So this is an adaptation of a derailleur hanger tool that allows it to roughly measure if a rear derailleur is out of whack.
Really just two pieces needed, an M10x1.0 coupler nut approx 2" long and the Park Tool DAG-1 or 2. The nut allows you to attach the derailleur to the tool, spaced sufficiently to use the feeler gauge to test alignment.
Once attached, the feeler will tell you if the pulleys are parallel & in the same plane.
There are a few ways to skin this cat, I'll try to list the options. Also a couple different ways to test derailleur function, that's all on the last pages.
Step 1: The Pieces
First off you'll need the Park DAG-2. The DAG-1 works also, just a little shorter. I'm using a DAG-1.
This tool is built to re-align rear derailleur hangers, we're going to adapt it to work on the actual derailleur. I've found this is an indispensible tool in the shop, most hangers are usually bent to some degree even if they haven't been traumatized. It's incredibly common. Point being, it's worth the money.
Step 2: The Pieces 2
Second piece, we need a coupler nut between the DAG and the derailleur. The thread is M10x1.0 - finding a coupler nut off the rack is next to impossible with this thread but you can make one pretty easily. This is the same thread for most rear hub axle cones & nuts so those pieces work nicely.
I'm using a handful of old axle parts. Some rear axles have a different thread, it's worth checking.
The DAG-1 has a shorter stand-off & threaded stud than the DAG-2, so I wanted a coupler nut that was approx 2" - it needs some length so the derailleur can swing freely once attached. The DAG-2 won't have that same problem, you can get away with a shorter coupler nut.
Step 3: Making the Coupler Nut - the Easy Way
Here's the easy way, and it's ideal for the DAG-2 (the longer stand-off version) -
- Large-ish washer, approx 1" OD - this gives a larger flat surface for the cone nuts to press against.
- small locknut - this takes up some space on the threaded stud
- longest cone nut you can find - it needs ample room to take the derailleur bolt threads while still holding the DAG threads.
There are good reasons why all those ingredients are needed, it'll make more sense when you see it. I learned that by trial & error.
Put them together in that order & make sure the washer isn't hanging on the small shoulder on the threaded DAG stud. Then attach the derailleur. They should all snug up tight & not rotate easily, also with no lateral play.
Step 4: Making the Coupler Nut - the Hard Way
This requires welding. Basically I'm making a taller stack of nuts by welding some together. This is ideal for the DAG-1 since it has a shorter stand-off.
Please pay no mind to the crappy looking welds, I was just monkeying around with dirty metal. Sticking to that excuse. I've made a few of these, some prettier than others.
If you go this route make sure the heat deformation doesn't get out of hand, the final coupler needs to be straight. I packed it all on an axle first for tacks, then finished in a clamp. DON'T do it all on the axle, the deformation will make it impossible to un-thread. Guess how I know that ;)
The washer got added last, used the DAG as a tack jig. There's a small shoulder on the DAG's stud bolt that the washer needs to be around, not on.
Once done welding you'll probably need to chase the threads with a 10x1 tap, the heat doesn't do them any favors.
Step 5: Test One
Once you've got the derailleur attached there are several planes to test.
All testing is basically lining the feeler rod up to the pulley teeth & then comparing to the other pulley. Usually the lower pulley is bent towards the DAG tool. Sometimes the pulley cage assembly will be twisted forward or back.
So test one is with the derailleur in a relaxed state. This will tell you if the pulley cage is mis-aligned. Check both the front & back of each pulley wheel for rotation in the cage.
Take it with a grain of salt - a few degrees out won't be noticeable on the bike & there's probably some inaccuracy in the tool.
Step 6: Test Two
The second test is with the derailleur tensioned some fixed amount. Do this AFTER it's passed test one.
Generally most derailleurs take abuse in the pulley cage, it'll get bent inwards typically and it's sometimes fixable. Less common is for the parallelogram body of the derailleur to be bent. That's probably never gonna get straight again, I'd trash that one.
If it's passed test one, putting it into a tensioned state will reveal body damage. So if it passes test one but is out of whack in test two, that means the body's twisted.
A couple ways to do this - either use a piece of cable to simulate pull from the shifter or use the limit screws. Either will move the body a fixed distance, the cable goes further & makes mis-alignment more obvious. I used a scrap of road brake cable.
The key is it has to be a fixed distance or you'll never get a good measurement.
Step 7: Alignment - Proceed at Your Own Risk!
So this last bit on alignment is really up to you. You may fix it, you may kill it. If the tool shows your derailleur is out of alignment it's done it's job. If you want to try repairing, here are some tips I've found but really there's no guarantee it's gonna work. Proceed at your own risk.
Then again, if it's really bent it's trash, so what do you have to lose?
So for bending the pulley cage, I like clamping onto the lower pulley bolt with vise-grips since I can compress bolt that w/o hurting anything. You can also use a lg crescent wrench but it'll likely bend the outer cage in the process.
It works really well grabbing both pulley bolts with two pairs of vise grips & working them against each other in various planes, that way you can kinda control where the bending occurs. Clamping the DAG into a bench vise makes the work much easier.
Probably some other good tricks for this, let me know what you like.