blinky

Have never tried shifting to small chain ring and either smallest or next up cog because years ago I heard it's a no-no, too much angle - but just recently had the bike in the shop for another issue and while on a trainer stand shifted through all the gears and sure enough got chain rub on both those cogs - a web search points to either the limit screws or installing a spacer on the BB but also I was thinking about loosening the clamp on the FD and turning it slightly .

What say you all ?

shelbyfv

Nope, Shimano says the four smallest may rub, though ymmv. https://si.shimano.com/api/publish/s...001-05-ENG.pdf

blinky

Thanks Shelbyfv , feel better now , will keep the info in that link - and btw It's a Shimano FD .

alcjphil

I say please tell us what components you have. It is common to get chain rub in the small/small gear combination if you don't trim your front derailleur. You don't do this by playing with limit screws. On some bikes, the sound is from the chain rubbing on the larger chainring. Knowing what bike you have will give us some clues to what is really happening.

shelbyfv

The Shimano site is good resource.

blinky

My local mechanic had the bike for a few days to look at another shifting issue I was having and he was able to get it so that the chain only rubs the outside of the cage when in the last two smallest cogs - I'll never be in any of those combinations with the small chain ring anyway but that information you provided from Shimano is comforting, thanks again !! .

Iride01 

You chain and cogs won't suddenly explode or be ruined if you cross chain. However for the longest life you shouldn't ride cross chained for a large portion of your time. If you are only going to be cross chained for a couple minutes or so while trying to top a hill, then no big deal.

IMO, more chains probably get worn out doing track stands at a stop than from cross chaining. But I have no evidence other than opinion.

SoSmellyAir

I think this is about as good as it gets. No matter how I adjust my FD I still get a slight rub on the third smallest cog (14T) which is only apparent when riding, not while my bike is on the stand.

Interesting that the manual (DM-FD0002-05) for the FD-9000, FD-6800, FD-5800, and FD4700 shows a similar picture as what shelbyfv posted above but only grays the 3 (not 4) smallest cogs.

https://si.shimano.com/pdfs/dm/DM-FD0002-05-ENG.pdf (see page 4)

JohnDThompson 

That's why you avoid those gear combinations. Chain rub on the front derailleur is a function of chainstay length and the number of chainrings and rear sprockets on your bike: short chainstays and lots of sprockets and chainrings make in more likely, and indexed front shifting makes it difficult to trim the derailleur to eliminate rub.

blinky

You're correct the manual for those earlier FD models does only show the three smallest but the link that shelbyfv provided is for models FD-R9100, FD-R8000, FD-5801, and FD-R7000 and does show the 4 smallest - I have FD-R8000 and so DM-RAFD001-05 is appropriate for my situation .

SoSmellyAir

Yes, it seems weird that Shimano implies that the trim function / chain rub situation got worse from the last generation to the new generation of Shimano front derailleurs!?

blinky

That dealer's manual doesn't mention the X-chaining from the large chainring over to the last few largest cogs in the back - the omission leads me to think it's similar to the recommendation for the small chainring .

shelbyfv

My own 8000 setup gives me noise in the largest two or three cogs with the big ring but I can use all but maybe the smallest cog while in the small ring. This would not be my preference. I'd like to have access to larger cogs with the big ring, suits my riding style and terrain better. Also, small cogs/small ring, the chain bounces around (no clutch, yet.) Anyway, I am still mostly clueless as to how the newest FDs work. I followed the instructions and got it working but I don't understand the subtleties.

oldschoolbike

Look for a way to move your chainrings inward a few millimetres, being careful to leave some clearance between the small ring and the chainstay. Another limitation may be whether the FD can be re-set inward enough. If you can't get all the movement you need, get what you can. It will help.

If you have square taper BB, try a cartridge that's 4-5 mm shorter. You could go cheap cartridge to try the fix, and upgrade once you know it works. If you have a two-piece crankset, look for spacers to remove from the drive side, but you might be stuck.

SoSmellyAir

Does this describe what is happening in the T-trim position?

Does this (i.e., the ability to use the second and third smallest cogs without noise) describe what is happening in the Low position?

The FD has four positions, from left (i.e., innermost) to right (i.e., outermost): L-trim, Low, T-trim, and Top. L-trim is set by the low limit screw. Top is set by the high limit screw. If you answer "yes" to both of my above questions, then it sounds like both Low and T-trim positions are a bit too far to the right. You can set these positions a bit to the left by slightly decreasing the tension of the FD shift cable.

Yes, the inline barrel adjuster in the FD shift cable works in mysterious ways. It took me forever to figure out the above.

shelbyfv

Thanks, and "yes" to both! I will ponder and try decreasing the cable tension. It's been a couple of years but IIRC I had to set a fair amount of tension to get the chain to the big ring. It's always been harder to shift than my 10 speed.

SoSmellyAir

Good luck. Hope that does it for you.

A new shift cable may help. My RD shift cable broke about two months ago. I had to remove the bottom bracket cable guide in order to thread the replacement RD shift cable through the guide, and so replaced my FD shift cable at the same time. IIRC, setting the FD positions with the new FD shift cable seemed much easier than my prior attempts to adjust them with the old FD shift cable.

cxwrench

There is no 'omission'. It's fine to cross chain big/big. I will increase drivetrain wear a bit but ALL modern drivetrains are designed to go big/big.

blinky

Thanks cxwrench !!

ClydeClydeson

Sometimes it is possible to get a new bike to shift into all gears without rubbing, but when assembling I would intentionally set it so it rubs in the most extreme X-chain scenarios. I intended it to act as a bit of a reminder to the owner that those combinations are not ideal.