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Computing seatpost size with a butt checker

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Computing seatpost size with a butt checker

Old 06-29-20, 10:26 AM
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smontanaro 
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Computing seatpost size with a butt checker

(warning - not a framebuilder...)

The Homemade Tools thread (specifically, the posts about butt checkers) got me thinking...

Could a butt checker be used to figure out the theoretical seatpost size for a frame? I ask because I've got this Eisentraut Limited:



A 26.8 seems to fit well (after a little bit of de-ovalizing with a wedge-bolt stem), but it also tends to slip a bit. The slot is parallel with the seatpost bolt tightened. I suspect the seatpost bolt might be a touch long. I'll toss a second washer in there to see if it does a bit better job securing the post. Still, it's possible that in the seat cluster area a bit of heat deformation might be at work. Riding with some friends yesterday, one of them (Billy Ketchum for those of you on CR) thought the frame would have originally taken a 27.2 seatpost. Given its size, I figured a 27.0 might have been common if it had something like a Columbus SP seat tube. The frame came with no tubing decals, and I haven't looked closely at the steerer tube yet to see if it has an embossed dove as a hint of the tubing used.

Side question: Is there a common mid-70s tubing Albert Eisentraut might have chosen for the Limiteds which would have taken a 26.8 post?
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Old 06-29-20, 10:36 AM
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unterhausen
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probably the most appropriate tool for that is a telescoping hole gauge and a micrometer or a caliper. I bought a Chinese set thinking that Starrett can't be all that much better, but I ended up getting a Starrett later. Too many sources of error to use a butt checker. One other problem with a butt checker is that you might not be able to trust the tube to be the right size. I noticed a Columbus SFL tube was tight in the seat lug, but didn't check the id. Never had a bad one. Turns out that bike now has a 27.4 seat post in it because the tube wasn't drawn properly. Live and learn, should have measured the ID before I brazed it in.

Using a telescoping hole gauge isn't foolproof either. Takes a light touch and seat tubes aren't usually round.

Last edited by unterhausen; 06-29-20 at 10:44 AM.
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Old 06-29-20, 09:54 PM
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Andrew R Stewart 
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I always have smiled at this type of question. The correct post is the one that works. Not always the one that math predicts.

Seat tubes vary is real IDs, fabrication heat distorts to some degree after that, Use further distorts form these initial post fits. Reaming at various points can help establish the roundness of the fit although not always be complete. Honing might deburr but not much more

Tube butt gages are for pre fabricating purposes. To cut and locate the butt length WRT the tube miters in a way that makes sense. To use them to calculate the theoretical ID is starting from the wrong side of the mirror. remember when hanging a mirror how it distorts if not done right? Bad analogy but make the point I hope. Andy
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Old 06-30-20, 07:27 AM
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On the frame I mentioned, a 27.2 appeared to fit. The end of the tube was not a cylinder. The problem was that the part that was 27.2 was not the first bit in the lug, so a 27.2 would slip when riding. It is also a butted tube on the top end, so the 27.2 section wasn't very long. I ended up reaming it all to 27.4. Hopefully it doesn't kersplode. I might end up replacing it.

I resolved to check this on all frames in the future -- before I build them, not after.
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