Join Date: Feb 2007
Location: Tariffville, CT
Bikes: Tsunami Bikes
Mentioned: 10 Post(s)
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If the chain didn't shift from the big ring to the small ring, that's because the derailleur spring wasn't strong enough to overcome something - only that spring works to move the chain to the small ring. (On the other hand, *you* move the chain to the big ring). Some potential causes of not being able to shift into the small ring:
1. Improperly adjusted front derailleur. Too high up on the seat tube or limit screws not right. I'd check height first, then limit screws. The further away a derailleur is from the teeth, the less easily it derails. This applies to the front and rear.
2. Too much cable friction. Either it wasn't routed right, there's too much cable/housing, it's kinked somewhere (just the cable could be kinked inside the housing - need to pull the cable all the way out to check), it's not seated in the downtube guide correctly.
3. Front derailleur problem. Damaged pivot or something. Unlikely on a new bike.
4. As I usually put it, "the nut that holds the seat down", or user error.
You can check all of these on your own.
1. Check height - shift the derailleur so the outer cage is just above the big ring. Measure the gap between the derailleur cage and the top teeth (full height ones, not the slightly shaved ones). It should be about 1-2 mm max.
1a. Check limit screws. In the small ring, big cog, the chain should have about 1mm clearance to the inner cage. In the big ring, small cog, ditto to the outer cage. Also, during this process, make sure the outer cage is somewhat parallel to the chainring. If it's pointing to the right (i.e. out to your right as you're sitting on the bike) then it won't want to shift down. Pointing to the left helps it shift down but you may lose the chain when you go back up.
1b. If you have a steel cage front derailleur, you can squeeze the front ends together a touch. But that's not checking, that's adjusting. But this is an excellent way of helping a chain down to the small ring - basically you're pointing the outside of the derailleur to the left (to help it shift down) and the inside of the derailleur to the right (to help it shift up). Win-win.
2. Purposely derail the chain to the inside of the small ring. Give the derailleur some room to move - you might have to move the chain away a bit. Then, with no chain on the chainring, shift the front derailleur up and down. It should be easy to shift into the small ring, the derailleur shouldn't hang up. If it does hang up, check the cable under the downtube and see if it's dangling. If it is, the problem is between where it's dangling and the front derailleur. If it's not, the problem is north of the dangling cable - up in the upper housing, bar area, or shifter (make sure you actually shifted it down).
It's possible that the cable is too thick for the housing, the cable might be kinked, the cable housing wasn't cut well so it's rubbing the cable, etc.
3. With the chain off, you should be able to move the derailleur by hand. It should feel extremely firm but smooth. If it's not smooth (and not due to the chain rubbing the cage etc - you're checking how smooth the pivots are) then the front derailleur has a problem.
4. I found out the hard way, in a race, that if you apply full power to the cranks and try and shift down to the small ring, it sometimes doesn't work. The only real thing you'll need to make sure of to eliminate yourself from the problem is to see if you're trying to downshift when pedaling hard - like on a hill. Front derailleurs simply do not work well in that type of situation. If you ease up on the pedals a hair at the top of the pedal stroke, the derailleur spring will have much less chain tension to overcome. Remember, only the derailleur spring is moving the chain, not you or the shifter. If you make life difficult for that tiny spring, it'll make life difficult for you. If you're shifting down from the big ring partway up a hard climb, that's usually a difficult shift for a front derailleur. If possible shift down earlier, before you get into the "super hard pedaling" zone, to ease the load on the front derailleur.
Hope this helps,