samkl 

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

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

Considering the extra force on the post coming from from the stoker’s 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 it’s fine, but just want to double check.

merlinextraligh

It’s fine

joeruge

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.

diabloridr

Then why does Co-Motion (to cite one example) specify carbon fiber posts on many of their tandems?

joeruge

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.

diabloridr

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.

joeruge

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.

merlinextraligh

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.

joeruge

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.

Paul J

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?

joeruge

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.

diabloridr

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.

Co-Motion uses an in-house design (undoubtedly manufactured elsewhere) on their higher end tandems. I believe Santana does the same.

YMMV

torger

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).

pacificpt

How many years / miles did you have on the bike? A failure of that sort is not what I want to have happen.

pacificpt

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.

diabloridr

15 miles and 30,000+ miles.

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