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Repair aluminium dropout on carbon fibre frame

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Repair aluminium dropout on carbon fibre frame

Old 10-17-22, 11:41 AM
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Silvek
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Repair aluminium dropout on carbon fibre frame

Hi everyone! I would like your advice on something, well... probably not that common.

A friend of mine had a little accident and managed to bend the aluminium rear dropout (together with the hanger, of course) on his carbon fibre Colnago AC-R 2014 frame.
We re-aligned the dropout (lateral alignment), found the hanger model in stock somewehere but we think that the dropout might be compromised for the following reason: the U shape the dropout had is now 'wider'. The distance between the the bolts is now 29 mm instead of 25.31 mm and a new hanger will not fit (please check the images).
What would be your aproach in re-bending the dropout to his original form? Is this doable and wise in this situation? Should he try finding a NOS aluminium dropout for this particular model and find someone good at welding it to the carbon chainstay? Is this something you would recommend?

Any kind of advice or idea will be very much appreciated! Thanks a lot!

(Unfortunately I can't attach photos)
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Old 10-17-22, 01:48 PM
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Without photos, all I can offer are general guidlines.

1- you cannot WELD a dropout to a CF chainstay, so replacing the dropout has to be done by someone familiar with CF condtruction.
2- Depending on the exact damage, a decent "hammer" mechanic might be able to correct it, but aluminum isn't super malleable, and there's then risk of cracks (or total breakage)
3- It might be possible and more practical to modify the new hanger to fit the frame than the other way around. At the very least, the consequences of failure would be less, since you can try again for small $$$,

I often approach issues like this with a "what's the worst that can happen" so I'd start by getting an estimate for the cost of replacing the dropout (if possible). Then, if it is possible I'd give working on the existing dropout a shot, knowing I'm OK if that fails. OTOH- if the dropout cannot be replaced, I'd stay clear and only consider options relating to modifying or making a hanger to fit what's there now.
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Old 10-17-22, 02:27 PM
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Conventional wisdom suggests you cannot/should not bend the aluminum dropout back into shape, but it's only 3mm... Maybe another way to go is to create a new mounting hole on the dropout for the hanger to bolt into?
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Old 10-17-22, 03:15 PM
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Welding aluminum to carbon? Not sure how you can even dream that would be possible.
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Old 10-18-22, 02:44 AM
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Originally Posted by cxwrench View Post
Welding aluminum to carbon? Not sure how you can even dream that would be possible.
Maybe they meant JB weld?
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Old 10-18-22, 10:56 AM
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Are these your pic's? If you just tell us that you uploaded some to the Gallery here on BF, then we'll know where to look. Sometimes they don't show up in your album on your profile, but looking in "recent" photos we can find them....









I didn't grab all of them. Let us know if there is another you think is needed.
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Old 10-18-22, 11:18 AM
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That might be time to talk with a carbon bike repair company.
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Old 10-18-22, 11:29 AM
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Your aluminum dropout is generally epoxied to the frame. Perhaps bolted to the seat stay, and epoxied to the chainstay.

Unfortunately, it can be difficult to source the dropouts. You can try contacting Colnago, but I wouldn't expect a lot.

Also try contacting some Carbon Fiber repair shops. Calfee and Ruckus Composites. Perhaps there are other companies that would do it.

What I'd seriously consider would be rebuilding the damaged derailleur hanger that you have.

Leave the general shape to the hanger, but straighten it the best you can (and make sure the hanger is still parallel to the axis)

Now weld a shim to the back side of the dropout slot on the hanger so that the shim is 90% of the full dropout thickness. And machine to take the axle in the right spot.

Unfortunately, my TIG welding Aluminum, it just never quite does what I want. But there are some people who are really good at it.
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Old 10-19-22, 11:19 AM
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Originally Posted by tFUnK View Post
Conventional wisdom suggests you cannot/should not bend the aluminum dropout back into shape, but it's only 3mm... Maybe another way to go is to create a new mounting hole on the dropout for the hanger to bolt into?
that would not resolve the issue as it is not the hanger that is the problem it is the aluminum dropout plate bonded to the frame, which got bent out by the accident so the axle doe not fit well

this suggests more that a "little" incident.

I concur the bending the plate could be problematic as aluminum react poorly, to bending. Problem is it could work, it could work and fail in future or it could just fail

I would check with a carbon repair or even talk to colnago

you can't weld to carbon, needs to be bonded

also heat from welding could impact the current bond (yes tig is more localized than say brazing steel, but still a risk factor)
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Old 10-19-22, 09:21 PM
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This falls under the category of, "If you have to ask, you probably shouldn't."
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Old 10-19-22, 11:43 PM
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If the dropout itself is bent, which it looks like it is, the frame is likely toast.

Aluminum doesn't like to be bent. It really, really doesn't like to be bent more than once. If you straighten the dropout, it's almost certainly gonna break.

The adhesives that are used to bond aluminum to carbon fiber are a one-way trip. There is no way to separate the dropout from the stays without destroying all three parts.

Call Craig Calfee. If he says "Yeah, I can fix that. It'll cost you $X," pay him $X and get it fixed. If he says "sorry, but your frame is borked," then your frame is borked, and you're bummed, and that's that.

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Old 10-20-22, 12:24 AM
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I agree that working with the CF joints isn't likely to be successful.

However, the most critical element, namely the top of the dropout slot which locates the axle and bears the load is still OK.

That means that a hanger can be fabricated to work with what's there, including a provision to fill in below the slot and take the compression from the QR.

The needed part can be made by hand, or the free end cleaned up and digitized, so the part can be drawn and made using CAD/CAM.

Last edited by FBinNY; 10-20-22 at 12:46 AM.
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Old 10-20-22, 12:36 AM
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Originally Posted by ShannonM View Post
Call Craig Calfee. If he says "Yeah, I can fix that. It'll cost you $X," pay him $X and get it fixed. If he says "sorry, but your frame is borked," then your frame is borked, and you're bummed, and that's that.

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This is really your answer. Unless someone shows up and had the same/similar issue and was able to do a permanent fix, they are all guesses.

Your only concern is having a catastrophic failure of the dropout, either the aluminum or the bond.

John
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Old 10-20-22, 05:48 AM
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Originally Posted by 70sSanO View Post
This is really your answer. Unless someone shows up and had the same/similar issue and was able to do a permanent fix, they are all guesses.

Your only concern is having a catastrophic failure of the dropout, either the aluminum or the bond.

John
Bingo. Given the deformation of the existing dropout, I'd worry about both the bond and internal damage in the stay(s).

While it wouldn't be as bad as a front dropout failure, I certainly wouldn't want to have the rear wheel come loose during a descent or in traffic (or at any other time, for that matter). Any of those could conceivably lead to a "life-altering event".

As suggested above, if it were mine I'd send the frame to a carbon frame repair specialist for eval and/or repair before I'd ride it (or maybe to Colnago, if they do such). And if they said it was toast, I'd start looking for a new frame.

Last edited by Hondo6; 10-20-22 at 05:53 AM.
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Old 10-23-22, 11:26 PM
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I'd really like to thank you all for your help! Super useful pieces of advice and really really appreciated!!
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Old 10-24-22, 01:32 AM
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That's Cast aluminum... once it bends, it's already cracked..... Bend it the other way and the Cracks open. Cast Aluminum is used because it is much stiffer than billet.. right up to yield point... then the grains separate and that separation is represented by the increase in distance between the dropout affixing holes.(edited)

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Old 10-24-22, 09:05 AM
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The bend doesn't appear to have gone through the chainstay/seatstay. So while they would have been put under stress during the incident, they wouldn't remain under continuous stress. Thus, if the joints are solid, then they're probably OK.
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Old 10-24-22, 09:13 AM
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Ultimately I think you have a few choices.
  1. Put it back together with the bent derailleur hanger and see how it rides. Your chain will naturally pull the wheel forward, and if put in the right spot, it may well stay put.
    .
  2. Modify the bent derailleur hanger to narrow the slot.
    .
  3. Modify the new derailleur hanger to move the hole. This may also mean some adjustment of the shelf on the back side that helps orient it.
    .
  4. Get a new "custom" hanger machined.
    .
  5. Contact Ruckus Composites or Calfee about getting it professionally repaired.
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