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Weird Seat Tube Size

Old 07-27-11, 08:49 PM
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Weird Seat Tube Size

Iíve been riding a Giant OCR 1 since 2005, and the short version of my problem is after breaking the original seatpost, the new seatpost broke the joint between the seat tube and the top tube. This was a catastrophic failure, but the frame was covered by warranty. So, now that I have the new frame fully put together, I want to find a seatpost that wonít destroy this new frame.

Where I run into trouble is the diameter of the seat tube. The Giant OCR 1 uses a 27.2 stock seatpost and a 27.2 to 31.7 shim. I havenít found anyone that makes a 31.7mm seatpost. So I started looking for shims first, but no one makes a particularly long one. I want to get as much seatpost into the seat tube as possible to spread that force out. Iím 6í6Ē, 325 lbs, and have long legs. So, thereís a lot of seatpost sticking out of the seat tube, and a lot of weight on that lever. This sort of problem would be handled very differently for a smaller rider.

Now, after all of that, my question is, can I safely use a 31.6mm or 31.8mm seatpost? Those are two reasonably common sizes, especially the 31.6mm.

I would love feedback or advice from anyone who has been in this sort of position before or who can give me more information.

Thanks,
Robu
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Old 07-28-11, 12:55 AM
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I added a brace part way up the seat tube that connected with the rack mounts on the frame on a too small bike that I had for awhile; after I had a seat post bend. Looked silly, but protected the seat tube. I got a round aluminum shaft collar from McMaster Carr, which I drilled and tapped. The braces were 1/2 x 1/8 aluminum angle.

Too bad you could not get a frame closer to the correct size for you. I am now careful to have less than 6" of seat post out of the frame.

You can make a shim - it is only a thin piece of metal wrapped around the seat post, with a feature to prevent it from sliding down inside your seat tube when you install it.

Keep in mind that the strength of the seat post is proportional to the diameter raised to the 4th power.

This post would work in the 31.6mm version:
https://www.amazon.com/Avenir-Seat-Po...835834&sr=1-23
with a .002" thick shim. You can stainless steel shim stock for any industrial supply store. Cut little slits around one edge, and fold over to make a crown, then form into a cylinder - you can use the seat post as a mandrel. Leave an 1/8 inch of so of clearance at the gap, and position the gap towards your handle bars.
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Old 07-28-11, 03:52 AM
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I don't think there is such a thing as a seatpost with an odd number after the decimal point. They increase or decrease in size in 0.2mm increments. That is your problem (as you probably already know).

Personally, I would doublecheck the diameter of the seat tube on the bike using a pair of vernier calipers (take three or four measurements and average them).

I've just done this on Machka's OCR3, which I think shares the same frame as yours, and certainly has the 27.2mm seatpost with shim. And my conclusion is that you will likely get away with fitting a 31.6mm seatpost.

I also probably don't need to point out to you that you need as long a seatpost as you can find in that diameter. Finding the diameter might be a challenge, and finding one longer than 350mm even more so.

I do have to suggest that your problems are unlikely to go away with that frame and seatpost interface. The leverage on the seat tube and top tube junction is going to be hefty. Minimising the setback on the seatpost might help a little to reduce the leverage, as will using a seatpost with a diameter closer to that of the tube.

But the compact frame design of the OCR design means you are always going to have a lot of seatpost exposed, and that always will mean lots and lots of leverage.

Last edited by Rowan; 07-28-11 at 04:00 AM.
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Old 07-28-11, 09:04 AM
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at 325 (my current weight kitted up)....i'd recommend a 410mm thomson seatpost in 31.6:

https://www.amazon.com/Thomson-Bicycl...1865249&sr=8-1

i also use a surly constrictor seatpost clamp just to keep things from sliding

best seatpost around (imo) and plenty of length for support. they also have 330 if that gets you past the bottom of the top tube by an a couple of inches.

take this with a grain of salt though...i see the ocr is carbon and i have 0 experience with anything but steel and aluminum.
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Old 07-28-11, 01:02 PM
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jgsatl
i'd recommend a 410mm thomson seatpost in 31.6
i also use a surly constrictor seatpost clamp just to keep things from sliding
Those made me grin. Those are the two components that made me finally post here to ask questions.

Rowan
Personally, I would doublecheck the diameter of the seat tube on the bike using a pair of vernier calipers (take three or four measurements and average them).
Can you recommend a place to get a cheap and functional set of calipers? I've never had to do this sort of thing on my own, because this is the first time my LBS hasn't been willing to work with me.
I do have to suggest that your problems are unlikely to go away with that frame and seatpost interface.
Yeah, I expect to break this one again in a few weeks. The original frame lasted me six years. I took it in for one of the full service cleans, and the crew installed the seatpost shim backwards, so when I went to adjust it, it broke the stock seatpost. I bought an Easton aluminum one (EA50 setback) and the frame failed just under four weeks later. It has not been a pleasant experience trying to get things back to good.

nfmisso
Keep in mind that the strength of the seat post is proportional to the diameter raised to the 4th power.
I've never heard that before, can you elaborate a bit? I don't doubt the accuracy of what you said, I just want to know why.

Thanks for the help folks. I'll keep an eye on this thread and start looking to implement your suggestions.
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Old 07-28-11, 03:39 PM
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Originally Posted by Robuchon
I’ve been riding a Giant OCR 1 since 2005, and the short version of my problem is after breaking the original seatpost, the new seatpost broke the joint between the seat tube and the top tube. This was a catastrophic failure, but the frame was covered by warranty. So, now that I have the new frame fully put together, I want to find a seatpost that won’t destroy this new frame.

Where I run into trouble is the diameter of the seat tube. The Giant OCR 1 uses a 27.2 stock seatpost and a 27.2 to 31.7 shim. I haven’t found anyone that makes a 31.7mm seatpost. So I started looking for shims first, but no one makes a particularly long one. I want to get as much seatpost into the seat tube as possible to spread that force out. I’m 6’6”, 325 lbs, and have long legs. So, there’s a lot of seatpost sticking out of the seat tube, and a lot of weight on that lever. This sort of problem would be handled very differently for a smaller rider.

Now, after all of that, my question is, can I safely use a 31.6mm or 31.8mm seatpost? Those are two reasonably common sizes, especially the 31.6mm.

I would love feedback or advice from anyone who has been in this sort of position before or who can give me more information.

Thanks,
Robu
Since you are a clyde the best thing to do is have a local machine shop turn a piece of solid round bar stock into a seat post for you.

Solid stainless steel round bar stock (so it won't rust in place inside the bike) is a fix I used to have my layback seat post made which works very well.
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Old 07-28-11, 03:54 PM
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https://www.northerntool.com/shop/too...4657_200334657 is the dial caliper I used to figure out I had a 26.8 MM diameter tube. My LBS ordered me a longer one that came in the next day. I hope that helps.
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Old 07-28-11, 06:17 PM
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Originally Posted by Robuchon

Can you recommend a place to get a cheap and functional set of calipers? I've never had to do this sort of thing on my own, because this is the first time my LBS hasn't been willing to work with me.
https://www.harborfreight.com/6-inch-...per-99639.html
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Old 07-29-11, 06:24 PM
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Thanks for the pointers on the calipers. I grabbed a decent set on the way home today and had a go at the seat tube. Not surprisingly, different cross sections gave me different diameters. The averages fell around of 31.7mm (31.76mm, 31.73mm, 31.67mm). The only 31.8mm I'm seeing out there are Kalloy. I've only heard one opinion about them and it was bad. Is there a component maker you know of that's a touch less common that I should check out? Thanks for your help.
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Old 07-29-11, 08:37 PM
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Originally Posted by Robuchon
......
nfmisso

I've never heard that before, can you elaborate a bit? I don't doubt the accuracy of what you said, I just want to know why.
Basic strength of materials; sophomore year Mechanical Engineering.
https://www.eformulae.com/engineering..._materials.php
https://www.rorty-design.com/content/...._strength.htm

For a rectangular section, it is base (width) x height ^ 3; for round sections diameter ^ 4 When you design things, these are among the most basic things that you always have to remember; right up there with gravity makes things fall.
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Old 08-11-11, 06:46 PM
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I don't know if anyone still cares about this, but I figured I should update this for completeness. I rounded up a Thomson Elite 31.6mm seat post and Thomson 34.9mm collar. They arrived today, and I installed them this afternoon. The hardware fits like a dream. The collar went on without having to strain to get it there, but it's obviously firmly seated. The seat post has that nice level of resistance in which it takes a reasonable push to get it to move without it being suspiciously difficult. The parts are greased, the torque is dialed in, and it's ready to go for the weekend rides.

P.S. Thanks nfmisso for the links. Some of that was pretty cool. You don't see much of that in the economics department.
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