Repairing cracked drop-out on aluminum frame?

[cadence90]

Originally Posted by Kontact

I get what you're saying, but the wheel isn't hanging from the dropout when you're sitting on it; the top of the dropout is sitting on the wheel. That is, until you close the QR properly, then they are clamped laterally and the dropout is effectively part of the axle.

Unless you know your dropout was already cracked, I'd be more inclined to believe something happened with the QR that caused it to become loose where the axle could move enough to crack the dropout, not the other way around.

That's fine, believe what you want.

In KJMUNC's case I have no idea what caused that crack, and so don't care to assume.

In my own case, I know 100% that there was no issue with the qr causing it to fail. The dropout/rd hanger (all one piece, obviously) split in half, and everything else followed.

No matter what the speculations are as to the cause, if I were KJMUNC I would not ride that bicycle in that condition. He has written that the frame is of high sentimental value to him, so I certainly hope he can have it properly repaired and then enjoy many more safe miles on it.

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[Mikej]

Originally Posted by Kontact

I get what you're saying, but the wheel isn't hanging from the dropout when you're sitting on it; the top of the dropout is sitting on the wheel. That is, until you close the QR properly, then they are clamped laterally and the dropout is effectively part of the axle.

Unless you know your dropout was already cracked, I'd be more inclined to believe something happened with the QR that caused it to become loose where the axle could move enough to crack the dropout, not the other way around.

There is a transfer of force from the wheel to the axle to the drop out. If you lowered the wheel in the drop out and clamped it with a gap, after a short ride or big bump it will transfer into the drop out and seat properly, transferring that same force directly to the drop out. The force is not magically disappearing because its clamped. A QR is about 700-1000 in/lbs. of force when properly closed. Its not going to prevent it from cracking, because it ALREADY cracked twice.

[shovelhd]

Originally Posted by Kontact

That's only if the drop out is one of the heat treatable alloys. It is just as likely that it re-broke because it wasn't flat when welded, warped when welded or the weld was only on the surface.

Or they had insufficient or improper cover gas. Lots of things can go wrong when welding.

[staggerwing]

Would think the logical repair would be to completely slice the old dropout off right at the point were the gusset between the chainstay and seatstay step up to the top clamping surface of the dropout. More area to get a full penetration bead.

Better yet would be to cut at the stays, and weld in a new dropout.

Both scenarios would require a new dropout, and most that I've seen don't quite look like what you have.

I have a junk, due to a cracked headtube, aluminum Raleigh frame in my basement. You are welcome to the dropout off of that, if it helps. Need to confirm it is similar.

[93KgBike]

I won't say I'm proud of it, but I repaired the swingarm of my Superlight with an alumiweld type product, XA-6000 or something?, because we were in the middle of nowhere. I never did warranty replace it. I rode it for another 10 years, and now it's waiting for its retirement rebuild. Point is, the weld material is stronger than the dropout. I would fab a little jig and have a go at it.

[KJMUNC]

Well that escalated quickly......

Thanks for all the input.....frankly it's probably more trouble than it's worth. The seller is offering it for a cheap price, reflecting the complexity of the problem, but I don't need any wall-hangers at this point so I'll prob just let it go.

In the spirit of full transparency (and because I used his pics above), the frame in question is for sale here: https://sfbay.craigslist.org/sfc/bik...415092904.html

Actually a pretty decent deal considering you get two frames. I just don't have the time or inclination to deal with both right now.

[dddd]

The stresses imposed by the chain's tension should not be underestimated, since the "hoop" of metal behind the axle acts flexibly in response to fluctuating chain tension.
This flex is what fatigues the dropout to the point of failure, more so than any stresses caused by the rear derailer imo.

Because this is a very highly-stressed area, as testified by the crack forming there, any repair will unfortunately also be subjected to what seems like this frame's most highly-stressed location.
Chain tension causes the axle to twist along it's length as the lower half of the dropout gets pulled forward elastically with every pedal stroke. The peak stress results from some of the chain's tension force acting on the curved part of the dropout, with peak tensile force apparently occurring in the middle of the curve.
Unfortunately, the QR's and locknut's grip on the upper run of the dropout cannot fully resist this cyclic, twisting-induced force (torque), though it is sufficient to keep the axle from sliding forward.

The metal there is now possibly contaminated by whatever possibly-incorrect welding material was used in the previous repair.
The choice of welding rod and any heat-treatment regimen will ideally be dictated by knowledge of what particular alloy this frame is made from.
I know of successful repairs to welded aluminum frames in highest-stress locations, but where the repair was carried out by the frame's designer/builder.

[Ralph]

Good luck whatever you do. I'm sure you know this but.....really really good production aluminum frames are so good and cheap these days.....like the CAAD 10's and 12's, no need to take a risk.

I currently ride a CAAD 10 a lot....got it last year for $590 new on E Bay, still impressed with it. Thinking about one of those new CAAD 12's on E Bay as well. They are really good frames. Their main fault is probably they are so inexpensive, folks just think there must be something wrong with a modern aluminum frame.

[Kontact]

Originally Posted by Mikej

There is a transfer of force from the wheel to the axle to the drop out. If you lowered the wheel in the drop out and clamped it with a gap, after a short ride or big bump it will transfer into the drop out and seat properly, transferring that same force directly to the drop out. The force is not magically disappearing because its clamped. A QR is about 700-1000 in/lbs. of force when properly closed. Its not going to prevent it from cracking, because it ALREADY cracked twice.

This simply isn't true. Once the QR is closed, the axles absolutely don't move in the dropouts. I mounted a wheel in a fork with tiny chunks of wood to move the axle down an extra 3mm to get tire clearance, and the wood didn't even have marks on it from riding.

We don't know why the Pinerello cracked the first time, and given the messy looking repair it probably cracked the second time because it wasn't square.



I don't see why anyone would think that derailleur forces are very high - most of the replaceable hangers used today are as soft as cheese.

[msl819]

Originally Posted by KJMUNC

Well that escalated quickly......

Thanks for all the input.....frankly it's probably more trouble than it's worth. The seller is offering it for a cheap price, reflecting the complexity of the problem, but I don't need any wall-hangers at this point so I'll prob just let it go.

In the spirit of full transparency (and because I used his pics above), the frame in question is for sale here: https://sfbay.craigslist.org/sfc/bik...415092904.html

Actually a pretty decent deal considering you get two frames. I just don't have the time or inclination to deal with both right now.

That fact that there is this much back and forth on the subject would be enough to give me pause and cause me to steer clear unless you want it to be a wall hanger. Would it fail catastrophically? Who knows. But there are so many great other frames, why take the chance unless there are no other options.

[mattsbeers]

Very cool frame, from my favorite favorite era in cycling. But those frames were meant to be the best tool available for a finite period of time and it seems that time has passed. So cool tho!