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Discussion Starter · #1 ·
I have a sliding seat post on one of my bikes and this issue may cover many of the sliding seat post problems others had are having or will have. I want to get the general publics opinion and see if we can get frame manufacturers to improve their quality standards and monitoring procedures. First a little history about me so you know where I'm coming from: I'v been a mechanical engineer for about 11 years now and a tool and die maker for 15 years before that. I have also been a quality engineer in the automotive industry and trained in QS, TS, PPAP preparation, SPC and many other quality areas.

The problem I've been having with one of my bikes is the seat post sliding in, even with a good bolt clamp on it. So I started to do a root cause analysis and here's what I found:

My bike is supposed to have a 27.2mm seat tube hole, but it measures 27.4mm, which is 0.2mm (.0078") oversize. I used telescoping gages to measure it and I've measured it numerous times in numerous locations. I've been hearing that the seat tube hole manufacturing method is reaming but no one has verified that yet. Here's a link for reaming standards and for a 27.2 hole the tolerance should be +0.021mm MAX. Mine being 0.2mm over already shows it's oversize, but I can only verify this with a copy of the print the frame manufacturer is working to, and the bike manufacturer either doesn't have this print or is unwilling to give out the specification and tolerance. If he doesn't have a copy of the print that really scares me because then what it says is many of our frames are being manufactured overseas and the bike manufacturers aren't monitoring or verifying their quality and methods.

Concerning the seat post here's what I found on the Sette APX Carbon post I'm trying to use, it measures 27.0mm, which is 0.2mm (.0078") to small.

Now here's a little blog from Surly on Why Seatposts Slip, You'll notice in that they mention a seat tube being over measuring 27.29, which is only 0.09mm over, which to the reamer standards would be acceptable.

So here are my questions: 1. Is my seat tube hole out of spec? and 2. How can we make the bike manufacturers recognize this specific problem?

Maybe I should try running this by some of the bike mags to see if they have looked at this problem or are interested in analyzing and writing about it.

As a side note I may have fixed my problem with a coke can, but this isn't a method many consumers or even lbs employees would be skilled at so it's not really a viable solution. After I ride it for a couple weeks I'll let you know if it works.
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