RoulezTandem

We found the single-bolt clamps failed to securely prevent rotation of the captain seatpost which holds the stoker handlerbar.
Yes, tubing is a good fit and we are grease-free.

Found these dual-bolt seatpost clamps and have used on two tandems now.
However to get adequate torque (I use torque wrench ±15Nm) takes them to the edge of stripping and we blow through one or two a season.

The steel bolt does not thread into the aluminum housing, as it would strip quickly.
They have cleverly inserted a piece of maybe 3/8" x 1" round steel bar stock, drilled and threaded crossways to receive the two steel bolts.
I find the bar stock threading is very sloppy and coarse for the fine-thread bolts.
One fix would be to have someone custom machine replacement bar stock.
At $10 a pop plus shipping sofar I keep several complete clamp sets on hand and carry one on the trail.

Very interested if others have found suitable alternatives?


Two-bolt 31.8mm seat post clamps: https://www.unicycle.com/kris-holm-do...t-clamp-black/

scycheng

We had this problem with our last tandem. The Salsa extra long armed single bolt clamp ended up deforming and cracking the seat tube. When our builder rebuilt the frame for me, I had him braze on double bolt seat clamp into the seat tube itself. No more problems after that. Not the cheapest solution but one that works.

canalligators

I’ve never really liked the usual design. Way back when I had a DF tandem, the captain’s post was only 1”. It usually stayed put, unless the bike fell and landed on the stoker bars. Though having the bars and seat move is better than breaking the frame.

BTW, those dual bolt clamps work well on recumbent risers.

bboy314

This homemade tandem sports a clamp made by Surly that doesn’t have 2 bolts, but one oversized bolt that you can tighten the heck out of without stripping threads. Though I ended up welding that second steer tube on behind the captain’s seatpost which negated the need for it anyway.

unikid

This handy stuff is made for carbon fiber seatposts and handlebars so you don't overtighten the clamp to get a proper grip:

Finish Line Fiber Grip Carbon Fiber Bicycle Assembly Gel: https://www.amazon.com/Finish-Line-C.../dp/B018FT9HOW

I would think if you put this on a regular seatpost it will also grab a lot better and require less torque. It is sticky and has little plastic beads or something in it that helps to grip the parts together.
It can't hurt to try the stuff out. It can be easily removed with rubbing alcohol. It works great on my mountain bike.

LV2TNDM

Something's amiss with your seat tube/post interface. Properly sized, you shouldn't have any issues even with stoker handlebars attached.

Have you taken calipers to both your seat tube and post? Confirmed they're correct? Your seat tube may very well be out-of-round, so be sure to measure at least two diameters (90 degrees or 45 degrees off). Another option is to try the next 0.02 mm thicker post to see if it fits. If out-of-round, you may find the larger post fits, even though it seems your, for example, 27.2mm post seems to fit snugly and properly. A Prestige tube hybrid bike I purchased came with a 26.8mm post and it didn't clamp very ideally - the seat tube pulled together FAR too much. So after properly de-burring the edges and flex-honing the seat tube, a 27.0 mm post fit perfectly. No issues whatsoever. Simply a manufacturing issue where they reamed the seat tube to the incorrect ID.

Another potential fix, despite being non-ideal, is to brinnell the seat post. This effectively increases seat post diameter and improves holding power of the clamp.

Personally, I'd NEVER go with a seat tube/post combination that could not utilize ample, waterproof grease. I don't like anti-sieze and "carbon prep" won't provide any corrosion resistance. Or at least not enough for my taste. And this point presents another example of pertinent experience: The Co-Motion tandem I purchased two years ago was in fantastic shape - but it was dirty and the owner hadn't really kept it up as one would expect. As a result, I discovered considerable rust in ALL of the frame tubes. The seat tubes were no exception. Luckily, the corrosion wasn't so bad as to cause hydrostatic welding. Luckily, this Co-Motion is coupled, so I was able to open up ALL the main triangle tubes and mechanically remove the rust. Then I used Naval jelly at the welds. Then applied Fluid Film & JP Wiegel throughout to prevent future rust. Both seat tubes, including the stoker post - with its shim for the Thudbuster (and therefore TWO surfaces capable of slipping), were liberally greased upon assembly. Had the owner done this, both seat tubes wouldn't have required flex-honing and rust removal.

Also consider that any steel frame will benefit from internal tube treatment. And all of these products are oils of some sort, so you're going to have lubricants in and around your seatposts anyway. Well, that is if you're concerned about future rust prevention.

I've noticed over the years that many seat collar clamps "over-clamp" before the seat post gets tight enough. In other words, their ID is too large for the OD of the seat tube. The remedy for this is very simple. Just shim your seat collar! I noticed my Cannondale collar pulled in very close when properly tightening. Although I had zero captin seat post movement issues, I was still unhappy with the obviously-incorrect collar fit. (This is now a 30 year old Cannondale road frame that was bead blasted and repainted when I obtained it used.) This solution worked wonders. The clamp now has a better "angle of clamp attack" while securing the seat collar binder bolt (it's a traditional slotted road bolt).

Also, seat clamps are designed with a certain frame tube and paint combination. You can see in your first picture where paint has come off of your frame and stuck to the inside surface of the collar. This shows where the layer of paint may be at issue. If your frame had very thick paint that has since rubbed off, then your collar may not be fitting the seat tube OD properly. Again, this is where a shim may be a simple and every effective fix. I'd even go further and reccommed you "over-shim" the collar a little bit so it has to stretch open a bit to simply slide over your bare seat tube. Then your clamp will have far more clamping range to properly grip your seat post.

I'd try this before trying to source the "perfect" seat collar with double bolts. A properly-functioning seat collar should easily clamp with enough force with one bolt AND while using lots of slippery, waterproof grease... AND not cause problems with stoker handlebars attached.

I currently own three tandems and none of them have had any sorts of issues with stoker bars causing the captain seat post to move. One tandem is a Cannondale mountain tandem that we ride pretty aggressively off road. Below is an example (yes, I'm bragging). In fact, this picture presents an excellent example. It took us a BUNCH of tries to get enough speed & momentum to get off the ground and onto the boards of the "Whiskeydrome." I slammed into the boards and had the front tire slip down over five times until I finally got up and onto the boards with enough speed where centrifugal/centripital force kicked in so we could stay up. Our stoker bars never twisted. But slamming my bars & shoulders into the boards repeatedly left me pretty sore!

LV2TNDM

Let me share one more anecdote...
My college roommate is now a professor of material science. We raced together at university. During his post-doc tenure at one of our national labs, he discovered that one of his Italian steel road bikes had a stuck seat post. He was a long-time cyclist and knew the importance of greasing one's seat tube. Plus, being a material scientist means he knew EXACTLY the molecular physics behind hydrostatic welding and how various metals interact, corrosion issues, galvanic electron transfer, you name it. So he and his lab mates figured, "Oh we'll get this frame fixed up, no problem!" Ha, ha, THINK AGAIN!!! They tried virtually every trick in the book and they simply COULD NOT FREE the seatpost. After many various attempts, finally those being destructive, they gave up in vain. Into the dumpster went the frame. (And BTW, he lived in a very hot & dry part of California, so it's not like he lived on a boat or in a salt-spray marine environment.)

So the moral of the story? Take seat post greasing seriously! Don't think it won't happen to you! Or at least pay some attention to the issue of hydrosatic/galvanic corrosion and welding. With a tandem it's so, so much more important, especially if it's a custom-painted frame with couplers!

merlinextraligh

I agree with LV2tandem. 2 bolts with 15nm to hold a seatpost. Typical torque spec for a seatpost collar is around 5nm. Something is not right with the seatpost frame interface.

15nm, and breaking one every year or so is not normal

RoulezTandem

Thanks all for the helpful feedback!
I agree, the diameters are off which is the root cause.

As I recall, due to unusual bike tubing diameters chosen for extra strength, standard seatposts did not fit well.
Builder machined a slip collar from aluminum stock which has a top collar 1/8" high that got powder coated to the steel frame. Not sure I want to remove it to make a thicker collar.
I looked at my torque ratchet and we are not tightening to 15nm, maybe 10 to 11 or 7 to 9 foot pounds. Not that much really.

Also, we used Framesaver product in all the steel frame sections and had product running out for a while, which lubricated the seatpost if the bike was laid down.
I drilled a tiny hole in the dust cap inside the S&S couplers so as to be able to blow compressed air through and push out/ dry out the framesaver.
I carefully clean and wipe the downtube and seatpost every so often but have not seen wet product for quite some time.

merlinextraligh

Shim from a beer can ought to work