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Discussion Starter · #1 ·
Hi - I'm trying to adjust my front deraileur (shimano lx), but the high limit adjust screw doesn't seem to be working. I'd appreciate any suggestions.

Here's what I did:
- Started in lowest gear (biggest chainring rear/ smallest chainring front)
- twisted cable tensioner at front shifter all the way clockwise (then backed out 2 clicks ccw) just so I have plenty of room to retighten later
- loosened allen bolt and released front deraileur cable
- pulled cage outward manually and verified about 5mm overhead clearance between cage and outer (biggest) chainring to verify height of deraileur
- adjusted low limit screw to 1.5mm clearance w/ inner side of cage
- hand pulled cable and tightened allen bolt
- adjusted cable tension at shifter so the chain shifts without hesitation up and down (though chain rubs against outer cage when at the big chainring)
- shifted the chain to highest gear (smallest rear chainring and biggest front chainring)
- tried to adjust high limit screw to move the cage outward to get rid of chain rub, but the cage doesn't move at all in response to screwing in either direction.

Any suggestions on what I may have missed would be greatly appreciated. Thanks in advance!
 

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Ok, here's the problem you have.

When you adjust the high limit screw without the cable attached then yes, it will work both ways - screwing in will limit the travel and screwing out will increase travel. When you attach a cable, however the screwing out direction no longer functions, as its the cable tension thats holding the derailleur in place, not the limit screw. What you need to do is to increase the cable tension (whilst in the outer position) on the barrel adjuster so that the derailleur moves across just enough to stop the rubbing, or hits the limit screw if the screw is in the right place.
 

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Discussion Starter · #4 ·
more on high limit adjust

Firstly, thanks to you both for the assistance!

Alex - I'm not sure I fully understand your point about the cable tension, except in the specific case when the cage has been extended outward, but is not actually stopped by the High Limit. In this case, I understand why screwing the High Limit Adjust wouldn't move the cage outward since its position was not limited by the screw in the first place. But I'm not sure why it wouldn't be able to adjust inward?

Since, I'm not sure I articulated the above well, allow me to reword my understanding of how things are *supposed* to work, and perhaps you or another kind soul can identify/explain what I'm missing.

1. The physical mounting (height and angle) of the front deraileur will determine the range of "possible" motion.
2. Increasing tension in the cable is what moves the front deraileur outward, and decreasing tension moves the deraileur inward toward its rest position (ie, no tension at all is fully inward).
3. The extent of outward motion is determined/limited by (whichever comes first), the amount of tension that's being applied, or has reached its "limit" as determined by the High Limit Adjust Screw, or finally, it reaches the full extension of the cage.

So I would think that, even when the cable has been tensioned, one could still adjust the High Limit screw to move the cage inward, because this would override the extension that the cage would otherwise try to reach based on the cable tension. Conversely, one could then still adjust the High Limit screw to move the cage outward, until such point that the position is "limited" by cable tension, limit screw, or full extension.

I'm not sure what the actual internal mechanism for the limit screws is, so perhaps this is why I can't actually visualize what's going on internally (ie, does it even make sense that the High Limit Adjust Screw "is not working?").

However, I did try to make adjustments using the barrel adjuster, and what I found is that when I increased the tension, the derailleur would hesitate when shifting downward to lower gears, and I haven't been able to dial in the correct middle point where it shifts up and down equally well, while still being able to reach the outer cog without rub. I could adjust the height/angle again, but I didn't have this problem before I released cable tension so I don't want to just move things around frivolously, and I also verified that when I manually pull the cage to the full extension, there is plenty of clearance for the outer chainring (if anything, there might even be *too much* clearance).

Any additional thoughts, tips, suggestions are very much appreciated! Thanks again!
 

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aybills said:
Hi - I'm trying to adjust my front deraileur (shimano lx), but the high limit adjust screw doesn't seem to be working. I'd appreciate any suggestions.

Here's what I did:
- Started in lowest gear (biggest chainring rear/ smallest chainring front)
- twisted cable tensioner at front shifter all the way clockwise (then backed out 2 clicks ccw) just so I have plenty of room to retighten later
- loosened allen bolt and released front deraileur cable
- pulled cage outward manually and verified about 5mm overhead clearance between cage and outer (biggest) chainring to verify height of deraileurTry 1mm to 2 mm clearance when outer edge of FD is over the big chainring
- adjusted low limit screw to 1.5mm clearance w/ inner side of cage
- hand pulled cable and tightened allen bolt
- adjusted cable tension at shifter so the chain shifts without hesitation up and down (though chain rubs against outer cage when at the big chainring)
- shifted the chain to highest gear (smallest rear chainring and biggest front chainring)
- tried to adjust high limit screw to move the cage outward to get rid of chain rub, but the cage doesn't move at all in response to screwing in either direction.

Any suggestions on what I may have missed would be greatly appreciated. Thanks in advance!
Also look closely at the shifting ramps on the FD, to make sure the height and toe in/out are okay...

Never said you lined the FD up so the cage is perpenticular to the crank axle.
 

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aybills said:
Hi - I'm trying to adjust my front deraileur (shimano lx), but the high limit adjust screw doesn't seem to be working. I'd appreciate any suggestions.

Here's what I did:
- Started in lowest gear (biggest chainring rear/ smallest chainring front)
- twisted cable tensioner at front shifter all the way clockwise (then backed out 2 clicks ccw) just so I have plenty of room to retighten later
- loosened allen bolt and released front deraileur cable
- pulled cage outward manually and verified about 5mm overhead clearance between cage and outer (biggest) chainring to verify height of deraileur
- adjusted low limit screw to 1.5mm clearance w/ inner side of cage
- hand pulled cable and tightened allen bolt
- adjusted cable tension at shifter so the chain shifts without hesitation up and down (though chain rubs against outer cage when at the big chainring)
- shifted the chain to highest gear (smallest rear chainring and biggest front chainring)
- tried to adjust high limit screw to move the cage outward to get rid of chain rub, but the cage doesn't move at all in response to screwing in either direction.

Any suggestions on what I may have missed would be greatly appreciated. Thanks in advance!
5mm of clearance is too much over the big ring. It should be about the thickness of a dime or penny, that's 1-2mm. 1.5mm clearance is too much for the low limit. Shimano specs call for 0-.5mm. I've found this can sometimes result in rub but I don't go a ton beyond 1mm because there's a better chance the chain will over shift onto the BB.

Like has been said, the cable is limiting the distance the cage can move out, not the limit screw. You can't make the cage move out farther if more tension isn't put on the cable. This can only be done by the barrel adjuster. The limit screw can keep the cage from moving out farther, regardless of how much tension is on the cable, but it can't make the cage move out farther by backing it out. Only cable tension can do this.

Edit: Besides low and high limit screws and FD height and angle there's another adjustment that needs to be performed when setting up a FD. Once the height, angle and limit screws are set, shift to the middle ring on the front and big cog on the back. Does the chain rub on the inside of the cage? If so increase tension on the cable with the barrel adjuster until the rub stops. Even though you've increased tension your derailleur will not move further out when shifting to the big ring because the limit screw prevents it from doing so.
 

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Discussion Starter · #7 ·
Thank you! Yes, now that I better understand the front deraileur, I'm pretty sure that the deraileur is too high, so I will adjust tonight (Sheldon Brown's site also says the lower, the better). I think that the deraileur is so high that the chain itself (when in the lowest rear cog) is now blocking the front deraileur cage from going outward further because the bottom of the cage hits the chain.

Thanks again everybody for all your helpful feedback!
 

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Discussion Starter · #8 ·
Front deraileur successfully adjusted

Mission accomplished! It took over an hour, but I'm quite glad I went through the exercise playing with heights, angles, tensions, etc. I have a much deeper understanding of the front derailleurs and chain movement (esp, when chain is going across big rear to big front and small rear to small front), and I now truly appreciate how tight the adjustments are for an indexed 27-speed system. My previous bike ('93 Bridgestone MB-3) had analog shifters for the front, so I never experienced problems w/ shifting up/down the front chainrings, or chain rub, etc.

Thanks again for everybody's feedback.
 
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