Author Topic: 30.9 to 31.8 seatpost shim?  (Read 2422 times)

rogerzilla

  • When n+1 gets out of hand
30.9 to 31.8 seatpost shim?
« on: 11 November, 2018, 06:38:23 pm »
Anyone know where to get one?  30.9 to 31.6 is common but Moulton (and Brompton) use the weird-ass 31.8 size. There are really good 30.9 posts available for not much (much less than 27.2 or 28.6 posts, for which 31.8 shims are readily available).

I suppose 31.6 plus a coke can shim would be 31.8 - cans are usually 0.1mm.  At least, I used one to get a JIS cup in an ISO head tube (30.0/30.2)
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

robgul

  • Cycle:End-to-End webmaster
  • cyclist, Cytech accredited mechanic & woodworker
    • Cycle:End-to-End
Re: 30.9 to 31.8 seatpost shim?
« Reply #1 on: 11 November, 2018, 07:10:00 pm »
Use a Diet Coke tin - much lighter than the regular

IGMC

Rob

Gattopardo

  • Lord of the sith
  • Overseaing the building of the death star
Re: 30.9 to 31.8 seatpost shim?
« Reply #2 on: 11 November, 2018, 09:17:01 pm »
I know that decathlon sell adaptors with their seat post https://www.decathlon.co.uk/272-33mm-suspension-seat-post-id_8350484.html  maybe there is a cycle shop selling something like that.

Chris N

Re: 30.9 to 31.8 seatpost shim?
« Reply #3 on: 11 November, 2018, 09:25:51 pm »
Make your own from some 26 gauge (0.45 mm) brass sheet.

LittleWheelsandBig

  • Whimsy Rider
Re: 30.9 to 31.8 seatpost shim?
« Reply #4 on: 11 November, 2018, 09:27:42 pm »
If you ask the factory, you may be able to get a TSR seatpost shim for a 31.6mm seatpost that replaces the normal 31.8mm shim.
Wheel meet again, don't know where, don't know when...

rogerzilla

  • When n+1 gets out of hand
Re: 30.9 to 31.8 seatpost shim?
« Reply #5 on: 11 November, 2018, 09:43:25 pm »
Thanks - have emailed them.  The only lightweight 31.8 post is a £100 Tune one!

Is it by any chance a 34.9mm seat tube without the shim?  30.9 to 34.9 shims are available although that would imply a 1.75mm wall thickness*, which is a bit excessive.  The existing shim may be glued in - it's certainly not interested in being hooked out, or I'd have measured it.

*powdercoat may be 0.5mm thick
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

LittleWheelsandBig

  • Whimsy Rider
Re: 30.9 to 31.8 seatpost shim?
« Reply #6 on: 11 November, 2018, 10:39:01 pm »
The TSR shim is quite thick and has an external flat at the front, to avoid problems with the bidon mounts. My TSR shim just pulls out.

I strongly suspect there would be enough heat distortion from the hairpins that a larger seatpost without shim would jam.

HKs Jubilee prototype has a larger seatpost and no shim but the frame is silver brazed, rather than the TSR's brass brazing.
Wheel meet again, don't know where, don't know when...

Re: 30.9 to 31.8 seatpost shim?
« Reply #7 on: 12 November, 2018, 12:20:47 am »
shim should come out with a slide hammer if it doesn't slide out easily. Seat tube can be reamed to remove any distortion from welding.

A 1.75 shim wall thickness wouldn't worry me.

cheers

LittleWheelsandBig

  • Whimsy Rider
Re: 30.9 to 31.8 seatpost shim?
« Reply #8 on: 12 November, 2018, 07:18:48 am »
Brass brazing the hairpins and monosphere housing tends to bend the seat tube. Reaming it quite often isn't enough.
Wheel meet again, don't know where, don't know when...

rogerzilla

  • When n+1 gets out of hand
Re: 30.9 to 31.8 seatpost shim?
« Reply #9 on: 12 November, 2018, 08:14:34 am »
I think a headset crown race remover under the lip will take the shim out neatly - as well as grabbing it, it will compress it and stop it gripping the seat tube.

I'm hoping it's engraved with the OD, as it would be convenient to just replace it with a 34.9 to 30.9 shim if 34.9 is the "raw" size.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

Re: 30.9 to 31.8 seatpost shim?
« Reply #10 on: 12 November, 2018, 10:13:59 am »
Brass brazing the hairpins and monosphere housing tends to bend the seat tube. Reaming it quite often isn't enough.

On a TSR the shim is what, about 100 mm long? If so it doesn't extend as far as the hairpin or monosphere mountings, only past the bottle braze-ons.

https://www.sjscycles.co.uk/seat-posts/316-use-seat-post-shim-309-mm-id/

is available in 34.9mm OD and 30.9mm ID. I think this one is aluminium (some other USE shims are plastic). Dunno if there are other options in this size combination or not.

cheers

LittleWheelsandBig

  • Whimsy Rider
Re: 30.9 to 31.8 seatpost shim?
« Reply #11 on: 12 November, 2018, 10:47:36 am »
A potentially bent seat tube at the top hairpin is why the shim stops above it. The fairly large difference between seatpost OD and seat tube ID allows the end of the seatpost to not jam against the front of the seat tube.
Wheel meet again, don't know where, don't know when...

Re: 30.9 to 31.8 seatpost shim?
« Reply #12 on: 12 November, 2018, 02:58:22 pm »
I have a two part shim fits any size that you can have for a few £. Its effectively two tapers. I will check tonight whether it will do your size.

rogerzilla

  • When n+1 gets out of hand
Re: 30.9 to 31.8 seatpost shim?
« Reply #13 on: 12 November, 2018, 06:56:21 pm »
The crown race remover took it out painlessly.  It's a 36mm seat tube and a 2.1mm shim, according to my calipers.  I was hoping it would be a 34.9 tube.  A 30.9 to 34.9 USE shim (same length as the OEM one) with an additional 0.5mm thick shim around it is probably the best option.  That's just a 100 x 100 sheet of aluminium rolled up.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.