New rear derailleur trouble

[micbm84]

Hope someone could give me some advice.

I've just installed a new rear derailleur on my bike, along with a new cassette, chain and cables.

My problem, it seems, is with the cable tightness. Starting at the highest gear (smallest cassette ring), when I tighten the cable to the point it will shift smoothly down (ie to larger cassette rings) everything is fine as I move to progressively higher gears.

However, when I'm in the lowest gear (largest cassette ring) it will now not shift up unless I loosen the cable, which means I can no longer shift down from my highest gears - and I'm back where I started. I've done this several times and seem to be going round in circles.

Any thoughts/advice would be greatly appreciated.

Michael

[nfmisso]

Hi Michael;

The derailleur spring is not strong enough to pull the cable through. Upshifting relies on the RD's spring to pull the cable through the system.

Debug: remove the cable from the RD; verify that the RD moves through its complete range freely.

Grab the end of the cable that was attached to the RD, hold on to it while upshifting and downshifting. Downshifting will pull cable into the house, upshifting will just let the cable go slack.

You need to find where in the system the cable is experiencing high friction.

Did you lubricate the cable? Is it getting hung up on a burr and the end of a housing?

[DaveM]

Check if derailleur hanger is bent. This would be pretty classic behavior of a misaligned hanger, even if shifting was fine with the old derailleur.

Also check the new cable housing (if you installed that). It a piece isn't fitting into the guide on the frame firm and straight or if the housing is compressing or flexing excessively. (And check you didn't use brake housing instead of gear housing)

[cny-man]

Yes, check for friction first. One way to check to see if that's the problem is when you shift to the next smaller cog and it does not go, "pluck" the cable (pull slightly and let go) at the two exposed sections. If the derailleur then shifts to the next cog friction is the problem. You need to check for smooth bends in the housing, no kinks in the cable, proper index compatible housing and properly cut and capped housing.

A bent hanger would not likely show the indicated symptoms. Rather it would shift OK in one part of the cassette but not on cogs further away, and would generally exhibit that behavior whether shifting up or down.