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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 06-07-12, 11:13 AM   #1
Mugre
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Seatpost shims?

The seat tube on our T-50 is a 29.8. This is now a not common size. The stock post has poor adjustment for angle and I'm thinking of a new one. I also want a Thudbuster for the little engine that could at my back. We ride in an urban environment and there's too many bumps and cracks to call though I will continue too call the bigger ones. Is there any big disadvantage or loss of strength when using a shim from the next size down? The Thudbuster doesn't come in 29.8 though I could order a Kalloy for the front post.


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Old 06-07-12, 11:40 AM   #2
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Originally Posted by Mugre View Post
The seat tube on our T-50 is a 29.8. This is now a not common size. The stock post has poor adjustment for angle and I'm thinking of a new one. I also want a Thudbuster for the little engine that could at my back. We ride in an urban environment and there's too many bumps and cracks to call though I will continue too call the bigger ones. Is there any big disadvantage or loss of strength when using a shim from the next size down? The Thudbuster doesn't come in 29.8 though I could order a Kalloy for the front post.


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http://problemsolversbike.com/products/seatpost_shims/
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Old 06-07-12, 12:02 PM   #3
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I rode 1,500 miles on a 0.4mm shim made from two rockstar cans with no issues whatsoever. I think with a purpose made shim you'll be perfectly fine.
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Old 06-07-12, 12:18 PM   #4
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I've been running a shim on my commuter with a suspension seatpost, and another on one of my mountain bikes with a Gravity Dropper seatpost for several years. No problems with either.
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Old 06-07-12, 01:27 PM   #5
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Seatpost shims?

We are running a ThudBuster & shim with no problems at all!
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Old 06-07-12, 02:32 PM   #6
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Our Speedster came from Co-Motion with a seatpost shim on the Thudbuster. No issues at all.
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Old 06-07-12, 03:55 PM   #7
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I would just make sure that the shims are 3" long or so. Some shims are shorter and don't provide enough contact area with the seatpost.
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Old 06-07-12, 04:20 PM   #8
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I would just make sure that the shims are 3" long or so. Some shims are shorter and don't provide enough contact area with the seatpost.
You also need to make sure that the shim extends below the junction of the seat tube and the top tube. Otherwise you will put too much stress on that joint and could end up with a failure there. Based on a picture I found of a T50, a 3" long shim should do the trick.

Note that the minimum insertion on a Thudbuster is 100mm (4").
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Old 06-07-12, 04:51 PM   #9
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Hi Mugre;

Should not be a problem; as long as you do not have too much seat post above the frame.

We're running a shim for the stoker, with no issues.

Note: strength of a seat post is proportional to the seat post diameter to the fourth power. (a 30.2mm seat post is twice as strong as 25.4mm seat post - same material and wall thickness for both). Stress on a seat post is highest where it meets the frame and is proportional to the distance from the frame to the center of mass of the load on the saddle.

I hope that you are enjoying your T50 as much as we enjoy ours.
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Old 06-07-12, 07:01 PM   #10
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You can make shim from your favorite beer/soda can.
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Old 06-07-12, 07:19 PM   #11
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I appreciate the input



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Old 06-07-12, 11:42 PM   #12
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On our Co-Motion, we run an old school Syncros ti post @ 27.2, and a Thudbuster brand shim. Same size you need. Works fine. The Thudbuster shims are the best quality and longest engagement I have found. Several years and a lot of miles with no problems.

I also bought the Problemsolvers but found them not to my liking but honestly did not try them. The length seemed short and not adequate compared to where the frames tubes / welds were are located.

PK
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Old 06-08-12, 05:08 AM   #13
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Originally Posted by Mugre View Post
The seat tube on our T-50 is a 29.8. This is now a not common size. The stock post has poor adjustment for angle and I'm thinking of a new one. I also want a Thudbuster for the little engine that could at my back. We ride in an urban environment and there's too many bumps and cracks to call though I will continue too call the bigger ones. Is there any big disadvantage or loss of strength when using a shim from the next size down? The Thudbuster doesn't come in 29.8 though I could order a Kalloy for the front post.


Mugre
You could run a shim 27.2x29.8. I wouldent make a shim from a can its not very safe

shims are cheap tandems east has a nice shim slection that they make and they are not expensive tandems east also had the thudbuster in stock the stoker should have one go to tandemseaat.com or call for them for help Mel if very helpful on the phone 856-451-5104
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Old 06-08-12, 11:26 PM   #14
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We're running 27.2 -> 29.8 mm shims on the front and back of our Co-motion tandem. The front works great, no problems at all, but the rear has occasionally allowed the seatpost to slide down a bit. Adding a bit of carbon mounting paste (even though it is an alu post in an alu shim, the paste adds a bit of friction) mostly solved the problem, but not completely. I'm thinking of trying one size of shim (0.2 mm) larger.
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Old 06-09-12, 05:47 PM   #15
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So I still haven't pulled the trigger on either seat post but it occurs to me, if I get a 27.2 seatpost, the stoker's bars will need a shim as well, Yes?

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Old 06-09-12, 06:54 PM   #16
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Originally Posted by nfmisso View Post
Hi Mugre;

Should not be a problem; as long as you do not have too much seat post above the frame.

We're running a shim for the stoker, with no issues.

Note: strength of a seat post is proportional to the seat post diameter to the fourth power. (a 30.2mm seat post is twice as strong as 25.4mm seat post - same material and wall thickness for both). Stress on a seat post is highest where it meets the frame and is proportional to the distance from the frame to the center of mass of the load on the saddle.

I hope that you are enjoying your T50 as much as we enjoy ours.
You must be a mech engineer! Very interesting.

I've used shims on my tandem for 3 yrs no problem.
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Old 06-10-12, 12:08 AM   #17
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Co-motion/Thudbuster combo with shim. No problemo.
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Old 06-10-12, 02:20 PM   #18
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Quote:
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So I still haven't pulled the trigger on either seat post but it occurs to me, if I get a 27.2 seatpost, the stoker's bars will need a shim as well, Yes?

Mugre
Yes, a shim for the stoker's stem will also be needed, but you can cut that one down to be only the height of the stem.
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Old 06-11-12, 05:03 AM   #19
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I would say if your seat tube is 29.8 go with a 29.8 front post and a 27.2 rear with a 27.2x29.8 seat post shim you will have no problems at all. Make sure u put greese on everything
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Old 06-18-12, 05:50 PM   #20
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You must be a mech engineer! Very interesting.
Yes; I am
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