Tandem Cycling - Seat post diameter poll

Hey folks,

What do you think is the most common diameters of seat posts on tandem bikes? Current bikes, or the past 7 years or so.

The actual answer for premium brand tandems is probably 29.8, given that Santana, Co-Motion and Burley (when they were making tandems) all adopted that size as their standard, followed closely by 27.2 since that's what Cannondale, Trek, and most of the boutique & custom builders like Rodriguez, Calfee, Seven, Ventana and others use. I'm just assuming that Santana & Co-Motion sell more tandems each year than C'dale, Trek and the others combined, but that may not necessarily be the case... hard to know since I'm still not sure how many tandems C'dale and Trek sell each year. The 'old' standard was 26.8 and that's what you'll still find on Bilenky's tandems.

Regardless, 27.2 still wins since you didn't include 29.8 in your poll.

27.2 also happens to be the most common size that's been found on our personal tandems:

28.9 = 95/96 Santana Arriva
27.2 = '98 Erickson Signature
27.2 = '98 Cannondale MT3000
27.2 = '00 Ventana El Conquistador
27.2 = '02 Erickson S&S
27.2 = '02 Ventana El Conquistador
27.2 = '08 Calfee Tetra S&S

The actual answer for premium brand tandems is probably 29.8, given that Santana, Co-Motion and Burley (when they were making tandems) all adopted that size as their standard, followed closely by 27.2 since that's what Cannondale, Trek, and most of the boutique & custom builders like Rodriguez, Calfee, Seven, Ventana and others use. I'm just assuming that Santana & Co-Motion sell more tandems each year than C'dale, Trek and the others combined, but that may not necessarily be the case... hard to know since I'm still not sure how many tandems C'dale and Trek sell each year. The 'old' standard was 26.8 and that's what you'll still find on Bilenky's tandems.

Regardless, 27.2 still wins since you didn't include 29.8 in your poll.

27.2 also happens to be the most common size that's been found on our personal tandems:

28.9 = 95/96 Santana Arriva
27.2 = '98 Erickson Signature
27.2 = '98 Cannondale MT3000
27.2 = '00 Ventana El Conquistador
27.2 = '02 Erickson S&S
27.2 = '02 Ventana El Conquistador
27.2 = '08 Calfee Tetra S&S


Wow! Okay, not much reason for a poll after a thorough answer like that! :thumb:

Our mountain bikes use 27.2, 30.9, and 31.6, and we see a tandem in our future one day, so your info is helpful before buying a nice seatpost. I guess I'll look for 27.2mm and can shim it later.

... too many seatpost sizes! :mad:

the follow up question would be whether one or the other has any advantage?

Given the tons of bikes with 27.2 seatposts, presumably there's an avilability advantge to 27.2.

The larger seatpost diameters would arguably have a strength/ rigidity adavantage (particularly for the Captain's seatpost attaching the stoker.) However, if in practice the larger opening is just shimmed down to fit a 27.2 post any theoretical advantage is gone.

All that really matters is that the seatpost fits the frame. That is the correct (and most popular) size.

^ true from the consumer's point of view. Just curious what's driving the manufacturer's decisions.

^Probably depends on what tubing diameter they use for the seat tube and whether or not they want to do anything special at the clamping area.

Wow! Okay, not much reason for a poll after a thorough answer like that!

What was the purpose of the poll before the answer?

Merlin: I think if you check you will find your bike has seatpost ID 31.8mm
with a sleeve allowing the AlphaQ 28.6mm to be used. I found this to be
the case on our Macchiatto when I changed the seatpost to the set
forward Thompson available in 27.2mm only so had to buy a new sleeve.
I covered the bases by buying a 31.8/27.2 and a 31.6/27.2 sleeve and
opted for the slightly smaller one as the other required more force to
install than I felt comfy applying (worried more about removal later).
The CoMo booklet sent with the bike says ST ID is 29.8mm but that
was clearly an error.

^ I believe you're correct. They're 28.6 seatposts with a shim.

For some reason when our bike shipped, they shipped 2 different shims, and obviously only one worked. The LBS discovered that about 2'oclock in the afternoon, and Co-Mo had the right overnighted and there by the next morning.

Hey folks,

Thanks for the discussion. Looks like life isn't much different concerning seatposts with tandems as it is with mtn bikes.

The purpose of the poll was just to get an idea of what is most common on people's tandems. I didn't realize someone would know so much about the variety of bikes to answer that well. I was imagining a bunch of you only knowing "well, I use a 27.2" for example.

Anyway, she wants a longer post and I've heard good things about CF posts absorbing high-frequency chatter. But those aren't very cheap so I figured it'd be nice to have the option to one day move that post onto the stoker's post of a tandem.

I bought a USE shim that claims to be 100mm long, so I think that should give it decent support, despite being shimmed.

Good to know 27.2 is common. I'll be going that route.

I think the idea of the larger diameter has to do with the front post also supporting a handlebar. Seems reasonable especially considering more expensive models have carbon posts. On the rear of our tandem, we ended up removing the suspension post and replacing it with a carbon post that was 27.2 using the appropriate sized adaptor shim.

While I know there are tandems out there that use 27.2 posts for the captains saddle and possibly a carbon post that is not too light weight without any safety issues, I'd imagine the larger post diameter would flex less while climbing out of the saddle.

I think the idea of the larger diameter has to do with the front post also supporting a handlebar. Seems reasonable especially considering more expensive models have carbon posts. On the rear of our tandem, we ended up removing the suspension post and replacing it with a carbon post that was 27.2 using the appropriate sized adaptor shim.

While I know there are tandems out there that use 27.2 posts for the captains saddle and possibly a carbon post that is not too light weight without any safety issues, I'd imagine the larger post diameter would flex less while climbing out of the saddle.

Huh, I didn't think of the larger seat tube size helping stiffness. I bet you're right! I always thought the larger seatposts were a marketing ploy that they were stronger, lighter overall, etc. But 27.2mm posts were always capable of being strong enough while being very light.

But I bet you're right that the seat tube size is determining the seat post size, not vice-versa like I assumed. Still not sure about for singles, but I guess a larger dia. captain's seat tube could really help firm up a tandem.