Metal strengths ?


I was just looking at the Koxx trials uni’s on

On the page they state:

“Unfortunately it occurs again and again that aluminum seat posts break. The move during jumping and pulling on the saddle fatigue the aluminium matrial. We recommend to use this unicycle with a normal steel seat post. There are no 27.2mm available at moment. We offer this unicycle incl. a 25.4mm seat post and an appropriate aluminium shim. We tested it and didn`t got any problems. Excess weight (depending upon the seat post length): 150 - 200g.”

I’ve seen a few broken KH/Onza seatposts on this forum, but never a broken steel one. But then I’ve never seen anyone doing trials with a steel post either.

I know nothing about metals other than steel is heavier than aluminium. And apparently stronger.

I believe what are saying but does anyone have any info that could verify this for me?

Or does anyone do trials with a steel seat post? Any reports?

Much appreciated.


Thomson offers 25.4mm and 27.2mm aluminum seatposts that will serve you fine.

You would need a rail adaptor then. It would be worth it though.

I have broken a steel seat post before. It happens I’m sure aluminum wouldn’t last as long though.

When you say aluminium wouldn’t last as long you don’t mean the tomson do you? They are built pretty sturdy. I had one once upon a time…

I think rail adaptors are a unicycle part in need of an overhaul (a view I think I share with Ken Looi). Current designs are far too heavy and bulky for the job that they serve. If seatbases could be built with rails incorporated into the design like on bikes it would eliminate the need for the adaptor. I’m too slack to do anything about it so I will just type a few words and hope someone else decides to do something.

I thought the question was about the materials, not specific posts, so I’ll have a go.
I think the problem is with fatigue, and it’s different effect on steel or aluminium. Steel can cope with fatigue (ie very slight, but repeated bending) better than aluminium. Steel can keep bending after the point that aluminium gives up and snaps, which is presumably what are saying.

Having said that, am I right in thinking that most broken posts have broken at a joint of some variety? I presume if that’s were aluminium has been welded than the different properties still apply, but the design of the post is quite important too. Is the Thomson post the one that’s made out of one machined piece, rather than several joined together? I suppose if you remove the weak point of a joint then you can get away with using a less ideal material.

Just eat less pies, and then the loads on your seatpost will be less. :smiley:


I broke the seatpost on my Schwinn. It was steel, but it broke at the weld at the top–I’d attribute it to the strength of the weld, not the strength of the metal.
I don’t know if Alum or Steel welds differently or better.

This is also a common concern with bicycle frames. New frames are commonly built from aluminum because it is cheap and lightweight, and withstands abuse for a long enough time. Abuse it too much though, and aluminum frames will snap where a steel frame would last conceivably forever. It is true, steel can be bent repeatedly without reaching the limit of it’s fatigue. Aluminum doesn’t have nearly the same limit of fatigue so aluminum parts and frames must be made so that they do not flex at all nearly as much as a steel one would. For this reason aluminum bike frames ride a lot less plush than steel ones, and sometimes they break if you ride real hard. I think with all the stress that a seatpost goes through, it would be wise to have an extra 150-200 gram in a seatpost that will be made of a more durable material. Of course, I haven’t broken any seatposts yet, so don’t even listen to me :slight_smile: Mine is definitely made of steel though.

It was about materials, and you answered it for me perfectly. Thank you.

Having said that, am I right in thinking that most broken posts have broken at a joint of some variety?

All the snapped posts I’ve seen snapped on the weld, apart from that recent one on the forum with its stiffener, and mine. (I reversed a camper van over my uni!)

So both alu and steel will break eventually and probably on the weld, but steel will last longer. I aint going for the rail adapter method, thats more weight and money! Thanks anyway, U-Turn!

Its all aesthetics, baby! I just got some silver pr1mo grind pedals and they would look good with the knurled steel seatpost, but I didn’t want to put a matching seatpost on if it was weaker.

Just gotta cut a shim and shorten the post now!

[B]Just eat less pies, and then the loads on your seatpost will be less. :smiley:

John [/B]

Cheers for that! I think.

Thanks for replying folks, I now know a bit more about the properties of metals. And my trials uni will look a bit better!


P.S Tim, did your steel seat post break doing something big or did it just break with time. How long did it last you?

Scott Wallis makes a carbon fiber seat base that does away with the need to use a rail adapter. Scott’s seat base is designed to be used with the Thomson seatpost. pics here

Scott’s design is currently the best option for a strong and light unicycle seatpost and seat base.

The current rail adapters do have their problems; heavier than necessary, bulky, and prone to failure because of small welds. But they’ve been the only solution for a while.

I use steel.

My old Miyata posts used to tear on the metal, not the weld. This was the curved part around where the tube attached to the plate on top, but separate from the weld. This was a result of the curve having too small a radius, and also of the low grade of steel that was used in those posts. They are aimed at an elementary school kid riding the uni around a playground, not hardcore freestyle or worse.

With today’s unicycles, we put more strain on the seats than ever before, with ever longer and stronger handles up front. So they’re eventually going to give at the stress points, each according to their design, material, and build/weld quality. The type of material used is only one of those factors. For example, if you get a Thompson post, it is unlikely to ever fail on you.

My projection of future unicycle post designs is a possible switch to an upside-down “L” shape, where the seat clamps onto the top part. The joint of the L will have to be strong, but if done properly should no longer be such a weak spot. Then the remaining tube might be useful for attaching brakes or other accessories.

I think Koxx decided weight was more important than strength, and made the seat stem as thin as possible to save weight, they are now selling a reinforced seat stem for $35.

The other solution is to use thicker aluminum like the UDC aluminum seat stem which uses twice the thickness of aluminum but is still 1/2 the weight of using a steel seat stem though more than a Koxx seat stem.

I guess you gotta choose which counts more; weight or strength


Am I the only one that questions the use of welded tubing for unicycle seatposts instead of the much stronger seamless tubing?

For thoes that dont know the difference welded tubing is when a sheet of metal is rolled into a tube and the joint has a weld on the inside of the tube. If you cut your seat post and feal the inside with your finger you can feel this ridge in the inside of the tube from the weld.

After breaking and bending a few unicycle seat posts I too made the transition to a rail adaptor so that I could use the much stronger BMX style seat posts that you can buy. Thompsons are nice but I cant justify that much money for a seat post.

The current design of standard unicycle seatposts with the mounting plate welded to the post is a problematic design. Both steel and aluminum posts can fail at the weld. Aluminum is more likely to fail at the weld than steel. One problem is the design of the weld joint. Just slapping a mounting plate on top of a post and welding the two together is a weak joint.

One solution would be to make unicycle seatposts the same way they make many standard bicycle seatposts. Bicycle seatposts are made by bonding the guts (the part that clamps the rails) to the post. The guts have a lollipop extension that fits very snugly inside the post and the two parts are bonded together.

The bonded design isn’t perfect because unicycling puts unique stresses on the seatpost that can cause the bond to fail. Unicycling puts a lot more torque stresses on the seatpost than bikes do. Unicycling also puts upward forces on the seatpost when you grab the handle and pull up. Both those forces combine to torture test the bonded joint.

I think the bonded joint can be strong enough for most use if tight tolerances can be maintained. I think sloppy tolerances or sloppy manufacturing may be the main reason that most (but not all) of the failures of bonded bicycle seatposts used on unicycles.

For aluminum seatposts I think a bonded seatpost would be stronger and more reliable than the current welded designs. A special mounting plate would need to be made that has a lollipop extension. The mounting plate would probably be cast and then the lollipop extension machined on a lathe to final tolerances. The problem there is that cast aluminum parts can be weaker which could cause problems with the mounting plate cracking. The person who designs the casting would need to know what they�re doing. The Taiwan factories that make the standard bonded bicycle seatposts could make the bonded unicycle seatposts.

I think a good aluminum bonded seatpost would end up working better than the current aluminum welded seatposts. You’d trade the problems of breaking the weld with the problems of breaking the bonded joint.

With some creativity I think it might also be possible to design a mounting plate (or guts) that allows for some angle adjustability. Maybe by the use of wedges. Using a cast part for the guts would allow for some creativity that could allow for a standard seatpost that allows you to adjust the seat angle.


I got this pic from a customer after a few days use this seat post.

I also break an aluminium TryAll (Koxx) seat post on my trials.




No way! I don’t share that view at all. I’ve always maintained that rail adaptors are an unnecessary complication that is NOT required on a unicycle. It adds weight and weakpoints which are not necessary. The best thing is to bolt the post directly to the seat, but also have an adaptor that allows angle adjustment.

The only anjustment you need is angle adjustment on a unicycle. Unlike a bike, you don’t need forward and backward adjustment- which is why bike seats have rails. You save weight, unnecessary complicating hardware, and increase strength by bolting a seatpost directly to the seat. Scott Wallis’s adaptor for the Thompson post is the perfect solution for this.

Thompson seatposts are nice- I’ve had one on my b*ke for the last 6yrs or so- it’s still working great! However, I can’t justify it on my unicycles because my height (5’5") and short legs mean that I end up cutting a $180 seatpost in half even for a 24" MUni.

Mike, I don’t like using welded tube for anything that is put under stress, especially unicycle parts. It’s a waste of weight to have a bead of weld going up the seatpost, not that you’d notice. And the more important point being that welded seams in tubing are generally weaker.

BTW, Mike, try ebay. I got one for $45, including shipping. I’ve seen them go for as little as $20.

Aluminum doesn’t have a fatigue threshold, so even the smallest forces that cause flex will damage the material, until it fails. Thomson posts are designed to make this flex so negligable that it won’t cause a post failure. Their design works.

I don’t use a rail adapter or Scott Wallis seatbase on my uni, but i do use a 27.2mm thomson post. I use the bottom plate from the thomson to interface with the cf base, and use the two mounting bolts from the thomson post to attach to the base. After a few rides it’s been working quite well, and it was cheap and light.

What I mean is aluminium doesnt have the streangth of steel. Aluminium Thomson would outlast a aluminium normal unicycle post and even the steel. The steel Thompson would outlast the aluminium though. Aluminium is just over all weaker then steel.

You do so share that view, I just worded it differently. When I said overhaul I meant reducing it or eliminating it. Thats cool how Scott has made one similar to how you envisioned it.

I’ve never bent a seatpost.

I’ve broken 3 steel seatposts.

1 Miyata, 1 coker, 1 old one.

All through fatigue from riding down too many too steep hills, on short cranks with no brakes. Two at the weld, one snapped plate. I don’t do trials or intentional big drops.

An interesting idea that came from a get you home bodge done at the british Muni Weekend was to attach a very strong cable to the crown of the frame, which would extend to the handle attachment point and take the stress from you pulling on the handle. This would probably be lighter than just beefing up the seatpost and seat loads to keep the handle strength.

If someone was making an L shaped seatpost, it might make sense to make it backwards, so the joint is directly where you pull on the handle to hop.