Rear shifting - Unable to shift to toughest gear

Question

I was adjusting my rear shifting following the advice in this video:

It was when I took off the wire tension and screwed the high limit all the way out that the chain wouldn't make it to the toughest gear. Instead it was stuck at the second-toughest gear.

So it seems that even without the wire pulling on the derailleur, it is too far on the left (i.e. towards the easier gears). What could I try to track down the problem, or better, what exactly is causing this and how can I fix this?

I was then continuing to adjust the derailleur, making the chain switch to my second-toughest gear when I try to shift to the toughest gear with the shifter at my handlebars and making the chain switch to my easiest gear when I try to shift to my second-easiest gear. This worked fine and the chain is currently not skipping gears on its way up or down, it just can't reach the toughest gear.

Answer 1

The first thing to do with any rear derailleur problem is to check if its bent. If it is bent, go to your LBS and see if it can be unbent (there is a special tool for checking unbent-ness, Park Tool DAG-2 or similar) -- bent derailleurs cannot be adjusted properly in most cases (and even a minor spill can cause a bent derailleur). If its not bent, then try adjusting it.

If you can't reach the smallest cog with the cable released (assuming a typical non-rapid rise derailleur, i.e. one which goes to the smallest with no cable tension), it likely means your high limit screw is mis-adjusted (too tight). [Typically, the problem upshifting into the smallest cog is usually a bent derailleur/cable friction, but you should have eliminated those two options before you got to this].

Some good tutorials for adjusting derailleurs are Sheldon Brown and Park tool.

Answer 2

Take a close look at the limit screw - is the derailleur actually hitting it? If not, then it's possible that the spring which pulls it to the right is worn out. If that's the case, you might be able to replace the spring, but unless it's a really expensive derailleur, you're better off just replacing the whole thing.

Answer 3

Just as Batman said, check the derailleur is not bent. If it isn't, but you still can't get the smallest cog I'd be looking to check if the mech hanger is bent. But if you're bike shop is checking the alignment of the mech, they should be able to spot if the mech hanger is bent or twisted at the same time.

Hope you get it sorted!