Break down, go ahead and give it to me.



U

Unibrier

Guest
The Seattle crew had a record for breakdowns at St. Ed's
yesterday.

Shortly into the ride we were playing on the stunts when
Harper snapped his SH square taper on a less than two
foot drop!

[image: http://www.unicyclist.com/gallery/albums/albux48/St-
Eds041804_078.thumb.jpg]

Later, G3o did a 4 foot drop and exacerbated his crank
allignment. The next picnic table 180-drop snapped the axle
of his Sem-Wide Hub.

[image: http://www.unicyclist.com/gallery/albums/albux48/St-
Eds041804_079.thumb.jpg]

Next, Pete tried the picnic table drop and bent his cranks!

[image: http://www.unicyclist.com/gallery/albums/albux48/St-
Eds041804_082.thumb.jpg]

Harper borrowed TB's Summit to finish the ride. G3o's
failure was at the end of the ride and had to walk back
to the cars.

The photos and a nice movie (2.2 MB) of independent cranks
are 'here.' (http://www.unicyclist.com/gallery/albux48)

JC's and TB's Profile hubs were unscathed. My SH splined hub
and cranks held up to my lame drops.

A record day indeed.

--
UniBrier - Its Time to Ride

Steve De Cuckoo

The Roadrunner or Ground Cuckoo can run at speeds of up to 15 miles per
hour (24 km per hour). It rarely flies and does not migrate. When it is
in danger, it runs or crouches to hide.

The RoadUnier or Cuckoo Cokier can ride at speeds of up to 15 miles per
hour (24 km per hour). It rarely flies and does not migrate. When it is
in danger it UPDs and slides.

------------------------------------------------------------------------
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UniBrier wrote:
> *JC's and TB's Profile hubs were unscathed. My SH splined
> hub and cranks held up to my lame drops.*

The KH splined hub on my Summit also held up well to the 4-foot-
ish drops it did from St. Ed's pedestal. In hindsight, I was
nuts to let Harper try that after the total lack of care he
showed for his own unicycle.

Here are some more shots from the day:
http://www.unicyclist.com/gallery/albux49

--
tomblackwood - Registered Nurtz

Tailgate at your own risk.....

------------------------------------------------------------------------
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I broke the first axle. I made the first pedestal drop. I
did it on a borrowed, unfamiliar unicycle. I get no respect.
JC has already broken a Profile crank, but he's JC.

The two pansies with the cameras can't even break their
own lenses.

Actually, watching Tom Blackwood chug up the lake trail was
a spectacular sight. Gee, I wish I were only 41 again.
Kids...they can do anything.

--
harper - Statuesque

-Greg Harper

B L U E S H I F T

"Stoke your own thread as much as you see fit, no problem." - Klaas
Bil

"Ugh, the Harpers; The worst neighbours in the world" - Tony Micelli
------------------------------------------------------------------------
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harper wrote:
> * JC has already broken a Profile crank, but he's JC. *

if you buy me a profile setup, i am more than willin' to
break them for you! yes, you made the first pedestal drop,
but my first pedestal drop was the pre-death drop. :D as far
as respect, should we just start callin' you "Rodney D"?? i
still like "Eustace"... :cool: -g3o {smug git}

--
g3orge - stupid git

happiness is a warm unicycle...
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Ben Plotkin-Swing wrote:
> *Harper, is that the same hub that Ryan couldn't break
> in Moab? *

No. The difference is that this one was on its maiden voyage
so it was essentially brand new. Also, it used crank bolts
rather than cranks nuts. the threads on the axle Ryan
couldn't break were male threads. The ones on this new axle
are female threads. As seen in the photo, the axle break
point is slightly inside the bolt-axle interface. I would
have expected it to snap right at the interface and
certainly not after just a two foot drop. Of course, I'm
much tougher than Ryan.

Did the girls ever give you a call?

--
harper - Statuesque

-Greg Harper

B L U E S H I F T

"Stoke your own thread as much as you see fit, no problem." - Klaas
Bil

"Ugh, the Harpers; The worst neighbours in the world" - Tony Micelli
------------------------------------------------------------------------
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Based on all of this, it looks like bolt-on cranks may not
be a good idea for unicycles. I was surprised at the size of
the hole in that first picture. There's a lot less metal
there to take up the stress compared to a solid axle.

What do you engineers think?

--
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:
> *
>
> What do you engineers think? *

Engineers don't think. We over-analyze and then have knee-
jerk reactions. Here's my knee-jerk reaction. The shear
point is at the interface of the crank surface and the axle.
The axle is clearly weakened inward beyond that point
because of, as you point out, the depth of the drilling. The
crank interface provided the shearing surface at a weak
region in the axle.

--
harper - Statuesque

-Greg Harper

B L U E S H I F T

"Stoke your own thread as much as you see fit, no problem." - Klaas
Bil

"Ugh, the Harpers; The worst neighbours in the world" - Tony Micelli
------------------------------------------------------------------------
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"harper" <[email protected]> writes:

> The ones on this new axle are female threads. As seen in
> the photo, the axle break point is slightly inside the bolt-
> axle interface. I would have expected it to snap right at
> the interface and certainly not after just a two foot
> drop. Of course, I'm much tougher than Ryan.

Am I visualizing this correctly: The bolt was shorter
then the depth of th crank? In other words, the bolt
threaded into the axle to some depth, b, and the crank
arm sat over the axle to a depth, with c > b (c closer to
the axle center).

If that's the case, I wouldn't expect it axle to break at b
because it was being protected by the crank taper. It seems
that stress would naturally occur at c, so I'm not surprised
the axle failed there.

A quick and easy way to beef up the design might be to
lengthen the crank bolt enough that it threads deeper than.
That way the bolt would also have to bend before the axle
could fail there. Then again, a bolt going through the
center would only feel a small portion of the stress - I
don't know how much it would help.

Ken
 
I think the short answer on Harper's last post was yes.

I don't know how much help you'd get (if any) from a longer
bolt. The problem in the usual axle breakage is metal
fatigue. With less metal to work with (the bolt is a
separate piece), I figure a taper with a hole through it is
going to weaken and break sooner.

On bikes this doesn't appear to be an issue. But for heavy
duty unicycling, it sounds to me like the old crank nut is
the better way to go for long-lasting axles.

Or could it just be a defect in that one axle?

--
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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View this thread: http://www.unicyclist.com/thread/31905
 
"johnfoss" <[email protected]> writes:

> Based on all of this, it looks like bolt-on cranks may
> not be a good idea for unicycles. I was surprised at the
> size of the hole in that first picture. There's a lot
> less metal there to take up the stress compared to a
> solid axle.

Oops. Should have read this before my last post. 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. Any loss of strength of standars square taper axle may
be unacceptable for hardcore use, but then the square taper
(in standard materials) may be unacceptable anyway.

Now, the question I have is whether cutting threads into the
axle weakened it.

[GEEK WARNING: Read on at your own risk]

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),

where T is torque (from the crank), od is the outside
diameter and id is the inside diameter of the axle. For
solid axles, it is just
1/od^4. Looking at the picture of the failed axle, I
estimate od=4.3w and id=2.3w, where w is the wall
thickness at its narrowest. Fixing maximum stress and
cancelling terms yields:

T_solid/0.0029 = T_hollow/0.0032, so

T_hollow = 0.92*T_solid

The maximum torque for the hollow axle is 92% of that for
the solid.

In reality, the situation for the hollow axle is even
better. This calculation is for a round axle, and doesn't
take into account the extra material forming the square
taper. That material effectively increases the outside
diameter, making the max torques even closer.

Cutting threads inside the hollow axle is an unknown to me.
But I know for sure that stresses love to concentrate at
sharp corners and if the threads go deep and are sharp, they
could easily weaken the axle. My earlier suggestion of using
a longer bolt might actually make matters worse if threads
have to be cut as deep as the crank sits.

Ken

P.S. I'm Chief Inspector of the Climbing Physics Police
(ret.), not an engineer.
 
It is foretelling that all of the square tapered unicycles
met with some sort of failure in the hub or the cranks.
Since all of this happened at an old Catholic seminary, I
can only conclude that it is a message from God that an
upgrade to a splined hub for these folks will be divine. Of
course, this same park is where my Profile crank broke in
half so I could be interpreting the days events all wrong.

George's broken hub was the most amusing. It broke
between the bearing and the flange rather than between
the bearing and the crank. That allowed the crank to spin
independently. Check out the video in 'UniBrier's
gallery' (http://www.unicyclist.com/gallery/albux48) to
see the effect.

The hub that Greg Harper was using is a prototype hub from
Steve Howard. Greg and Steve will have to look at it and
try to figure out why it broke so easily while the previous
hub survived 6 foot drops by Ryan without snapping. It
could be that the broken hub wasn't heat treated correctly,
maybe the raw material was somehow inferior or forged
differently, maybe the way the internal threads were cut
caused a stress riser in exactly the wrong spot, lots of
potential reasons why it failed. That's why it's a
prototype hub. You don't really know these things until you
go out and try to break it.

--
john_childs - Guinness Mojo

john_childs (at) hotmail (dot) com
Gallery: '' (http://www.unicyclist.com/gallery/john_childs)
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john_childs wrote:
> * You don't really know these things until you go out and
> try to break
> it. *

But Harper wasn't even trying to break it and he did. He has
a gift that way....

--
tomblackwood - Registered Nurtz

Tailgate at your own risk.....

------------------------------------------------------------------------
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UniBrier wrote:
> *... profile hubs were unscathed*

It warms my heart... I've laready gone over 6' multiple
times on them without even hearing a creak. I'm happy.
Sadly, I no longer have an excuse for not doing the 10
footer near my house, though.

Hey, does anyone else think that the Kooka crank rotated on
the taper causing it to round out before it broke for
Harper? It looks like that kooka's now trash. Shame, I reme
mber Harper talking at moab about how hard he looked for it.
It even matched his frame.

edit: Are those profiles that uniskier/pete bent?!:(
:confused:

--
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:
> *Hey, does anyone else think that the Kooka crank rotated
> on the taper causing it to round out before it broke for
> Harper? It looks like that kooka's now trash. Shame, I
> remember Harper talking at moab about how hard he looked
> for it. It even matched his frame.
>
> edit: Are those profiles that uniskier/pete bent?!:(
> :confused: *

The cranks that Pete bent are Odyssey Black Widow Euro's.
Nice lightweight cranks for road use and XC, but not really
meant for drops.

It does look like Greg's Kooka rotated on the taper, but
that could also be the hub rotating before snapping. We
won't know until Greg takes it apart. I'm guessing that it
was the hub rotating before snapping and that the Kooka's
are going to be fine.

Greg managed to accomplish what Ryan could not -- breaking a
Steve Howard hub. I don't think Greg is ever going to let us
forget that now.

--
john_childs - Guinness Mojo

john_childs (at) hotmail (dot) com
Gallery: '' (http://www.unicyclist.com/gallery/john_childs)
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gerblefranklin wrote:
> *
>
> Sadly, I no longer have an excuse for not doing the 10
> footer near my house, though.
>
> *

Except for......YOU'RE ON A UNICYCLE!

:)

--
Sofa - you - pee - dee

'Unicycle Product Reviews' (http://tinyurl.com/368h6) *107* reviews on
*72* products

'London Unicycling Club Website ' (http://www.brianmackenzie.com/LUC/)
version 3.02

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