OP: The problem may be not only excessive fatigue loads for the frame material and near a weld zone, it may also lay in the design of the seatpost clamping slot.
EDIT: I NOW SEE THAT THE CLAMPING SLOT IS ON THE REAR, REVERSED FROM ON MY BIKE. SO MUCH OF THE BELOW MAY NOT APPLY, UNLESS, THE CRACK BEGAN AT THE SLOT AFT (DUE TO CLAMPING STRESS, NOT BENDING STRESS FROM THE SEATPOST LOADS) AND WORKED AROUND FORWARD TO THE WELD. Either way, file a letter with the Consumer Product Safety Commission. My letter below with regard to a very similar failure on a different brand of bike, after the bike maker blew me off:
I own a <my model bike> folding bicycle. There are structural cracks in the frame.
I, the sole rider of the bike, am well below the bicycle design maximums of 231 pounds weight and 6'3 height (I am 170 lbs. and 5'8 height.) The bike has been ridden solely on smooth roads, and has low mileage (I am estimating 2000-3000 miles based on tire wear.)
The frame has cracks at the top of the seat tube, already half-way around the tube on both sides, below the seatpost clamp. If the cracks continue, it will result in separation of the top portion of the tube, and seatpost, from the frame. The cracks also lead to the welds joining the top of the seat stays to the seat tube, possibly causing failure at that joint as well. Either of the above failures risk fall hazard and/or loss of bicycle control and risk of severe injury or death.
As reference, please see recall number 16-270; This recall of a different bike brand involved The top clamp of the bicycle's seat post can crack, posing a fall hazard to the user. While due to a different part, this failure mode is similar to the failure mode above, and thus proves precedence with regard to it being a safety issue and the necessity of a recall if the failure proven. Please see link:
https://www.cpsc.gov/Recalls/2016/Ad...tional-Recalls
This frame design is used on multiple <my model bike> bicycles. A quick search on the internet shows this failure mode to be common on this frame design.
I contend that the frame cracks are the result of a design defect. (I am a retired mechanical engineer and manager, and have authored patents of, among other things, clamped structural connections loaded in fatigue.) At the top of the seat tube is a slot parallel with the tube axis, to allow the seat post clamp to close the seat tube tightly around the seat post. All bikes have a similar slot, but <my model bike> positioned the slot at the front of the tube, rather than the back of the tube as on all other bikes I have seen. The cracks initiate from the radius at the bottom of the slot, and propagate on each side circumferentially, progressing rapidly, as the crack increases the local stresses in a vicious cycle. When the rider sits on the bike, most of the force on the seat post is down, but due to the seat post angle, some is aft, which causes a bending moment (torque) into the seat tube, and this moment is especially high on <my model bike> due to the extremely long seat post, due to the folding design. That bending moment stresses the seat tube in tension in front, and compression in the back. This crack is a fatigue failure, and those only happen in tension, not compression. Thus, <my model bike> putting the clamping slot on the front of the seat tube introduced a weak point for a crack to initiate. Every other bike in the world puts this slot at the back of the seat tube, even other <my model bike> like the <other model>, which would put the slot in compression. Placing the slot on the front of the tube is a design defect. However, having the slot on the back, while definitely improving durability, would not guarantee no failure, if the stress in front still exceeded the fatigue life limits of the material. The seat tube is highly stressed; Notably, later models of this same bike are equipped with an aluminum bushing at this interface between the seat post and the seat tube, I believe to strengthen this area. The failed frame in question is equipped with the earlier plastic bushing, which is lower strength and less rigid, which may also allow excessive deflection of the stressed area under seatpost clamp loads.
I communicated the frame failure, with detailed description as noted above, and photographs, to the local <my model bike> dealer, who forwarded them to the manufacturer, <my model bike>. The manufacturer has refused to replace or repair the frame, saying the failure is due to excessive seat post extension (my seat post is 1.5 below maximum extension, it would be impossible for me to ride with the seatpost higher because I am 5'8), and the bike is out of warranty, despite this being a safety issue.
What I am requesting from CPSC:
Ask <my model bike manufacturer>:
What was the reason for the change from plastic to aluminum for the seat post bushing, and,
Has <my model bike manufacturer> completed cyclic fatigue testing, at maximum design rider weight (231 lbs.) and maximum design seatpost extension (equal to 6'3 rider height, maximum mark on the seat post), to confirm if this is a valid solution to the seat tube failures on this frame design.
If the aluminum seat post bushing is proven to prevent frame failures, I would require the manufacturer, <my model bike>, to:
Replace any frames with cracks at the upper seatpost, and,
Replace the plastic seatpost bushing with the newer aluminum seat post bushing, including on frames with no cracks, as a preventive measure.
If, however, the aluminum seatpost bushing is not proven to prevent the frame failure as noted, I would request that CPSC require <my model bike manufacturer> to:
- Replace completely, all frames of this design, with frames proven to not fail in this manner.
Best regards,
(me)
Mechanical Engineering Manager (retired)
Attachments: photographs of failure