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Cracked front hollow axle
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pago cruiser
Posted 6/11/2017 12:06 PM (#93811)
Subject: Cracked front hollow axle



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Posts: 34
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Location: Tucson - its a dry heat
Posted this on the VALK forum, as I was first thinking this was bike specific. But after some thinking, I think this is HACK specific.

Have been trying to get my WASP Leading Axle forks installed for the last couple weeks, and work keeps conspiring to steal my wrenching time. Had the LL's on the bike a couple weekends ago, and this afternoon got back on the project to pull the front wheel off and re-engineer the caliper mounts - but that's another story. The WASP stuff fits tight; as in you have to tap the axle into the bearings - with a hammer. A little too tight a fit, but that's also another story.

Tapped the WASP axle almost out, and needed to use the VALK axle to tap it out the other side. Picked it up from where I left it a couple weeks ago, tapped it out, set both axles on the ground, pulled the wheel, took a sip of beer and ... the thought occurred: why did my thumb feel a sharp ridge on the VALK axle?

Dang. I have been riding 50 years, wrenching for most of that, and have never, ever, seen or heard of a stock axle - hollow or solid - failure on a street bike.





The crack line starts on the left in the pic, then works its way up and at angle to the point where the large diameter part that rides in the fork slider boss meets the axle diameter - about 90 degrees from crack start to finish.
It almost hits larger diameter end, but stops about 0.10" shy. I'm going to find some dye penetrant this week to highlight this for a photo. While stress risers at rapid transitions are typical failure points - still... a LOT of bikes have this axle design...

My theory (thought about this a lot last night) is overloading/over-stressing.

The stock VALK is about 750 lbs with a full tank of gas, at about a 50% front-rear weight distribution. With a Motorvation F2 steel frame, sidecar body, battery, air horn/compressor, tools, spares, gear for a week trip, canine, and wife, we are prolly real close to 1400 lbs., albeit spread over 3 wheels. I really need to get this rig weighed...

Combine that with:
1. A 6 degree rake to the triples in order to make the VALK steer a bit better due to the hack, and
2. A 1.5" fork extensions to make up for the height lost due to the triple tree rake, and
3. A really rough hard-pack washboard, rock embedded, and potholed 3 mile driveway, that I usually deal with by speed. I can ride it at a walking speed, taking an hour to go three miles; or at about 40-45 mph, providing a relative smooth ride, but which works because it is too fast for the forks to respond to the rough road, thereby transferring the rapid loads/unloads to the "hard parts" of the bike. Like axles and bearings.

With all the above, I think it just overloaded the hollow axle. A hollow axle is typically a good design on a motorcycle. Without getting into a "which oil is best" discussion, a hollow axle, when compared to a solid of same diameter, provides good weight savings, with little loss of strength. BUT. It also changes the failure mode. A solid axle will bend before failure. A hollow axle cracks, as the load carrying outer diameter does not have a solid core to help absorb forces. It has almost the same load carrying ability, but has lost its toughness, for lack of a better word. Not a good failure mode on a part that is difficult to visually inspect on a regular basis. Just what is the inspection interval on an axle???

All that said, on some machining forums I visited last night, JAP axle material is generally not highly regarded. Other materials (4130/4140, etc) are much stronger and tougher. The VALK LL's from WASP came with their own axle - I have an email to them to see what material they use, as their axle is about 2" longer than the VALK stock axle - making a dicey situation worse.

Anybody seen something like this?
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Al Olme
Posted 6/11/2017 1:26 PM (#93812 - in reply to #93811)
Subject: RE: Cracked front hollow axle


Expert

Posts: 1732
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Location: Minneapolis, Minnesota
Mitch,

Two things...
1. Is there any chance that the crack is the result of you using the Valk axle as a drift? The crack seems to be an unusual failure mode to have resulted from use where the load is at 90 degrees to the surface.

2. Like many folks on this forum, you don't say where you are located. We're not asking for your social security number, just city and state. it really helps when you're trying to help someone or might need help or maybe just a fellow sidecarist to ride with. ANYWAY... If you come to the USCA rally in Corning, one of the tech sessions will weigh your rig, give you the load on each wheel both in pounds and as a percentage and give you an idea about how you might want to load the rig as a best practice.

Good luck,
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pago cruiser
Posted 6/11/2017 5:18 PM (#93815 - in reply to #93811)
Subject: Re: Cracked front hollow axle



Member

Posts: 34
25
Location: Tucson - its a dry heat
Hm.
I remember putting my location when I originally registered a long time ago.
But then a couple years ago I had to re-register, and all my older posts just went into the ether. Guess I'll fill in the missing info - thanks. We're just outside Tucson.
No chance on the failure due to using the axle as a drift. I felt the crack before I drifted the other out. Besides, a 1 lb mallet with maybe a 3" swing is hardly at the limits of a 20mm axle. Hell, my thumbs and occasional fingers have absorbed way more than that abuse, I'm sorry to say...

I was thinking it was a classic stress riser crack; it starts at the riser - about 0.10" away, runs parallel but slightly diverging from the change in dimension about 3/4", then turns and runs another 3/4" down the length of the axle. The entirety of this crack is between the change in dimension (cradled within the fork slider boss) and where the bearing rides on the axle.
Just saw you are in Minneapolis; we're heading a bit south of there on the rig this summer trip - Rochester.
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Al Olme
Posted 6/11/2017 7:57 PM (#93817 - in reply to #93815)
Subject: Re: Cracked front hollow axle


Expert

Posts: 1732
100050010010025
Location: Minneapolis, Minnesota
pago cruiser - 6/11/2017 4:18 PM

<>
But then a couple years ago I had to re-register, and all my older posts just went into the ether. <>
Just saw you are in Minneapolis; we're heading a bit south of there on the rig this summer trip - Rochester.


Mitch,

I'm really surprised to hear that your posts went away. We seldom delete anything and it's a pain to erase posts of just one member. Not sure how that happened. Rochester isn't so far away Maybe we could do lunch. If you come here, I'll buy.
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pago cruiser
Posted 6/11/2017 11:58 PM (#93819 - in reply to #93811)
Subject: RE: Cracked front hollow axle



Member

Posts: 34
25
Location: Tucson - its a dry heat
No sweat on the posts... stuff just happens sometimes.
You got a deal on lunch; the 2nd round is on me.
Here is the current project that led me to the cracked axle:



Re-designing the caliper mounts is a pain - that pattern took me all friggen day... with a hacksaw, a belt sander, and a drill press. Kinda irritating: I mailed the calipers to WASP, and then they shipped them back with the Leading Links themselves. Somehow, the caliper mount holes are short by about 8MM, meaning they used a rotor diameter 16mm less than stock... ??? They also designed them so the hoses face forward... Sigh.

Dropping these off at a machinist tomorrow to make a bearing pocket out of 6061 and then weld onto a 6061x1/2" caliper mount the water jet guys are doin tomorrow.

I need a road trip...
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