Most comfortable light seatpost 31.6mm

Discuss light weight issues concerning road bikes & parts.
youngs_modulus
Posts: 575
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

TimF wrote:
Thu Mar 14, 2019 12:06 am
I have a Canyon VCLS S15 on one of my bikes, which I find really comfortable and not too bad for weight. I could potentially pick up another one and shim this to 31.6 as they only come in 27.2 however I'm wondering (as I'm not sure how good that'll look) what else is comfortable and light?
Keep in mind that you’re asking for things that are mutually contradictory. For compliance, you want a skinny seatpost. (Because seatposts are cantilevered beams, bending stiffness for round posts is proportional to the fourth power of the diameter. In other words, diameter is easily the biggest single contributor to stiffness/compliance).

That’s why these flexy seatposts are always offered in 27.2mm diameters, and why (IMHO) some new Cannondales come with 25.0mm seatposts. The smaller the diameter, the better these compliant seatposts work.

Because of that fourth-power relationship between diameter and stiffness, a 4.4mm (13%) decrease in diameter (from 31.6mm to 27.2mm) results in a relatively huge increase in compliance. Under the same conditions (seatpost extension, material, wall thickness and loading), a 27.2mm post will flex (displace) about 58% more than a 31.6mm post.

One way to think of this is that for a given seatpost weight, a smaller-diameter seatpost will flex more than a larger-diameter one. You asked for the most comfortable light post, so it sounds like you want the most flex per mass, right? If so, go for a shimmed 25.0mm or 27.2mm post. But some people won’t like the idea of a shimmed post, which I understand. If you’re among them, you’re looking at a stiffer 31.6mm post.

I hope that helps!

youngs_modulus
Posts: 575
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

StevieB wrote:
Thu Mar 14, 2019 2:04 pm
Take a look at the Niner RDO Carbon Setback Seatpost. I have a 31.6 on my mtb and pushing down on the seat, while it's not crazy-flexy, you can easily see it deflects a lot more than the 25.4 carbon SAVE post does on my '15 Synapse (newest SAVE posts are redesigned to flex more).
I’m guessing that the post on your Synapse has much less exposed length than does the post on your mountain bike.

It’s worth remembering that you can’t really compare seatpost flex between different bikes unless the length of the exposed post (from the saddle rails to the frame) is identical between the bikes. For cantilever beams like seatposts, compliance is proportional to the cube of the free length. That’s a strong dependence!

by Weenie


Kazyole
Posts: 140
Joined: Tue Apr 02, 2013 1:45 am
Location: NYC

by Kazyole

Darimo. I have a 27.2 with the older style clamping system. Total weight is 72g. I have it paired with an unpadded Berk Lupina and it's a damn comfortable combo. 200m days no problem.

Wookski
Posts: 1099
Joined: Tue Feb 23, 2016 5:51 am

by Wookski

TimF wrote:
Thu Mar 14, 2019 9:34 am
Wookski wrote:
Thu Mar 14, 2019 2:51 am
Darimo. Game over.
Is this comfortable, I can see it's very expensive and light?
As comfortable as any carbon post I have used (McFK, Schmolke, Enve, 3T etc) and under 90g

jeanjacques
Posts: 192
Joined: Thu Apr 14, 2016 11:01 am
Location: France

by jeanjacques

Ritchey Flexlogic, some article show a flex gain. 0 offset is 140g for 31,6x300. With 25/15mm offset, there is the Superlogic one, maybe 20g lighter.

tele
Posts: 4
Joined: Sat Dec 06, 2008 2:25 am
Location: Western Mass. USA

by tele

If you go with a 27.2 seatpost and shim it, do you lose the advantage of the skinnier diameter because of the shim?

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onemanpeloton
Posts: 283
Joined: Wed Jul 12, 2017 11:30 am
Location: Edinburgh, UK

by onemanpeloton

tele wrote:
Fri Mar 15, 2019 1:38 pm
If you go with a 27.2 seatpost and shim it, do you lose the advantage of the skinnier diameter because of the shim?
no because its the exposed seatpost that is flexing, creating comfort, not the part in the frame
2017 Giant TCR Disc
2015 Giant TCX
2016 Cube Stereo 140

youngs_modulus
Posts: 575
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

That’s exactly right. The part of the post within the frame is effectively “locked” to move with the frame, while the free length is (obviously) not.

Here’s a link to the closed-form solution to this kind of loading situation:
https://en.wikipedia.org/wiki/Deflectio ... ever_beams

This same equation describes the deflection of handlebars, stems and rigid forks. Science is fun!

TimF
Posts: 108
Joined: Wed May 09, 2012 11:18 pm

by TimF

Now leaning strongly towards a shim. Anyone done this? I'm interested in what it looks like!

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C36
Posts: 643
Joined: Fri Mar 03, 2017 3:24 am

by C36

TimF wrote:
Wookski wrote:
Thu Mar 14, 2019 2:51 am
Darimo. Game over.
Is this comfortable, I can see it's very expensive and light?
I have it in 25.4 that replaced a cannondale Save that is supposed to be engineered with some flex.
While I was not expecting anything in terms of confort I got really surprised to notice some confort improvement.


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KCookie
Posts: 1495
Joined: Thu Feb 12, 2015 9:40 am
Location: Pom living in Australia

by KCookie

TimF wrote:Now leaning strongly towards a shim. Anyone done this? I'm interested in what it looks like!
I have. Pics attached. ImageImage

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spdntrxi
Posts: 3116
Joined: Sat Jul 20, 2013 6:11 pm

by spdntrxi

I use a cane creek sleeve with a 27.2 Canyon S14 post (Ergon Cf3) on my Parlee...

TimF
Posts: 108
Joined: Wed May 09, 2012 11:18 pm

by TimF

Looks good! Thanks

StevieB
Posts: 17
Joined: Wed Apr 09, 2014 2:54 pm

by StevieB

youngs_modulus wrote:
Thu Mar 14, 2019 6:21 pm
StevieB wrote:
Thu Mar 14, 2019 2:04 pm
Take a look at the Niner RDO Carbon Setback Seatpost. I have a 31.6 on my mtb and pushing down on the seat, while it's not crazy-flexy, you can easily see it deflects a lot more than the 25.4 carbon SAVE post does on my '15 Synapse (newest SAVE posts are redesigned to flex more).
I’m guessing that the post on your Synapse has much less exposed length than does the post on your mountain bike.

It’s worth remembering that you can’t really compare seatpost flex between different bikes unless the length of the exposed post (from the saddle rails to the frame) is identical between the bikes. For cantilever beams like seatposts, compliance is proportional to the cube of the free length. That’s a strong dependence!
Actually not by much, since both frames are sloping. Maybe 3cm tops, and you can visibly see where the RDO post is flexing, which is up by where the bend in the post is, far above the seat clamp. I was surprised to see it flex that much at that diameter, even though Niner do tout their "unstiff" layup. It definitely deflects more than my '15 SAVE post, despite being much larger in diameter. Diameter's less of a predictor in anisotropic materials like carbon fiber in any case. FWIW, the current-style SAVE posts with the scalloped rear profile have a good bit more flex in them than mine.

youngs_modulus
Posts: 575
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

30 mm out of, say, 200 mm of exposed post is indeed quite a big difference when you consider the cubic relationship I mentioned above. You’re looking at just over 50% more deflection for 230 mm of exposed post as opposed to 200 mm for the same wall thickness/laminate schedule.

And a seat post with a small-radius curve can create a local hinge, but don’t fool yourself into thinking that the rest of the exposed post length is unimportant.

StevieB wrote:
Sun Mar 17, 2019 9:37 pm
Diameter's less of a predictor in anisotropic materials like carbon fiber in any case.
Oh, this is fascinating. Do go on.

by Weenie


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