Metal strengths ?

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

http://www.lhthomson.com/bikes.htm

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U-Turn said:

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

http://www.lhthomson.com/bikes.htm *

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.

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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
municycle.com 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. 😄

John

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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.

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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
:-) Mine is definitely made of steel though.

--
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johnhimsworth said:

*I thought the question was about the materials, not specific posts,
so I'll have a go.*

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!

JUST EAT LESS PIES, AND THEN THE LOADS ON YOUR SEATPOST WILL BE LESS. 😄

JOHN

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!

T.

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?

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Rowan said:

*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. *

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'
(http://www.wallisdesign.com/HANDLEpics.html)

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.

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I use steel.

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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.

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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' (http://tinyurl.com/dexju) 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

Jim

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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.

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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.

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Hi,

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.

Cheers

Roland

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Rowan said:

*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. *

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.

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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.

--
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Rowan said:

*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...*

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.

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I've broken 3 steel seatposts.

1 Miyata, 1 coker, 1 old unicycle.com 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.

Joe

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