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Old 08-10-22, 07:06 PM
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sweeks
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Join Date: Sep 2004
Location: Chicago area
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Bikes: Airborne "Carpe Diem", Motobecane "Mirage", Trek 6000, Strida 2, Dahon "Helios XL", Dahon "Mu XL", Tern "Verge S11i"

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Originally Posted by Gods lonely man
Would it be safe to ride this frame with this crack on the seat tube?.
I have a couple thoughts. First, a crack like that would most likely form or, at least, propagate as a result of tensile stresses. But this particular location should be mostly under compressive stress unless there are some unusual circumstances. So I wouldn't expect this crack to go anywhere very fast if at all. Could it have formed as a result of brazing the lug?
Second, even if it did propagate, that crack isn't likely to cause a catastrophic failure anyway.

Here's an extreme example of a seat tube slot crack gone wild. A folding bike I bought in 2006 developed cracks extending circumferentially from the stress-breaker hole at the bottom of the slot. This seat tube was exposed to at least four cycles of latch opening/closing every work day for several years. I failed to notice it in time to increase the radius of the stress-breaker, and it eventually circled the top part of the seat tube at the weld joining the seat tube to the frame tube. This left the latch and its collar attached to the seatpost. It didn't interfere with riding the bike, as my weight held the seatpost in place and the crack didn't propagate anywhere else. Of course, if I lifted the bike by the saddle, the seatpost would come out. A couple of home-made aluminum stays connecting the seatpost clamp to the frame took care of this. I've ridden this bike for ten years since this repair without any problems. See the attached image.

My "winter" bike, with the jury-rigged stays to hold the seatpost in place when lifting the bike by the saddle.
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