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Seatpost shims - any particular disadvantages?

Old 01-31-18, 01:31 AM
  #1  
Sito
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Seatpost shims - any particular disadvantages?

Hi there, I have a CX bike with alu frame and 31.6mm seatpost diameter, currently using an alu seat post. I love the bike, but, as you'd expect, the ride is a little harsh. So I've been contemplating the idea of getting a 27.2mm carbon seatpost and using it with a seatpost shim to soften things up a little bit.

Do you think it'll be worth the effort? And are there any potential drawbacks to such a setup?
Any advice much appreciated, thanks!
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Old 01-31-18, 01:43 AM
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I am wondering the same thing. All my bikes have 27.2 seatpost and I have a Boris X7 FatBike on the way with a 30.4 searpost. I have a few higher end seatposts in my parts bin that are 27.2 and a shim is cheap but I wonder if it would comprise strength and stability or just rattle and not seat properly.
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Old 01-31-18, 06:21 AM
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How much compliance do you think you are going to get from a carbon seat post? I'm thinking very, very little if any.
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Old 01-31-18, 06:43 AM
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I tend to agree, unless its very long. Have any tests been performed with an accelerometer? Maybe a plastic shim may add to the compliance. Im betting a more compliant seat and fatter tyres with lower pressure is likely to do more for comfort. At least that is what my experience tells me.
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Old 01-31-18, 06:53 AM
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I used one on my mtn bike for a bit then swapped it to my GF's CX bike so she could use a nice Thompson i had sitting around. It works fine and you won't notice it's there unless you think about it. weather the other post will help or not is up to you. cheap to try though.
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Old 01-31-18, 07:19 AM
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Seatpost shims are generally OK. Heck, I even have a bike (a '72 Peugeot UO-8) that came from the factory with one.

But... With a shim, the load from the seat post may be concentrated at the top 2" or so of the seat tube. Without a shim, the post is likely to be in contact with the seat tube for a longer length, so it would be better supported and less likely to cause the seat tube to crack or bend.

For the CX bike, if you're going to purchase a seatpost, why not purchase one that's the right size?
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Old 01-31-18, 07:48 AM
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They add another opportunity for 'mystery creaking sound'

and the seatpost feels harsh, because you're sitting on it for situations where you ought to be out of the saddle
arms and legs have way better compliance
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Old 01-31-18, 10:56 AM
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Originally Posted by Retro Grouch View Post
How much compliance do you think you are going to get from a carbon seat post? I'm thinking very, very little if any.
Exactly. Though there *is* the "placebo effect"...
The OP would be better off, IMO, getting a suspension seatpost such as the Cane Creek "ThudBuster". I have the "long-travel" (LT) version on my winter bike, and it does a pretty good job of smoothing out bumps. It's a little big, though, and I suspect the "short-travel" (ST) version would work as well. They are a little pricey, but there's a lot more compliance than a carbon fiber post would provide. This video gives an idea of how it works (strange point of view!):
Steve

EDIT: Also maybe look into lowering your tire pressure, or even fitting slightly wider tires.
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Old 01-31-18, 10:59 AM
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I have several bikes with seat post sizing shims, all nicely machined, my latest bike , a Bike Friday Season Tikit,

I used my 25.0mm seat post with a shim sleeve to 27.2, inside the existing, shim sleeve, so it has 2 in it.

It works very well ..

you may want a suspension seat post such as the Cane Creek Thudbuster, (2 of my bikes have those, 1,LT, 1, ST)

In Madrid, the EU, , German Air Wings posts will be available .. It came on my Koga WTR.. a 7005 alloy frame

I replaced it primarily because it was a zero set back design and I wanted to move my saddle back more than it allowed..

It was fitted with a shim sleeve at the NL factory..





...

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Old 01-31-18, 11:30 AM
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I also had a bike come with a shim as standard - I suspect they only wanted to stock one seat post size for multiple bike styles.

It was nicer than I would've expected, it was 3 or 4 inches long, and caused no problems - it was used with an aluminum seat post.



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Old 01-31-18, 11:43 AM
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USE, a British company made spring & elastomer seat posts in 25.0 then a 27.2, and made shim sleeves for other frame needs..

I used 1 , keeping my <C> Super record seat post when I sold the AlAn frame it fit in. reused 1st in a steel frame..




...
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Old 01-31-18, 12:46 PM
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I have moved a favorite CF seatpost from bike to bike several times using a USE thermoplastic shim. It's inexpensive and works perfectly. Another benefit is not having to worry about the galvanic welding issue with a CF/Al interface.
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Old 02-01-18, 02:17 PM
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Thanks for all the replies. Seems like generally the shims don't cause too much trouble, so might be worth a try. And yes, I know you have to get out of the saddle a lot when riding in the dirt and I also use wide and soft tires, but hay, there's always room for improvement and I like trying out stuff
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