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Help with front derailleur adjustment

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Help with front derailleur adjustment

Old 08-06-14, 07:28 AM
  #1  
Randi38
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Help with front derailleur adjustment

I recently had a bike shop replace my rear cassette.
On my first few rides everything went well, but now I'm having an issue with the front derailleur shifting properly.
This bike has 7 sprockets on the rear and 3 on the front and also is an older road bike with down tube shifters.
Anyway the problem that has developed is getting the front derailleur to shift onto the smallest chain ring on the front.
9 times out of ten it will not go onto this sprocket when in shifted fully into the top position where the lever is parallel to the frame tube.
I can however get it to shift into the smallest chain ring by overextending it beyond its normal stop position tho.
I've looked at videos on how to adjust this derailleur, but they all go thru a somewhat lengthy process on getting this adjustment correct, when all I want to do is just make a slight adjustment to correct my problem.
I'm just not sure which of the high/low screws to adjust to fix my issue?
I'm trying to avoid having to take my bike back in to the shop when I think this is an easy fix that I can do at home with some guidance.
Thanks for any advice!!
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Old 08-06-14, 08:18 AM
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Andrew R Stewart 
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With so little info and no photos we have to make assumptions. There are a number of factors controlling frt der performance. The der cage's height above the large ring, usually about 2-4mm. The cage's rotational alignment WRT the ring's plane. The cage should be pretty parallel to the plane of the rings. The range of in and outward swing. This is controlled by the limit screws. One can look at the screw ends and see which is contacted by the stop tab with in or outward movement and figure which screw does which limit. It is likely that the outer screw limits the outer movement and the inner screw controls the inward travel, but confirm this first. The cable needs to have enough tension so that as the lever is rotated the der cage moves but not so much cable tension that when the lever is fully forward/down/released the cage is limited by the cable and not the inward limit screw. The lever needs to have enough cable wrap so that the cable can move enough to fully move the der through the needed range. The shape of the cage WRT the size and diameter differences of the rings also comes into play. The chain's side play/flexibility and links' contouring help control shifting. The rings' teeth contours do the same.

I would first learn to play with the frt der using only the cable, with the bike up in the air. This way the condition of the lever is left out of the "system". One can tell very quickly if the cable is too tight or loose by watching the cage movement and the cable's being pulled. The location of the der on the seat tube WRT the rings can be observed as the cage is roughly centered over each ring. The limit screws can be turned, about 1/4 turn at a time, and the change in cage travel can be observed. Only when the der and cable aspects are working well would I use the lever to repeat the above exercise.

Remember that the frt der shifting when the bike is up in the air (on a stand) is different then when you're pedaling. There is virtually no chain tension until you pedal. Trying to shift the frt der with too much pedal pressure will cause the chain to get taught and not want to flex sideways and shift off one ring and onto another ring. So "soft pedaling' is a needed technique when riding and shifting.

The OP mentions that the cassette was replaced recently. Was the chain replaced too? It is possible that the old chain is too flexible to work well. It is possible that a new chain is too narrow to not get trapped between the rings. Modern chains are narrower then older designs. It's even possible the the crank/rings are bent or the Bb is so loose that the der cage has a "moving" target to work with.

There are so many questions that I have as to the entire system that I can't tell the Op what the simple fix is (if there is a simple fix). If this bike were in front of me many understandings and likely paths of improvement would be known in only a few minutes. Andy.
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Old 08-06-14, 08:22 AM
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Sometimes the lengthy process is required, but it could just be a binding cable/housing. Try shifting the derailleur by pulling up on the cable at the down tube and then letting go. If it shifts down well (or at least more easily) then you need to resolve a friction issue - lubrication, kink, broken cable strand, etc.
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Old 08-06-14, 01:12 PM
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use friction shifting? where is the lever? how long is the housing and cable ?
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Old 08-06-14, 01:20 PM
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lube the cable where it passes under the bottom bracket, lube the pivot points on the fd
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Old 08-06-14, 01:25 PM
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IF the limit screw needs adjusting it would be the low limit screw. You would turn it counter-clockwise a very small amount. You could try a 1/16th turn. As others have already said, it could be something else so remember where the screw is set now in case you want to turn it back to the same spot.
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