Broken axle on KS 16X? Worst crash ever!

[RoboFixIt]

Should the 16X be recalled? This seems to be made from cast metal! I sent King Song emails about issues with the wheel rubbing months before this break and now we know it was the axle but I found out the hard and painful way. I think maybe we should report some serious safety and quality concerns to King Song...

 

P.S. I will upload a video on the crash soon, I'm hurt pretty bad so taking it easy for a while...

See email from King Song from 5 months ago bellow:

 

 

Email from King Song from 5 months ago

 

My dear friend,

 

we also found some cases about this issue, we are sorry to make you have this bad experience, please contact your retailer of your country, they will be happy to support you, thanks and best regards,

 

KINGSONG SUPPORT

 

To: <info@szkingsong.com>;

Sent: 2021-12-02 05:48

Subject: Solutions to repair KS 16X Tire Rubbing?

 

Hi I wonder if you have a solutions for customers that purchased the KS16x because many have a tire rubbing on the shell?

 

Please see attached .jpg I took this morning after removing the shell"

 

I tried to tighten all screws that attaches the wheel to the frame but they were 100% perfectly secure so I'm wonder what else could be the issue and how it could be fixed?

 

Before emailing King Song I researched the following thread and no one has come up with a solution even after changing the shell.

 

 

 

 

 

 

 

My dear friend,

 

we also found some cases about this issue, we are sorry to make you have this bad experience, please contact your retailer of your country, they will be happy to support you, thanks and best regards,

 

KINGSONG SUPPORT

 

[supercurio]

Sorry you got injured @ElectronxCycles. Definitely recommend to always ride with gear, no exception.

The recommendation to replace the inner shell for tire rubbing makes sense since there is a known weak point on the earlier KS-16X inner shells around the pedal hangers, which was a little bit strengthened in the newer mold.
But the motor axle is not that strong. I bought my 16X second hand and the axle was bent, I know of another one with bent axle as well. Aside from bending, the axle breaking on 16X doesn't appear to be a common issue - therefore I don't see a recall happening here.

The photo you sent is missing, it could be relevant to see it so we can observe how badly bent was the axle back then. I'm inferring that it's what was causing the tire rubbing? There was no follow-up on your previous post mentioning progress.

[0000]

Just going to throw out a guess here, but it looks like remelted pot metal from god knows what. Paperclips? Rusty nails? Who knows. From the break you can see there was no stretch or deformation, just the dirtiest shearing possible (maybe some combination bending stress too depending on where the load was supported) at the weakest point, where the axle was hollowed out to fit the motor's wiring harness.

Not only is this an inappropriate design anyways for what would have been a lifetime of fatigue stress from repeated impulse forces through normal use. That is bearing the rider's weight while hitting bumps at speed, taking jumps, hopping curbs, etc., but the very nature of hollowing out the axle and leaving sharp edges creates stress concentrations which increases the chances of what you experienced.

So no, it's clearly not safe for a weight bearing axle. It doesn't matter if King Song controls the manufacturing process directly or if they subcontract their motor builds to a 3rd party supplier. They bear full responsibility for both the bad design and this catastrophic outcome. Savvy consumers beware.

Other 16X owners may have been getting away without failure thus far, and you just happened to be the unlucky one to receive "top-shelf" pot metal possibly due to "supply chain issues" and QA failures (do they have these? LOL), but that doesn't take away from the fact that this motor axle is a poor design in the first place for an EUC. I'm not familiar with all manufacturers designs, but I know some Begode wheels use much larger bearings and solid aluminum weight bearing mounts which don't rely upon a hollowed out pot-metal axle to support the rider.

This axle design flaw has been present in direct drive hub motors used on e-bikes for years, and I am aware it was an issue reported by others in the DIY community at endless-sphere in years past.

Good luck on the recovery. This is both a bad and good lesson that supports the All The Gear All The Time position as Supercurio said, and hopefully you don't let one bad wheel design and manufacturer keep you from experiencing the bliss of riding EUC in the future.
 

Edited June 30, 2022 by Vanturion

[Tawpie]

The evolution of a product...

Until the introduction of the V11 and RS, all unicycles were made with hollow axles fashioned from drilled out metallic rods. It was an inexpensive way to build a toy—these things weren't intended for anything other than occasional use on nice smooth pavement (in fact, the 16X manual explicitly says "Do not ride on rough or bumpy surfaces, such as muddy or pebbled roads" and "Don't jump with the EUC" among other things.

The move to hollow bore motors was driven in part by a need to strengthen the axle system because the capabilities of the wheels and how they were used was outpacing the durability of the hollow axles. The old way was fine for back then but it's not really up to what we do with our wheels today... everything new is hollow bore. Would you be surprised if your Schwinn Apple Krate (https://bikehistory.org/bikes/krates/) broke something on the Whistler mountain bike trails? And yeah, I am really that old.

I guess I'm saying that yes, the old design isn't the best or the safest, but to be fair it was consistent with the industry standard practice at the time. Don't expect it to be a modern wheel because it's not... it's one of the first 'not-entirely-a-toy' wheels and much has been learned in the last 3 years.

@ElectronxCycles I hope you recover quickly, any crash is bad news, worse when it's a wheel failure. Is this the same wheel that dumped you (lift switch?) and then you had to hammer the pedal rod out? If so, it's time to part it out. It's either cursed or just plain wore out.

(if you were a resident here below the 47th and passed a background check, and any red flag checks, and waited the appropriate cool down period I'd advise you to do the humane thing and put that poor hoss out of its misery in the time honored fashion using a low capacity magazine of course. But it's probably more fiscally prudent to get what you can for the bits and pieces... if the batteries are any good, they'll bring hundreds and assuming it doesn't have a case of bad juju I might give you 50USD for the control board)

Edited June 30, 2022 by Tawpie

[0000]

Lol, and if you don't want to put down ye olde 16X so the King can Song sing once again... I remember some e-bike builders had success pressing out their cast axles and machining a proper forged steel axle to fit. Time and money either way you go.

[.....]

Looks like standard chinesium to me. Hell, it looks like its high quality chinesium, as its at least not full of voids the size of marbles. The best way to make sure you dont get anything made with these exotic paperclip metals is to not buy ANY euc's. If you arent willing to do that, youll have to do like the rest of us and ignore the obvious while lowering ANY standards you may have once had.

Good luck getting good after sales support from the maufacturer. Theyll be more than happy to sell you more replacement chinesium tho. Now you know where your old cars and junk materials went. On a ship to China so it could be sold back to us. Canada makes GREAT metals, these arent made from that.

Edited June 30, 2022 by ShanesPlanet

[techyiam]

Very sorry to hear that you got injured. 

From the video that you posted, I believe what is shown is the fractured surfaces of of a fatigued fracture. Crack initiation usually occurs at points in a high tensile stress field. The cut out in the axle, creates stress concentrations due to corners (stress risers due to geometry), and the reduction of load bearing material. And that is where we see the crack propagated. The fine, smoother texture of the fracture surfaces on either side of the notch in the axle is the area where the crack propagated. The coarse fracture area is where the remaining undamaged portion of the axle giving out (catastrophic failure) because there is not enough material left to support the load.

Your wheel was rubbing the side of the shell not because the axle was bent, but rather the crack was opening up. This also can explain why the hub to axle area cracked at the toe of the weld.

There is nothing here that shows who is to blame, whether it is the manufacturer, or the rider. At least for me, I can't make out any features from your video to suggest that there was a manufacturer defect, as opposed to rider abuse.

 

Edited June 30, 2022 by techyiam

[supercurio]

@techyiam tanks for the detailed explanation.
Maybe @ElectronxCycles has pictures/videos of this axle which would allow a post-mortem analysis confirming your theory.

I'll inspect the (moderately bent) axle on my 16X to make sure it doesn't show any trace of crack!

[Funky]

To much cut out material.. Makes whole axle so much weaker.  Axle should be without any holes! If you need to pass true wires - it should be 2x the thickness. (Because cutting out that hole, you just made the axle 2x smaller/weaker.) How euc manufacturers make these things are mind boggling..

@ElectronxCycles May i ask how much do you weight? Also asking kingsong for any help, etc are pointless.. Like most manufacturers they don't want to deal with any customers, they want only your money.

 

Off topic: Quick question. Does the ks18xl have the same axle design as 16x?

Edited June 30, 2022 by Funky

[Chriull]

8 hours ago, techyiam said:

There is nothing here that shows who is to blame, whether it is the manufacturer, or the rider

Just did a quick google and looked at pictures of fractured steel/metal parts - very interesting!

Seems to be a very own and very exciting field!

[supercurio]

Ah yes interesting indeed, this Wikipedia page gives a good intro as well.

After looking at the video again, I realize that the axle is cracked behind the bearings, in an area that's not visible unless removing the motor side covers. Not trivial to look at.

[techyiam]

48 minutes ago, supercurio said:

After looking at the video again, I realize that the axle is cracked behind the bearings, in an area that's not visible unless removing the motor side covers. Not trivial to look at.

Right between the inner side of the bearing and the hub. 

It almost seems like a rider did a big drop off or jump, and on the landing, the weight of the rider would cause a large bending and shearing stress right at the top of the axle. The shearing stress would be "magnified" right at the inner bearing and hub interface. And on top of that, at the very precise moment, the cut out side of the axle could also be at the top position by chance. All of this could add up to create a high enough tensile stress field, and started the crack initiation.  However, it would be interesting to see whether any part of the axle was bent also.

Edited June 30, 2022 by techyiam

[0000]

2 hours ago, Funky said:

To much cut out material.. Makes whole axle so much weaker.  Axle should be without any holes! If you need to pass true wires - it should be 2x the thickness. (Because cutting out that hole, you just made the axle 2x smaller/weaker.) How euc manufacturers make these things are mind boggling..

To be fair, you can get away with including a small channel for wire pass-thru in the exact center of the axle without losing too much strength, and most importantly not have the very bad "C" shape which has the stress concentrations leading to the crack propagation then catastrophic failure @techyiam described, but King Song did not spec their motor design that way.
 

10 hours ago, techyiam said:

There is nothing here that shows who is to blame, whether it is the manufacturer, or the rider. At least for me, I can't make out any features from your video to suggest that there was a manufacturer defect, as opposed to rider abuse.

Here's where I disagree. Not only is the axle design bad for the reasons we both mentioned, but it's the manufacturer's responsibility for designing the wheel for intended use, which should include a reasonable degree of abuse in calculating intended maximum loads and fatigue stress. @ElectronxCycles isn't the only one who has ended up with axle failures from this design so it's not an isolated incident. Here's an example from endless-sphere a decade ago where a machinist took matters into their own hand after a similar axle failure.

Also, look at the size of the motor wiring harness vs the size of the hollowed out channel from @ElectronxCycles video. It's way bigger than it needs to be so they could have easily spec'd a smaller center bore. And that's the key here, this is a common off-the-shelf motor not specifically built for EUC, it's just a lazy design where I would question if mechanical stress analysis was even considered in the first place. Speculating, but it's probable King Song simply chose this motor entirely for reasons of convenience and price without running any stress calcs.

Anyway, the situation is as @Tawpie described, the need for a more robust design has already been realized.



 

Edited June 30, 2022 by Vanturion

[techyiam]

4 minutes ago, Vanturion said:

Here's where I disagree. Not only is the axle design bad for the reasons we both mentioned, but it's the manufacturer's responsibility for designing the wheel for intended use, which should include a reasonable degree of abuse in calculating intended maximum loads and fatigue stress. @ElectronxCycles isn't the only one who has ended up with axle failures from this design so it's not an isolated incident. Here's an example from endless-sphere a decade ago where a machinist took matters into their own hand after a similar axle failure.

The context of which I was speaking from pertains only to what I saw in the posted video. In another words, I didn't find a smoking gun in the video that would point to a manufacturer defect.

[techyiam]

2 hours ago, Vanturion said:

Anyway, the situation is as @Tawpie described, the need for a more robust design has already been realized.

If you are referring to the S22, then I would agree that the hollow motor axle would be more robust. However, look at the slider design. It may be be sturdy, but were they too cheap to hire a suspension guy? The extremely high stiction, and exposed sliding surfaces facing the wheel, where it effectively become part of wall of the wheel well. Mind you, to their credit, people having saying good things about the action of the rising rate links and the shock, especially when compared to the other suspension wheels.

Edited June 30, 2022 by techyiam

[0000]

2 hours ago, techyiam said:

The context of which I was speaking from pertains only to what I saw in the posted video. In another words, I didn't find a smoking gun in the video that would point to a manufacturer defect.

Yes, we're talking about the same video.

The defect I'm referring to is the design choice to use an axle that removes material from outer edges resulting in a "C" shape with the cut-out channel. You said it yourself, the sharp edges leave stress concentrations which propagated a crack in the axle leading to catastrophic failure. If you're going to hollow out a load bearing axle, the best way to do this is to do an axis bore about the centroid, and make it only as big as you need to pass the wires.

The other way in which the manufactured failed is by not compensating for this design deficiency with a higher quality metal, not that that's the best solution anyway. I don't know what the material properties are, but the label Chinesium and it's obvious connotations is probably not the worst assessment of the quality of metal being used here. Anyway, this configuration is known to result in these types of axle failures speaking from lurking endless-sphere years ago when messing with DIY e-bike stuff, and using it "off-the-shelf" without changing design parameters (such as larger diameter shaft and/or axis bore to compensate for not properly calculating maximum design load and fatigue stress in the first place) for use on an EUC is the defect.

Wasn't talking about suspension design at all.

Edited June 30, 2022 by Vanturion

[Funky]

Will ask again.

Off topic: (Because i'm twice the weight of some of you guys.. 280lbs.) 

Does the ks18xl have the same axle design as 16x? Asking - so i know, if i need to ride "extra" careful.

Thanks. <3

[0000]

25 minutes ago, Funky said:

Will ask again.

Off topic: (Because i'm twice the weight of some of you guys.. 280lbs.) 

Does the ks18xl have the same axle design as 16x? Asking - so i know, if i need to ride "extra" careful.

Thanks. <3

If you look at this clip, it looks like this particular 18XL they've shown used the center-bore method to King Song's credit and the bearing rides outbound of the wiring harness pass-thru, which if you look again at @ElectronxCycles's video, the bearing rides inbound of the hollowed out pass-thru putting the weakened portion of axle directly in the load path.

But, and that's a big butt, we all know that all of the manufacturers tend to "tweak" things between batches/runs and so the real answer, for your wheel, is I couldn't say if it's like the one in the link or not. Unless someone more knowledgeable about King Song's manufacturing history of the 18XL chimes in, I would say you can't know 100% unless you physically inspect your own wheel.

Regardless, based on your weight, if I were you I'd still be more careful with bumps at speed and everything else that'll impart higher loads over time than say someone whose 150 lbs or less. It's still a relatively small shaft, and there could be inconsistency in the quality of the metals they use between batches as well (just to reference all of the global supply chain nonsense that's been going on since our respective countries' ruling classes decided to start fucking with everything).

Edited June 30, 2022 by Vanturion

[Funky]

42 minutes ago, Vanturion said:

If you look at this clip, it looks like this particular 18XL they've shown used the center-bore method to King Song's credit.

But, and that's a big butt, we all know that all of the manufacturers tend to "tweak" things between batches/runs and so the real answer, for your wheel, is I couldn't say if it's like the one in the link or not. Unless someone more knowledgeable about King Song's manufacturing history of the 18XL chimes in, I would say you can't know 100% unless you physically inspect your own wheel.

Regardless, based on your weight, if I were you I'd still be more careful with bumps at speed and everything else that'll impart higher loads over time than say someone whose 150 lbs or less. It's still a relatively small shaft, and there could be inconsistency in the quality of the metals they use between batches as well (just to reference all of the global supply chain nonsense that's been going on since our respective countries' ruling classes decided to start fucking with everything).

I got these two photos of mine wheel insides - when i was changing bearings/tire/waterproofing the wheel. Those wires "red arrows" go into the axle. Question would be how big are the "C" cutouts.. (Sadly didn't take side shots..)

One "zip-tie" meaning both wires goes into one "hole". So it should have centered "hole"? Or would it have 2 "C" cutouts. Who knows..  

Edited June 30, 2022 by Funky

[0000]

36 minutes ago, Funky said:

I got these two photos of mine wheel insides - when i was changing bearings/tire/waterproofing the wheel. Those wires "red arrows" go into the axle. Question would be how big are the "C" cutouts.. (Sadly didn't take side shots..)

One "zip-tie" meaning both wires goes into one "hole". So it should have centered "hole"? Or would it have 2 "C" cutouts. Who knows..  

 

I dunno man, unfortunately all the bits of interest aren't really shown in either of the pics. Tough to say what's going on. They all have cutouts to pass the wires, but it just depends on whether the bearing rides inbound or outbound of the wire pass-through bore opening.

Interesting that King Song uses hall sensors too vs on my RS, it's just the motor wires and no hall sensors.

[0000]

I looked at it again.

In the blue box, it mostly looks like the motor wires are passing through the thicker diameter section of the axle like in the clip I linked so it would follow that the bearing would ride outbound of this. So you're probably good in that it's not the worst possible design case like the 16X. That said, everything else I mentioned still applies if you want to ensure long-term structural integrity

[Funky]

11 minutes ago, Vanturion said:

They all have cutouts to pass the wires, but it just depends on whether the bearing rides inbound or outbound of the wire pass-through bore opening.

What do you mean by that? Where the "hole" is before or after the bearings? The axle itself doesn't move - it's welded. The bearings sit much further away from wires. (More outwards to the paddle hangers.)

Sorry i don't get what you meant with that. 

I think if i remember right the "hall sensor" wires went into the axle it self - i could see the white silicone on tha axle.. I think..

Edited June 30, 2022 by Funky

[0000]

20 minutes ago, Funky said:

What do you mean by that? Where the "hole" is before or after the bearings? The axle itself doesn't move - it's welded. The bearings sit much further away from wires. (More outwards to the paddle hangers.)

These hub motors are also called an out-runners in electric motor terminology. You're right in that the axle isn't rotating, it's the outer shell/magnets/rim/tire of the motor that rotates about the motor's inner stator. Think about the load path.

Rider weight --> Feet = Load

Load Path = Load --> Pedal --> Pedal Hangar --> Axle --> Bearing --> Motor Shell --> Rim --> Tire --> Ground

So where the load travels between the Axle --> Bearing is what we are interested in here.

In the 16X design, the load "travels" through the weakened part of the axle before it's supported by the motor case bearing.

In the 18XL design, the load "travels through the bearing before reaching the weakened part of the axle that passes the motor wires through.

18XL design >>> 16X design. Check both video clips again for visuals of this.

Edited June 30, 2022 by Vanturion
words

[Funky]

24 minutes ago, Vanturion said:

These hub motors are also called an out-runners in electric motor terminology. You're right in that the axle isn't rotating, it's the outer shell/magnets/rim/tire of the motor that rotates about the motor's inner stator. Think about the load path.

Rider weight --> Feet = Load

Load Path = Load --> Pedal --> Pedal Hangar --> Axle --> Bearing --> Motor Shell --> Rim --> Tire --> Ground

So where the load travels between the Axle --> Bearing is what we are interested in here.

In the 16X design, the load "travels" through the weakened part of the axle before it's supported by the motor case bearing.

In the 18XL design, the load "travels through the bearing before reaching the weakened part of the axle that passes the motor wires through.

18XL design >>> 16X design. Check both video clips again for visuals of this.

Check 16x video again the "first" 12 second and then check again at 0:43 seconds.(It's the motor cover with bearing inside.) The break happened after the bearing. (In the motor itself.) > "travels through the bearing before reaching the weakened part of the axle that passes the motor wires through. 

They both seem same. Maybe 18xl has the wires little bit more inwards..

Or i'm wrong?  (Or maybe i'm looking from the wrong side.. )

Edited June 30, 2022 by Funky

[0000]

12 minutes ago, Funky said:

They both seem same. Maybe 18xl has the wires little bit more inwards..

Or i'm wrong?  (Or maybe i'm looking from the wrong side.. )

Ergh, making me work here. See if these help.