Cable tension for Shimano 105 front derailleur is very tight

Question

I just got a Merida Scultura 400.

On the biggest chainring, there's rubbing on the outside of the front derailleur cage. The only way that I can lower this is by tightening the cable by a lot. Is this normal on a road bike?

Answer 1

First you should make sure your "high limit screw" is loose enough. This may be marked H on the derailleur, or it may be one of two adjacent unmarked screws. By loosening this you allow the derailleur to travel a bit more outboard.

Then, using some combination of adjusting the cable tension with the clamp bolt and with the barrel adjuster, set the tension so that the cage stays outboard enough (but not too much). On your particular bike the barrel adjuster is inline with the shifter cable near the handlebars.

If your question is whether it's normal for there to be significant tension on the shift cable when the front derailleur is in the outer position, the answer is yes. This tension must overcome the (fairly strong) return spring in the derailleur.

Answer 2

Your high limit screw is the most likely problem. Less likely is debris preventing the derailleur from moving all the way out. It's always good to keep your drive train clean though, and turning a dirty screw can damage the threads; clean the area thoroughly before making adjustments.

The book probably recommends not turning the high adjustment screw while it is loaded (shifted to the large chain ring). That's a good idea. I'll confess sometimes I shift to the small ring to make sure the screw turns easily, then make adjustments while on the large ring. If the screw won't turn easily, see below.

Shift to the small sprocket and the big chain ring and adjust the screw to leave a small gap between the derailleur and the chain. Then shift to the tallest sprocket (second tallest is good enough) and make sure there is a gap on the inside. After adjustment, the screw should still be bottomed out (when shifted to the large ring). Verify by turning the screw counterclockwise. If the derailleur doesn't move outward, something else is wrong, hopefully insufficient cable tension.

Test the adjustment while shifted to the shortest and second tallest rear sprockets to make sure it shifts smoothly and the chain doesn't rub on either side. Don't worry too much about the tallest sprocket--you shouldn't use that combination anyway.

If shifting is unreliable, not smooth, or you drop your chain occasionally under load, tweak the adjustment. Dropping the chain under load can be caused by the derailleur moving too far out, even though it doesn't rub on the chain after shifting. Take a screwdriver with you on your next couple of rides. It's already in your gear bag, next to your Allen wrenches, pen knife, spoke wrench, and tire tools, right?

A spring keeps the screw nominally loaded so it won't turn while shifted to a smaller ring, but mine will still wander, so I use locktite once I'm satisfied with the adjustment. Mark the head of the screw wrt the derailleur (ink will rub off; scribe with a knife or chisle) and remove it. Clean male and female threads with a degreaser, apply a tiny dab of locktite to the screw tip, and reinstall the screw. The screw will bottom out close to the adjusted position, so it's not necessary to count revolutions while backing the screw out. The locktite I use is not so strong that I cannot turn the screw after it sets. If the screw won't budge next time you need to adjust it, try penetrating oil, and if that doesn't work, warm up the area with a hair dryer or (very carefully) with a heat gun to soften the locktite. Use a heat shield if you are worried about blistering your paint or damaging a composite frame.

Edit: I recently broke a 20+ year old index shifter (brifter) while adjusting it. I think I overloaded it by pulling sideways on the cable as a shortcut to see if additional cable adjustment was needed. Pushing the cable sideways has a very large mechanical advantage, i.e. it greatly increases the load on both the shifter and derailleur when the derailleur is at end of travel (already pushing against the high limit screw or the derailleur is jammed).