Rear derailleur bent?

robthehungrymonkey

So, i've got a bent (in some way) Rear derailleur. I had a crash and moved the groupset over to a new frame. It took me a while to diagnose the problem as the issue was only apparent when I was riding in the smaller chainring. No issues with the new frame and a new derailleur was purchased on ebay and works fine.

Now, can i fix it? I'd like to re-use the mech on another build (it's Di2) and at first thought I could just replace the cage with a 105 one off another bike and see if that fixes it... Turns out you can't swap the cages between Ultegra and 105.

So, is there a way of working out exactly which part is knackered? (it's hard to tell from looking at it) If it's definitely the cage, then I could buy a non-di2 mech on ebay and swap the cages over. If it's something inside the Rear derailleur then i'll probably have to write it off.

svetty

If the cage is bent enough to impact shifting it should apparent by eye. On the other hand if it shifts OK in the big ring it is presumably moving correctly implying no significant internal derangement. All this presupposes that the mech hanger is correctly aligned - which is implied as you state that the new mech works OK.......

robthehungrymonkey

Svetty wrote:

If the cage is bent enough to impact shifting it should apparent by eye. On the other hand if it shifts OK in the big ring it is presumably moving correctly implying no significant internal derangement. All this presupposes that the mech hanger is correctly aligned - which is implied as you state that the new mech works OK.......

Yep, completely new frame too. The new frame has very short chain stays (I removed quite a few links from the chain when I swapped it over from my Trek Madone), so maybe that exaggerates the issue? i'm tempted to just build up the new bike I want to put this broken mech on (actually, my girlfriends) and try it.

svetty

You can't assume that a new bike will have a correctly aligned hanger - I always check.
I would try it as you suggest. If the cage is either out of true vertically or is twisted you should be able to see this and gently correct any deviation.

robthehungrymonkey

Svetty wrote:

You can't assume that a new bike will have a correctly aligned hanger - I always check.
I would try it as you suggest. If the cage is either out of true vertically or is twisted you should be able to see this and gently correct any deviation.

TO Clarify, I have replaced the derailleur and the new one works flawlessly on the new frame. However, i'd like to try and re-use the old one if possible as I've collected enough spares to put Di2 on my girlfriends bike.

Would it definitely be the cage? As I really can't distinguish any difference.

sandyballs

Can you clarify what the actual issue is, you state that it only occurs when you are in the small chainring, what is the derailleur doing, rubbing, intermittent shifts, other?

Seems strange that it only occurs in one chainring if your original post is correct.

robthehungrymonkey

Sandyballs wrote:

Can you clarify what the actual issue is, you state that it only occurs when you are in the small chainring, what is the derailleur doing, rubbing, intermittent shifts, other?

Seems strange that it only occurs in one chainring if your original post is correct.

On my Cervelo S5
In the small chainring, it is noisy (noisier in the top half of the cassette). And it skips under power.

A new derailleur (well, off ebay) seems to work fine.

Cannondale Supersix EVO

Last night, I spent some time installing Di2 onto my girlfriends bike (a Cannondal Supersixe Evo). And it seems to work ok!!

I am now super confused and can only guess this is due to the super short chainstays on the Cervelo and there being little margin for error???

svetty

Skipping suggests chain may be a factor?

robthehungrymonkey

Svetty wrote:

Skipping suggests chain may be a factor?

I thought that at first. Replaced the chain and the same chain works fine on the new mech.