XXLHardrock

Looking at possibly shimming a 27.2 seatpost to a 29.8 hole. Cane Creek makes a shim thats 100mm long. Thought I would use 2 of them and cut the collar off one so that there would be more seatpost in the seat tube.

Your thoughts?

3alarmer 

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...I don't like them, personally won't use them. But if you are going to do so, extending the length of it would be the way to go.

My issue is that there is so much going on in the seat post / seat tube interface that I don't want to further complicate things, and that you can buy the proper sized, relatively functional and well made seat post online for less than 20 bucks if you go with one of the Kalloy posts.... so why bother.

hockey

No Problem. In fact my Giant mountain bike came standard with a seatpost shim. Most shims are the length of the safe insert point on most seatposts.

DiabloScott

I don't understand the part about using two shims - how would you keep the one without the collar from slipping off?

Otherwise, if you have the perfect shim size it's a fine solution and you'll probably forget it's even in there.

xenologer

Shim is fine, But only use ONE that is the correct size
don't try and double up because it's the wrong thickness.
for one, the curvature will be wrong....
and cutting the collar off, I'd worry about it slipping down.

FBinNY 

There's no reason to double up on the shims. They're longer than the minimum insertion depth so you'll get no benefit from the second, lower one.

Your post in a 100mm shim (3 diameters +) is the same as any post inserted 3 or more inches into any frame. These guidelines aren't pulled from thin air, and are based on the depth where close to 100% of the theoretical strength is attained. Or to put it another way, there's no gain (none, zero, zip nada) in support or rigidity if you go deeper.

fietsbob

I have done precisely this, my Bike Frriday Tikit came with a machined shim in its steel seat mast, to use a 27.2 seatpost..

they used a Kalloy Uno post that slipped its toothed angle grip,, AKA craptastic.. it had to go..

I had left from my prior AlAn frame .. they have a 25.0 seat tube ID. For that bike I bought a Campagnolo Seat Post..

sold the AlAn, to use that post in another bike, for a while, I got a 25.0 to 27.2 machined shim made by the British USE company.

It later went in a drawer, and was retrieved to put inside the 27.2 ID shim , with the seat post ..the post a Campag super record

A much better execution of design, its 1 bolt design does not slip .. it now works well ..


Both are machined, with a lip.. this is not what I understand the OP is thinking of..

perhaps they want to hire a shim machined that is 200mm long?





....

3alarmer 

...this makes little sense. A long seatpost inserted into a seat tube will increase the stiffness (rigidity) of the seat tube/seat post combination for whatever distance it is inserted. (If it is the correct size and slip fit in the the seat tube). If you are talking about something else, like the rigidity of the seat post and saddle position, that's a different horse color.

Other than not having to cut the post, it's a good reason for keeping a post long, if you're interested in further stiffening what is already one of the stiffer tubes in the frame.

maddog34

how much will those shims cost? a decent post of the correct size is like maybe $20.

and the frame's seat tube is plenty strong enough without a seat tube rammed all the way down into it... first time you re-adjust seat height, the trimmed shim will be forever lost into the frame... bad plan.

one shim is fine,,,, two is a joke, plain and simple.

FBinNY 

Yes, the seat post will stiffen the tube somewhat, but not as much as you expect because of the mismatch in the modulus of elasticity. An example showing how little the post helps can be seen in the number of tall seat tube frames that crack despite the post being inserted well past the break.

In any case, frames and posts are (supposed to be) engineered such that the worst case (tall seat post inserted the minimum amount) functions well withing the design objective. If the designers were counting on any boost in stiffness from post/tube overlap, they'd shortchanging their taller and/or heavier riders who need that added stiffness the most.

XXLHardrock

Thanks for all the replies.

The thought of using two shims was to use them end to end with a longer than necessary seatpost in order to hopefully strengthen the whole assembly.

The whole reason for going down this road is that I need a 29.8 post with sufficient length and enough setback to accommodate the short rails/ adjustment range of a Brooks saddle.

What I'm looking for is easily found in a 27.2 diameter but in a 29.8, not so much. Not at all actually. Except for the OEM post which works but has a reputation for bending.