SalsaShark

EDIT: This is a damaged seattube, not seatpost. I am unable to edit the incorrect thread title. If an admin could correct the title, that would be cool. 😉

So my 2015 Trek 520 has developed a crack in the seattube, below the binder assembly, stemming from both sides of the circular hole in the cinch cut of the rear of the tube.
This is the second frame I have had this happen on. The original 520 had the same failure after about 3 years and 8000 miles. Trek warranted the frame.
This frame made it about 7000 miles before suffering the same failure. The crack has traveled about a third of the way around the circumference of the tube.



FYI, seatpost is a 300mm Velo Orange Grand Cru 27.2mm, inserted well beyond the minimum insertion. In fact, the end of the post extends almost 4" below the bottom of the seatstay/top-tube cluster. I weigh just south of 160lbs. Binder assembly was never excessively overtorqued.
Trek has offered to replace the frame again, but the canti version with steel fork is no longer available, so the new frame would be the current version with aluminum fork and disc brakes. I would really rather try to salvage my current frame and wheels, brakeset, geometry, etc... I just really like this bike, and have had it dialed in for years.
I was planning on having a local framebuilder attempt a repair on the seattube, probably welding a bead to seal the crack, and cleaning up the interior to make sure the 27.2 post fit is retained. I don't really need it to look perfect, but need it to be functionally reliable.
Does anybody here have experience with a similar repair, and does this course of action seem like it could be successful? If anybody has any other (better) suggestions they are welcomed.
I figured that if this repair attempt was not successful in the long term, I could lop off the binder assembly, modify a 27.2 post into an expanding (quill) post and keep the frame on the road.
Any guidance here would be appreciated.
Thanks!

sweeks

I would drill "crack stop" holes at the ends of the cracks. At least 1/4" in diameter, or a little bigger. then keep an eye on them for further cracking.
The cracks are in the seat tube, not the seat post.

GamblerGORD53

I would chop the top off and braze a 1 1/4 tube all the way around.
Use a collar like you said and make sure the slot is narrower than now and DON'T drill a big keyhole. If the double thick is too stiff, then make a short cut on the front/ inside.
My custom bike was like this. Then I had to cut it shorter to hold the tapered post end.

79pmooney

Is that a 3rd crack just below the left seatpin ear? Looks identical to the two coming off the seat lug slot except not as good focus.

SalsaShark

Nope, that is just a piece of dust or something lit up by the flash from the photo.

dedhed

If you repair it you'll never get a replacement again from trek

SalsaShark

Understood. But if the goal was a replacement frame, that would already be accomplished. Problem is the replacement would not be the desired frame - thus the intention to repair.

Sy Reene

Possibly from over-torquing the seatpost clamp? Surprised a bit that Trek didn't try and claim this.

smd4

How would over-torquing, which would force the cracks closed, cause cracking??

icemilkcoffee 

Are you sure the 27.2 seatpost is in fact 27.2mm? It makes no sense that two frames would fail in exactly the same way

70sSanO

If you are sure you don't ever want a replacement, take it to a frame builder and see if a more permanent fix can be made. I'm sure it is not common, but a quick search does show it is not a completely rare occurrence.

It doesn't make a lot of sense, but if the 27.2mm seatpost is small, as mentioned above, or the ID of the seat tube is a bit large there might be enough leverage over time to cause the break. It is definitely looks like an issue with stress/flex, at that point of the seat tube.

Many vintage, horizontal top tube, bikes don't have that much of the seat tube extending above the top tube. That may be a better fix, but I'd talk to a frame builder to figure out the best way to do it.

John

SalsaShark

The seatpost is correct and fits perfectly. I am running a layback post with a Brooks Pro, so there is leverage, but no more than any one of the millions of riders out there using a similar setup.

The clamp does not need to be torqued very tightly to keep the post from slipping. There is no issue with the clamp.
As I said, Trek has already approved the warranty claim if I want to go that route, but for me, the options of a new style frame or credit towards a new complete bike are not as enticing as keeping this one on the road.

I have a couple buddies working as framebuilders, and have discussed some repair options, but this is new territory for them, also. I was just trying to gather as much information as possible, and maybe find somebody who has has an experience with a similar repair in the past.
Thanks!

sweeks

This ^^.
Aluminum is pretty strong under compressive loading; it's alternation between compression and tension that causes cyclic fatigue. I've seen this in several aluminum frames on folding commuter bikes, which have the seatpost clamp cycled several times a day. It seems unusual to find the same kind of failure in a seatpost, or even two consecutive seatposts, that are tightened and mostly left alone. Still, the mode of failure seems similar.