Bicycle Mechanics - Rear Derailler will shift up but not down?

Hi,

While trying to fix my bike i have come across a problem:

The rear derailler will allow me to shift up but not down. What have I done or is the rear derailler worn out?

Does the cable need tightening? I have tried tightening it. In top gear it is tight but in the lowest there is no tension yet the rear mech wont shift down? are the springs worn out. (N.B. the bike is a few years old)

Thanks for any replies.

Hi,

While trying to fix my bike i have come across a problem:

The rear derailler will allow me to shift up but not down. What have I done or is the rear derailler worn out?

Does the cable need tightening? I have tried tightening it. In top gear it is tight but in the lowest there is no tension yet the rear mech wont shift down? are the springs worn out. (N.B. the bike is a few years old)

Thanks for any replies.Define what you mean by up and down. Exept for rapid rise RD which are low normal,rather than hi normal, downshifting to lower gears or bigger cogs is controlled by the shifer positively pulling cable.

Try this: Work the shifter (you didn't say what kind you have...) to the "high" position.

Now make sure your chain is on the smallest rear cog. (that's the highest gear) There should be little or no slack in your derailleur cable at this point. If you can feel a lot of slack, you need to loosen the attachment at the rear DR and pull it snug. That should put you in the ballpark; fine adjustments can be made at the shifter or rear DR cable adjusters.

What I mean't was that when the rear dr will shift up to the smallest cog but not back down again when I use my gripshift.

When I change to the smallest cog there is no slack but when I change down there is alot less tension in the cable but it doesn't change down.

Thanks