Front Derailler rubbing chain
The inner plate of my bikes front derailler rubs the chain when I am on the smallest chain ring (of a double) and the largest rear cog. If I upshift to any other cog, it doesn't rub.
Should I be adjusting the limit screws, the cable tension or rotating the FD?
The low limit screw of the front derailleur needs to be adjusted so that the inner plate moves in a little bit. Adjust it while the chain is on the small ring, so that you can see what is happening. Or, the FD might need to be rotated. Or, a combination of the two. Probably just adjusting the low limit will help. (The inner and outer plates should be roughly parallel to the chainrings.)
So does the cable tension adjustment just effect the shift from the small to big chain ring? Does the fact that the rubbing only occurs in the lowest gear (largest cog) suggest that the FD is mis aligned?
There should be no tension on the cable when the chain is on the smallest chainring - it should be just barely slack. The low limit adjustment determines where the derailleur cage stops. The most common reason for rubbing as described is low limit mis-adjustment or FD mis-positioning.
Cable tension must be correct for best shifting to middle and/or largest ring. Hi limit adjustment will determine how far away from midline the cage will travel, altho that can be affected by inadequate cable tension.
Take a look at the Park Tool site for step-by-step adjustment advice.
So if the cable isn't slack I should loosen it a bit? Presumably, the FD won't reach the low limit otherwise?
Originally Posted by JanMM
Licensed Bike Geek
As another poster suggested, the Park Tool site http://www.parktool.com/repair/readhowto.asp?id=75 has a comprehensive section on bike repairs. The provided link is to the part about the front dérailleur adjustments. It is strongly suggested that you follow the directions, in order, one step at a time. Doing one dérailleur adjustment can affect other parts of the dérailleur adjustments.
Adjusting dérailleurs isn't hard but until you understand how to do it, they can drive you nuts!
Thanks, I have read the Park Tool section on front dérailleurs. It is not clear to me from those directions what effect adjusting the shift cable tension would have on the inner plate.
Originally Posted by Davet
BTW I would prefer not to reposition the FD if I can avoid it. I am making the, possibly naive, assumption that the bike mechanics in the LBS I purchased the bike from are more likely to have positioned the FD correctly than I would be likely too. It seems likely from reading the Park tools website (and the Zinn book) that I need to adjust the limit screw. I am curious about the cable tension though.
Licensed Bike Geek
OK. You won't have to re-position the dérailleur IF the gap between the outer cage plate and big ring teeth is satisfactory AND the cage is parallel with the chainrings. If the former are OK, then when the front der has put the chain in the small ring and the rear der has put the chain on the largest cog, check that there is no tension on the front der cable. If there is, turn the cable adjuster so there is a very little slack in the cable. Now look to see if there is clearance between the inner cage plate of the front der and the chain. Ideally there should be about 1MM distance between the two. If not, you need to turn the low-limit screw counter-clockwise until the 1MM clearance appears. When the clearance is satisfactory, re-adjust the cable adjuster so there is very little slack in the cable.
Originally Posted by adgrant
If you have a method of keeping the rear wheel off the ground (a repair stand or a Y-stand) it's easy to check for proper shifting and cable rub.
Hope this helps.
Short answer. Yes if the cable is too tight it will keep the derailleur from reaching the limit screw (assuming the limit screw is set correctly). It would basically be akin to holding down the upshift lever on your shifters just a wee bit and then riding around like that.
Thanks Davet and BikEthan, that was exactly the information I was looking for. I actually just bought a repair stand which is how I noticed the chain rub (I don't spend much time in the bottom gear).
The bike mechanics could adjust it this way if you have the left shifter without the trimming capability that allows to move the front derailleur slightly to the left or right without shifting to another chainring (for example, SRAM Rival or Force).
If you loosen the cable from the FD while the chain is on the small ring, does it still rub? If so, then the problem is likely the low limit.
Looks like it was the low limit screw. I just need to do some more testing to ensure I didn't over adjust.
The easiest way I've found to set your limit screws is to loosen the cable pinch bolt so the cable goes slack, adjust the inner limit so the inner plate doesn't rub (while on the largest rear cog), and the outer limit so the outer plate clears the chain (while on the smallest rear cog). Before retighetning the cable pinch bolt, reset your barrel adjuster towards the middle of it's travel. Now just adjust your shiftline (if needed) and you're good to go. Hope this helps..
Yes that is helpful thanks. I actually ended up adjusting both screws and I also had to tighten the cable tension. The FD now shifts much more quickly than it used to and I have no chain rub. Its basically working perfectly on the repair stand. Hopefully it works equally well on the road when I take it out this weekend.
My RD didn't seem to require any adjustment.
Here are your possibilities:
1. Your cable is too tight.
2. Your limit screw needs adjusted to allow for more inward travel of the mech.
After reading the solutions and your final response, it sounds like you have the problem licked. Let me add as a point of information:
Originally Posted by adgrant
ALWAYS adjust your rear der properly BEFORE adjusting the front der as the rear der operation will affect the operation of the front der.
ALSO, to eliminate as many variables as posssible, it is a BEST PRACTICE to shift gears by pulling the cables themselves when checking hi-lo limits initially. This eliminates the possibility that a sloppy shifting mechanism ( excess play, worn mechanism ) will mislead you while making adjustments.
While not addressing your specific problem, please keep in mind that MOST shifting and braking problems result from cable issues, usually inadequate lubrication and/or poor cable routing. Check the easiest things first... If your adjustments are good and it still won't shift or brake properly, see above. New cables and housings often cure many a stubborn problem.
Well I took the bike out today. The FD now shifts much more quickly into the big ring and the down shift is also faster. Only problem is I managed to throw the chain shifting into the small ring.
Here's some additional advice. Hope it helps.
Originally Posted by adgrant
ALL RDER ADJUSTMENTS MUST BE MADE BEFORE ADJUSTING FDER
Set front der. shifter in small chainring position.
Shift chain to largest cog in rear cassette.
Release tension on front der. w/ chain on small ring, largest cog in rear cassette.
Adjust low limit screw ( right to tighten, clockwise ) until it begins to rub chain as you are spinning w/bike in stand.
Readjust as follows:
Turn low limit screw left to loosen, counterclockwise until it stops rubbing. You want APPPROX. 1mm clearance.
IF you have a front derailleur adjustment screw on your handlebar, adjust it so you are able to change cable tension in either direction ( tighten or loosen ) by positioning it halfway between either extreme. This is for FINE cable tension adjustment and should be used to fine-tune tension if necessary, AFTER the GROSS CABLE TENSION has been set w/ the pinch bolt as follows.
Reattach front derailleur cable. This cable should be snug and under some tension but should not be as tight as a banjo string. Pulling the cable taut w/ one hand while tightening the pinch bolt w/ the other hand will generally yield acceptable tension. Applying TOO MUCH TENSION will prevent the der. from reaching the limit screw stops you set earlier. THIS is why cable tension is so important. Addititonally, proper cable tension will allow for quick and accurate shifting action.
Spin the cranks and check operation of fder for rubbing in small chainring. If rubbing occurs, adjust low limit counterclockwise slightly until it stops.
Check cable tension. Should be snug.
Shift chain to smallest rear cog to check if chain rubs on fder in this position. This is called cross-chaining and is not recommended because of the extreme chain angle and stresses placed on the drivetrain as well as premature wear of the fder itself. It is possible to use this combination but shifting to a different chain ring is preferrable.
To set fder high limit screw to prevent rubbing, first adjust clockwise until rubbing is detected and then back off adjustment until rubbing no longer occurs.
IF YOU HAVE 8-10 REAR COGS, IT MAY BE IMPOSSIBLE to eliminate chain rub at this extreme angle w/out increasing chain/fder gap to an unacceptable level which allows chain derailment to the outside. Excessive gap can be seen but can ultimately only be determined by making the fder adjustment to eliminate rubbing and then carefully shifting the fder full range to test. DERAILMENT is to be avoided at all costs. A slight rub at extreme chain angles is much better but if you experience this you should be in a diff. chainring anyway.
Shift fder between front chainrings and note performance. Adjust as needed to prevent derailment. Sluggish downshifting is often symptomatic of excessive cable tension. Decrease tension w/handlebar barrel adjuster to correct.
Shift rear cogs up and down in each chainring. Note performance and adjust as needed.
Last edited by rickowensis; 11-18-07 at 11:24 PM.
Thanks for the additional info. My low limit screw is now set to position the FD as close to the chain as possible in the smallest chain ring but lowest cog. Chain does not rub but does drop off the small chain ring if I shift to it while the RD is shifted to a low cog. The only solution to this issue appears to be to rotate the FD so the leading edge is further out.
I suggest you check tension of front deraileur. Excessive tension can cause the fder to snap from ring to ring derailing the chain. If this is so, decrease tension. Depending on type of shifter - Gripshift, RapidFire etc., or wear of unit - it may be necessary to have less tension to achieve desired results.
Originally Posted by adgrant
Fder alignment should be parallel w/chainrings as baseline adjustment. Satisfactory shifting results can usually be obtained from here. Also, ensure fder is only approx. 1-2 mm above largest chainring to mitigate chances of derailment. Slight tweaking of fders can sometimes help if all else fails but it is important to understand what the underlying problem is.
Good luck. Keep me posted. If this doesn't work, give me particulars about your bike, shifting components, wear etc. and I will continue the analysis if so desired.
I also have a problem with chain rub, but it is when I'm in the big ring and small sprocket. I believe it is a problem with cable tension and not with the limit screw. The bike shifts slightly sluggishly from the middle ring to the big ring. When I put the chain on the big ring and the smallest sprocket, I can eliminate the rub by pulling on the cable with my hand. I don't know how to use the barrel adjuster to tighten the cable. It is a Jagwire Rocket adjuster. Sitting on the bike, do I turn it CW or CCW?
I looked in the Troubleshooting section of the manual for my FD and it says the solution for chain rub on the outerplate in the small cog is to loosen the H-limit screw by 1/8th turn. Does that make sense to you experienced technicians?
Has nothing to do with experience, but with logic. You said "I can eliminate the rub by pulling on the cable with my hand." Doing so moves the derailleur against the H limit screw. Therefore the limit screw cannot be the problem. You are back to cable tension OR derailleur position, which must be proper as to rotation and height, per Park Tool site. As for which way to turn an adjustment barrel, one normally can figure it out by looking but all you have to do is twist it a few turns in one direction and see what happens.
Originally Posted by Equinox