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Tig welding an aluminum seat tube back together?

Old 02-13-20, 07:20 PM
  #51  
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erileykc,
I am not sure about the diameter of the original seat post, but given that the bike is such a good unit I would suggest that if you can find a steel seat post that fits into the frame to use it. If you need to have the adjustable function then you will need to create another clamping system. If the seat does not need to be adjusted, I would epoxy a steel/aluminium seat post in place and ride on. If you choose to epoxy a post in place and need some small adjust-ability use an old school seat post clamp that allows you to make adjustments on the tube by small increments. HTH, MH
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Old 02-13-20, 08:13 PM
  #52  
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It seems that we have two sets of respondents: those who see a mechanical problem, and those who see a metallurgical problem. The first group doesn’t seem to be addressing the second group...But if the second group is correct, they pretty much trump the first group.
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Old 02-13-20, 08:37 PM
  #53  
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Originally Posted by WizardOfBoz

If your bike can take joints that are only 30% of the strength of base metal, you don't need to heat treat. But because modern bike frames must already be beefy enough to keep stresses low to avoid fatigue failure, no modern frame will have the added mass to allow it to survive without proper heat treatment. As the OP found. I agree with the aerospace welder: the original bike welder may have done a great job (its certainly aesthetically adequate) and was let down by his heat-treating colleagues.
...with respect, take a look at the wall thickness of the aluminum tubing in the original pictures. Rad Bikes doesn't care about frame weight savings, because their products are all electric bikes.

The portion of the seat tube above the top tube is not an especially high stress area if properly designed and the seat post fit is a proper slip fit the length of the seat tube. Ideally, all it does is provide someplace to hang the clamp that keeps the seat post from slipping down or up in the tube once you have proper positioning. The stresses ought to be, properly, taken mainly by the seat post, and distributed along the length of the seat tube that are in contact with it.

Which is why you have so many of us questioning what is going on mechanically. It doesn't much matter how well you heat treat the frame if there is a constant rocking back and forth of the post and saddle, and the bulk of those forces are absorbed by that poor little aluminum alloy collar...it's gonna break off in the manner seen in the pictures.
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Old 02-13-20, 09:27 PM
  #54  
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Originally Posted by 3alarmer
...with respect, take a look at the wall thickness of the aluminum tubing in the original pictures. Rad Bikes doesn't care about frame weight savings, because their products are all electric bikes.

The portion of the seat tube above the top tube is not an especially high stress area if properly designed and the seat post fit is a proper slip fit the length of the seat tube. Ideally, all it does is provide someplace to hang the clamp that keeps the seat post from slipping down or up in the tube once you have proper positioning. The stresses ought to be, properly, taken mainly by the seat post, and distributed along the length of the seat tube that are in contact with it.

Which is why you have so many of us questioning what is going on mechanically. It doesn't much matter how well you heat treat the frame if there is a constant rocking back and forth of the post and saddle, and the bulk of those forces are absorbed by that poor little aluminum alloy collar...it's gonna break off in the manner seen in the pictures.
3alarmer, I think I understand your point. We are both wondering what happened, with one fact that is clear being that the frame broke. And it was in an area where a thick tube was used. And it was right at the weld. And the member above who does aerospace welding thought that the bead looks good. That leaves me to believe that it was a heat treating issue, or a poor quality material issue, or (possibly but unlikely in a factory environment) the wrong choice of welding rod.

Your point about distributing the force along the seatpost has some merit, however, wouldn't this make such a break that was observed less likely?

Your point about rocking back and forth is what engineers refer to as fatigue stress. That's why the collar should be so thick: the same forces spread over a thicker tube mean lower local stress and less fatigue effect. Speaking as an engineer (BS/MS/PhD, Registered Professional Mechanical Engineer), I suppose that the design engineer could have underestimated the forces so that even that thick collar wasn't enough. So the failure mode could have been design-related. But that tube looks honkin' thick, man! One other data point has been others saying that their bike from that mfr had no problems. Which leads me to suspect that the true issue was likely the post weld heat treat. What is certain is that if it was a weld that hadn't been heat treated, welding it again with no heat-treat is almost certain to lead to disaster.

My main point (in case I rambled, which I do) has been that a weld fix that is not too costly is likely to be ineffective and will break again or (put another way) that a weld fix that will work will involve proper heat treating and refinishing and will be less cost effective than purchasing a new frame.
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Old 02-14-20, 07:35 AM
  #55  
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My current plan is to epoxy one of these shims in the tube and probably epoxy the seat post into that. One thing I looked at after the suggestions in this thread was the difference between how tight the post is in my Jamis Coda and in the Rad. The Jamis post is so tight that it's difficult to even turn in the tube while the Rad is so loose that the saddle can rock half an inch when the tube is inserted at the limit line into remaining seat tube. It seems clear that the only 'grab' of the post was at the collar alone and that the post was able to rock ever so slightly and perhaps that eventually broke the tube just above the welds.

https://www.jensonusa.com/Cane-Creek-Seatpost-Shim
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Old 02-14-20, 07:57 AM
  #56  
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Originally Posted by erileykc
My current plan is to epoxy one of these shims in the tube and probably epoxy the seat post into that. One thing I looked at after the suggestions in this thread was the difference between how tight the post is in my Jamis Coda and in the Rad. The Jamis post is so tight that it's difficult to even turn in the tube while the Rad is so loose that the saddle can rock half an inch when the tube is inserted at the limit line into remaining seat tube. It seems clear that the only 'grab' of the post was at the collar alone and that the post was able to rock ever so slightly and perhaps that eventually broke the tube just above the welds.

https://www.jensonusa.com/Cane-Creek-Seatpost-Shim
I'm not quite sure what the shim will do for you. It does require a smaller post. If you go with the epoxy, it should fill the space around the post for the length.

Some people have used soda can shims which are thinner.

I had wondered if the post was loose. Although, i think in general the posts are now reamed that they are easily removed rather than having to twist them out like in the past. Nonetheless, a bit late now, but 27.4 or 27.6 seatposts should be available.

Since both the post and the frame were supplied by the same company, this might come back as a manufacturing defect.
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Old 02-14-20, 08:01 AM
  #57  
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Originally Posted by Koyote
It seems that we have two sets of respondents: those who see a mechanical problem, and those who see a metallurgical problem. The first group doesn’t seem to be addressing the second group...But if the second group is correct, they pretty much trump the first group.
If we concentrate on the one joint that failed, one can look at possible causes. A couple of posts down, the idea of a loose fitting post comes up (although I've wondered about it, and did notice the chunk of clamp on the post shows some squishing of the metal.

We don't know about the rest of the frame, and that isn't the issue at this point.
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Old 02-14-20, 08:17 AM
  #58  
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To all those suggesting epoxying the seatpost with or without a shim, into the seat tube. What happens when the OP hits a big bump or a pothole? How likely is that epoxy join to fail and have the seat post slide further into the seat tube?

Cheers
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Old 02-14-20, 08:47 AM
  #59  
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Originally Posted by Miele Man
To all those suggesting epoxying the seatpost with or without a shim, into the seat tube. What happens when the OP hits a big bump or a pothole? How likely is that epoxy join to fail and have the seat post slide further into the seat tube?
Agree. So, first, let's think about the "shim". As I understand it, the idea is to 1) put a shim "tube" into the broken post with epoxy. And then 2) put the original collar on to that with epoxy(?) And then to 3) insert a smaller seat post to fit in the shimmed seat tube. First, putting the collar back on adds no bending strength - epoxy has a tensile strength of about 3500psi and 6061-T6 is supposed to be around 42,000 yield strength. So all the new strength must be in the shim and the seatpost. The collar may add some strength to the top of the shim to keep the seatpost from ripping through the edge of the collar. But it also would effectively take some of the bending stress and concentrate it in the shim at the point of the broken joint. As far as transmitting force from the seat to the bike, it's all going to be in the seatpost and shim. So why not just use the original seatpost (or one that's longer with the same diameter) if it fits, and lose the collar?

Fit is the key. Epoxy has been successfully used to glue tubes in lugs and its even the matrix in CF. The key in its application lies in ensuring that the parts joined (or the carbon fibers used) match up very tightly. Epoxy's job is not to take a lot of force itself, its to distribute the force to nearby structural (aluminum or a carbon fiber) structural members. This distributes force to the elements that can take it. This only works if you have a very good fit between (in this case) tube and shim and seatpost. Again, since adding the collar doesn't add any bending strength I'd just find a long seat tube that fits the tube well without shim. If you have to use shims to match diameter, fine, but it should be a fairly snug fit, and everything could be epoxied together.

The statement above about the seat tube moving 1/2 inch is not an issue that is gonna be handled by any epoxy! Without a tight fit, I suspect that Miele Man's fear of the post breaking loose is a prophetic one.
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Old 02-14-20, 09:02 AM
  #60  
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I wonder if an external collar on t he seatpost resting against the seat tube after the seatpost itself is epoxied in place (no shim used) would help to keep the seatpost from sliding out should the join with the epoxy fail in the future?

Cheers
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Old 02-14-20, 09:10 AM
  #61  
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Originally Posted by Miele Man
To all those suggesting epoxying the seatpost with or without a shim, into the seat tube. What happens when the OP hits a big bump or a pothole? How likely is that epoxy join to fail and have the seat post slide further into the seat tube?

Cheers
A lot will depend on the materials prep, and overall quality of the joint. If done well, it is unlikely that the post will suddenly drop 1 foot. But, more likely the risk of twisting, or a drop of an inch or so. Annoying yes, but probably little more than an annoyance.

And the repair for a failed joint will be quite simple..

We have a lot of bonded materials in life, and a quality tube-in-tube bond would be one of the strongest possible bonded joints.

If the current seatpost is loose, I'd evaluate whether a larger, and perhaps longer one would be appropriate.

Get it right the first time.

Oh, and the vintage steel posts were mentioned. Not necessarily my first choice, but the twisting post toppers may have an advantage of reducing any twisting forces that get transmitted to the seatpost. Anyway, at least worth considering the implications.
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Old 02-14-20, 10:36 AM
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Originally Posted by Miele Man
I wonder if an external collar on t he seatpost resting against the seat tube after the seatpost itself is epoxied in place (no shim used) would help to keep the seatpost from sliding out should the join with the epoxy fail in the future?

Cheers
...yes, that's what I would do in the spirit of belt and suspenders. I've used those clamps in creative ways over the years.

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Old 02-14-20, 10:41 AM
  #63  
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Originally Posted by erileykc
... the Rad is so loose that the saddle can rock half an inch when the tube is inserted at the limit line into remaining seat tube. It seems clear that the only 'grab' of the post was at the collar alone and that the post was able to rock ever so slightly and perhaps that eventually broke the tube just above the welds.

https://www.jensonusa.com/Cane-Creek-Seatpost-Shim
...you probably ought to address this. I suspect that using the same epoxy as a fill along the length of the replacement seat post will be adequate. All you're really trying to do is to stop that rocking motion. The problem with most seat post shims is that they are not long enough to adequately support the inserted length of the post.
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Old 02-14-20, 11:40 AM
  #64  
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Ummmh...you have an excuse, reason to buy a new bike...
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Old 02-14-20, 01:12 PM
  #65  
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Originally Posted by CliffordK
A lot will depend on the materials prep, and overall quality of the joint. If done well, it is unlikely that the post will suddenly drop 1 foot. But, more likely the risk of twisting, or a drop of an inch or so. Annoying yes, but probably little more than an annoyance.

And the repair for a failed joint will be quite simple..

We have a lot of bonded materials in life, and a quality tube-in-tube bond would be one of the strongest possible bonded joints.

If the current seatpost is loose, I'd evaluate whether a larger, and perhaps longer one would be appropriate.

Get it right the first time.

Oh, and the vintage steel posts were mentioned. Not necessarily my first choice, but the twisting post toppers may have an advantage of reducing any twisting forces that get transmitted to the seatpost. Anyway, at least worth considering the implications.
I remember well the time I adjusted the height of my alloy seat post and thought I'd tightened the binder bolt enough. I was a bout 25 miles from home when I hit a pothole and the seat post slid down about one inch. Riding with the cleats was awkward. I was passing a small hamlet a few miles down the road and saw a fellow working in his garage. I asked if I could borrow a 5mm hex key. He loaned it to me and I was able to get the saddle back to the right height for that bike. I figured it happened because it was one of the very few times when I didn't have my hex keys with me.

Cheers
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Old 02-14-20, 02:28 PM
  #66  
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Originally Posted by erileykc
My current plan is to epoxy one of these shims in the tube and probably epoxy the seat post into that.
DON'T USE A SHIM. You add an additional layer which may increase the likelihood of failure. If the epoxy is thicker, it does not matter. A loose fit actually makes it easer to apply an even coating. You need to prepare surfaces properly.

Use an extra long tube, preferably steel, for your repair. If it is too short, you may have another failure. The longer it is, the stronger it is.

I would put a clamp around this tube, just above the frame. So if the epoxy does fail, it can't slide down.

I would still make the seat adjustable. Use a seat post which fits inside or outside the repair tube. Cut a slit, and use a clamp.

So before beginning, seek out an appropriate tube for the repair, and an appropriate seat post.
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Old 02-14-20, 02:56 PM
  #67  
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Properly prepared and properly applied epoxy is highly unlikely to fail in this case. Still, fitting the largest diameter seat post that fits makes a lot of sense; measure carefully. A narrower "glue line" is better if it doesn't inhibit getting the glue into place. Consider texturing the post and inside seat tube with abrasives.
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Old 02-14-20, 03:23 PM
  #68  
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Originally Posted by Miele Man
To all those suggesting epoxying the seatpost with or without a shim, into the seat tube. What happens when the OP hits a big bump or a pothole? How likely is that epoxy join to fail and have the seat post slide further into the seat tube?

Cheers
Miele Man,
The epoxies I am using have a shear strength of 2800 psi, and need only fit into the joint about an inch down. 2 inches would be overkill, but then extra strong bond. To break the epoxy bond you will need to apply a heat above 350*f and use some twisting pressure, while lifting the post out. Smiles, MH

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Old 02-15-20, 12:28 PM
  #69  
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Originally Posted by Mad Honk
Miele Man,
The epoxies I am using have a shear strength of 2800 psi, and need only fit into the joint about an inch down. 2 inches would be overkill, but then extra strong bond. To break the epoxy bond you will need to apply a heat above $350*f and use some twisting pressure, while lifting the post out. Smiles, MH
Heya, Mad Honk! Just a hint I've found useful. Holding down Alt and typing 0176 on the numeric keypad gets you the magic degree circle: °
And I agree that epoxy should hold a seatpost. IF the glue layer is reasonably thin, which we've discussed before and I think you had some extensive experience with in making golf clubs, right?
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Old 02-15-20, 12:45 PM
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To reiterate a point: If the OP has a half-inch play in the seatpost and uses epoxy to fill the gap, I suspect it will fail. In this scenario, the glue doesn't act to quickly distribute force to the metal structrural members, it becomes a structural member itself and will have to absorb normal frame stress. As Mad Honk points out, there already was a failure of aluminum. Had that aluminum been properly heat treated, it would have strength figures around 40,000 psi. If it wasn't properly treated, around 12,000 psi. And it broke. Epoxy (strength figures around 3000 psi) will surely fail.

If the OP finds a post that is a close fit, then the epoxy transfers stress and distributes it to the stronger aluminum frame elements. Your mileage may vary, and the consequences of trying this (I suggested a new frame) are on you, but a close-fitting (not too tight, but certainly not rattling around) post expoxied into the frame would likely work.

I use an industrial quality flexible epoxy (West Systems G-Flex) to get some toughness in joints. Here is a case where I'd probably not use Harbor Freight's epoxy. BTW, if the best post that can be found still presents a little gap, you can add fillers to epoxy to increase its bulk strength a bit (e.g. West Systems High Density Adhesive Filler). This stuff is all not super cheap, but it probably gives you the best chance of success. Remember to clean the surface per instructions.
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Old 02-15-20, 05:29 PM
  #71  
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Wiz and 'specially the OP,
The rads are cool bikes and we have a bunch of them here in B-town. So so saving yours is gonna be a good thing.
Wiz, Yes the last 35 years in the golf repair industry has taught me a bit. And I have a bunch of white papers and mag inputs which has made me well aware that I need a transciptionist to help me out with the printed stuff that comes out of my mind. But here on the forum I'm "on my own in the wild" so to speak.
Now epoxy bonding: .005" is the optimum for adhesion .010" is an outside limit even with glass beads used as filler to help keep the epoxy from becoming the matrix to hold everything together. So yes a good bonding between a new steel sleeve for a seat post and the old aluminum seat tube is a must. But as I see it, a steel sleeve post epoxied in about two inches down the old Al tube will be a good solution. Cut a relief in the top of the steel post for a clamp and install a new seat post. Any BMX post clamp should work and a 1" BMX post would help make the rad bike even more rad! Just again my opinion, but it will save the bike and make a $1500 bike ride-able again. Smiles, MH
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Old 02-15-20, 10:37 PM
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Mad Honk, I wish I shared your optimism. The original design, which was a thick solid tube that was welded and (we think) not properly heat treated, broke. The seat post was extended properly (minimum insertion distance was observed). I'm skeptical about the remaining welds and or tube material.

Also, the tube is distorted, so I'm unsure that one could find a good steel tube or post that would fit into the existing seat tube, past the top tube and seat stay welds, with the close clearances you mentioned.

Of course (as others have said), calling up Rad and asking for a warranty replacement would be optimal (and if they really want to come out looking good, they'd offer one).

If the OP must try to fix the frame, I'd recommend grinding down and smoothing the jagged break, and would use a very long aluminum seatpost glued in 6 -10 inches (I'd prefer ten: this provides more lever arm for the thing). I'd use flexible epoxy (the West Systems G-flex). If the original collar was swaged down (so that the ID of the collar was 27.2mm and the ID of the tube is a bit larger) then you'd have to glue in 27.2mm ID shim tubes at the top of the remaining seat tube, and near the bottom of the new seatpost. Then glue the seat post in.

Getting this all set up to have glue joints that have enough surface area with that 0.005" clearance is going to a real bugger.

I guess you could try having someone weld it, with the gusset plates that dedhed suggested. But those gussets are welded on: you might end up with something weaker than the original.

I'd really, really, really suggest calling Rad and letting them know of this thread. If they get the OP a new frame, great. If not, well, the OP should share that info too.
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Old 02-16-20, 10:30 AM
  #73  
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Devil's Advocate position here: Ride it.
The fracture is completely circumferential around the seat tube. If there is no evidence of the fracture propagating off in another direction along the tube or into the seat stays, there is no down-side to riding the bike except 1) the saddle is somewhat rotationally unstable (though not completely, because of the rough interfaces between the two parts) and 2) you can't lift the bike by the saddle without the seatpost coming out.

This part of the frame, especially at the front of the seat tube where it joins the top tube, is under a lot of tensile stress. Even more so with a heavier rider. Trying to re-construct the frame by any combination of welding or adding tubing elements is likely to be unsuccessful, as outlined by several posts above.

The seatpost can't drop into the seat tube as long as the clamp is tight. The tendency of the saddle to rotate while riding is minimal, because of the rough interface. The saddle can be somewhat stabilized by constructing one or more "stays" connecting the seatpost clamp to some part of the frame. It's not ideal, but it's not dangerous. Of course, you should be vigilant about watching for fractures elsewhere on the frame, but this is a particularly high-stress area.

Here's my reason for this suggestion (note that it's only a suggestion!):
I had the same exact frame failure on a folding bike after about 7,000 miles. This was a bike (Dahon Mu XL) with a very long seatpost. I'm not a lightweight rider, so I reasoned that welding would not be a good fix, and that a new frame would be likely to fail in the same way. So I opted to continue riding the bike, but with the addition of two stays connecting the seatpost clamp to the frame's rack mounts. The bike now has 12,600 miles on it and no further frame cracks have appeared. You can see the fractured stub of the seat tube and the stays in the image below.
Leaving Devil's Advocate Mode!

Stabilized Seat Tube Fracture
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Old 02-16-20, 10:54 AM
  #74  
CliffordK
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Originally Posted by sweeks
Devil's Advocate position here: Ride it.
The fracture is completely circumferential around the seat tube. If there is no evidence of the fracture propagating off in another direction along the tube or into the seat stays, there is no down-side to riding the bike except 1) the saddle is somewhat rotationally unstable (though not completely, because of the rough interfaces between the two parts) and 2) you can't lift the bike by the saddle without the seatpost coming out.
The one comment is that there may be a benefit of filing down the damaged part of the frame so it is smooth. That will help prevent a place for cracks to nucleate.

The OP still has an issue with the steatpost being unstable in the hole which should be addressed. Undersized seatpost? Find a larger seatpost?
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Old 02-16-20, 02:30 PM
  #75  
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I agree with everyone saying this probably isn't economical to repair by the book but you might get away with a bodge for a bit.

Speculating about the cause of the failure what's that lumpy bit on the front of the broken-off piece? It almost looks like it might be a sanded-down weld as if this was already repaired from some mishap in the factory?

Another possible cause of this is lack of deburring around the hole at the bottom of the slot causing stress to be concentrated and a crack to start.
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