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Another delay seat post size

Old 09-30-18, 10:08 AM
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Another delay seat post size

Ordered a 27.2 seat post for the frame I just finished, however it came through at 27.12. Do any of you see an issue of using this post without a pop can shim? Henry James cast seat lug with ears.
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Old 09-30-18, 10:14 AM
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I would not think that would be an issue at all, but let's see what some of the more seasoned wrenches have to say. When you put it in the seat tube, does it freefall until it bottoms out or is there light friction?
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Old 09-30-18, 10:36 AM
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No mention of actually trying the post yet. All posts have a tolerance in diameter. .08mm is about .003", a very small amount and often less then measuring error in using a caliper VS a micrometer.

Question- Was the post chosen for it's price or looks? Or was it chosen for it's precision? Andy
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Old 09-30-18, 10:43 AM
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A loose fit of the seat post is more likely to cause point loading on the seat tube. This may or may not be a be a problem well down the road. If it were a carbon bike I would do something about it. If the bike and post were carbon, I'd soften the contact area by cutting a slot across the bottom of the seat post from left to right. Consequently, the post instead of ending abruptly would a softer contact area.
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Old 09-30-18, 06:56 PM
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Originally Posted by berner
A loose fit of the seat post is more likely to cause point loading on the seat tube. This may or may not be a be a problem well down the road. If it were a carbon bike I would do something about it. If the bike and post were carbon, I'd soften the contact area by cutting a slot across the bottom of the seat post from left to right. Consequently, the post instead of ending abruptly would a softer contact area.
What I think is being said here is that with a loose fitting post the bottom end of the post can cause a crack/damage to the seat tube ID. An interesting idea but not one I've seen in 45 years of shop work, no mater what the frame material. Now I have seen steel frames with a expanded "ring" around the seat tube right at the post's end. But this is from rust taking up more space then no rust does. The rust bulges the seat tube bigger. I have seen the tops of a seat tube broken off because the post wasn't inserted far enough to clear the lug or other tube junctions.

If the post is held so loose that it rocks back and forth within the seat tube while riding that I suspect other issues will show far sooner that and seat tube damage (at the post's bottom point). Andy
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Old 09-30-18, 10:30 PM
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Stick it in and try it out. What guarantee to you have that the frame tube ID is exactly 27.2mm or +/- some tolerance? Won't know if the seat post/seat tube combination works until you actually try them.
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Old 10-01-18, 06:47 AM
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Ditto--forget about the numbers if it fits right. The proof of the pudding....

At the co-op, we volunteers measure and sort posts, and the actual measurements we come up with are often off by 0.1 mm, easily.
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Old 10-06-18, 02:33 PM
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So I just measured (with a Starrett #2 micrometer) 6 different "27.2mm" seat posts. 2 Bontragers, 1 each Campy NR, SR, American Classic and Ritchey. Each was measured at the aprox same locations along their lengths and around their diameters. I've "averaged" the results. 5 were undersized by between .0018" to .0038" (aprox 0.05mm to 0.1mm). One was aprox .0002" oversized. A few had slight tapers along their lengths of no more then .00003" and a few had slight ovalization of less then .00002". That the 5 posts are so closely grouped in average diameters speaks to the "standard" of slightly undersizing of a post in general.

The op's post measurement of 27.12 (or .08mm/.0003" undersized) suggests it's within the "usual" range of seat post tolerances.

BTW I also measured a cheap Asian (Kalloy? the cut off had no brand but was carded as a Sun Lite). It's 26.4 stated diameter was actually about 26.3mm. Again about tat one end of the "standard range". Andy
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Old 10-06-18, 05:35 PM
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^^^^as already stated above, seat post diameters are one of the less exactly defined factors in bicycling science.
I have a whole milk crate full of them, in stated diameters I most commonly use like 26.8, 27.0, and 27.2.

Sometimes on a project I have to try out a couple to get the best slide and fit. It's just the way it is.
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Old 10-07-18, 07:20 AM
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Originally Posted by andrewclaus
Ditto--forget about the numbers if it fits right. The proof of the pudding....

At the co-op, we volunteers measure and sort posts, and the actual measurements we come up with are often off by 0.1 mm, easily.
Another analogue guy. It's good to know there are still at least 2 of us.
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Old 10-07-18, 08:00 AM
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I'd test-fit the post before despairing. A Henry James seat lug suggests this is not a production frame, and the seat tube may not have been reamed post-brazing. Seat tubes are frequently distorted at the seat lug because the top tube is fixed at both ends and expands when heated for brazing. Steel is soft at brazing temperatures, particularly if brass is used, and the expanding top tube pushes into the seat tube causing distortion. This makes the inside diameter slightly undersize at the seat lug.

So, long story short, try the post. I wouldn't be surprised if it was tight and seat tube needs reaming even with a slightly undersize post.
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Old 10-07-18, 08:39 AM
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Originally Posted by Retro Grouch
Another analogue guy. It's good to know there are still at least 2 of us.
The count is at least 3 of us... I have adapted to being good with numbers/dimensions/measurements because that's the language when sharing info with another. But the actual fit is the meaning. I am less and less surprised at how so many newbies rely on the numbers and not the meaning behind them. Good pedigree and a beautiful coat mean nothing if the dog won't hunt Andy
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Old 10-07-18, 09:03 AM
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Did the OP bail out on us?
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Old 10-07-18, 10:12 AM
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Originally Posted by Paul Barnard
Did the OP bail out on us?
Maybe he tried to fit the post and went for a ride. Andy
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