Anyone out there have experience running a smaller diameter post in a larger frame with a seat post shim?

My SuperSix came with a 31.6 post which I'm about to replace. Two questions:

**Edit**
1) In general does a 27.2 post weigh less than the same post in 31.6? Hard to tell browsing the web as the claimed weights are usually for just one size. Seeing as how the USE thermoplastic shim is a claimed 25g, it would seem that a 27.2 post w/ shim would be the way to go if the weights are substantially different.

2) Will it suck? My LBS said "don't do it", claiming in their experience the bottom of the post (I assume the bit sticking out past the shim) will rattle around in the frame.

I have a nice EC90 post from an old steel bike I'd love to run as it is 171g (uncut) + 25g shim would give me a decent weight post for "free".

Looking for feedback!

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Fear is healthy, panic can be deadly

I have a 08 Cervelo R3 which was a real wiered size. I went to 27.2 and have never had a issue with it. Never had heard any ratteling that your LBS has described. I do have a Di2 battery in seat post maybe that prevents it from ratteling.

I've been using a USE thermoplastic 31.6 to 27.2 shim for 3 years in one of my frames without any problems. My frames aluminium and I didn't want to use an aluminium shim in case I had problems with it bonding to the frame and / or seatpin. I've had no bother with it. No creaking at all or rattling. I use carbon paste between the pin and shim.

I used to do in back in 90-tees with my first Giant TCR (silver and still have it) but as I loved that bike I would not use the shim now.

No need and do not see any reason. It gathers dust, dirt etc...since then no more shims.

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IRONMAN 70,3 TARGET TIME 03:55

I also use the USE themoplastic shim and have being doing so for a year. No issues at all!

I have the USE shim in my PedalForce QS2. I prefer 27.2mm posts and the shim works great. The one drawback is that there's a small gap in the shim and if you ride in the rain small amounts of water and grit can go down it. It's no big deal just ends up with more crud than a regular non shim installation. I've infrequently run rags down the seat tube to clean it up.

Excellent - thanks for all the replies - I will order up a USE shim and see how it goes.

Thanks!

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Fear is healthy, panic can be deadly

The biggest advantage to a 27.2 post isn't even the weight savings. It's the added comfort. 31.6 posts are much harsher riding than 27.2 (in an equal post, please don't compare a carbon 31.6 to an alloy 27.2)
I used to use the USE shim on my scapin. No issues at all. If you have fibre grip or another such carbon paste around, use it. It helps. Also, if you turn the slot so it faces the front of the frame, you won't get as much crud in it.

Where are you getting a weight of 5g?
That's a manufacturer's claimed weight of 26g for a 27.2 to 31.6 shim.

So 25g per FWB. Either way, that's not 5 g.

Your figure of 5g appears to be based on the 25.0 to 25.4 or 27.2 to 27.8 aluminum shims, which are listed at 5g.

My USE thermoplastic shim (31.6-27.2) worked like crap. Had to tigthen it in ways that didn't feel safe to prevent tbe seatpost from slipping. I am now using a no name alu shim and it works great.

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Thermoplastic shim is slippery, especially against use's own hard anodised shockposts. But now there is grip paste (carbon paste, fiber grip, whatever), so that should be a non-issue now.
31.6-27.2 should pose zero "rattling" issues inside the frame, the gap inside is too big to have the post touch the seattube inner walls. It sometimes happens with long posts in certain frames without any shim...
Another reason the USE shim is a good choice is its insert depth-100mm i think.

I liked the 5g weight better . But as they say, reality is what does not go away even if you choose to ignore it. Thanks for the correction.

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Fear is healthy, panic can be deadly

I'll try the paste for sure. I'm trying to reuse an old post here, so my investment is limited to the $10 shim. If it does not work out I'll sell the post. As for he "rattling" I thought the same thing - the gap will be way to big for that post to ever flex enough to hit the seat tube.

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Fear is healthy, panic can be deadly

About the "rattling" a few members have been talking about. The length of seatpost under the shim, where there could be "rattling", is useless isn't it? It doesn't help support anything, so why not cut it of? Free weight saving...

You read my mind - that's what I'll be doing. Seeing as how I made a 20g miscalculation on the shim weight I'll be trimming the post and ordering a Ti EC90 bolt kit.

Update: just got the shim and it came in at 20g, 5g lighter than advertised. The surface is textured, plus carbo grip and we should be good.

Trimmed the post for another 10g off, bringing the whole thing to 181g. Ti hardware will bring it down to 170g. She ain't Schmolke light, but I can put the savings into other parts.

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Fear is healthy, panic can be deadly