Originally Posted by
Amt0571
I had a Dahon that developed the same issue. Dahon didn't honour the warranty saying it was my fault for not respecting minimum post insertion (which was false).
I finally drilled both holes at the end of the crack and it worked for 3 years until I finally sold the bike.
Of course, I never bought a Dahon again.
My experience exactly on same problem with Dahon, I'm nowhere near max rider height or weight. That bike also had the early plastic seatpost shim so the seat tube distorted more when clamping, this was replaced with aluminum, less elastic, and also reinforces the area. For the tiny bit of weight, I'd prefer a stainless steel shim as it would be 3X as stiff. Clamping slot was also on front of seat tube so loaded in tension, crack happened at T slot at bottom. Not noticed quick enough to mitigate.
Originally Posted by
Ron Damon
Isn't this an issue or an issue aggravated by the hole drilled on the front rather than rear of the seat tube?
Yes. They ignored proper convention. The slot was later moved aft, along with the shim materal improvement. Still they refused to admit poor design. I did a detailed engineering analysis and writeup, sent to them.
Originally Posted by
FBinNY
Yes, that would be another contributing factor, since cracking is more likely on the side in tension vs. compression.
I have no idea whether this slot is front or back, so I didn't mention that consideration.
However, no matter how you slot or slice it, extended seat tube designs of this type are problematic, and need extra care in choosing material, and detail specs.
As a manufacturing engineer, I'm saddened by how some people are so quick to ignore design principles that served so well for 100 years.
That's not to say we can't change things, just that we need to consider why things were done certain ways, and think things through more carefully.
Exactly.
REMEDIATION:
Drill hole at end of each crack, large enough so that afterward you can fit something round and hard inside of the hole and "peen" the surface of the radius, both for work hardening and to introduce residual compressive stresses on the surface, this improves fatigue life. Someone else on here did exactly that, they worked in dental restorations so understood the value of "burnishing".
Make sure you're not missing a shim between the seatpost and seat tube, and if you buy one, buy the longest one possible. Be sure about the inner and outer diameter, they come in many sizes.