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I'm replacing a broken seat post but my new one is 100mm shorter. The insertion into the frame will only be 70mm, the old one (supplied with the bike) is 170mm. Am I putting my frame at risk?
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I was told the bottom of the post should extend below the bottom of the weld between the top tube & seat tube - in your case - good question?
I'd try get it below the bottom of the weld on the bracing piece.
Quick email to the tech dept of the manufacturer will sort you out. As a Guide Turner told me that 100mm was the minimum insertion for their bikes.
My specialized has a small hole in the seat tube (Stumpy with interrupted design like yours) - that hols is the minimum insertion, maybe worth a check if you can see your bike.
I think it will be advised for it to be below the bottom weld on the top tube, i.e. not the gusset. I asked the same question for my Cove GSpot (which as a bracing peice like yours) and was told that... however with the design you have there that's going to make it impossible to lower your seat without using a dropper post.
A quick mail / phonecall would put your mind at rest though.