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Are seatpost shims safe to use from the perspective of the frame, seatpost, and the rider?

My concern is that seatpost shims only support the seatpost for a few centimeters - while a properly matching seattube will support the seattube for the entire depth. Would this increase stresses on the seatpost or the frame?

Seatpost shims

And would this be an argument for avoiding short shims and trying to get as long as possible?

Seat shims

Related but with no relevant answers: Is it safe to use aluminium seatpost adapter (27.2mm to 31.6mm) for a carbon frame?

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    My experience is limited to only using them on steel frames, but your collar, on commonly available TT and MTB and Road Bikes of any frame material only constricts over a very limited area. The rest of the seatpost/seat-tube has no contact. (e.g. no strength there, if there is, your seatpost is stuck and if they intermittently--touch it'll rub a holein your seattube!) It really is more of a nuisance, IMHO... get the right size from the beginning for least headaches... I've had some problems with the seatpost creeping down with heavy impacts (hitting a pothole hard, etc...) – david1024 Feb 20 '17 at 21:56
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Longer is better – it helps stop rattling if the seat post is longer than the shim and banging the seat tube, although if that happens you can always cut down the set post.

As far as safe to use – as long as the shim is at least as long as the minimum seat post insertion required for the frame, in theory you are stressing the frame no more than using the correct diameter seat post. Many frames do require 100mm or more, and many seat post shims are not 100mm.

On carbon I would hesitate – if you spend that much on a frame, you can afford a new seat post, but some posts (e.g. MTB Droppers) come in limited sizes so you don't get much choice: new bike or shim.

Factor in your own weight (I am a lighty) and riding style (I am a wimp on the MTB) and decide how much stress you put on the frame compared to others. Would I use shims? Certainly, on steel and aluminum frames, and probably on carbon if I had no other option. If a 100kg 20yo downhill rider asked if he should shim his off-season Carbon XC race bike to put on a dropper, I think I would suggest not to.

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