Bent Rear Mech - or Frame?

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Mick F
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Bent Rear Mech - or Frame?

Post by Mick F »

After my accident on Tuesday, and checking out the Moulton, I see (and hear) the rear mech is out of alignment. ie no longer parallel to the drop-out. It's a Deore XT 10sp.

Photos by way of illustration.
IMG_0985.jpg
IMG_0987.jpg
No matter how I twist the mech (using a large adjustable spanner) it doesn't seem to want to know. Indexing up the block, the alignment isn't an issue, but on the smallest cog, the chain clicks and almost de-rails off the bottom jockey.

Any ideas what I should be doing about this?
Could it be that the drop-out is bent? If so, how do I check?
Mick F. Cornwall
TheBomber
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Re: Bent Rear Mech - or Frame?

Post by TheBomber »

To check the hanger - and assuming you don’t have a hanger alignment gauge - screw an old rear hub axle in and sight it in the various planes to check if all is well. Is the hanger steel? If so the axle will also give you the necessary leverage in case any bending is required. Better than using the derailleur for leverage and stressing its pivots. Obviously your mech may already be damaged.
Jdsk
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Re: Bent Rear Mech - or Frame?

Post by Jdsk »

"Derailleur alignment tool recommendations?":
viewtopic.php?t=146971

includes Brucey's advice, including how to do it if you only need to do it once in a blue moon.

Jonathan
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Mick F
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Re: Bent Rear Mech - or Frame?

Post by Mick F »

What an excellent idea!

I do have another XT but I'm unwilling to remove the chain and the mech, to replace it and the chain, and check again, only to find the hanger is bent.

It's steel frame with steel dropouts, so an axle screwed into the hanger threads could show up any errors.
All I would need to do is undo the rear mech off the hanger and let it dangle on its cable.

Back later! :D
Mick F. Cornwall
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NATURAL ANKLING
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Re: Bent Rear Mech - or Frame?

Post by NATURAL ANKLING »

Hi,
What I normally do is put a straight edge like a 12 inch rule on the hanger surface on the outside of the frame, after removing the derailleur from the bolt. The edge not the face of the rule!
Ideally if you have a say 24 inch aluminium rule, remember to use the edge not the flat face.
And aluminium one is rather thicker and it is possible to actually clamp the edge of the rule To the mounting face with suitable soft faced woodworkers type clamps, metal clamps won't work as you won't be able to keep the rule on the face without it slipping.

Bike needs to be upside down ideally sitting on the handlebars and perpendicular to the ground.
you need to be able to stand at the back of the bike and look at the alignment.
Check vertical and horizontal alignment.

I commonly use a 24 inch aluminium rule, clamped to my pedal face, to check that the pedal face is true to the bike, In both vertical and horizontal positions.
It's tricky but it can be done and if you use a 24 inch rule say, then your eyes can see any obvious misalignment.
I wouldn't attempt it with a steel rule of any description, and don't be tempted to clamp on the face of the rule as it will distort!

Then as it's obviously bent?
Use a large adjustable spanner clamp on the hanger and tweak it.
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Mick F
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Re: Bent Rear Mech - or Frame?

Post by Mick F »

The frame seems fine to me.
I have an old cheapo pair of hubs, so it was only a mater of stripping down the rear one for its axle.

The axle went in rather too easily, so I used a hub locknut to seat it properly.
IMG_0988.jpg
Comparing the "bent" XT rear mech with a brand new one ............... probably the last brand new one in the whole world ............ it's possible (but only just) to see the hanger bolt at a different angle in the same direction of the alignment problem. It's not something I could show clearly enough with a camera, but with the naked eye, you can just make it out.

What to do with it, I'm not sure, but at least I can replace the whole thing if I can't twist the thing with brute force.
The good thing about these mechs, it that they don't use a top pivot. This is the main disadvantage of "normal" rear mechs in that when the top pivot is worn, the mech is scrap. I've lost an otherwise excellent Chorus rear mech like that. Only "cure" was to use friction levers and not indexed ones.

Also lost an otherwise good Tiagra in the same way. Hence why I bought the Deore XT .............. and a spare one too as it's as rare as hens' teeth.

The XT has a hinged part above the parallelogram, and that is removable so I could take it out and give it a whack or three in the workshop vice. :wink:
Mick F. Cornwall
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Mick F
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Re: Bent Rear Mech - or Frame?

Post by Mick F »

Mick F wrote: 27 Jan 2022, 10:36am The XT has a hinged part above the parallelogram, and that is removable so I could take it out and give it a whack or three in the workshop vice. :wink:
Got it out, but it's a bit small for receiving a whack.

IMG_0989.jpg
Mick F. Cornwall
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Re: Bent Rear Mech - or Frame?

Post by rjb »

Mick F wrote: 27 Jan 2022, 11:11am
Mick F wrote: 27 Jan 2022, 10:36am The XT has a hinged part above the parallelogram, and that is removable so I could take it out and give it a whack or three in the workshop vice. :wink:
Got it out, but it's a bit small for receiving a whack.


IMG_0989.jpg
"Whack". Have you been playing wordle. viewtopic.php?t=149467 :lol:
Can you get a longbolt in both holes then you could straighten it out until the bolts are parallel. Possibly pack with a few washers as well but a craftsman like you doesn't need me to tell you the finer details as each task is unique. :D
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 :D
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Re: Bent Rear Mech - or Frame?

Post by rjb »

I had to straighten out the bent hanger on my recent purchase. Unscrewed the mech and left it hanging with the rear wheel in situ, then screwed a spare rear wheel in and used this as a lever to get it aligned. viewtopic.php?t=149442#p1668552
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 :D
TheBomber
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Re: Bent Rear Mech - or Frame?

Post by TheBomber »

If that part is the culprit, and it resists attempts to straighten it, you could check with a Madison dealer to see if it is available as a spare part.
Mick F wrote: 27 Jan 2022, 9:53am I do have another XT but I'm unwilling to remove the chain and the mech, to replace it and the chain, and check again, only to find the hanger is bent.
If you remove the jockey wheels from both mechs you can get the existing chain through the spare one and try it out.
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Mick F
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Re: Bent Rear Mech - or Frame?

Post by Mick F »

Hi guys, been thinking and looking.
I took the top piece off the old bent mech, and did the same to the brand new one.
The differences are quite marked.

I spent ages trying the illustrate it and perhaps quantifying it. Took a photo of each, laid the shots side-by-side to show.
Old and Bent on the left, and Brand New and Straight on the right. I supported the mechs with a screwdriver and laid an Allen key in the empty slots to show the parallel-ness or not.

What to do with Old and Bent, I don't know. Thick solid alu.
Bent Left New Right.jpg
The mechs are adjusted on the outer stops the same. It's the top part of the mech parallelogram that seems bent as you can tell how the Allen key lays with the outer XT piece.
Mick F. Cornwall
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Re: Bent Rear Mech - or Frame?

Post by slowster »

Mick F wrote: 27 Jan 2022, 2:10pm Took a photo of each, laid the shots side-by-side to show.
Old and Bent on the left, and Brand New and Straight on the right. I supported the mechs with a screwdriver and laid an Allen key in the empty slots to show the parallel-ness or not.
Do the Low limit screws make any difference to the apparent difference in alignment between old and new derailleurs? The screw is presumably completely backed out in the new mech, and presumably screwed in partly in the old mech.
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Re: Bent Rear Mech - or Frame?

Post by Mick F »

I screwed it in and out on both. The cage is on a parallelogram and follows a straight a parallel path. It's just that on Bent, the cage isn't aligned correctly. Going in and out doesn't alter that ............. or that's the way it seems on New and on Bent.
No wobbly play on Bent, as it's not floppy or rubbish at all. It's just not straight.

Having a beer and siting in the livingroom, and looking and thinking.
Part of me thinks that I should just chuck it into the parts bin, and then fit New and forget about Bent.
Another part of me, likes to fix things.

It's not the top unit or the bottom unit, or the parallelogram specifically, or play in the the whole thing.
It's just "bent" and "stretched" perhaps, making the cage not be inline with the chain. No damage to the cage at all.

The whole thing took a whack when I slammed into the tarmac and it became "twisted" perhaps.
I think I have nothing to lose by attempting to "un-twist" it.
Sort of diagonally mounting it into the vice and giving it a squeeze.

Tomorrow job but only after I've been to the nurse to have my dressings changed. I wonder if she knows anything about bike mechanicals? :wink:
Mick F. Cornwall
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Re: Bent Rear Mech - or Frame?

Post by fausto99 »

Sorry to hear about your spill. Hope you're better soon.

If it was me I'd buy a new one and then spend way too much time trying to repair the old one anyway. :lol: I find it very difficult to throw anything away! !
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Re: Bent Rear Mech - or Frame?

Post by fausto99 »

Mick F wrote: 27 Jan 2022, 11:11am
Mick F wrote: 27 Jan 2022, 10:36am The XT has a hinged part above the parallelogram, and that is removable so I could take it out and give it a whack or three in the workshop vice. :wink:
Got it out, but it's a bit small for receiving a whack.


IMG_0989.jpg
Maybe a trick of the light but do I spot 2 cracks in this item? In which case, into the recycling bin with it (no pun intended) :lol:
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