Tandem seatpost sizes. GAAAAAH!

[roubaixtuesday]

Or... assumption really is the mother of all ****ups.

So, having had our rather wonderful tandem completely refurbed, my rather wonderful stoker has a conversation with another couple with a tandem.

The subject of suspension seatposts comes up. Our stoker seatpost has an ancient "post moderne" suspension post. Theirs has a Cane Creek Thudbuster, apparently the Rolls Royce of such gear.

I duly promise to get one. I am wary of seatpost sizes, so purchase a micrometer to properly check. The post moderne has a 25.4mm post, but with a shim. I check the pilot seatpost to get the right size. 27.2mm. Thudbuster of same size purchased.

Turns out said Thudbuster needs elastomers changing for my stoker, too svelte for standard. I spend some time this morning doing that. I remove old seatpost, and fight with annoying clamp to remove saddle. I fit saddle to new seatpost, carefully measure insert length and thoroughly grease.

I fit new seatpost to frame. Or, rather, I don't.

BECAUSE THE REAR SEATPOST IS A DIFFERENT SODDING SIZE TO THE FRONT ONE.

Who on earth ever thought that was a good idea??

The rear appears to be 26.4mm, a size I wasn't even aware existed.

GAAAAAAH!

[Brucey]

unlucky that, but not unheard of.

If you can't send it back it need not be a dead loss; someone with a lathe could turn it down to the exact correct size for the frame (which is possibly not 26.4mm, quite)

cheers

[Clipper_2018]

26.4mm is not unheard of.

I have a 26.4mm fluted alu seatpost branded ITM.

It was bought circa 1983 for a 1960s Carlton renovation, and I keep it in the hope of finding another, similar Carlton one day.

[peetee]

I expect the frame builder thought it was a good idea. I would imagine the tubes have been individually chosen to suit the riders and the bikes intended use. Granted, It's a shame that wasn't pointed out to you.

[roubaixtuesday]

Granted, It's a shame that wasn't pointed out to you.

The original frame designer did the refurb; a 30 second phone call would doubtless have given me the right answer. Alas, the sole person deserving of my ire is myself. Which makes it all the more galling, naturally.

[iandusud]

26.4mm is almost certainly a plain guage 531 seat tube. A very common size when PG 531 was in common use. 27.2 will be a double butted 531 seat tube. The frame builder probably thought is judicious to use the stiffer PG 531 on the rear seat tube on a tandem.

[iandusud]

26.4mm is almost certainly a plain guage 531 seat tube. A very common size when PG 531 was in common use. 27.2 will be a double butted 531 seat tube. The frame builder probably thought is judicious to use the stiffer PG 531 on the rear seat tube on a tandem.

[roubaixtuesday]

The frame builder probably thought is judicious to use the stiffer PG 531 on the rear seat tube on a tandem.

If I read your post correctly, it's the other way round; pilot is 27.2, stoker is 26.4.

I'm sure there was a good reason for it, doubtless along the lines of what you suggest.

Just not what I was expecting.

[AndyA]

unlucky that, but not unheard of.

If you can't send it back it need not be a dead loss; someone with a lathe could turn it down to the exact correct size for the frame (which is possibly not 26.4mm, quite)

cheers

I've done this with a Suntour suspension seatpost using a Colchester Student lathe. Tricky, and the result wasn't perfectly concentric because of my shoddy workholding setup. Seatpost works great and stoker is very happy with it however

[roubaixtuesday]

Alas, I lack either a lathe or the skills to use one.

More happily, Bikester have said they'll take the post back.

I've ordered a new 25.4mm one, and will use the shim I already have from the old 25.4mm poste moderne one.

What could possibly go wrong?

(If anyone wants the 25.4mm poste moderne one, it's free to the first taker)

[iandusud]

The frame builder probably thought is judicious to use the stiffer PG 531 on the rear seat tube on a tandem.

If I read your post correctly, it's the other way round; pilot is 27.2, stoker is 26.4.

I'm sure there was a good reason for it, doubtless along the lines of what you suggest.

Just not what I was expecting.

No you read my post correctly. 36.4 is PG 531 which is stiffer than DB 531.

[roubaixtuesday]

Finally, job done and gets top rating from stoker too.

[Raleigh Steve]

All’s well that ends well!

Lovely looking frame that.

[reohn2]

All’s well that ends well!

Lovely looking frame that.

+1
Glad you're finally sorted Brian

[roubaixtuesday]

All’s well that ends well!

Lovely looking frame that.

Thank you!

'tis a thing of beauty I think.



Second hand to us, originally supplied by Swallows but Rob at the Tandem Company (who also ran Swallows) tells me it was probably actually made by Chas Roberts as they subcontracted a few.

The paint job is brand new, the bike about 25 years old. The "Toucan" decals are a copy of the original designed by Rob's mum.