stilltooslow

I'm trying to be a bit thrifty by using a 31.6 - 27.2 shim to save the cost of a new seat post for a new frame. Do they have a greater tendency to shift or creak? I hate to compromise, but if it holds just as well then what the heck.

I have a nagging sense that with the smaller contact area, there may be an issue.

Opinions?

redtires

I've used them extensively with a U.S.E. suspension post. This was on mountain bike and lot's of good, bumpy and technical trails and never had any issues. Just get a quality one and you should be just fine.

rdtompki

Just don't DIY. The purpose-made shims provide more than enough support for the seatpost.

Bob Dopolina 

What? No tin cans?

jan nikolajsen 

The big jump from 27.2 to 31.6 should give you no problems. The paper thin shims that add a mm or less are, on the other hand, a load of trouble. But you don't need one of those.

A good quality shim will have a lip on top keeping it from descending into the seat tube.

sbxx1985 

You'll be fine.

fietsbob

Machined shims used several

after I found a buyer for my AlAn I had 25.0 seatpost left
shimmed it to 27 to use in an RB1

now I put a thudbuster in my Koga, 31.4 to 27,2 shim was bought..

make sure its long enough insert , in the seat tube like the Min Insert line on seatposts.

Brannigan

HonestOne

I used the outside of an old 9V battery to make a seat shim and padded it with duct-tape to make it thicker. I never had a problem with it.
- Aaron

rpenmanparker 

Beauty of using the shim is retaining the extra flexibility (if you can call it that) of the smaller diameter seat post. Reputed to be more comfortable than the 31.6 mm post. It took me a long time to get accustomed to the feel of a larger diameter post (same model post and saddle as all my others) on a new bike a while ago. Frame stiffness was not all that different, but the larger diameter setup felt like I sitting on a brick. Finally adjusted to it. You will avoid all that with the shim. As advised above, just use a commercially supplied model.

ultraman6970

+1

mihlbach

I have used beer cans on numerous occasions for slight mismatches....seems to have always worked to me

RollCNY

Corollary question:

If you use a shim on a carbon frame to adapt a smaller aluminium post, do you eliminate the chance of galvanic corrosion seizing the seatpost to the frame? Would you only seize the shim to the frame? Seems like it might be a nice means of eliminating a potential problem.

merlinextraligh

Our aluminum Co-Motion tandem came with carbon fiber seat posts, and aluminum shims. Captain's seatpost got stuck to the point that I had to destroy the CF post to get it out. Problem was partially that the fit was just too tight, and partially corrossion.

Based on that experience, I don't think the shim is going to help in that regard.

rpenmanparker 

That's a good question. Dunno. It could work that way. What are folks doing now with just an Al post in direct contact with the frame. Is there a passivating agent available to coat the post? Is the black anodizing on many posts sufficient protection? My 27.2 mm carbon frame has a 27.2 carbon post, so I have never had to worry about it, but I can't get that post any more. I have been wondering what to do since the angle adjustment in the clamp has been giving me some trouble and the post may need to be replaced. I like the weight and ebay price of the Thomson Masterpiece, but it is Al.

merlinextraligh

^ Carbon fiber assembly paste. Reduces the need for torque to hold the seatpost in place, and protects against Galvonic corrossion.

rpenmanparker 

Thanks. That's what I was guessing, but I wanted an authoritative answer. Appreciated.

Looigi

There's a lot of bending load at the seat post clamp. What's important is that there is sufficient length of support along the seat post below that. The shim needs to be a good fit and long enough to provide adequate support. What's acceptable? IDK, but I'd try to get 4" or more.

Campag4life

You will get divided comments and experience on this. Personally, I would never do it. For the simple reason that I can look at a design and see the stresses because of my experience. A seatpost actually toggles within a seat tube. There is effectively 2 pt contact...up top at the collar circumferentially and at the tip of the post fore and aft. When a post is shimmed up top, it basically toggles/pivots with greater displacement about the seat collar. The bottom of the post will have little restraint. This causes a lot more stress on the seat tube up top.
Again, some will do it and it will work. I wouldn't.
Many times economies chosen are false.

rpenmanparker 

No doubt that seat posts would be more stable and do less damage to the seat tube if they were clamped at both the top and bottom, or at least at top and the minimum insertion point (which would eliminate the issue of variable insertion). There is no particularly obvious design concept to accomplish that. I should mention that despite my dismay at the stiffness of the 31.6 mm seat post that I referenced above AND the fact that I purchased a shim to 27.2, I never pulled the trigger. I too decided that the correct fit was the best course, and by and by I got used to the stiffer post. Maybe I am recommending the shim to OP, because I am hoping he will take the shim off my hands. Seriously, your analysis is helpful.

RollCNY

If the shim was the same length as the minimum insertion length, wouldn't this become somewhat less of a design concern? The two contact distances would be the same spread as the minimum specified by the seat post designer.

rpenmanparker 

That would seem to be an improvement, but the readily available one I bought was quite a bit shorter than that. Perhaps 3 inches instead of 4.

Campag4life

I am no expert on available shims because I believe it is a flawed concept but believe that your definition is a sleeve and not a shim...latter by definition creating a stress riser and if used for example on a carbon fiber post, will dent the post because of lack of compressive strength of carbon.
Keep in mind, that min. insertion is not ideal by any means. If in combination with a 99% rider weight, even more stress.
A seat tube is not vertical but rather has a lateral force component comprised of about 20% the vertical component. That means if the bottom of the seat post isn't flapping inside a larger seat tube it is supported by design.
This isn't a right or wrong thing as much as a false economy to shim a skinnier seatpost in a larger seat tube. I personally am loathe to even buy a bike with a 31mm seat tube because of the stiffer companion post and compromise to ride quality...one of the reasons I like Specialized bikes for example. They design with a 27.2mm post many of their bikes with this intent. Shimming is bad on a couple of levels. But again, there are likely thousands of bikes out there...perhaps tens of thousand with shims.

merlinextraligh

It's pretty common on aluminum tandems, so that you can use a 28.6 seatpost, that fits with a 1 1/8 stem for the stoker, and fits into oversized aluminum tubing.

Other than getting a seat post stuck because the shim was too tight, we haven't had a problem with it. Admittedly shimming from 28.6 to 31.6 might be a bit different from 27.2 to 31.6.

If it can take my weight, as well as a stoker tugging on the bars out of the saddle sprinting and climbing, I'm thinking it will hold up in single bike usage.

Campag4life

Curious if design intent is to run a 31.6mm seat post on your tandem, why do you run an undersize seatpost?
Also, is the stoker handlebar connected to your shimmed seatpost?