Weird seatpost sizing

[Bluetree]

Question for you wrenches out there....


My CF frame requires a 31.0mm seatpost. I'd like to get the Thomson Masterpiece but their stock size is 30.9.

Will that .01 variance make any difference of should I be looking for a smaller-diameter seatpost and use a shim?

Thanks!

[wolfak]

Ok, just to be clear the difference is 0.1, not 0.01 - a lot larger and will likely make a difference. I think there may be some sort of insert that could help. I'd say go visit your friendly LBS to see what they might know if someone on here doesn't know.

[Pedal Wench]

You can try it, but be careful about overtightening to get it to work -- you could crack your frame.

I have a bike that takes an incredibly rare 31.4 post. Even with a shim, some seatposts worked, some didn't. You might get away without needing a shim, because I've seen a great variety within the same sizes.

[aicabsolut]

I've seen 31.6 but not 31.0 ...

This was the only shim I could find http://www.excelsports.com/new.asp?page=8&description=SX+Seatpost+Shim&vendorCode=USE&major=1&minor=19
Not very helpful.

[Bluetree]

You can try it, but be careful about overtightening to get it to work -- you could crack your frame.

I was afraid of that too, not sure if a new collar would work, either. I've been eyeing a new Campy collar and may try that out.

Yeah, 31.0mm is weird. I don't know why they didn't just make it 30.9mm but some boutique builders seem to relish their "non-standardness" :rolleyes:

I may ask around my club to see if anyone has a spare 31.0mm to test out before I drop coin on a new one.

'Nother question... how can I tell if it would work? .1mm is less than the width of a human hair. Can something "appear" to be on tight enough then slip once you're 50 miles from home? Can a torque wrench give you the most accurate reading?
:confused:

[aicabsolut]

I was afraid of that too, not sure if a new collar would work, either.

'Nother question... how can I tell if it would work? .1mm is less than the width of a human hair. Can something "appear" to be on tight enough then slip once you're 50 miles from home? Can a torque wrench give you the most accurate reading?
:confused:

The collar goes on the outside of the frame, and while some collars may extend up higher on the post than others, tightening the clamp down on the post will also tighten it on the tube. There is a very good risk of cracking your CF seat tube.

As for the other questions, yes. Even a correct sized post may slip over time for one reason or another. A torque wrench is the best, but there are degrees of accuracy among different wrenches. The best one I've seen is one (I think Park Tools makes it) where you pre-set the torque, and then when you start tightening the bolt, the wrench itself will not let you tighten past the set torque. Some people say just to go to Sears and get a Craftsman torque wrench.

Then there's a mini torque key for things like CF bars and stems where minute readings and low torque are more crucial.