Philly Tandem

I searched the archives but didn't find anything related to this type of issue: I just noticed that the seaststay welds on my son's Niner EMD MTB (XS size) are basically lifting off the adjoining seat tube (photo attached). So the weld itself is cracking at the joint line, but the frame itself is not cracked. The crack does not go all the way around, and I'm guessing my son's been riding it like this for a while. It's an otherwise great frame in good condition, and is ridden by a fairly light youth NICA rider. Can another bead be laid down to reinforce this area, or is the frame bound for the recycling bin? I'm not so worried about cosmetics. Thanks for any advice...

dsaul

It looks like they were made from 7005, so it can be welded. I would contact Niner first and see if they will help with a replacement frame.

Andrew R Stewart 

I suspect that the cracks are in the ST and not restricted to the weld only. This is pretty classic weld bead root cracking IMO. I wonder if there was a lot of reaming of the ST after welding, too thin a ST wall was chosen or just poor welding that resulted in a stress riser at the root. Anyway about it I think any repair is to be considered a short term solution. Best is to replace, worst is to fix and wonder when the other shoe will drop. Andy

unterhausen

sure looks like the tube is cracking, as would be expected. Take the seatpost out and see.

TiHabanero

I have seen weld cracks at the head tube and BB, but never at the seat stays/seat tube junction. Is this common? Just how much stress is taking place there? Is the stress from the seat post being bounced on while hammering the trail or is it from the shock coming up from the rear wheel?

Andrew R Stewart 

I suspect there's a few things contributing to these cracks. First is the use of a frame material that has a limited fatigue life. Weld beads can end up with a slight undercutting of the tube's surface right at the bead's edges. The ST ID might have been reamed to regain a round shape and of a wanted dimension, this would thin the tube about the welded area.

Not that it matters in this discussion but there's cracks in the paint radiating from the binder slot's bottom "relief" hole. These I suspect are paint only. But it would be interesting to find out if they are in the tube also.

IIRC 7005 alloy will continue to age harden for a long time. It does this without elevated temps (as in heat treating). It was my understanding that the Asian factories chose 7005 because no after construction heat treating would be needed and the amount of time between welding the frame and the customer riding the bike was long enough to bring the after welding strength to acceptable amounts WITH NO ADDITIONAL COST.

dsaul

I suspect, from the cracks around the seat tube slot, that the seatpost was slightly undersized and doesn't fully support the seat tube at the junction with the seat stays. That allowed the seat tube to flex from the loads being transferred up the seat stays and into the seat tube.

Andrew R Stewart 

Perhaps after welding ST ID reaming produced an ID that was too large for the speced post. I didn't want to claim that the binder slot base's cracks are other then only paint (which is a fairly common condition). I was hoping my comments would motivate the OP to pull the post and take a look. But someone said something once about a horse and water Andy

unterhausen

It's really common on that kind of attachment. In this case, Andy might be right about the cause. Builders that dont' reinforce the seat tube often cause this failure. Surly Pugslys had it for a number of years.

Philly Tandem

Thanks for the comments everyone, and sorry I haven't been able to check back in before now. I did, in fact, pull the seatpost (photos attached), and don't see any radiating cracks inside. However, there is a small hole inside around the area of each weld. Not sure if this was done purposefully during construction to allow for the relief of gasses during the welding process, or if they are the result of burning through the thin aluminum while welding. The outside cracks appear to only be at the weld joints, and not in the tubing itself. The correct size seatpost was always used (OEM). What appear as "cracks around the seat tube slot" are really just scratches that were touched up. I don't see any other cracks besides those in the welds as already noted. I haven't contacted Niner yet, but as 1) I'm not the original owner, 2) frame was only originally warrantied for 2 years, and 3) Niner got bought out and isn't really interested in helping with issues from previous ownership, I guess it's just wall art at this point. Sigh.

unterhausen

If it was my bike, I would ride it. Those are definitely vent holes.

dsaul

If you're planning on hanging it up anyway, bring it to me and I'll have a go at welding it for you. I'm in South Jersey, so not too far if you're in Philly. I welded a cracked aluminum seat tube for a friend, last year, and it still hasn't cracked again.