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enter image description hereI recently bought this old road bike and just noticed this little crack. Any advice on how to handle this? Can I reinforce it somehow? It's aluminum and carbon fiber frame.

Mahalo, Casey

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  • FYI, the seat stays are the tubes that are attached to the part of the bike you photographed. I think most people would call this the seat cluster or the seat clamp area.
    – Weiwen Ng
    Oct 13 at 16:48
  • In some cases you'd be able to use a spiral hose clamp to close the crack, but there doesn't appear to be enough room in the above picture. Oct 13 at 17:07
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    Call me crazy but as long as the seat post is fully inserted I'd ride that and just keep an eye on it.
    – David D
    Oct 13 at 17:09
  • Yeah, if the post is always inserted at least 6 inches into the fitting (more is better) and if the bike is not subjected to excessive strain, and if the crack is regularly inspected (mark the end of the crack), then the bike should be fine for casual riding. Oct 13 at 20:23
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    @DavidD the problem is that cracks like this tend to grow even from small flexing, which will happen because the area is also under load from the seatstays, not just from the seat tube. The further it grows, the less chance of fixing it. Oct 14 at 9:45
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The problem you're up against here is the cracked area of the seat lug is probably taking significant load in the original configuration of the seat cluster design. That implies you have to make it strong again and/or change how the part is being loaded. Just drilling the end of it to stop the propagation probably isn't enough to avoid problems with it continuing to open or other areas getting overloaded. It may even not be enough to still allow you to properly secure the seatpost, since the seatpost binder will be trying to pull the crack open.

Likely candidates for how this happened would be simple fatigue, over-torquing the binder bolt, installing an undersize seatpost, and/or installing a seatpost above the minimum insertion. Looking at it, it seems likely to be some combination of the first three. Riding for a long time with a nominally correct but physically undersize post could easily do this, as could trying to tighten down on an incorrect size post. Chunky and strong aluminum parts have very low tolerance for that kind of thing, and will tend to crack rather than yield.

If one had to get more life out of this frame using relatively simple means, one approach would be get a thick-walled, strong, not too expensive correctly sized seatpost (a Kalloy Uno would be a good choice), roughen and clean the seatpost bore, remove the binder bolt, drill the end of the crack, and bond the seatpost in at a fixed height with epoxy. The idea here is that you're adding frame structure to take stress off the cracked area and also eliminating the need for the seat lug to be flexed closed. It's a little like simply adding a bunch of material to the lug. All the bonded contact area you're creating may be able to reinforce the area sufficiently that the stress riser of the crack and all the previous fatigue is less of a concern. Drilling the end of the crack to prevent propagation is still critical.

A much more involved take on the same concept would be turn it into a seatmast frame to still have height adjustment. There are aftermarket seatmast toppers for round tubes that could be adapted.

I'm not saying any of the above is a good idea per se, but it's something that could be done if trashing it isn't an option. The major concern would be that rider weight on the saddle is going to want to open up the crack further no matter what you do, and also that frames like this are simply all very old now and trying to get more out of it may be unrealistic, especially if the crack occurred as a result of fatigue and not abuse.

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  • Is there any need to consider the dissimilar materials if you bond a seatpost in? There's the aluminum lug and a carbon tube involved. I'm not sure if the same epoxy bonds to both aluminum and carbon. Of course, just bonding to the aluminum section could be sufficient. That was a good point which I hadn't thought of.
    – Weiwen Ng
    Oct 13 at 20:51
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    The carbon is likely a bigger ID such that there's no contact. Frames like this were made with epoxy to join the aluminum lugs and tubes. There may be an epoxy choice that's best for this because some ability to be flexible may be good, but most epoxy can join aluminum and cfrp. Oct 13 at 21:36
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    I'd also fill the crack and hole with epoxy, paste some more around the lug and wrap the entire joint with some fibreglass. It's going to be a mess but, hey, what do you have to lose. Oct 14 at 9:51
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That lug has already failed, and is in danger of cracking completely through.

I would not ride that bike as-is.

Things you could do with the bike:

  • Part the bike out for the good components, and replace the frame. Safe, practical, and probably the best answer.
  • Clean the bike, polish it, and hang it on a wall as "art"

Your possible repair methods might be

  • Get a steel sleeve made for the outside of the lug, such that the tolerance needs heat in the ring to expand it and go on the outside, and the ring will clamp down on the lug once it cools. This will likely cause corrosion over time between dissimilar metals. An aluminium ring is probably unsuitable here. Downside is the existing fillets will get in the way of a plain ring, so machining would be needed which could weaken the ring or the lug.
  • I doubt the lug would survive any TIG welding, but you should get the opinion of a skilled aluminium welder on that.
  • Epoxy is probably not a good solution either. The forces on the crack will change constantly, flexing the epoxy and the metal/glue bond will likely fail quickly.
  • Get a replacement lug cast at a foundry. Depends what kind of facilities are available in your area, but if you can send the old lug to use as a pattern then that would help.
    The foundry would mold a clay layer all around the old lug to make it thicker, then pack it in sand, remove the template, add molten aluminium, and let it cool.
    Then you need the lug machined in a workshop to make the inside diameters the same as original, and then the threads retapped and the outside cleaned up.
    Ideally the new lug would have more meat/meat in the areas that had cracked.
    Finally, a frame builder would use the correct epoxies to secure the carbon fibre tubes into the lug.
    Expect to pay a lot for this work, potentially more than the cost of a new frame.

Upshot: I think your bike is dead, and you've done well to notice before you suffered a catastrophic failure.

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    That frame would make a nice light fixture with the rear drop-outs and the seat-stays against the wall and the lampshade under the headtube.
    – Carel
    Oct 14 at 15:18
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Unfortunately, the frame is dead. Carbon tubes can be repaired - a repairer would (from what I know of the process through podcasts) sand off the damaged area, then apply and bond carbon fiber sheets to the area. Aluminum can't be cost-effectively repaired; in theory, you could weld the crack closed, but you'd need to heat treat the frame to restore its strength, and that bit is the part that would make it not cost effective, as you'd need to strip the parts and paint first.

Here, you have the additional complication that the carbon tubes are bonded into the aluminum lug. This style of construction was, as I recall, popular on earlier carbon frames. If you could weld that lug or even source a replacement, you would need to disbond the carbon tubes first, then re-bond them.

I have heard that in the aerospace and defense industries, it is possible to bond carbon sheets over aluminum. I don't think that type of repair is a common thing in the cycling world, however. If you have a local carbon fiber repairer who you know has some aerospace or defense experience, you could ask.

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  • This is a frame built with carbon tubes that are epoxy bonded in metal lugs. LOOK and some Italian brands built some of these. Unfortunately the photos don't show the make, but a dealer might be able to have it sent to the builder and have the lug de-bonded and replaced. I'd dread the costs, though. Might still be worth asking.
    – Carel
    Oct 14 at 9:49
  • Maybe wrapping the lug in carbon fiber would work to reinforce it? Or putting a clamp on top?
    – Michael
    Oct 14 at 11:32

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