Break down, go ahead and give it to me.



Ken Cline wrote:
> *In response to your question, I calculated the (relative)
> strength of the axle. The short answer is tubes are really
> quite strong and you only lose about 8% of the strength by
> hollowing out the axle.*
Does this mean that by hollowing out the center of a forged
or machined piece of cold rolled or other material that you
should only lose about 8% of strength?

I think I can accept that number as believable. But I also
believe it's 8% too much. Square taper axles can't afford to
give anything away.

> *The long answer is that bending strength is limited by
> the maximum stress exterted on the axle, which will be at
> the outside of the bend (in a smooth walled tube). For
> hollow axles, that stress is proportional to
> T / (od^4 -id^4) *
All those numbers were very cool, and I won't claim to have
followed it all. But it seems like you were calculating
strength based on some fixed set of parameters.

Unicycle axles break gradually. That's why the age-old
question of "how high of drops can I do?" never makes sense.
It's not how high, it's how many (x how hard).

As was described in later threads, many axles have been
known to "twist" before breaking off. I've seen this on my
own and others. When cotterless cranks are suddenly no
longer at 180 degrees to each other, it usually means your
axle is going to let go in the near future.

--
johnfoss - Walkin' on the edge

John Foss, the Uni-Cyclone
"jfoss" at "unicycling.com"
www.unicycling.com

"Hey, could I have some of that spinach? I need to get this pork rind
taste out of my mouth." -- Ryan Atkins to Kris Holm, on the way back
from Moab after sampling some of my pork rinds. They grossed out the
whole van!
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johnfoss wrote:
> *As was described in later threads, many axles have been
> known to "twist" before breaking off. I've seen this on my
> own and others. When cotterless cranks are suddenly no
> longer at 180 degrees to each other, it usually means your
> axle is going to let go in the near future. *

I wonder if there aren't already cracks by the time twisting
becomes noticable. I.e. Is the twisting happening because
the spindle is no longer a solid piece due to the presence
of cracks? Or, is the spindle still solid, but just so
weakened (by repeated stresses) that it is deforming?

--
duaner - -

duaner.
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"johnfoss" <[email protected]> writes:

> Does this mean that by hollowing out the center of a
> forged or machined piece of cold rolled or other material
> that you should only lose about 8% of strength?

Yes, that's my extimate for that axle. Of couurse if you
change the size of the hole, the strength loss will
change too.

There is another I didn't mention before. My calculation is
for the torque twisting the axle on its axis. There is also
a component twisting it down towards the ground (between
bearing and crank). Still, when we land with the cranks
horizontal, there should be much more torque along the axle
than down towards the ground.

> I think I can accept that number as believable. But I also
> believe it's 8% too much. Square taper axles can't afford
> to give anything away.

True. I think I said something to that effect. We don't need
equations to come to this comclusion - the results of this
multiple break down are ample evidence.

Of course if manufacturers used exotic ultra-high-strength
steel, hubs could be twice as strong or more. But then they
wouldn't sell for $25.

> > *The long answer is that bending strength is limited by
> > the maximum stress exterted on the axle, which will be
> > at the outside of the bend (in a smooth walled tube).
> > For hollow axles, that stress is proportional to
> > T / (od^4 -id^4) *
> All those numbers were very cool, and I won't claim to
> have followed it all. But it seems like you were
> calculating strength based on some fixed set of
> parameters.

Sorry if I didn't make it properly clear. I enlarged the
picture of the broken axle and measured the inner- and outer-
diameters on my computer screen. Crude, but sufficient for a
ballpark estimate. That where I came up with 4.3w and 2.3w
for the outside and inside diameters of the axle. The
formula T / (od^4 - id^4) is a simplification of an
engineering formula made by removing constant factors. The
resulting comparison is identical to the more complex form
since the factors cancel during division.

> Unicycle axles break gradually. That's why the age-old
> question of "how high of drops can I do?" never makes
> sense. It's not how high, it's how many (x how hard).

Right. I was intentionally vague about what "max stress" I
was talking about. There's yield stress, the force the axle
can take without fracturing. Then there's the elastic limit,
the amount of stress required to permanently bend the axle.
Exceed these enough and the part fails. As you point out, we
rarely create enough stress to cause a new axle to fail in a
single event, but, conveniently, the strength of the part is
the same regardless of which of these stresses you pick as
the maximum.

Ken
 
Sofa wrote:
> *
>
> Except for......YOU'RE ON A UNICYCLE!
>
> :) *

Oh, god! Thanks, Sofa. That excuse'll hold for at least a
month. Phew! Until the gras dies and I can see the ground
before I land. The thing that was holding me back was the
tall grass all over the rollout making it so I really
couldn't tell if the landing would be any good. Mid-late
summer it all dies, though, and it'll just be more padding
for the landing. Any other excuses that'll assist in my
procrastination?

--
gerblefranklin - Trials Unicyclist

Don't you think it's a cruel irony that acting like a G.I. Joe in the
army can get you a Medal, while playing with one can get you thrown out?
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First of all I'd like to thank Harper for being a guinea
pig, testing these hubs, and sorry the new lame hub failed
so early and wrecked your ride. But I think I've learned
something from it.

The first batch of hubs I made had external threads like a
Suzue. That was on purpose because I knew the solid square
taper would be stronger than one with a hole in it. My line
of thinking was "why make an already weak hub weaker by
drilling a hole in it?" It's one of those original hubs that
Harper has been riding for almost two years and Ryan tried
to break in Moab.

As we know, there's a problem with the external thread type
hubs: it's tough to keep the cranks on and tight enough. I
made a hub with internal threads to address that problem. I
used standard 8mm bicycle crank fixing bolts and drilled the
holes plenty deep to ensure that the fixing bolts wouldn't
bottom out in the hole. I also threaded the hole deeper than
necessary just to make sure the fixing bolt didn't bottom
out in the threads. Now I know that was WRONG!

I'm sure the hole extending beyond (deeper) than the back of
the crank weakened the axle significantly (I calculate about
21% weaker than solid) and the threads going beyond (deeper)
than the back of the crank only gave the axle a nice place
to start cracking internally.

I've come to a conclusion about my making square taper hubs:
why bother?

There's two things to consider when talking about unicycle
hubs. One is material strength and the other is the shape
and size of the axle at it's weakest point. I think the
weakest point on a square axle is just inboard of the crank
- exactly where Harper's axle broke. At this point I measure
the square to be .530". On a Profile or KH splined axle this
dimension is almost .750" *round*. The round shape and
larger size makes a huge difference in strength over a
square taper. Notice that "splines" per se don't make a
stronger axle ... it's the larger "root diameter" of a
splined axle that makes all the difference.

I've done some calculations and made a drawing showing the
result. I'll stick my neck out here and attach the drawing.
I think it *might* be correct but be aware that I've
simplified the forces acting on the axles to that of simple
twisting (not to mention that I probably don't know what
I'm doing!). If nothing else, the drawing shows pretty well
what the relative size difference is between the Profile,
KH and square taper axles. Also, I think I'm conservative
on my comparison of the KH to the Profile because I'm
ignoring the eight beefy splines that surely add a lot of
strength to the KH axle.

In conclusion I'll say that Harper's axle broke because
the crank fixing bolt holes were too deep and especially
that the threads were too deep. I don't think the 17-4ph
stainless steel was faulty or heat treated wrong.
Actually, I think that G3orge's broken axle is more
interesting. Why did it break inboard of the bearing and
in a spot that's greater in diameter? Perhaps the welded-
on flange is to blame?

SH

+-------------------------------------------------------
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| Attachment filename: spindle ends.jpg |
|Download attachment:
http://www.unicyclist.com/attachment/204891| +-------------------------------------------------------
---------+

--
showard - ------
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I'm just glad there are people like you that can do the thinking for
people like me! And I have confidence that someday you'll invent the
"unbreakable" hub/axle/crank combo. Really, very interesting reading,
and nice illustrations Steve:cool:

--
elmer - uniimpaired

"At 40 life begins...to show."
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showard wrote:
> *
>
> I've come to a conclusion about my making square taper
> hubs: why bother?
>
> *
LOL,i got a good chuckle out of that.:D

--
forget_your_life - #3649 plus_your_life

[image:
http://www.unicyclist.com/gallery/albums/albup23/bunnyjumping.gif]
dream one dream many....
*'R.I.P' (http://tinyurl.com/vr86)*
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showard wrote:
> * Actually, I think that G3orge's broken axle is more
> interesting. Why did it break inboard of the bearing
> and in a spot that's greater in diameter? Perhaps the
> welded-on flange is to blame? SH *

one could hazard a guess at "poor technique". however, if
you were to need a test subject for a splined hub, i just
happen to have need for a splined hub. :D in exchange, i'd
be happy to send you my broken hub for analysis. does the
sem wide hub have a welded-on flange?? from the looks o' it,
it looks as if the axle is press-fit into a separate hub
piece... -g3o

--
g3orge - stupid git

happiness is a warm unicycle...
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Not that a picnic table drop is anything by the group's
standards, but for what it's worth, whereas the Odyssey
cranks were insufficient (both bent), my Uni.com hardened
square taper hub is holding up just fine for now...but from
what I've seen so far, the spines are soon to arrive and
take their rightful place in the center of my wheel.

--
unisk8r - All this for just cross-training?
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unisk8r wrote:
> *Not that a picnic table drop is anything by the group's
> standards, but for what it's worth, whereas the Odyssey
> cranks were insufficient (both bent), my Uni.com hardened
> square taper hub is holding up just fine for now...but
> from what I've seen so far, the spines are soon to arrive
> and take their rightful place in the center of my wheel. *

If you showed up at the 1997 CA Muni Weekend and started
jumping off picnic tables you'd have been in an elite group.
Back in the "old" days that was not the style of riding
people were doing. Things are different now and the
equipment has changed to keep up. Back then a standard
unicycle with a 2" knobby tire was a muni. The riding style
was XC with no jumps and drops. Now a basic muni has a fat
DH tire, stronger hub and stronger cranks. Jumps and drops
are considered a normal part of riding.

Back in '98, '99 the Odyssey Black Widow Euro cranks worked
just fine for me. I never worried about them bending. My
riding was XC style on a Pashely MUni and a 2" or 2.1" XC
tire. In August of 2000 I bought a DM Vortex just one month
before the 2000 CA Muni Weekend and my riding style changed.

--
john_childs - Guinness Mojo

john_childs (at) hotmail (dot) com
Gallery: '' (http://www.unicyclist.com/gallery/john_childs)
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You wouldn't happen to still have the frame, would you? If
so, would you ever consider selling it?

--
gerblefranklin - Trials Unicyclist

Don't you think it's a cruel irony that acting like a G.I. Joe in the
army can get you a Medal, while playing with one can get you thrown out?
------------------------------------------------------------------------
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gerblefranklin wrote:
> *You wouldn't happen to still have the frame, would you?
> If so, would you ever consider selling it? *

Which post are you replying to? If this is about my Vortex
muni I've still got it and it's still a complete unicycle.
It's a 26x3. It still gets used, but it doesn't get ridden
as much now that I have a KH 24 because I prefer the 24x3 to
the 26x3. The 24x3 is more manageable and maneuverable
through technical bits and the 24x3 is easier to hop with.
But when I want to go faster I use the 26x3 Vortex.

--
john_childs - Guinness Mojo

john_childs (at) hotmail (dot) com
Gallery: '' (http://www.unicyclist.com/gallery/john_childs)
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"showard" <[email protected]> writes:

> I'm sure the hole extending beyond (deeper) than the back
> of the crank weakened the axle significantly (I calculate
> about 21% weaker than solid) and the threads going beyond
> (deeper) than the back of the crank only gave the axle a
> nice place to start cracking internally.
...
> I've done some calculations and made a drawing showing the
> result. I'll stick my neck out here and attach the
> drawing. I think it *might* be correct but be aware that
> I've simplified the forces acting on the axles to that of
> simple twisting (not to mention that I probably don't know
> what I'm doing!). If nothing else, the drawing shows
> pretty well what the relative size difference is between
> the Profile, KH and square taper axles. Also, I think I'm
> conservative on my comparison of the KH to the Profile
> because I'm ignoring the eight beefy splines that surely
> add a lot of strength to the KH axle.

Thanks for the drawing! I should have done my homework
before posting my estimate. Even so, my calculations end up
with different results than yours: Drilling should weaken
the taper by 13%, and the drilled taper should be only 36%
the strength fo the Profiles. If you want, email me
([email protected]) and we can figure out which of us is
mistaken - I don't want to bore people in the forum
figuring this out.

> In conclusion I'll say that Harper's axle broke because
> the crank fixing bolt holes were too deep and especially
> that the threads were too deep. I don't think the 17-4ph
> stainless steel was faulty or heat treated wrong.
> Actually, I think that G3orge's broken axle is more
> interesting. Why did it break inboard of the bearing and
> in a spot that's greater in diameter? Perhaps the welded-
> on flange is to blame?

Yep. The weld is my #1 suspect.

Ken
 
On Wed, 21 Apr 2004 01:02:20 -0500, "showard" wrote:

>Notice that "splines" per se don't make a stronger axle ...
>it's the larger "root diameter" of a splined axle that
>makes all the difference.

It's a pity that the ubiquitous square taper design is so
small. Were it twice the size they might be a fine concept
also for heavy duty.

Klaas Bil - Newsgroup Addict
--
Clearly a system of 1/14 and 1/16 is not decimal - Mikefule
on the English weight system
 
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