Seat Post Shim Slipping

Hi everyone!

As I mentioned in another thread, I have recently acquired a used Torker AX 29” from C-list. I purchased this uni to be a road machine, and for this purpose it seems perfect.

My issue is that the seat post was shortened by the original owner, and I’m a relatively tall guy who likes a high seat. I needed to replace the seat post, but the frame has in inside diameter of 26.8 mm, which is apparently an awkward frame size in the world of unicycles.

First, I looked at the Kris Holm rail adapter which joins a standard 4-bolt uni seat to a rail bicycle post. It turns out, these are not so easy to find anymore, and they weren’t very well reviewed even when they were available.

Next, I purchased an aluminum seat post shim which is 26.8mm on the outside and 25.4mm on the inside. Everything fit together perfectly with no wobble and I tightened down the double-bolt clamp as much as I was comfortable. I tested this setup on the bike trail last night, and while I love the higher seat, I found that the seat post twists under normal riding conditions. I especially noticed the seat twisting while mounting and when executing sharp “torso-turns”.

I’ve tried cleaning out the seat post tube on the frame and applying a very light layer of grease to the outside of the shim, but this did not seem to have much effect.

I have three questions:

Does anyone have any suggestions as to how I can stop the seat post and shim from twisting while riding?

Does anyone have any suggestions for another way I can fit a longer seat post to this uni?

I have seen an older post on this forum suggesting that the Kris Holm seat post (I assume the 27.2mm) might fit my frame. Is this still true, or would it require modification?

Thanks for your time everyone!

Hi Geolojas,

even if the KH rail adapter is no longer available, it served its purpose and I have a friend still using mine to this day. The main downside in my opinion was the weight penalty.

In the past, I ran several times a 25.4 post in a 27.2 frame with a shim and never got a slipping issue. The main thing was the use of a double clamp for a good grip. The second thing was to run the clamp tight.

From what you described from your frame measurements, the worst thing that can happen is that you have to file the seatpost a bit to make it fit (depends how patient you feel…)

The Torker AX is a bit of an odd beast. I wouldn’t expect a 27.2mm seat post to fit without mangling the seat tube.

You could try to get a better seat post clamp; it might be able to hold the shimmed seat post. I like the Salsa clamp myself.

Or, you could look at getting a KH Zero seat and use a Pivotal seatpost; there are tons of those available in all sizes (and they tend to be lightweight and inexpensive).

My Torker DX seatpost always slipped, even though it was the right size. I had a bunch of other similar problems with that uni that gave me the general impression that tolerances on Torker stuff aren’t very tight.

I ended up making a seatpost shim from a layer or two of aluminum can, and it never slipped again. So you might try shimming the shim…

hmmm, would a surly constrictor fit your frame? They come in 30 and 32mm… and they’re quite the clamp. The salsa and thompson seatpost clamps are both good as well. If the slipping is just side to side, you may want to roughen up the shim with some heavy grit sandpaper? Don’t know if that’s a good idea or not really. I’ve never used grease on my seatposts but haven’t had an issue unless I was using a quick release. I have a KH rail adapter laying around I got the from UK store when they still stocked them. Never used it, but it’s a tank. It looks similar to the old T-bar that nimbus used to make, only much much beefier.

You might try swapping grease with assembly paste, which is what is often used when clamping carbon fiber seat posts, stems or bars on bikes. Secondly, if the post extends far enough into the frame, perhaps a second clamp may also help, so you’re not overly stressing one clamp.

How much is “as much as I was comfortable” when you tightened the bolts?

My first suggestion is to tighten the bolts until the post stops slipping. You can get them retry tight before something strip or breaks.

I say “ditto” to roughing up the shim, inside and out, and make sure it, the seatpost, and the seatpost tube are clean and free of grease. I’ve seen cross-hatch patterns on seatposts before for the same purpose. I also might try a double seat post clamp if you can get the correct size. I had a similar issue with a crappy seatpost clamp, and the saddle was rotating a little. I replaced it with a double seatpost clamp with hex (allen) bolts, and it’s perfect now.

While I doubt this is a problem, I thought I’d throw it out there just in case, especially since you’re dealing with some odd sizes and pieces, and I’m not sure what all you may have done to make things fit (I know I get pretty “creative” when I’m making odd sizes fit). Make sure that when you tighten the clamp there is still a gap left in the shim. If the gap closes all the way, it won’t compress further, and the seatpost could still be a little loose inside the shim. That’s probably unlikely, but after looking at photos of a few shims online, some of the gaps in them don’t seem very big, and I could see that happening under certain circumstances, although I’m sure that is rare.

My two cents…

The carbon assembly paste is a good suggestion. Also, a rail adapter is probably easier to get than you think. A while back I posted a wanted ad for one in the trading post, and someone offered me one for the cost of shipping. That would be my suggestion. It is heavy, but overall tou won’t notice the extra weight.

Thanks!

Wow! Thanks for all the great feedback everyone.

I got home last night and figured I would try the easiest suggestion first. Killian, I think you win the prize. I am almost embarrassed to say it, but i just wasn’t putting enough muscle into tightening the clamp. :roll_eyes:

Before i tightened it I noticed that while the outside edge of the ends of the clamp were in contact, the inside closest to the seat post still had a gap. Also, there was still a small visible gap in the shim. I cranked down the bolts, went for a 4 mile ride, and had no problems with the seat moving while I rode. It did get crooked during a UPD, but once i straightened it out it stayed in place for the rest of the ride.

I’ll hang on to all the other suggestions, and will probably enact some of them to ensure that things really stay put in the future. Its amazing how disorienting it was to ride a unicycle with a twisty seat. Its like the uni turned to water beneath me every time the seat moved.

Thanks again everyone!