.

oldguy00 · #3 08-11-2020, 12:22 PM

Quote:

Originally Posted by yinzerniner

That FD looks like it's set up WAAAAAYYY too high off the big ring. 1-3mm is usually the rule of thumb, and your pics look like close to 7-8mm.

Also, the bottom of the FD cage hitting the chain could be the cause of all your noise and chainsuck issues.

I'm not sure who set up the FD but it looks like a pretty poor job.

OK, so please explain, how does the chain touching the bottom of the FD cage cause chain suck? I don't think I understand what it is. Is the chain getting jammed into the chainstay from the top, or up from the bottom??

yinzerniner · #4 08-11-2020, 01:10 PM

Quote:

Originally Posted by oldguy00

OK, so please explain, how does the chain touching the bottom of the FD cage cause chain suck? I don't think I understand what it is. Is the chain getting jammed into the chainstay from the top, or up from the bottom??

I might've misstyped earlier - meant to say that the improper setup of the FD could be the cause of the bottom of the cage hitting the chain and the chain suck.

If the FD isn't set up properly any host of issues can pop up, chain suck being one of them. If the cage is too high then it could cause slower shifts. When shifting up or down, if the chain isn't "derailed" or pushed from one ring to the other fully and quickly it can get jammed on one or both rings and "sucked" up.

Your best bet is to try to clean the chainrings and chain and setup the FD from scratch and see if that helps. As Mike Ehrmantraut said - "No More Half Measures"