Front Derailleur Problem
 

Hi I'm having a weird problem with a front derailleur and cant find any help.

Its a 3 ring Shimano and i've managed to get it working but whenever I shift down from the largest ring it automatically shifts all the way down to the smallest ring (skipping the middle). Doesn't seemed to be a cable/limit screw issue. Anyone know whats up?

What shifter(make and model) and what derailleur do you have? Limit screws only limit how far the derailleur travels from large to small chainrings.

Is it an STI setup, and are you using the stock derailleur and crankset/rings? I had this same issue when I changed to smaller rings. The issue is/was caused by the triple derailleur hump not lining up to lift the chain onto the big ring. Due to this, more tension has to be put onto the cable to get ti to shift up, and that ends up causing a big shift down when the tension is released. I have not solved this issue yet, but one of the fixes is to use a derailleur for a double without the hump:

https://www.renehersecycles.com/trou...h-sti-triples/

With just one post you aren't yet allowed to post pictures. That would get the fastest and best answer. The derailleur itself only does what the shift mechanism lets it do so most likely there is a problem inside the shifter. If you had a bar end shifter this would be a very easy problem of using a screw driver to tighten the bolt on the side of the shift mechanism. If you have SRAM Grip Shifters the repair is much more difficult. You can see what holds the shifter in each position here

You Tube

. If the index spring is broken or bent your shift mechanism is probably toast. The Grip shifters are very common on bikes.

What type of shifter (specific model number, if possible).

Loosen the cable binding bolt at the derailleur. Feed the inner cable back through the bottom bracket guide and any downtube guides, so that you can hold the inner cable just beyond where it exits the initial downtube cable stop. Apply finger tension to the cable, enough to slightly resist movement initiated by the shifter. By feel, how many shift "clicks" can you feel through the cable when starting in the most slack, "lowest" shifter setting? Now, from the highest or "tightest" setting, how many clicks can you feel down to the end?

Can you feel the cable pull length changing as you shift up and down at the shifter?

With the cable unattached, can you manually swing the derailleur from the lowest desired set point to outward and centered over the largest chain ring? Return the derailleur to its sprung inner position. Shift the rear to its innermost cog. Is there any chain rubbing on the inner edge inside of the front derailleur cage? Conversely, is there more than 1mm air gap between the chain and the inner cage? If so, you'll need to adjust the "L" limit screw on the front derailleur to make the gap correct without rubbing. I like as small a gap as I can dial in.

This is your starting point. Now we'll know how many trim settings you should expect to have. We'll also be able to tell you how taut to have the cable set up initially, in the starting (lowest) ring. From there, it's simply a tension adjustment

It's always a good idea to start with the official Shimano instructions.

Note the parts I've highlighted:


https://cimg4.ibsrv.net/gimg/bikefor...4e194be515.png
Shimano FD-5703 triple derailleur instructions