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I recently purchased a used tandem.

The stoker's seatpost is 27.2 mm diameter, and I'm not having any trouble with that.

The captain's seatpost is a weird suspension seatpost that is less than 27.2 mm, and is shimmed to fit the seat tube; the stoker stem also has a shim between it and the captain's seatpost, and doesn't get a good grip on it.

I am investigating replacing the captain's seatpost with a conventional one, but it seems that after 20+ years, the seatpost binder has compressed the opening down to 27.0 mm--it will not fit a 27.2-mm seatpost, and my micrometer reads closer to 27.0 than 27.2. It's a brazed-on binder, not a separate collar, and the binder "ears" are extremely close together.

I'd like to loosen this up to fit a 27.2-mm seatpost, but am not sure of the best way to do that. I suppose it is possible that the framebuilder (no longer in business) really did build this for the captain to use a 27.0-mm seatpost, but that seems very unlikely. The frame is steel (853).

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  • I'd bin the suspension seatpost - they tend to be utterly dreadful. It could be elastomer-based too.
    – Criggie
    Dec 31, 2021 at 20:58
  • @Criggie: I only know the Thudbuster suspension seatposts, I thought they were solid quality?
    – Michael
    Dec 31, 2021 at 21:12
  • I've got a Redshift suspension seatpost on my distance bike that's quite nice, but this one…I could write a short dissertation on how weird it is.
    – Adam Rice
    Dec 31, 2021 at 21:20
  • @Michael the one I had was all inside the cylinder of the seatpost with none of the geometry you'd see in a thudbuster. Something like "how do suspension seatposts work and for what scenarios are they useful ?" might be a good question.
    – Criggie
    Dec 31, 2021 at 21:35

2 Answers 2

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The frame is steel, so it should survive the binder being stretched open by 0.2mm to 27.2.

Putting a "27.0mm" seatpost in is a bad idea because the post is clamped by the clamp, but also contacts down inside the seat tube. A smaller seatpost will loose contact earlier.

Also consider that the stoker's handlebars are probably attached to this seatpost, so there's more potential force to be applied here than a regular bike seatpost.
I bet the stoker felt weird when their handlebar's suspension flexed.

Worst case possibilities, the clamp snaps when you stretch it or the frame cracks at the top (very unlikely). Then I'd attach a normal "over the top" seatpost clamp, or get it repaired by a competent frame builder.

You might even choose to take the clamp off with heat (or get that done) and fit a conventional clamp - depends how much seattube you have to work with.

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Your more likely failure is the front seatpost slides while riding, upsetting both riders. The frame may be taking gradual damage too due to flex and friction.

Shims are a necessary evil sometimes, but the right-sized seatpost is always best where possible. Tandems support more weight and higher power than separate bikes, they do require good parts done right. That means thre brakes, higher spoke wheels, good chains/cassettes/chainrings, and seatposts that do their job all the time.

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Pry the slot apart with a large flat screwdriver or small cold chisel, inserted from the inside, until the slot is parallel again or slightly flared. Take off any rough edges with a half round file and wrestle the 27.2 seat post in (don't hammer it). You may need to wiggle it around when slightly inserted to correct any ovality in the tube, but there shouldn't be much distortion. If it takes too much effort to fit the seat post you might want to have a frame builder (or well equipped mechanic) run a reamer down the tube, but they shouldn't need to remove much material.

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