I have done a whole load of stuff to my new bikes (changed compact crank for standard, swapped chains, used quick link instead of Shimano pin, but a narrower handlebar on my Trek, etc) but the thing that is or was causing me grief was the damage to my Trek derailleur hanger -- which can not be changed other than at the Trek factory.
It had clearly been bent in after a crash. It does not seem to be something that is going to happen all that often so I used the spare rear wheel method to get the derailleur hanger back to being parallel to the wheel.
A spare rear wheel will screw into the derailleur hanger hole. The task is then to make the spare wheel and the wheel on the bike parallel. As shown in the photo below
Rear Wheel and Squared Vernier Callipers by
Timothy Takemoto, on Flick
instead of using a ruler (which can result in non horizontal measuring since the two wheels are not concentrically aligned) I adapted some cheap vernier calipers to ad a square as in the photo below.
Squared Vernier Calliper by
Timothy Takemoto, on Flickr
It seemed to be that I was getting within 2 to 3mm of parallel and that if anything the rear derailleur was pointing very slightly outwards.
Even so I could only just get to the smallest cog with a totally slack wire and no delimiter screw. There was no way I could get the derailleur to go out too far and then delimit using the screw. This meant that part of the change up to the next larger, 13 tooth cog was to take up the slack of the wire. This meant that the derailleur was not quite changing into the bigger cogs.
It seems to me I need a
little bit of tension on the wire in the lowest cog, and a little bit of delimiter screw (i.e. that in the lowest cog position the derailleur is resting on the delimiter screw rather than resting on the wire tension) for smooth, and stable (irrespective of wire stretch or ferrule compaction) shift up the cassette.
I think that the crash had caused not only a down and inwards bend in the hanger but also simply pushed the whole hanger a little bit too far inwards. Having been readjusted, it now appears to be parallel to the wheel but just not out far enough. Pushing the hanger outwards outwards without at the same time bending it any direction seems to be very difficult. Is there a way of doing this? I thought of using a the top of the handle of a G clamp somehow to push outwards.
Not wanting to put undue stress on the hanger, I put a washer of about 1.5mm thickness in between the derailleur and the hanger. This means that I can set the wire to have just a little bit of tension, and for the derailleur to rest on the derailleur limiting screw in the top (smallest gear cog). Now I think my gears are really well synchronized all the way up. Here is a picture, with the bike upside down, showing the washer (made of brass?).
Derailleur Hanger Washer to Cure Crash Damage by
Timothy Takemoto, on Flickr
There is a danger of course that I may strip the derailleur hanger thread using the above technique but it feels strong, for the time being, till the next crash.
The washer worked. It is shifting to all gears. A derailleur washer (or spacer in effect) could be used when the cables are getting old, or the spring in the derailleur is getting weak too. If the hanger is replaceable it would not matter if the hole were stripped.