As posted on many other threads, bifold frames have a notorious tendency to form a circumferential crack on the front of the seat tube, just above the weld to the main tube, or originating at the bottom of the T-slot if that is in the front, not the back of the tube. This is due to any or all of:
- hugely long seatpost, so exerts a higher bending moment aft than standard frames
- stress there due to clamping force (much increased on bikes with a plastic (instead of aluminum) bushing/shim there between the seatpost and seat tube)
- front T-slot causing a stress concentration on the tension stress side of the tube under rider weight, and fatigue stress happens in tension; T-slot on back of seat tube is better, because that is the compression stress side under rider weight
- stress concentration at transition of weld to tube, less if that is well blended
- lower material strength due to effect of weld heat zone; minimized if frame is heat treated after welding, or of a material that retains full strength after welding without post heat-treating.

I had to toss a frame where I did not discover the crack early enough (it had the earlier plastic bushing), and it had progressed all the way around to the seatstays on both sides and had started to go down the seat tube.

On my replacement frame, I've monitored things closely there because of this.

It has the newer aluminum bushing for less clamping distortion.

I added a soda-can material shim between the aluminum bushing and frame, to further minimize tube distortion under clamping. This makes the seatpost tight even without clamping, so is much more difficult to raise and lower the seatpost, however I do that very seldom, so not a big issue.

Recently, I discovered a small crack, about 1/8"/3mm long, emanating laterally from the bottom of the T-slot. Removing the seatpost and bushing and examining the inside of the seat tube confirmed this, and that it was not simply a crack in the paint. This is early enough to mitigate, hopefully. Longer, and I would have drilled a small hole at the end of the crack, as stress relief, to hopefully stop it. But in this case, I removed the crack area of metal. I don't have a round file, but a neighbor had a power drill and the right size bit. The key is to remove the crack as the tiny end of the crack is a continual stress concentration, and replace that with a smooth, gentle radius, at least as large a radius as that originally on the T-slot, that reduces stress concentration. If the crack is short, remove it completely. If the crack is long, just drill a small hole at the end of it to impede crack propagation.

I removed the seatpost and bushing. Drilling into the crack would have removed more material than necessary, because the crack was shorter that the drill diameter, so I could not drill partly over an edge. So instead of drilling, I put the drill on low speed and used the side of the drill bit (turning forward, right hand), to slowly cut into the crack area, like a milling cutter. I first poked the drill in to the far side of the seat tube, then backed off some to make sure I was not drilling into the far side of the tube. Then slowly, with lateral pressure, began to cut away the crack area. Slow and patient, this took about 20 minutes I think, including stopping multiple times to remove chips with a magnet. Then deburring inside. Then burnishing the new edge smooth; I wanted to do this with the small end of a knife honing steel, but hard to get in there. Instead, I reversed the drill direction to run it backwards, and used more lateral pressure with the drill, to burnish the edge without metal removal. This introduces residual compressive stress on the surface, like shot-peening, to improve fatigue life.

Then reassembled. Hopefully the repair holds up.

The soda can shim may not have helped, but didn't hurt.

What I *really* want is a stainless steel bushing in place of the aluminum one, I've only seen aluminum ones for sale; Steel has 3X the elastic stiffness of aluminum, so it should help reduce the stress on the seat tube. I wish I still had access to a metal lathe, I'd make one. If taking more stress on the bushing causes it to crack, I'd still much rather that part, easily replaced at low cost, be replaced, than a crack in the seat tube turning the frame to junk.

This was a 4130 chrome-moly steel frame. Procedure is same for aluminum frames.

Note: This failure mode has also shown up on aluminum adjustable height handleposts at the clamping slot; Mitigation procedue is the same.

So far I've only seen these failures on brand D folders, both mine, and posts by others. I'm curious as to whether it shows up on other brands and the details, such as T-slot location, or if crack was instead at weld zone, etc.

Cheers.