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Seat post questions...

Old 06-30-22, 10:23 AM
  #1  
ArgoMan
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Seat post questions...

Good morning everyone!

Continuing with my build questions , I have an early 90's "Geoid" Scapin frame that I'm building out. Using my Swiss made vernier calipers, I get an interior tube reading of 26.6. So I then go onto the various websites looking for an appropriate seat post, which to my dismay is a confusing topic. So many post dimensions! Several manufacturers make a 26.6mm post. Some make a 26.5 and 26.4mm post. I'm assuming that I need a 26.6mm post, but I saw some commentary that I may wish to go with a slightly smaller diameter. To that issue, what to you all say?

Next, 26.6mm seems to be a rather obscure diameter. But I found some cheaper-priced posts that seem to fit the bill (aluminum) by a company named "Kalloy." But they are single bolt posts. I don't know what that means for fit and performance. I also found a guy on eBay selling Kalloy posts that are "micro adjustable". Sound interesting. I also found a number of sellers on eBay selling used or NOS posts that have a "double bolt attachment" (I think that's the phrase), made in either Japan or Italy, which I like the idea of. I am weary of a Chinese manufactured aluminum post that may fracture, as I've read some stories online of this happening and am familiar with Chinese manufacturing practice compared with others, like Japanese or Taiwanese. Any thoughts and suggestions would be very welcome. I'd especially appreciate Smelly's input, as he is really a store of great knowledge.
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Old 06-30-22, 10:35 AM
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Hit the red button under your post and have the thread moved to C & V. You'll get a lot more interest/answers there.
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Old 06-30-22, 10:38 AM
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Depends on a lot more stuff than what you measured with your Swiss made Vernier calipers. Actually they aren't the precise way to measure the inside diameter of any round tube needing real accuracy. Nor do they get deep enough into the tube to be where you can get a measurement that hasn't been affected by the years of clamping and unclamping the seat post clamp.

You do want the post to be smaller in diameter than the part of the tube below the clamp. Since your seat tube probably has a slit cut in it, it will be okay if the size of it actually measures the same or even smaller than your seat post as that part of the tube will open more when you insert the seat post.

So it's really a try it and see sort of thing. Possibly you might need to ream the tube if it's been mucked up by years of abuse.
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Old 06-30-22, 10:50 AM
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Originally Posted by Iride01 View Post
Depends on a lot more stuff than what you measured with your Swiss made Vernier calipers. Actually they aren't the precise way to measure the inside diameter of any round tube needing real accuracy. Nor do they get deep enough into the tube to be where you can get a measurement that hasn't been affected by the years of clamping and unclamping the seat post clamp.

You do want the post to be smaller in diameter than the part of the tube below the clamp. Since your seat tube probably has a slit cut in it, it will be okay if the size of it actually measures the same or even smaller than your seat post as that part of the tube will open more when you insert the seat post.

So it's really a try it and see sort of thing. Possibly you might need to ream the tube if it's been mucked up by years of abuse.
Yes, thank you! I don't recall right now if the post as a slit. I can ream it easily enough and will have to polish the inside a bit, as there's some "muck" therein. I can also "shim" a post is needed. What are your thoughts of a single or double post, and the micro adjustable posts?

Thanks again!
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Old 06-30-22, 11:57 AM
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Originally Posted by ArgoMan View Post
Yes, thank you! I don't recall right now if the post as a slit. I can ream it easily enough and will have to polish the inside a bit, as there's some "muck" therein. I can also "shim" a post is needed. What are your thoughts of a single or double post, and the micro adjustable posts?

Thanks again!
Though I mentioned reaming, I wouldn't ream the tube unless the inside surface is so bad that it makes inserting the seat post smoothly difficult. As to double or single I guess you are talking about the clamp for the saddle.

I like the two bolt clamps that let me have infinite adjustment of the tilt of the saddle. Some people find they are more difficult to use. You'll have to make your own personal assessment of what you prefer. But there are newer seat posts with a single bolt that work entirely different than those of the era your bike is from. My newest bike has a single bolt saddle clamp on the seat post and it works very well. Don't have any reason to change it to the two bolt that I've long preferred.
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Old 06-30-22, 12:21 PM
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Thomson makes a quality seatpost - including 26.6

mighy be able to find an old Suntour Superbe 26.6 seatpost ?

Ritchey ... Kalloy ... American Classic ? ...

Origin 8 ? ...
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Old 06-30-22, 12:29 PM
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Old 06-30-22, 12:43 PM
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There are two ways to determine seatpost size. One is to try seatposts of various sizes and see which one fits. The other is to use a specialized stepped tool which does effectively the same thing. Since you probably don't have various sized seatposts or the tool, your best bet is to take the frame to a shop which can do this for you.

Using the wrong size seatpost will cause problems. I wouldn't recommend guessing.
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Old 06-30-22, 12:59 PM
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26.6mm is the post diameter on my '72 Zeus with metric 531 tubing; 26.4 pinches the lug's ears too much.
Make a post in the 'for trade' thread and get a vintage post for your vintage bike. It will not have been made in China, if that is a concern of yours.

Anyone who suggests getting any seatpost diameter but the correct one for your frame - should not be taken seriously.

Last edited by Wildwood; 06-30-22 at 01:02 PM.
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Old 06-30-22, 01:15 PM
  #10  
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When measuring a seat post cavity, I measure in five locations, add the fire measurements up and divide by five. This method helps to account for any ovality that might have occurred during years of use. When installing a seat post, I install one that is tight, not loose at all. A slightly loose post will usually alloy the clamp assembly to distort and, more often than not, the post will slip.

Before measuring, I use emery cloth to clean the inside of the seat post cavity. Up and down, round and round, and repeat as often as necessary to get things clean. If there is any ovality, I try to eliminate it as best I can. Usually, the distortion is around the clamp ears. Finally, I use a small smooth round file to clean up the slot in the seat tube. I try to slightly camphor the edges of the slot, reducing the chances of scoring the alloy seat post. With all that done, I find the tightest seat post and install it. A proper sized seat post done not have to be super duper snugged up. Super duper snugging will, often times, just bend the seat clamp bolt and/or distort the clamp ears

It all might sound like a lot of work, but a half hour's effort will, usually, do the trick. When ever attempting to fit the post, always ensure that it is greased, even for trial fits. Best of luck.
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Old 06-30-22, 01:19 PM
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Kalloy makes several "nothing fancy but perfectly fine" seat posts. I have used them on many builds, a big plus is the big range of size these are available in, I consider them a good value, if not great lookers.
But you can get into trouble with a caliper (yes, even a Swiss digital model) when trying to get accurate internal seat tube dims. You're better off taking it to a LBS that has a post sizer "mandrel" they can use, maybe even a few donor posts they can try out for the final word on fit.
But do you know exactly what tubing this Scapin is built with, esp. that seat tube?
I never heard of this Geoid model, but Scapin used a lot of the lower priced Columbus tubes (as well as the highest grades) like Gara, Zeta, etc. to cover all the price points

Last edited by unworthy1; 06-30-22 at 01:22 PM.
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Old 06-30-22, 01:50 PM
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I too was not familiar with 26.6 mm diameter post either till I started building up my 1983 Gitane Pro.
Was surprised to find out the Simplex SLJ seatpost on it was a 26.6 diameter as I thought most French bikes with metric tubing uses 26.4 diameter posts.
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Old 06-30-22, 01:52 PM
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Vernier calipers are not designed well for the task of measuring a seat tube.

a telescoping bore gauge would be the reasonable cost way to go.

I would want to measure 3-4 cm below the opening, beyond the heat effected zone is the point.

then a number of other radial measures at a few stations. My hunch you will find variance and smaller at the top.

review why that might be so, if an undersized post had been inserted prior, easily closed up.

an example - a 1972 49cm Colnago super I bought had a 27.0 post. Even after truing up the slot region a 27.2 would not go.
the tube was 27.2 below the lug. Time for the reamer. Now 27.2 happy.
should have been done at the original assembly- all paint in the tube was there still.

the odds of such a small frame being built with SP tubes? Very low.

there are nice high quality digital bore gauges...
$$$, but really nice and quick to get the real answer. They use three points to assess the diameter.
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Old 06-30-22, 02:00 PM
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You can come on by and check with my gauge.



Post your location. There's a reasonable chance you're nearby a BF member with the tool you need.
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Old 06-30-22, 02:09 PM
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+1 on the Thomson - it is so choice. If you have the means, I highly recommend picking one up.

I got a Thomson Elite to replace the single bolt seatpost on my hardtail 20 years ago. It's now the post that I use on my Bianchi. I have a Masterpiece on my Lynskey, as well.

The problems with a single-bolt post are that it relies on serrations in the clamp to dictate saddle angle, so you're unable to make very-fine tweaks. Additionally, if your weight is resting on the rear of the saddle, behind the bolt, it acts as a lever - making that bolt a single point of failure, which is what happened with my single-bolt post on my hardtail.
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Old 06-30-22, 02:11 PM
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People tend to bash China made product on every forum. Often they do it on their Apple computer or Apple phone that is made in China..... Go figure....

Anyway, in my own manufacturing experience, China makes some outstanding stuff, some mediocre stuff, and everything in between. Its more about what the customer wants, rather than China making crappy stuff.

BTW, in the world of cheap manufacturing, China is no longer "cheap". Check out various well known branded bikes that are now made in Cambodia.

On seat posts, I keep a box full. Some 26.6 posts will measure close to 26.6, others will be closer to 26.4, even though they are marked 26.6.

The Stein tool above is a good one.

I tend to run a brake hone into the seat tube, particularly if the existing seat post has zigzag marks on it. I don't get carried away with the home, its just knocking down any high spots inside the tube.
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Old 06-30-22, 04:21 PM
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pretty sure Scapin would not have built with any type metric tubing, Columbus or other.
But yeah, the metric "good stuff" DB tubing could take either 26.4 or 26.6, depending on how it was reamed.
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Old 06-30-22, 04:38 PM
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Originally Posted by randyjawa View Post
When measuring a seat post cavity, I measure in five locations, add the fire measurements up and divide by five. This method helps to account for any ovality that might have occurred during years of use. When installing a seat post, I install one that is tight, not loose at all. A slightly loose post will usually alloy the clamp assembly to distort and, more often than not, the post will slip.

Before measuring, I use emery cloth to clean the inside of the seat post cavity. Up and down, round and round, and repeat as often as necessary to get things clean. If there is any ovality, I try to eliminate it as best I can. Usually, the distortion is around the clamp ears. Finally, I use a small smooth round file to clean up the slot in the seat tube. I try to slightly camphor the edges of the slot, reducing the chances of scoring the alloy seat post. With all that done, I find the tightest seat post and install it. A proper sized seat post done not have to be super duper snugged up. Super duper snugging will, often times, just bend the seat clamp bolt and/or distort the clamp ears

It all might sound like a lot of work, but a half hour's effort will, usually, do the trick. When ever attempting to fit the post, always ensure that it is greased, even for trial fits. Best of luck.
Especially for all trial fitting IMO, and all others too.
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Old 06-30-22, 06:11 PM
  #19  
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As to the 2-bolt vs 1-bolt question: I prefer 2-bolt posts because the tilt of the saddle on them is truly infinitely adjustable. The ones with the two bolts on top (Campagnolo and its imitators) are something of a PITA to adjust, but a 10mm ratcheting box-end wrench in through the back of the saddle between the rails gets it done without too much fuss or bother. If you look hard enough (eBay is your friend here), you can find Campy 2-bolt posts in pretty much any conceivable size - 26.4mm and 26.6mm very much included.

Having said that, if you need a 26.8mm or 27.2mm post, you can't do better than a Nitto S83 (two allen bolts on the bottom, easier to work with) but they ain't cheap (about $125 from Rivendell).

My issue with single-bolt designs is they all rely on serrations to keep everything in place, tilt-wise, and I usually find I want the adjustment to end up exactly between two of the serrations. It isn't the end of the world, and I am currently successfully using one or two 1-bolt posts, but I mostly have 2-bolt posts and, as I said, I prefer them.

Full disclosure: When I started getting serious about cycling as a teenager in the 1970s, a two-bolt Campy seat post was one immediate indicator of a good bike worthy of closer examination. Because of that, I wanted one and I grew to love the look of them. I still do. The fact that they work great is why I still use them when possible, but the scratch of my nostalgia itch is icing on the cake.
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Old 07-01-22, 07:18 AM
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Geoid was simply Scapin's terminology for Columbus' teardrop shaped Megatube concept. Megatube was introduced for the 1995 model year and was a Bianchi exclusive for that year. Consequently, a Scapin with Geoid tubing is no earlier than 1996.

As for the post diameter, I can't imagine it being anything other than 27.2mm. The vast majority of the standard outside diameter (28.6mm), steel, seat tubes offered by Columbus in the late 1990s used a 27.2mm post. The only exceptions of which I'm aware were Utrafoco, which used a 27.4mm post, and Aelle and Gara, both of which used a 26.8mm post. I certainly wouldn't expect Aelle or Gara to be used on a high end Scapin. Besides, they weren't offered with a Megatube option.

That still leaves several possibilities:

1, Caliper cleanliness and calibration: Clean the jaws and slide of the caliper and make sure it zeroes properly with the jaws closed. Examine the jaws for nicks.

2. Build-up on the inside of the tube: Check for burrs around the top edge of the tube, cinch slot cut and vent hole. Remove as necessary. Remove any foreign material on the inside of the tube.

3, Distorted seat tube: Distorted seat tubes can result from the manufacturing process or an undersize post being installed. Distortion created during manufacturing typically results in varying measurements when taken across different angles. Also, take several measurements of the outside diameter of the post just below the top tube to see if the tube is still distorted at that point. If so, the inside of the tube will have to be reamed.

If the outside of the tube is round when measured just below the top tube, the issue likely results from an undersize post or at least can be treated like an undersize post issue. Undersize posts typically result in a cinch slot with non-parallel sides. For this amount of variation, the cinch slot would be visibly narrower at the top of the slot, than at the bottom. If so, gently pry the slot open with a wide, flat bladed screwdriver of similar tool, so that the cinch slot is slightly wider at the top than the bottom. This will open up the top of the seat tube, allowing you to test post fit posts past the bottom of the cinch slot, where the tube should be not be distorted.

Last edited by T-Mar; 07-01-22 at 07:23 AM.
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Old 07-01-22, 07:27 AM
  #21  
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Originally Posted by aliasfox View Post
+1 on the Thomson - it is so choice. If you have the means, I highly recommend picking one up.
+1 for the FB reference.
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