Go Back  Bike Forums > Bike Forums > Tandem Cycling
Reload this Page >

Seatpost considerations

Notices
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!

Seatpost considerations

Old 02-19-20, 03:44 PM
  #1  
samkl 
Senior Member
Thread Starter
 
Join Date: Feb 2017
Location: Chicago, IL
Posts: 501

Bikes: 2004 Trek 520, resto-modded 1987 Cannondale SR400, rando-modded 1976 AD Vent Noir; 2019 Wabi Classic; 1989? Burley Duet

Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 206 Post(s)
Liked 89 Times in 47 Posts
Seatpost considerations

I was lucky enough to have been given an old Burley tandem which Iím fixing up.

The previous owner was shorter than me, so I had to raise the captainís seat about two inches, right up against the minimum insertion line. This is a 25.4mm seatpost, so pretty narrow, and the stokerís stem is bolted to it as well. The seatpost is an SR Laprade.

Considering the extra force on the post coming from from the stokerís handlebars, is this seatpost safe to ride at the minimum insertion? Or should I get a longer seatpost? I weigh 170-175lbs for reference.

I assume itís fine, but just want to double check.
samkl is offline  
Old 02-19-20, 08:17 PM
  #2  
merlinextraligh
pan y agua
 
merlinextraligh's Avatar
 
Join Date: Aug 2005
Location: Jacksonville
Posts: 30,993

Bikes: Willier Zero 7; Merlin Extralight; Calfee Dragonfly tandem, Calfee Adventure tandem; Cervelo P2; Motebecane Ti Fly 29er; Motebecanne Phantom Cross; Schwinn Paramount Track bike

Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 1275 Post(s)
Liked 447 Times in 238 Posts
It’s fine
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.
merlinextraligh is offline  
Likes For merlinextraligh:
Old 02-20-20, 09:39 AM
  #3  
joeruge
Full Member
 
Join Date: Jan 2006
Posts: 210
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 77 Post(s)
Liked 72 Times in 40 Posts
Minimum Insertion

I've always wondered about this. My sense is that the minimum insertion is more about protecting the frame than the seat post. Think about the mechanical advantage that a long seat post would have on the top of a seat tube if it was only inserted 1/2" As long as the seat post makes it below the crossing of top tube, you are probably safe. Remember, a captain's seat tube has more support than a single bike's seat tube. Still, I guess I would feel more confident if would have a bit more insertion.

​​​​​If you do decide to get a longer post, it is generally recommended that the captain's seat post not be carbon fiber because of the attachment of the stoker's handlebars.
joeruge is offline  
Old 02-24-20, 09:46 AM
  #4  
diabloridr
Full Member
 
diabloridr's Avatar
 
Join Date: Dec 2010
Location: Central Coast, California, USA
Posts: 434

Bikes: Co-Motion Macchiato, Calfee Dragonfly, Ancient Sun Fixie, Trek 5900

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 49 Post(s)
Liked 12 Times in 11 Posts
Originally Posted by joeruge View Post
​​​​​If you do decide to get a longer post, it is generally recommended that the captain's seat post not be carbon fiber because of the attachment of the stoker's handlebars.
Then why does Co-Motion (to cite one example) specify carbon fiber posts on many of their tandems?
diabloridr is offline  
Old 02-24-20, 11:45 AM
  #5  
joeruge
Full Member
 
Join Date: Jan 2006
Posts: 210
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 77 Post(s)
Liked 72 Times in 40 Posts
Originally Posted by diabloridr View Post
Then why does Co-Motion (to cite one example) specify carbon fiber posts on many of their tandems?

Well, one answer is that Co-Motion 'specifies' a particular carbon seatpost and not some random piece of crap that comes from China, which some guy who might want to save a few bucks as well as a few grams might consider. I didn't say a carbon seatpost should never be used, just that it may not be generally recommended. A carbon seatpost may be adequate for some teams, but if you are a big or very strong team, what does it get you for the potential risks? 20 or 30 grams on a bike that weighs up around 14,000 (not to mention riders weight of around 140.000)? Think about the kind of forces a captain seatpost must withstand that a single bike seatpost never sees - all that twisting and yanking from the stoker's bars. You know from other carbon elements on a bike how careful you have to be with them; no star-nuts down in the steerer tube and you have to inspect every nick and crack. Rodriguez Cycles in Seattle (https://www.rodbikes.com/index.html), who knows a little bit about making tandems, doesn't even use a carbon seatpost on their super-light, sub 30lbs steel tandem. A quote from their website:


"A captain's seat post has to be extremely strong. On a tandem, the captain's weight, as well as a good part of the stoker's weight is on that seat post. When a powerful tandem team is standing and climbing, the stoker is putting a lot of force on that seat post that isn't there on a single bike. A carbon fiber seat post is easily damaged by clamping things onto it.....such as a stoker stem. When a carbon fiber seat post has crack in the finish, it can eventually become a break. This process is accelerated when it is used as a captain seat post. I would consider that a captain seat post shearing off while riding would be a catastrophic failure."


Since we got rid of our suspension seatpost, we do use a carbon post for the stoker. It did save us maybe 200 grams off the suspension post. But moving to carbon was more to give the stoker a little 'buzz kill' rather than trying to save weight.

Last edited by joeruge; 02-24-20 at 07:47 PM.
joeruge is offline  
Old 02-26-20, 04:55 PM
  #6  
diabloridr
Full Member
 
diabloridr's Avatar
 
Join Date: Dec 2010
Location: Central Coast, California, USA
Posts: 434

Bikes: Co-Motion Macchiato, Calfee Dragonfly, Ancient Sun Fixie, Trek 5900

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 49 Post(s)
Liked 12 Times in 11 Posts
Originally Posted by joeruge View Post
Well, one answer is that Co-Motion 'specifies' a particular carbon seatpost and not some random piece of crap that comes from China, which some guy who might want to save a few bucks as well as a few grams might consider. I didn't say a carbon seatpost should never be used, just that it may not be generally recommended. A carbon seatpost may be adequate for some teams, but if you are a big or very strong team, what does it get you for the potential risks? 20 or 30 grams on a bike that weighs up around 14,000 (not to mention riders weight of around 140.000)? Think about the kind of forces a captain seatpost must withstand that a single bike seatpost never sees - all that twisting and yanking from the stoker's bars. You know from other carbon elements on a bike how careful you have to be with them; no star-nuts down in the steerer tube and you have to inspect every nick and crack. .
Interestingly, I broke the aluminum captain's seatpost on our old Speedster, likely through from high-cycle fatigue or corrosion-fatigue.

I'd worry more about component design than about the material it is made of.
diabloridr is offline  
Old 02-26-20, 08:03 PM
  #7  
joeruge
Full Member
 
Join Date: Jan 2006
Posts: 210
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 77 Post(s)
Liked 72 Times in 40 Posts
Originally Posted by diabloridr View Post
Interestingly, I broke the aluminum captain's seatpost on our old Speedster, likely through from high-cycle fatigue or corrosion-fatigue.

I'd worry more about component design than about the material it is made of.

Wow! That's pretty amazing. Never heard of an aluminum seatpost breaking. Was it completely unexpected or did you have any warning? Where did it break, where it went into the frame or near where the stoker bars attached?

Joe R.
joeruge is offline  
Old 02-27-20, 08:36 AM
  #8  
merlinextraligh
pan y agua
 
merlinextraligh's Avatar
 
Join Date: Aug 2005
Location: Jacksonville
Posts: 30,993

Bikes: Willier Zero 7; Merlin Extralight; Calfee Dragonfly tandem, Calfee Adventure tandem; Cervelo P2; Motebecane Ti Fly 29er; Motebecanne Phantom Cross; Schwinn Paramount Track bike

Mentioned: 16 Post(s)
Tagged: 0 Thread(s)
Quoted: 1275 Post(s)
Liked 447 Times in 238 Posts
It appears Rodriguez is in the minority. Co-Motion, Santana, and Calfee all build bikes with CF seat posts.

Also, consider that Rodriguez is in the business of selling steel tandems, and doesn't even use CF forks on tandems, according to their website.

IMHO, the concern about clamping force of the stoker stem is misplaced. Certainly, as with any bike part, you don't want to over torque it, but a seat post has to be designed to withstand forces sufficient to clamp it down anywhere along its length given that the seat post can be clamped into the frame anywhere along its length above the minimum insertion point.


Personally, we're a big team ( 350 pounds), and ride hard doing competitive group rides and racing. In 12 years of using various CF seat posts, we've never had an issue, including full on out of the saddle sprints.

I would trust my safety to an ENVE CF seat post over some aluminum posts, particularly a lighter older aluminum post.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.

Last edited by merlinextraligh; 02-27-20 at 08:48 AM.
merlinextraligh is offline  
Likes For merlinextraligh:
Old 02-27-20, 09:22 PM
  #9  
joeruge
Full Member
 
Join Date: Jan 2006
Posts: 210
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 77 Post(s)
Liked 72 Times in 40 Posts
Rodriguez Fan

I hear ya. I guess I'm just a fan of Rodriguez philosophy, which, if you read their technical articles (and they have a lot of 'em), make a lot of sense. - to me.
joeruge is offline  
Old 02-28-20, 08:26 AM
  #10  
Paul J
Senior Member
 
Paul J's Avatar
 
Join Date: Sep 2012
Location: Upstate South Carolina
Posts: 1,066

Bikes: 1980's Spectrum 10 sp Campagnolo Centaur, 1990 Eddy Merckx 10 sp Campagnolo Centaur, Bushnell Tandem, Co-Motion Speedster Tandem

Mentioned: 5 Post(s)
Tagged: 0 Thread(s)
Quoted: 177 Post(s)
Liked 98 Times in 61 Posts
I've always been concerned about using a CF captain seat post as it has the stress of the stoker's stem. Something that just hit me is a full CF fork has the significant stress of a rider's stem on the CF steerer tube. That's basically the same type stress, now I need to be reconsidering all this. It would be interesting to know the engineering difference between a CF seat post and a CF steer tube. Any thoughts on that?
Paul J is offline  
Old 02-28-20, 12:05 PM
  #11  
joeruge
Full Member
 
Join Date: Jan 2006
Posts: 210
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 77 Post(s)
Liked 72 Times in 40 Posts
Maybe a new thread? Tandem rated carbon fork

Originally Posted by Paul J View Post
I've always been concerned about using a CF captain seat post as it has the stress of the stoker's stem. Something that just hit me is a full CF fork has the significant stress of a rider's stem on the CF steerer tube. That's basically the same type stress, now I need to be reconsidering all this. It would be interesting to know the engineering difference between a CF seat post and a CF steer tube. Any thoughts on that?
I don't have any experience with a carbon fork on any of my tandems. I was given an Mtb. tandem that came with a CF fork but was warned by the gifter that the fork was not rated for use on a tandem, so I haven't used it. So, once again, I would defer to a Rodriguez article on the use of carbon forks on tandems;
The lightest fork for a tandem bicycle - carbon fiber tandem forks

Now I know there will be some strong and divergent opinions on this, but their article makes a lot of sense. According to the article, at the time of its writing, there were no tandem rated forks. Any company making them, removed them from the market. Rodriguez seems to be most worried about braking forces, more so than the any damage caused by a stem. I would think that any carbon fiber fork used on a tandem would have an aluminum steerer. I supposed some custom tandem builders might decide to use a really light fork in some circumstances to make the weights of their bikes look a little more appealing. I don't where it stands now, but it's interesting reading.
joeruge is offline  
Old 03-02-20, 12:36 PM
  #12  
diabloridr
Full Member
 
diabloridr's Avatar
 
Join Date: Dec 2010
Location: Central Coast, California, USA
Posts: 434

Bikes: Co-Motion Macchiato, Calfee Dragonfly, Ancient Sun Fixie, Trek 5900

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 49 Post(s)
Liked 12 Times in 11 Posts
Originally Posted by joeruge View Post
Wow! That's pretty amazing. Never heard of an aluminum seatpost breaking. Was it completely unexpected or did you have any warning? Where did it break, where it went into the frame or near where the stoker bars attached?
Broke at the frame interface when I sat my fat @$$ down on it. Bemused stoker ended up holding her handlebar attached to the failed post. We were just cruising along through a neighborhood and were able to stop without any excitement.

Originally Posted by joeruge View Post
So, once again, I would defer to a Rodriguez article on the use of carbon forks on tandems;
The lightest fork for a tandem bicycle - carbon fiber tandem forks

Now I know there will be some strong and divergent opinions on this, but their article makes a lot of sense. According to the article, at the time of its writing, there were no tandem rated forks.
Co-Motion uses an in-house design (undoubtedly manufactured elsewhere) on their higher end tandems. I believe Santana does the same.

YMMV
diabloridr is offline  
Old 03-04-20, 01:58 AM
  #13  
torger
Senior Member
 
Join Date: Mar 2018
Posts: 130
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 32 Post(s)
Liked 22 Times in 10 Posts
A few seatpost comments. Lightweight seatposts may be butted, ie have thinner walls in the middle, alloy as well as carbon. That type of seat postis probably not an ideal choice when clamping a stoker handlebar. Carbon seatposts designed to be sturdy can be sturdy enough. About cheap Chinese seatposts - they are typically overbuilt and definitely does not have any wall thinning tricks, so I'd think that they hold up really well.

We've chosen to use high end alloy seat posts designed for mountain biking, without butting, the shannon hardcore seat post (the non-light version) which exist in several diameters and lengths, even up to 500mm.

Seat posts have minimum insert recommendation, and so have many frames. The frame minimum insert is not so easy to come by, can sometimes be found in the frame manufacturer's technical documentation. It's usually around 8-10cm. But obviously the heavier you are and the longer seat post extension you have, the more force is put on the seat tube / seat tube interface. The minimum insert has some margin, so a light rider with little extension can use a bit less insert.

Personally I like to have some margin though and like to be able to tighten down the stoker stem hard (to avoid slippage) without risking cracking the post, so I use sturdy long seat posts with lots of insert, especially since the tandem frame we have is really low making seatpost extension rather long (like on a MTB).
torger is offline  
Old 03-09-20, 04:02 PM
  #14  
pacificpt
Newbie
 
Join Date: Mar 2020
Location: Central Coast CA.
Posts: 2

Bikes: Burley Rock and Roll

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by diabloridr View Post
Interestingly, I broke the aluminum captain's seatpost on our old Speedster, likely through from high-cycle fatigue or corrosion-fatigue.

I'd worry more about component design than about the material it is made of.
How many years / miles did you have on the bike? A failure of that sort is not what I want to have happen.
pacificpt is offline  
Old 03-09-20, 04:17 PM
  #15  
pacificpt
Newbie
 
Join Date: Mar 2020
Location: Central Coast CA.
Posts: 2

Bikes: Burley Rock and Roll

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
For cruising around on our Burley Rock and Roll my wife prefers the elastomer seatpost. Not sure if we are losing efficiency, but if it makes it more comfortable she will ride more.
pacificpt is offline  
Old 03-10-20, 10:07 AM
  #16  
diabloridr
Full Member
 
diabloridr's Avatar
 
Join Date: Dec 2010
Location: Central Coast, California, USA
Posts: 434

Bikes: Co-Motion Macchiato, Calfee Dragonfly, Ancient Sun Fixie, Trek 5900

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 49 Post(s)
Liked 12 Times in 11 Posts
Originally Posted by pacificpt View Post
How many years / miles did you have on the bike? A failure of that sort is not what I want to have happen.
15 miles and 30,000+ miles.

We were (are?) an aggressive team so lots of standing and sitting to load the post.
diabloridr is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell or Share My Personal Information -

Copyright © 2023 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.