Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 20 Post(s)
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9 out of 10 times (maybe more like 99 out of 100) when a seat tube breaks, it's because the seat post wasn't inserted deep enough. This is true regardless of the frame material.
Many people think that inserting a post up to the minimum insertion line is enough, but that isn't always the case. The min-insert line is related to the post flexing (camming) out of the frame, but doesn't address the needs of modern frame design where the seat tube extends above the top tube. The second consideration which is key to seat tube survival is that the seat post must extend deep enough to end below the bottom of the top tube, preferably an inch or so below that.
Seat tubes break when the post ends above the top tube joint, so all the post's stress is placed on the unsupported section of seat tube above the joint. To protect your next frame , get a longer post, make sure it fits well, and that way the post will buttress the seat tube and take it's fair share of the stress of the rider's shifting weight.
An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
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