Would you shim a carbon seat tube?

tim_wand

My Carbon frame has a 31.6mm post, I thought I would shim it to 27.2 and fit my Spesh Pave Sl seatpost as it takes out a lot of buzz and gives great compliance. ( I m an old man ).

However the Mech in my LBS reckons I run the risk of splitting the seat tube.

Any one done this , or have any thoughts which will make me a bit more confident about stepping it down.

pinno

[Edit]

http://www.ebay.co.uk/itm/Cycle-Seatpost-Shim-Bicycle-Seatpin-Shim-Alloy-RRP-8-99-/400254651791?pt=UK_sportsleisure_cycling_bikeparts_SR&var=&hash=item5d31094d8f

tim_wand

Shimmed down a 31.6 seat tube to take a 27.2 seat post, but mechanic in the LBS told me this would stress the seat tube. I ve got a 31.6 carbon seat post ( OEM ) but find it a bit harsh. Always got on with the Zertz insert Spesh Pave Sl seat post in other frames , so got the shim to fit this in current frame.

Now worried it might cause the seat tube un due stress because of what the grease monkey said.

The Shim inserts about 50mm, the actual seat post about 100mm when set at the right ride height for me.


Yep got your edit now Pina, What you ve shown above is what I ve used. But the LBS doesn't recommend it, says shiming the seat tube will cause it stress . Not looking to rectify a fault , just looking to run a narrower circumference seat post, I thought this was common practice.

pinno

tim wand wrote:

Shimmed down a 31.6 seat tube to take a 27.2 seat post, but mechanic in the LBS told me this would stress the seat tube. I ve got a 31.6 carbon seat post ( OEM ) but find it a bit harsh. Always got on with the Zertz insert Spesh Pave Sl seat post in other frames , so got the shim to fit this in current frame.

Now worried it might cause the seat tube un due stress because of what the grease monkey said.

The Shim inserts about 50mm, the actual seat post about 100mm when set at the right ride height for me.

I edited my drivvle.

I may have a spare 27.2mm carbon seat post.

keef66

I have a CR1-Sl with a 31.6 mm seat tube and was thinking along the same lines as you, but in their literature Scott specifically advise against using shims. Not sure I want to risk it...

yakk

Hi there, just my experience. I use a shim for a 30.9 seatpost up to 31.6 (carbon frame and post) without problems and have used a shim for a Ti seatpost in a carbon frame (27.2 up to 31.6) without problems.
Yak

crankycrank

Try contacting Specialized directly. They may have a different opinion than your LBS. I wouldn't think a shim would be a problem but possibly a short shim will not give the same structural support as a longer seatpost matched to the same dia. seatube.

fleshtuxedo

If the shim is shorter than the seat post, use 2 shims? You could remove the lip from the second one if necessary. That gets the correct width on the whole post. I would happily do that on a frame of mine and I wouldn't ask the frame manufacturer or anything like that. As long as there is a sufficient length at the full diameter it will surely be fine.

nicklouse

shims are made to the general minimum insert length so there is no issue using one.

often it can be advantageous as it keeps the carbon away from any irregularity that can create failure.

it also can help with any removal issues.