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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 03-25-14, 09:50 AM   #1
qspencer
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Stoker stems and non-standard seatposts

I have a 2002 Raleigh SC Tandem, which uses 29.2 mm seatposts. I may need to get a different stoker stem, but looking around at my options, it appears that this is a non-standard seatpost diameter and that 29.8 is the closest "standard" diameter for a tandem. What are my options? The size differential seems just small enough that a shim may not be practical and just big enough to make me worry about trying to just tighten the clamp down a little extra. It may depend on the clamp mechanism. The difference in circumfrence would be 1.9 mm by my calculation. It doesn't seem out of the question for a clamp to have a gap at least that big.
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Old 03-25-14, 10:09 AM   #2
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If you shim to a 28.6 seatpost you'll be able to use any standard 1-1/8" stem.
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Old 03-25-14, 11:23 AM   #3
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I'm not sure you're going to find a shim that thin.

We're using a Thompson 28.6mm, with a shim on our Robusta. I believe the opening on the Robusta frame is 29.4, and I had to shave down the shim with a dremel tool to keep it from being overly tight.
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Old 03-25-14, 12:11 PM   #4
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29.2 to 28.6 is 0.6mm on the diameter, or 0.3mm on the radius => .012". Pretty easy to find .012" aluminum stock to make a shim; here is a source: Plastic and Aluminum Shim Stock and your local hobby store probably has some too.
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Old 03-25-14, 02:11 PM   #5
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There is a seller on ebay named Gobike88tony who sells .3mm seatpost shims for $4.90 including shipping from China. I recently installed one on a 25.4 seat post in a bike with 26.0 tubing. Worked fine.
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Old 03-25-14, 05:33 PM   #6
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I've made shims out of aluminum cans (beer/softdrink). One or two layers of that thin alu can do the trick.
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Old 03-25-14, 07:41 PM   #7
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Shims are available to solve this in many ways:
Shim the 29.2 down to 28.6, use a 28.6 post and a 28.6 stoker stem.
Shim the 29.2 down to 28.6, use a 28.6 post. Use a 29.8 stoker stem with a second shim to 28.6.
Shim the 29.2 down to 27.2, use a 27.2 post. Use a second shim to go from the 27.2 post to the stoker stem size.
Etc.

One source for shims is here: http://www.niagaracycle.com/categori...eatured&page=1
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Old 03-25-14, 08:45 PM   #8
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Originally Posted by WheelsNT View Post
Shim the 29.2 down to 28.6, use a 28.6 post and a 28.6 stoker stem.
This seems the most straightforward approach but I don't see any 29.2 to 28.6 shims. I'm guessing that .3 isn't thick enough to keep its shape before installation. If you want to roll your own shim use 30 gauge (0.01") brass sheet.

I have a 2012 Companion (seems to be the same frame) that I'm planning to replace the stoker seat post. I'll be using a 29.2 to 27.2 shim which is readily available.
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Old 03-25-14, 08:54 PM   #9
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Originally Posted by qspencer View Post
I have a 2002 Raleigh SC Tandem, which uses 29.2 mm seatposts. I may need to get a different stoker stem, but looking around at my options, it appears that this is a non-standard seatpost diameter ...
What are you trying to accomplish with a different stem? You might be able to just replace the extension.
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Old 03-25-14, 10:06 PM   #10
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What are you trying to accomplish with a different stem? You might be able to just replace the extension.
The goal is to get it closer to the stoker than currently possible, but you are right, it might be possible by just replacing the extension.
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Old 03-26-14, 07:21 AM   #11
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The goal is to get it closer to the stoker than currently possible, but you are right, it might be possible by just replacing the extension.
Shimming the seatpost down to 28.6 probably is not the best way to go then. Most standard stems top out at 130mm, with some at 140, even 150, but those are hard to find.

Co-Motion adjustable stoker stem, and the Co-Motion Max adjust stem are both 29.8 and can be shimmed smaller.
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Old 03-26-14, 01:22 PM   #12
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The goal is to get it closer to the stoker than currently possible...
Measure the male extending part of the stoker stem. If it is 28.6 or larger, you are all set. Buy a cheap 28.6 seatpost and cut clamp part off the top of the post. Insert the 28.6 seatpost into the female part of your existing stoker stem with a shim if necessary. Clamp a normal 1-1/8" stem on the seatpost (1-1/8" = 28.6mm).

If the male part of your existing stoker stem is smaller than 28.6, then you need a smaller cheap seatpost for the extension, and you then need to shim the 1-1/8" stem on that seatpost as well.
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Old 03-26-14, 02:02 PM   #13
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Or buy an adjustable stoker stem that is at least 29.2, and shim as necessary.
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Old 03-26-14, 03:04 PM   #14
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I don't see any 29.2 to 28.6 shims. .... I'll be using a 29.2 to 27.2 shim which is readily available.
You're right, I was sloppy. Looks like there are 25.4 to 26.0 shims, though, and there is also a 30.9 to 31.6 shim that would be 0.1mm larger and might fit.
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Old 03-26-14, 04:31 PM   #15
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All of our bikes now have 27.2 seatposts to keep life simple and compatible, with shims used where necessary. For the tandem captain's post, there is one 27.2-29.8mm shim in the frame, and then a 27.2-28.6mm shim under the stoker's stem (a 35 degree, 150 mm fixed model sold by SJS in the UK, who also sell 165 and 180mm versions). You could easily do the same thing with a 29.2 mm frame.
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