Cracked frame
 

I recently cracked the frame tube that contains the seat post it's just above the welds , wondering if I can have it repaired anybody knows anything I'd love to hear it
Yes the frame is steel does anybody know if there a way I can post a picture on this forum?

More info is needed to say for sure, but likely no, a repair in such a high stress area is not likely to be reliable.

Yes... Not so hard a repair, If its a steel bike. A simple braze and a grind will do. If the crack is well approximated all you may need is a little silver solder...

If its aluminium or carbon then its another story...

I had a crack that migrated from the bottom of the slot around the circumference of the tube. It was repaired by cutting off the existing seat post clamp tabs, grinding out the crack and welding a new seat post clamp over the tube.

If it is steel, it can be repaired. Even without a picture, I can almost guarantee it was caused by your seatpost not being inserted far enough into the frame.

If you simply click on the picture link, it will upload a photo to your album:
https://www.bikeforums.net/g/user/541473

From there, refer back to my post #6, and people will find it.

Pictures of crack
 

Here are the images the OP needed to post. Cracked frame at seatpost insertion point.

https://cimg1.ibsrv.net/gimg/bikefor...49627ffc73.jpg
https://cimg6.ibsrv.net/gimg/bikefor...e143e9ff60.jpg

That's a 'NO'. Sorry, JasonJas . ANy weld repair will not be as strong as the original, which was already not strong enough.

THis was likely caused by a too-short seatpost, though. Seatposts have a 'minimum insertion' mark, but I am never sure if the seatpost is designed for the mark to be at the top of an unsupported extension on top of the seat tube, or if it is better to make sure the mark is down near the tube junction where there is support.

Also, welding will deform the seat tube and it may make the seatpost fit unreliable.

Your “NO” is far too hard. A weld at the crack would hold up for a while but the problem isn’t the weld but the lack of annealing the frame afterwards. It could be welded to get you down the road but it’s not a long term fix.

As for the break being caused by the seatpost not being inserted far enough, that’s a possibility but I broke a frame in a similar place because I was using a very long set back seatpost. The post was inserted far enough into the frame but the setback cause too much stress above the frame junction. The seat stays on the bike came in a lot lower than this frame, however.


A competent welder should cause any kind of deformation of the tube when welding. Heat transfer for MIG welding is relatively localized.

Just to be clear, I probably wouldn’t try to fix the frame for years of riding but as a short term fix, it’s viable. It’s probably not worth the cost of the fix, however. Frames are fairly cheap.

I've seen this type of failure quite a few times, and seen a handful of repair attempts. Any welded repair will not likely finish the ride home from the welding shop.

I had a BMX frame crack somewhat similar to that in high school. But it was opposite, cracked on the side with the slot.

Took it into our welding/metal shop teacher. He cleaned it up and tig welded it in maybe 10 minutes.

I think the crack was caused by a QR cam to adjust the seat post on a bike that maybe wasn’t designed for it.

Anyway. Beat on the bike another year or so. It didn’t crack again.

That doesn't look like a steel frame, it looks aluminum.

And it shouldn’t warp to bad. Steel pulls to the side heated up. A crack like that is already hard up against where it would go.

I’ve used steel warping to my advantage, not in a bike context though.

I’d say you got not much to lose if you like the frame.

Depending on the tube size, there are a few internal locking seatposts. Mostly vintage, and often not real long. 27.2mm? However, it might be possible to make one too.

This. Also, the welds in your pic look too big for steel. I suspect it's an aluminum frame, in which case NO you shouldn't get it repaired.

Is your frame under a lifetime warranty like most frames are?

Again, to be clear, I probably wouldn’t fix this bike. However, I broke the seat mast on a bike like this one at where the arrow is pointing.

https://cimg9.ibsrv.net/gimg/bikefor...0b0457e36.jpeg

The area is unsupported and broke because of a seatpost with massive setback (a Titec Hellbent post with 1.5” of offset). A machinist friend at work welded it and it lasted for many years…and I’m not a light guy. I didn’t use the Hellbent post again, however. The machinist is a master welder whose had no problem with doing the weld and wasn’t concerned about it failing. It certainly lasted more “than the ride home from the welding shop”.

It would be better to get a new frame…and probably cost just about as much…but welding would hold up for quite a while even for aluminum.

Stick a magnet by the frame and you will instantly confirm whether or not it is steel versus aluminum.

I build bike frames and that is definitely an aluminum frame. As for welding it, if it were mine, I would weld it and make sure I had a long enough seat post in the future. I have personally welded an aluminum MTB frame with a similar failure due to a short seatpost. I advised the owner(a friend) that he needed a longer seat post and that the weld may crack again in the future. He is a large rider, over 6' tall and over 250lbs and that frame is still going 2 years later.

With a proper length seatpost that extends a couple of inches below the top tube, the worst that can happen from a future failure of the repair would be an annoying creaking sound and maybe some side to side rotation of the saddle.

Thank you. Straight from the horses mouth.

Yes, weld it. If the frame is 7000 series aluminum it will age harden on it's own. Even if it's 6000 series that's not a super risky place for a repair. As mentioned be sure you have a nice long seat post that goes well down the tube below the weld zone (just in case.)

There is a reason why bikes and seat posts have a " minimum insertion point " , you need to follow that and then your frame wouldn't crack.

That’s not necessarily true, not at all. That should be obvious for a lot of reasons, not the least of which is the fact that minimum insertion marks are specific to seatposts and have nothing to do with the frame they are used in