Hi all,

A buddy gave me a decent seatpost, much better than my current one. It has an od of 27mm and my present one has an od of 28.6mm. Jenson has shims and I'm wondering which one to get. There is one that fits a 27.2mm post and a 28.6mm frame. Is this the one I need?

thanks
charlie

No, you need a shim to go from 28.6 mm to 27.0 mm. There is a reason seatposts are made in 0.2 mm increments.

.2 of a mm will make that much of a difference? I could see a problem if the shim was smaller. I haven't been able to find a shim for a 27mm post.

Yes, 0.2 mm does make a difference. The 27.2 mm shim will probably not hold the seatpost firmly enough.

Tightening the frame around an undersized seatpost causes permanent distortion of the frame
For a 28,6 id Seat tube
a 28.6 OD shim with a 27.0mm ID will be .8mm thick
(28.6-27.0=1.6/2 =0.8..

free is not always a bargain, this may be one of those cases ..

have jenson Or your local bike shop, sell you the right sized seatpost.

that being the 28.6..

.2mm makes the difference between wiil fit and won't fit.

What exactly will be undersized here? The outside diameter of the shim is the right size.

The shim has a slit in it just like the seat tube on the bike. When clamped against a seat post that is too small, it will reduce in diameter and the effect is basically the same as clamping a too-small seat post in place.

You ask : that is what I responded to.

In a machine shop that size of discrepancy matters.. 0.008"

Good luck ..

In seatposts it matters too as several of us have responded.

J&B has a shim that size. Find a shop to order one for you.

Thanks everyone for the replies. Honestly I find it hard to believe .2mm will make that much of a difference in this application. We're not talking about a machined surface here. I'm willing to bet if one was to measure the interior of the seat tube it would vary by way more than .2mm. Some guys even make shims out of beer cans and have no problems. No way is this area of the average bicycle that precise.

charlie

OK, try it your way and let us know how it works out. It's your bike and your seatpost.

However, as several of us have noted, seatposts are made in 0.2 mm increments for a reason and it's not because the manufacturers like to have a lot of stock numbers.

Well Charlie IF you buy a precision measuring tool like a digital vernier caliper .

And you can make up beer can shims that will come out to 0.008" thick,
then you can have it your way , but as I understand you had a seatpost that fit already ,

just the freebie looked nicer , I'm sure a higher quality post with a beer can shim
will raise your aesthetic standards immeasurably..
over just leaving what you have in service, which one assumes does the job fine..

Le sigh. Running a precision reamer down the seat tube is an essential step in frame building. Yes, it is that precise (and really, 0.2 mm = 8/1000 inch = rather sloppy by machinist standards)

A beer can shim though, is just about the right thickness to make up a 0.2mm discrepancy.

Hey Guys,

Received the seat post shim that wasn't supposed to work today and guess what, it works perfect. Seriously people it's a bicycle not a Swiss watch. No way is .2 mm critical in a seat tube application.

You know what? Because it's pinched at the top and is only seeing the proper pressure in a thin band rather than the more spread out correct size you're likely going to find that the seat post walks down the seat tube over time because it is only tight around the top and is too sloppy lower in the seat tube which allows it to move side to side a little. You'll be regularly having to loosen and lift it out a half inch or so when you notice that you're legs are getting more bent. And THAT is why the right size is needed.

Cutting a beer can shim and slipping it in between the seat post and shim would likely aid a lot in avoiding this. I know you don't believe this at the moment but after you have to re-adjust your seat post a couple of times think back to this.

I accidentally bought a 26.8 seatpost instead of a 28.6. I could only find a 25-26.8 shim, which was the right thickness. It worked perfectly, although it stays with the seatpost rather than staying in the seattube when the seatpost is loosened. So, probably any shim of the right thickness would be better than one that is set for one of the correct diameters but the wrong thickness.

Well good for you. So today, you saved a few bucks but let me tell you how many of these stories end. You may end up ok but you asked bicycle mechanics for their opinions and you got some great responses. you may well be a poster here in a few months or a few years where this .2mm discrepancy has caused damage to your frame or cool free seat post. At that point, a few dollars saved won't be such a big deal. That may or may not matter to you but when the folks here respond, they respond for your benefit as well as the benefit of others who may be reading the posts to find a solution to a similar problem. Sometimes the question is about saving a few dollars without much of a downside and other times its about saving a few dollars where safety may be a concern. The answers here are read by all. So please show a little respect for the opinions of those you thought enough of to ask this advice. It's a bit immature to come back here and wave that .2mm difference in the faces of those who have answered you as they made it quite clear that it does matter to those that want it done right.

Yes, I think this is true. For our tandem, I had to fit a standard 1 1/8" stem for the stoker to an odd/small-sized captain seatpost. Found a shim of the correct thickness and cut it to size.

OP, if/when you discover the shim you got isn't working out, you'd be better off getting a 28.8 to 27.2mm shim, which is the same thickness as 28.6 to 27.0mm. You'd have to squeeze it into the seat tube a little more, but it'll be the right thickness. May have to cut the gap a bit wider to accommodate for the smaller circumferences of the seat tube and seatpost.

Edit: forgot the link: https://www.chainreactioncycles.com/M...?ModelID=43709

Slow down here pal. I show plenty of respect for other posters here and have taken their advice on many other questions that I had. Advice that I highly valued and let them know as much. Nobody is rubbing anybodies nose in anything, one poster wanted to know how it turned out and I came back to let everyone know. I have been a machinist for over 30 years and I know the difference. There are many areas of a bike that sub-millimeter differences make a difference but I contend that in this area it is not that critical. I rode 30 miles today and never had a hitch with the seatpost. Please get down off your high horse.

It sounds a little bit like the allowable tolerances of seattube, shim, and seatpost may have added up in your favour this time.

There's also huge difference between bikes. Take an old Hi-Ten Schwinn frame and you can probably get away with something half a cm too small and just cinch it down. Take a higher end aluminum frame and tighten it enough to hold a seatpost .2mm too small and it'll probably crank the frame at some point. I've seen frames ruined by using .2mm too small and too large posts. I've also seen people riding around with pieces of water pipe holding up their seat. The answer that you "can't do this" is "correct", but sometimes you can get away with things. But it's up to you to decide how forgiving the parts you're working with are and how much it would suck if you ruined your frame...no single right answer to those questions.

op: will work?

bfbm: no work.

op: yeah, ok. i do anyway

bfbm: dummy

op: u mad? i ride thurty mile and sp no move

classic bike forums.