alias5000

Hi!
I'm currently fixing up a Roland Tandem Team Sport CX (something like this). 26mm seatposts seem to be too small and 26.2mm doesn't fit. Are there 26.1mm seatposts? Any other ideas on how to get this sorted out?

The issue with using 26mm seatposts is that stokers can very easily turn the captain's seat, despite tightening the seat clamps REALLY strong.

Thanks!
alias5000

dsbrantjr

You could make a shim from 0.002" shim stock (approximately 0.05mm) which would bring a 26.0mm post up to very near 26.1mm, like this in stainless: https://www.mcmaster.com/2317K52, or brass is slightly cheaper but a little less corrosion-resistant: https://www.mcmaster.com/9011K201 Just remember to leave a couple of tabs to fold over the seat tube so you don't lose the shim down the tube.

Andrew R Stewart 

I wonder if the top of the captain's seat tube has some distortion or other reason to restrict the 28.2 post to fit. If someone with frame repair/building experience could get their hands on this bike they might be able to improve the post's fit. Andy

alias5000

Ha, everything in one answer that I have just hesitantly been pondering about. Thank you!

dsbrantjr

I live to serve. Checking for a distorted seat tube as Andrew Stewart suggests might get to the root cause, but if you have to pay someone to do it a shim might be more cost effective and would do no harm, unlike reaming out the tube as some might suggest..

zacster

My experience with shims hasn't been good. My bike uses a 30.0 seatpost, not an easy thing to find, and I tried using a 27.2? post with the proper sized shim but it would not stay put. The thing is that it was a carbon post, an aluminum shim and a carbon tube. It was just a bad combo. I tried carbon paste, and stripped a few bolts in the process. Ultimately I found an original post for my frame and haven't had any problems since.

Andrew R Stewart 

Different people, different methods and different results drive the advice here. Since we repliers are not where the bikes are we shoot in the dark to a degree.

My thought about a reaming method is really more to find out about the roundness or burred condition of the seat tube clamping area. A light scraping with a reamer will show the "high" spots long before any significant material is removed. Of course many here think of swarth spiraling off, like a drill bit produces. But like any therapy reaming is also a discovery technique if one is sensitive and experienced.

The reason I return to this method is that much of the clamping "force" that a post sees is from the amount of surface contact between the tube and post. When the tube ID isn't really round a post will slip far easier as the tube still has some elasticness it how it surrounds the post. And a stoker stem on the captain's post is close to a worst case situation for that post to stay put.

I, too, have found shims to be temperamental at times. They add a layer of slip potential and can't make up roundness fitting problems. Think of shims as sometimes using a pain killer instead of dealing with the root cause. When all is properly treated a pain killer has it's place. But there are side effects sometimes. Andy

63rickert

Totally agree with all of the above. One more point is seatposts are not accurately sized. And they too can be less than round. Check the seatposts you are already working with against a good vernier caliper. Measuring to the degree of accuracy required is a little tricky and is a good skill to have.

alias5000

I guess, I should add some more context. I'm working on this tandem at our local coop which will be used as rental tandem. For that reason, we're a little cost-sensitive (thus, shim sounds attractive here). I have tried our entire stash of new 26.0mm and 26.2mm seatposts, as well as a few used ones I could find - all with the same result and same feeling. I can push a 26.2mm seatpost down the tube a bit, but eventually it gets stuck and it suffers quite noticeable scratch-marks. A 26.0mm does give me a tiny wiggle room in all directions. It's really hard to measure this, but my feeling is that the tube is fairly round.

CliffordK

You might try honing or reaming the seattubes slightly. That should help with burrs and reduce scratching. A solid reamer should help get it round to depth too.

I'm not a big grease fan, but a little lube might help in a few circumstances.

Another thing to try is to sand down a 26.2 seatpost until it fits. If it is painted, then perhaps just taking off the paint. But, you want to reduce by about 0.1mm, or reducing the radius by about 0.05mm... Which isn't very much.

Keep in mind that a lot about the posts clamping react like the circumference of a circle, so a 0.1mm difference in diameter is about a 0.31mm difference in circumference. Not a lot, but it does add up, especially if you're off by a few tenths.

Mad Honk 

Alias5000,
In this case you won't really need a $200 reamer to do the work. Get a piece of 3/8" wood doweling and cut a slit in the end of dowel about two inches into the dowel. Put the dowel in a drill, and put a piece of 2" sand paper into the slit lengthwise and wrap it in the correct direction of the drill (RH). Use the drill, just like a reamer to clean out the tube and remove any rust and debris in the tube and knock down any abnormalities, then try the right size seat post again. HTH, MH

CliffordK

I'd do a larger dowel, if you can get it to all work. 3/4? But, I like your thoughts.

Brake hones are also cheap. Or, at least they used to be.

The reamer should get it close to perfect. Hand reamers are typically adjustable, and it would augment a co-op's tools quite well, at least if anybody can figure out how to use them.

But, the soft dowel/sandpaper might be good for knocking off the burrs, and working around the slot.

63rickert

https://www.grainger.com/product/WES...d-Reamer-4LGT5

This is a fairly basic cheap tool. Don't expect it to last forever. Fortunately reaming out a seat tube is not that severe and in any case take your time when working with these. A little at a time. Once I got to work with a 1920s reamer that said 'Torrington'. Haven't seen many bike tools that nice. But the cheapo will work.

Referring back to first post since the bike has already been used with an undersize post and tightened too hard, the hole is not round. You want it round. It would be better to take it out to 26.4 than continue to mash metal. If the lug ears show any deformation bend those straight before starting the cut.