How to fix? - Derailleur got chewed!

[redmat]

Hi CTC, newbie here... stumped.

Almost before I'd accelerated away from some traffic lights I had a nasty graunch from my rear wheel - my rear derailleur was inside my back wheel spokes!

I couldn't understand it at all until I noticed it wasn't the derailleur springs... the hanger bracket the derailleur screws into was bent!

Does anyone know how I can fix this in a meaningful way? Or if it can be fixed at all? or if it's even 'worth' fixing?!

Any and all input appreciated!

Thanks in advance from redmat

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[andrewjoseph]

is that a steel frame or aluminium? I'm guessing steel. might be ok to bend back in with heat. if alu, i don't think bending back will work, most likely snap it off.

[andrew_s]

Bending it back is the only option, as the gear hanger is non-replaceable. What you want is to get it straight with as little back and forth bending as possible (i.e. don't bend too far, then have to bend back again).

Coarse straightening can be done with a big adjustable snugged down over the hanger, or by supporting the hanger on a brick, paving slab or similar, and hitting it with a lump hammer.
There are tools that screw into the derailleur threads for final alignment, with an arm that will swing round so you can check it's the same distance from the wheel rim all round, and which is strong enough to heave on the end of to actually do the alignment.
The Cyclus from Rose is the cheap option (£20 + shipping), the home Park tool is about £55, and Chainreaction have one that looks similar to the Park, but is about £38. Google derailleur hanger alignment tool (or gear hanger).

I'd guess that the bike had fallen over or crashed at some point, bending the hanger in a little, then the next time you got out of the saddle whilst in bottom gear the derailleur cage snagged on the spokes and got dragged into the wheel

[foxyrider]

take it to a shop - they will have the proper alignment tool.

If you try a DIY without having the correct gear/expertise you could easily wreck that which is then a potentially expensive trip to a frame builder! It is fixable but please don't just 'have a go'!

[fastpedaller]

If it's steel then you'll need someone with oxy-acetylene to heat it, bend it straight and recover the thread. If it's aluminium, then it could be repaired at home, depending on your skills - I had the same happen to a cheap aluminium frame, and fixed is and used it for a further 5 years before I gave it away. So, (if it is aluminium) scrape the paint off the dropout and rub a bar of soap on it! Heat with a blowlamp until he soap goes brown, and let it cool naturally (ie don't quench it) This has Anealled the aluminium alloy, making it less likely to break when bending. Gently bend it a little, but I'd suggest only a little (don't ty to bend it all the way in one go) - I think about 4 repeats of the procedure should do it. make sure it's in line using a straigh edge. To recover the thread I used 'Lumiweld' or similar - just clean the thread and follow the instructions, again using your blowlamp to heat. Put plenty on to make the hole smaller than the 9mm needed, and then drill carefully with 9mm drill (or use a round file) You'll then need the correct m10x1 tap to get the thread good. Hope you save it!

[Brucey]

that looks like steel to me (check with a magnet) and it looks quite badly bent.

I'm guessing you don't have oxy-acetylene to hand so applying enough heat to be useful would mean a trip to the local welder. I'd therefore suggest that you could try straightening that cold, on the basis that if you fail, you will need some heating or welding done anyway.

The hanger thread is 10x1mm which is the same as most QR rear axles. To straighten, you should

1) fit a solid axle wheel/ hub (or length of studding etc) into the dropouts ( a QR hub isn't strong enough) and

2) fit a (pref 10x1 threaded) axle to the hanger, with a nut each side.

In place of 2) you can use a wheel with a 10x1 solid axle (there are a few like this, but most are 3/8 x 26); this will allow good leverage and it will allow you to check the alignment.

If you fit the hub/wheel 1) in the frame reversed you will have more clearance for the 2) part on the hanger.

The purpose of 2) using something threaded through the hanger is that this way, the threads won't be any more damaged by the straightening process; if you just stick an adjustable spanner over the dropout, sometimes the hanger will bend in the middle of the hole, as it were. This is much less likely to happen if you use an axle as described.

Once the dropout has taken a new set, it is a good idea to check that all the nuts etc are still tight before doing any more.

I am a little concerned that the hanger has stretched slightly, so that even once straightened, the derailleur may not be secure. If this happens there are various solutions (including a thin bolt through the whole thing) that can work out OK.

Worst case is that you need a new hanger welded on. I have made a new hanger before now by taking a 10x1 threaded locknut, modifying it (grinding down and building up as necessary) and welding it to the frame. This isn't that difficult to do; anyone with a MIG welder ought to be able to manage that, and once the parts are dressed, you would never know it had been done.

good luck!

cheers

[Brucey]

....I couldn't understand it at all until I noticed it wasn't the derailleur springs... the hanger bracket the derailleur screws into was bent!....

BTW just in case there is any confusion over cause and effect here; the bend in the dropout is certainly because the mech went into the rear wheel. However whilst the initial cause of that cannot be certain, there are two common causes;

1) that the mech wore or went out of adjustment, and thus was able to get into the back wheel, or

2) that your bike fell over or took a knock and the hanger was already slightly bent; this similarly can allow the mech to enter the back wheel.

BTW shimano have (perversely IMHO) engineered many of their mechs in recent years so that they will instantly snag on the spokes if there is the slightest problem. In the 1980s they built them differently (near the top pulley) so that they would clatter the spokes and make a noise well before they would actually get snarled up badly. All this means that you need to be much more careful with mech adjustment these days.... so much for progress!

cheers

[BigG]

In my experience, another possible cause of the mech going into the spokes is back pedalling when the mech is not aligned with the sprocket on which the chain sits. The chain will then sometimes derail from the top guide pulley. This can happen if you change gear (or move the lever) after you have stopped or stopped pedalling and then do no more than reposition your cranks for the restart. The old Huret Duopar was particularly prone to this problem, but modern mechs can suffer this way too.

[redmat]

@everyone - Great help thanks! To sum up answers: steel frame, bend has damage thread beyond repair, it's a 10 year old inherited bike so maybe things are a bit worn... think I either get brazing/welding or go single gear... thoughts?

@BigG - you could be right with the slight back pedal theory

@Brucey - thanks for clearing up cause & effect and the good instructable... the annoying thing is I'd just spent ages (+£) 're-rigging' the damn thing!

@fastpedaller - good to know that a bend fix can last a good few years

@foxyrider - probably a good shout

@andrew_s - right, won't over bend - I forgot about metal fatigue!

@andrewjoseph - steel frame so could bend as you suggest but the thread will be a nightmare

[fastpedaller]

If it's steel, a framebuilder could braze a new dropout in, or heat and bend to get enough of it straight, chop off the gear hanger and braze or weld on a new one, or if they can get it all straight, then drop some weld in the thread and redrill & tap. May not cost much at all, so worth considering.

[Brucey]

BTW a temporary (or perhaps permanent) easy fix is to use a bolt-on gear hanger, of the kind that was commonplace some years ago. I think you can still buy them.

cheers

[Vorpal]

I think if it was me, I would just have a go at bending it back. Park tool make an alignment tool, though I think it wouldn't be too hard to rig something. If the threads are damaged, you can try to clean up the threads with a tap of the correct size. If it doesn't work, you aren't out much at this point.

Then, you can try cutting it off & putting in a replaceable hanger. It will probably require milling the rear drop out, and drilling & tapping a mounting hole for the new hanger. There are many replaceable hangers available, the hardest part will be figuring out which one to get.

There are a also a couple of companies that make custom replacements just for this purpose.

[breakwellmz]

http://www.ebay.co.uk/itm/Shimano-rear- ... h2w1hsoBGQ

[redmat]

@everyone - think I know what to do now so thank you all

@fastpedaller - great idea

@Brucey - didn't know that; definitely a possibility!

@vorpal - good shout

@breakwellmz - helps to see!

Thanks again!

[breakwellmz]

Sorry, i thought you would be able to see it by clicking on the link(in blue)