draft for new seatpost adapter


I truly do like the Moocow Unicycle company for it Open Source approach.
I once ordered their Saddledapter, however, ended up never using it; it’s bolts are too tiny, my tools will bend. And the nuts to tighten the seat are unreachable, because of the walls that are meant for strength.
Last week I discovered there already is a v3, but I already spotted a design-flaw; the sink’ed-in bolts will never make much contact; the tighter you spin them, the less contact area it will have.

And so I thought about drawing the one I have in mind for long:

  • Make it compact and smooth.
  • Have the bolts go very tight along the seatpost
  • Allow to reach the seat-nuts.
  • Have decent size bolts, such that you can use bigger tools to tighten.
  • Don't have thread, but use a nut, such that you can replace it.
  • Have bolts on both sides, for nicer -more direct- application of force; and to be able to apply it straight (as in symetric).
  • As it consist of two separate halfs, I think it's unlikely to break 100% at once.
  • the post will go in for 29mm (in stead of 20.something)
See rendering images below.

I don’t know how much of a fortune it will cost to realize these, probably much as I did a lot of round edges (which aren’t that obvious in the renderings). But I wonder if there is any interest in such.
So please let me know what you think of it, especially if you think they could be better.





One more picture of a slightly different version:

The design appears to have a serious stress concentration between the square corner at the top of the the post socket and the square corner nearby on the outside. The cross-section also seems very thin there.

I personally have no interest in these, as I haven’t been breaking seatposts much in the last years, and if I wanted one, I’d take the proven strength of the Moocow design over the elegance of yours. I think it’s pretty cool though that you make use of the open source approach, as that isn’t really a common concept for mechanical parts, at least outside of 3d printers.

Two points I marked:


  • OTM pointed this one out already, the way the diameter decreases there is pretty bad from a structural point of view. I see the radii, but you are still going to see an increase in stress there. The effect of stress risers is unfortunately even bigger when looking at fatigue strength, which seems to be the issue with most seatposts.
  • Those two internal chamfers don't serve much function (tiny bit of weight decrease maybe), and will be pretty expensive to make. I'd leave them out. [/LIST]

    I’d need to check in CAD (or better a 3d printed prototype), but I think your design will make it impossible to use seats with allen bolts, at least in the more “extreme” seat angles, as it will be hard to get an allen wrench in there. Mad4one uses these:

    Which is what I would use if I used a moocow seatpost adapter, or regular socket head allen bolts, depending on your seat.


  • It’s then called Open Hardware.

    Mine need to fit a 28mm tube, so the shown post is now option.
    Probably the nuts will solve the tightening problem that the Moocow gives me, but then still it’s bolts to clamp the clamp are far too tiny.
    And drilling the hole to 28mm will make it scary thin.

    Based on above feedback (plural) I’ve adjusted my design.
    However there will ever and ever be a weakest area.
    I guess with this one the top plate edges are the first to break.
    But being significant thicker than the posts I mentioned before, I guess the whole thing -times two- is strong enough for my own kind of use.






    The sharp corner at the end of the post hole will still focus stress.

    You could radius this and put a small step as the post stop.

    I might be a little slow this morning but what’s the point of a seat post adapter that’s more expensive than a seatpost?

    This is a genuine question, not sarcasm. I just don’t get it.

    Assuming the adaptor is more durable than the tube, it can be reused on many posts that consist of any length cut from plain tube.

    but will you be able to tighten the adapter enough on the seat tube so the seat won’t spin around. After I UPD, it does already sometimes happen that the seat is twisted and then I only have to unscrew the bolts at the frame to turn it back. With the adapter there are 2 places where the seat can twist and I would need to loosen 4 additional bolts.

    LOL. Either of the connections can correct the alignment.

    If you put the split 2mm off centre you could add a central rib to the base. Then make the other side interlock underneath it so when it is fitted to the saddle base both sides see advantage of that rib. However small it is it will help considerably with strength.

    Ah right! Good point.

    Because actually I need it in odd sizes, that aren’t available.

    I don’t understand what you mean. In my case you release 4 bolts if you want to twist one way OR the other 4 bolts if you wish to move the other angle.
    (or -in my case- simply twist the tube on the other end).

    With the Pitfighter-III laying in front of me I was thinking the same.
    In deed has 1 mm ofset (times 2 halves), I will play with that.

    One different approach I thought of is doing it like below BMX seatpost;
    it’s super-neat as it also cuts the need for a seatpost-clamp; look how finger-friendly that could be.


    Just my two cents:

    The sharp edges would probably cut the crap out of my hand. Not a huge worry for muni, but my hand would be touching it all the time on my street/trials uni. I own one of the Mad4One setups, and I slightly cut my finger when trying something SIF. I ended up wrapping tape around the adapter.

    I am no longer breaking seat posts like I once did. I am a more gentle rider now, I infrequently UPD, and when I do it’s normally on grass or dirt. When I UPD, I typically either catch the uni or slow the fall with my hand before letting go. But, beginners seem to have the kind of UPDs where the saddle slams into the street. At a certain point, that stopped happening to me.

    Side note: Some riders were complaining about the weld on the Nimbus seat post…in another thread. I looked on my trials uni, and I have that seat post. I also noticed that the slight inward tapering of the post in the final inch helps me get my fingers around the saddle in the middle section.

    Breaking? I just don’t want it to twist.

    Is this clamp equal or superior to the standard nimbus clamp for the seat post?

    If I have to take out a tool after each time I fall to straighten it out? I don’t want it. It has to work like a collet. Good luck

    I think that is the post where they were talking about it breaking when ridden hard… the seatpost has been redesigned since then and they are considerably stronger now. We changed the press tool to add a small rib down the centre.


    How about a standard seatpost, a shim and a good double bolt seatclamp?

    example for shims:

    Shims are never an optimum solution as they have 2 surfaces that can create problems.

    I am going to throw in another thought.

    Currently we use (possibly without realising it) our seatpost and the interface with the frame as the failpoint. It is the stress relieve valve for our frame system. If everything is made super rigid… what will fail next?

    You are right with both points. To me it comes down to the usage of the uni. Is is just an older frame with a non std size and it shall be used for riding around instead of hanging in the garage without seatpost? Then a shim might be the easy fix.
    Or will it be used for hardcore riding? In that case the old frame might anyhow not survive for long…

    Emphasis on CAN. Good shim’s shouldn’t be much of an issue. Unfortunately, most cheap shims that are available aren’t exactly high precision, and that combined with the tolerances seatposts and tubes have means that it comes down to a 50% chance of either seattubes deforming, or nothing fitting.

    If you are considering a very expensive custom part however, having someone make a quality shim sounds like an alternative to consider.

    That’s why I don’t crank my seatpost down as much as possible on my 19", because experience has told me that when riding flatland, there will always come a bail where you step on the seattube, and the seat will twist. How much you tighten the seattube only determines how horrible the creaking sound the twisting makes is, and if you’ll need an allen key to correct it back.

    I leave it so I can still twist the seat without losening the clamp, but it will take a lot more force than normal riding/tricks could exert.

    Aside from the twisting, I don’t think a weaker seatpost will make the frame last much longer. While there is some effects of taking out a bit of shock loading, I at least doubt you could make a financial case for breaking seatposts instead of frames. (On common setups, at least).