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Seatpost Shims... Bad Idea?

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Seatpost Shims... Bad Idea?

Old 10-05-05, 06:42 PM
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Seatpost Shims... Bad Idea?

Does anybody have any experience with seatpost shims?... I am currently awaiting the arival of my new Litespeed Siena and I have an FSA SL-K carbon seatpost that I was hoping to use on it... Problem is the post that I have is a 27.2mm and the Siena uses a 31.6mm post... Would it be a bad idea to use a shim in there?... I figure it might be a bad idea since I would just be adding complications to the mix but I just wanted to see if anybody has any real experience with these things...

Thanks,
-Jason-
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Old 10-05-05, 06:48 PM
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There are proper 36.1 to 27.2 shims. Correct ones I have used have not been an issue...YMMV.
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Old 10-05-05, 07:29 PM
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I use a shim on my track bike. No issues. Just remember: do not grease the seatpost, grease the shim.

Cane Creek, Wheels Manufacturing make appropriate shims for your use.
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Old 10-05-05, 08:19 PM
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another vote: shims ok, no problems.
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Old 10-05-05, 08:50 PM
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cool - thanks guys... another worry that I had was that the shim might get stuck in the seat tube - I guess thats where the grease comes in... I have never used or even seen how one of these shims works but I guess I can figure things out when it arrives...

One order for a Cane Creek seatpost shim made!

-Jason-
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Old 07-23-15, 09:14 PM
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Just in case someone reads only the good bits about shims here, please also see this thread and know that shim removal may be fraught. I did not use grease or anything, and perhaps if I had I would be fine but I found that I can not get my shim out of my old carbon frame even when gripping the shim in a vice. I tried accross the diameter to no avail. Then I tried gripping bits of the shim. I could not budge it. Twishing resulted in the bits held in the vice shearing away.
Stuck seatpost adapter shim (old frame, no worries) by Timothy Takemoto, on Flickr

To remove it I would need to cut it with a hacksaw blade as recommend as method of last resort by Sheldon Brown.
14 Ways to Unstick a Seatpost

Not that this matters since I do not want to use either the frame (old and creaking) or the shim (since this difficult to remove). I thought I would try removing it in response to a question about shim removal elsewhere.
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Old 07-23-15, 11:29 PM
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One advantage is that the skinnier seat post with shim will be lots more comfortable than the full diameter post.
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Old 07-24-15, 03:18 AM
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Zombie zombie zombie.

But sadly, it won't bring sydney back to life.
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Old 07-24-15, 11:16 AM
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Originally Posted by rpenmanparker View Post
One advantage is that the skinnier seat post with shim will be lots more comfortable than the full diameter post.
I'd love to hear more about this...
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Old 07-24-15, 11:17 AM
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Originally Posted by Rowan View Post
Zombie zombie zombie.

But sadly, it won't bring sydney back to life.
Good catch, as he used to say...
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Old 07-24-15, 02:54 PM
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Originally Posted by rpenmanparker View Post
One advantage is that the skinnier seat post with shim will be lots more comfortable than the full diameter post.
Originally Posted by rmfnla View Post
I'd love to hear more about this...
Maybe Timtak will help us with this. Perhaps more "goodly flex".
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Old 07-24-15, 04:00 PM
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Originally Posted by rmfnla View Post
I'd love to hear more about this...
Spesh has spec'ed a 25.4 mm post on some of their bikes specifically to improve rider comfort. Stiffness of a tube increases proportionally to the cube of the diameter. Even a small reduction in seat post diameter makes a big improvement in its flexibility and shock absorption. Conversely oversize seat posts are less comfy due to the increased stiffness.
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Old 07-24-15, 04:04 PM
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Originally Posted by rpenmanparker View Post
Spesh has spec'ed a 25.4 mm post on some of their bikes specifically to improve rider comfort. Stiffness of a tube increases proportionally to the cube of the diameter. Even a small reduction in seat post diameter makes a big improvement in its flexibility and shock absorption. Conversely oversize seat posts are less comfy due to the increased stiffness.
I think you mean Cannondale? 25.4 seatpost on my Synpase. I understand the new CAAD 12's are sporting the thinner diameter as well.
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Old 07-24-15, 04:11 PM
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Originally Posted by dtrain View Post
I think you mean Cannondale? 25.4 seattube on my Synpase. I understand the new CAAD 12's are sporting the thinner diameter as well.
Thanks for the correction. Yes, you're right.
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Old 07-24-15, 04:20 PM
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Originally Posted by rpenmanparker View Post
Thanks for the correction. Yes, you're right.
I'll admit that it looks a little silly at first. The bike my Synapse replaced had a 31.6 seatpost. Even my thin-tubed steel Bianchi had a larger diameter post (27.2). More comfortable? I guess, but IDK that I can honestly tell. It's such a different frame that it's hard to pinpoint.
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Old 07-24-15, 06:49 PM
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Originally Posted by rpenmanparker View Post
Spesh has spec'ed a 25.4 mm post on some of their bikes specifically to improve rider comfort. Stiffness of a tube increases proportionally to the cube of the diameter. Even a small reduction in seat post diameter makes a big improvement in its flexibility and shock absorption. Conversely oversize seat posts are less comfy due to the increased stiffness.
As a recovering mechanical engineer who has been riding as an adult (so thy say) for about 340 years I have trouble believing that anyone would notice any difference.

Most of the impact on a seatpost puts it into compression and I just can't see such small differences having any noticeable effect.

Maybe you know of some evidence that backs this up..?
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Old 07-24-15, 07:13 PM
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Originally Posted by rmfnla View Post
As a recovering mechanical engineer who has been riding as aan adult (so thy say) for about 340 years I have trouble believing that anyone would notice any difference.

Most of the impact on a seatpost puts it into compression and I just can't see such small differences having any noticeable effect.

Maybe you know of some evidence that backs this up..?
Define "most". You have a 72-74° STA and 25mm offset seatpost. Where there is compression, there is also a significant bending element.

I have one frame, a Cinelli Xperience, which is brutal with a 31.6 aluminum post, tolerable with a 27.2 aluminum post, and borderline comfortable with a 27.2 carbon post. Both Al posts were Deda RS01 posts, carbon was Deda Zero100. Criteria was whether I could move after a 60 mile ride, and if I could do consecutive 60 mile days. Only variable changed was seatpost.

That being said, I have other frames that I doubt I could tell a difference with post size or material. But those frames are comfortable out of the gate, so my criteria would fail.
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Old 07-24-15, 08:26 PM
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Originally Posted by rmfnla View Post
As a recovering mechanical engineer who has been riding as aan adult (so thy say) for about 340 years I have trouble believing that anyone would notice any difference.

Most of the impact on a seatpost puts it into compression and I just can't see such small differences having any noticeable effect.

Maybe you know of some evidence that backs this up..?
The bending mode is very significant.
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Old 07-24-15, 08:35 PM
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I have a seatpost shim on my Rockhopper frame because it took an odd size, and I happened to have a 27.2 post laying around that I wanted to use instead of ordering a new post... So I ordered a shim instead...

Never the slightest issue, even when I was 230lbs (166 this morning). More comfort? I don't know... all my bikes have 27.2 in them
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Old 07-24-15, 11:27 PM
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Originally Posted by rpenmanparker View Post
The bending mode is very significant.
Goodly flex @timtak
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Old 07-25-15, 04:09 PM
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Originally Posted by BoSoxYacht View Post
Goodly flex @timtak
Originally Posted by BoSoxYacht View Post
Maybe Timtak will help us with this. Perhaps more "goodly flex".
As BoSoxYacht (who seems quite obsessed) says, I liked riding with my seat offset foward, tip of saddle over bottom bracket partly due to a lack of flexibility, partly in immitation of triathlon position as promoted by Profile Design Fast Forward seatposts.

And it seemed to me that the forward offset on top of the 27.2 seat post made for ride comfort through that which I termed "goodly flex," which can also be achieved through long handlebars stem which encourages carbon forks to flex more.


Goodly Flex
by Timothy Takemoto, on Flickr
So if it were just about compression then as rmfnla says
Originally Posted by rmfnla View Post
Most of the impact on a seatpost puts it into compression and I just can't see such small differences having any noticeable effect.
I doubt it would have a noticable effect. But it is the rpenmanparker "bending mode" or RollCNY "bending element" rather than the compression that matters, and this flex/bending mode/bending element can be increased to become even more "goodly" with saddle offset (and stem length in the case of the forks).

The flex may also have contributed to the shim getting well and truly stuck. But there was no need for me to remove the shim so perhaps I will get another and put the 27.2 in my new (Velobuild R022) frame.

That said, these days I tend to ride with a less forward offset position in the drops and the R022 frame is comfortable anyway.

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Old 07-25-15, 07:03 PM
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I needed a 24mm seatpost to complete a project bike for my wife. Obviously, money was no object. I had to pay $40 for one on eBay; somebody else must've had the same bike. (I paid $25 for the frame itself)

I've used pieces of an aluminum can to shim a 27mm seatpost into a 27.2mm seat tube. Since the pieces were small enough to leave gaps, removing them was never a problem.

I did have a bottom bracket sleeve seized up that required judicious use of a hacksaw to remove so do whatever you can to ensure your seatpost shim doesn't bond itself to the frame.
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Old 07-27-15, 11:50 AM
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Goodly Flex; interesting concept.

Yes, I get that because the seat tube is at an angle there will be some non-compressive forces, just can't see it making that much difference.

Of course, I have never used two different sized seatposts on the same frame so I have never compared.

Guess I have to take your word for it until I do...
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Old 07-27-15, 03:00 PM
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Originally Posted by rmfnla View Post
Goodly Flex; interesting concept.
Yeah, I'll pass on that. The last thing I want is my CF steerer tube flexing.
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Old 07-28-15, 12:34 AM
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Originally Posted by rmfnla View Post
Goodly Flex; interesting concept.
Yes, I get that because the seat tube is at an angle there will be some non-compressive forces, just can't see it making that much difference. Of course, I have never used two different sized seatposts on the same frame so I have never compared.
Guess I have to take your word for it until I do...
Thank you :-) but the should go to rpenmanparker and RollCNY who provide the theory (the cube of diameter no less!) and experimentation behind using lower seatpost diameters. I had not thought about diameter, only offset. Thanks to their information I have ordered a shim and am going to change my 31.6 mm 15mm offset, back to my 27.2mm with 30mm offset, to get my G.F. back :-)

And I had wondered why I was not enjoying the new frame....

Doing the math (comparing the cubes) the difference between a 27.2 and a 31.6 seatpost, cubed, is a ratio of about 156%. The 27.2 mm diameter post should be 1.56 more goodly flexible than the 31.6 seat post even if offset were the same. I have no idea by what power the offset matters but even if it is linear then the double offset should result in three times more flex.

The shim I have ordered is better quality too. (I think that going from 27.2 to 31.6m may be too much to achieve with cans, but I have not tried).

Originally Posted by BoSoxYacht View Post
The last thing I want is my CF steerer tube flexing.
I think that by far the greater part of the goodly flex at the front is in the curved part of the forks. My graphic is very poor

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