Break down, go ahead and give it to me.

General
1

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!

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.

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

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.

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.

2

Re: Break down, go ahead and give it to me.

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

3

jenn kept askin’ if anyone made a free hub.
i can now say i do…
-g3o

4

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.

5

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. :smiley:
as far as respect, should we just start callin’ you “Rodney D”??
i still like “Eustace”… :sunglasses:
-g3o
{smug git}

6

Harper, is that the same hub that Ryan couldn’t break in Moab?

7

Me too… :sunglasses:

8

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 at the end of the bolt thread and certainly not after just a two foot drop. Of course, I’m much tougher than Ryan.

As you saw, Ryan was making a drop of 6 to 6.5 feet onto flat concrete, not 2 feet onto dirt. He also tried until the cranks were so hopelessly bent that they were unusable and he could no longer land the drop.

Did the girls ever give you a call?

9

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?

10

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.

11

Re: Break down, go ahead and give it to me.

“harper” <harper@NoEmail.Message.Poster.at.Unicyclist.com> 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

12

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?

13

Re: Break down, go ahead and give it to me.

“johnfoss” <johnfoss@NoEmail.Message.Poster.at.Unicyclist.com> 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.

14

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 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.

15

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

16

Re: Break down, go ahead and give it to me.

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?!:frowning: :thinking:

17

Re: Re: Break down, go ahead and give it to me.

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.

18

numbers/equations etc.

42.7% of all statistics are made up on the spot.

19

Re: Re: Break down, go ahead and give it to me.

Except for…YOU’RE ON A UNICYCLE!

:slight_smile:

20

http://www.unicyclist.com/gallery/albux50

i took some photos o’ the nubbin. looks like the cranks are totally intact.
-g3o