Snapped axle on Crystalyte TC100 hub motor, almost died

almost killed myself, luckily I was just taking off from a stop light when the left axle of my hub motor snapped off.

My initial shock has led me to think of scrapping the whole thing cause I could have easily been dead if it happened at 45pmh.

I am pretty sure there was just way too much reverse torque going on, cause I had my regen setup to kick in when the throttle is released, so there was pretty frequent torquing going on for the last few months, plus the bike is pretty heavy ~120lbs plus me at 220lbs which I imagine wasn’t helping. The frame is all steel (EEB Full Suspension frame), so i didn't think about also strapping on some torque arms, which may have helped.

Anyways, curious if this happened to anyone else.

I _may_ consider replacing the axle (any advice on that would be appreciated) and then just turn regen down a ton, or turn it off all together. I’m also considering just selling everything cause I don’t want to be driving a death trap, and maybe I can convince my wife to let me buy a motorcycle since that’s really why I built this big ass bike in the first place.

So if anyone is interested in buying parts, I might list them out and give a good discount. Not sure yet.

That's the wrong way to anchor the hub's reaction torque, and now you understand why. How does it feel that this obvious bad design was for the purpose of the manufacturer saving maybe one dollar?

Edit:

I can see how the above would be interpreted as chastising the OP for not using torque arms. That's not what I was trying to say, though. I was calling out the manufacturer along with all the other Chinese hub motor manufacturers for using a flatted axle to transmit torque. Its dumb.

Double sided strong torque arms can help keep the torque forces balanced between both ends of the axle. That snap looks like it may be corrosion initiated fracture propagation.

liveforphysics said:

Double sided strong torque arms can help keep the torque forces balanced between both ends of the axle. That snap looks like it may be corrosion initiated fracture propagation.

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That's what I was thinking as well. The perimeter of the break is rust colored which makes me think this failure has been in progress for some time now.

400% digital magnification here. It would be better to get a close-up photo and not use the digital processing. Nonetheless, rust preceding the break seems pretty obvious. So did it crack first, rust a bit, and then break or did corrosion on the outside spread inward? That's well beyond my expertise level.

wturber said:

liveforphysics said:

That snap looks like it may be corrosion initiated fracture propagation.

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That's what I was thinking as well. The perimeter of the break is rust colored which makes me think this failure has been in progress for some time now.

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That's how metal fatigue works. You get a microscopic crack, and it progresses at a slow but exponentially accelerating pace as the part is stressed. When there isn't enough material left to carry the load, it breaks.

Corrosion could have been the cause of the initial crack, but then so could have poor workmanship (e.g. sharp inside corner, flawed surface finish). But the real cause is the needlessly high material stress level due to the torque carrying interface being too small for the job it's being asked to do.

There is no valid excuse for this now pervasive bad design. Heinzmann was in the business first, and the way they solved the problem is both effective and cheap:

I am no chemist, but in my research into electroplating and oxidation, I've come to understand that oxidation only happens at the surface, where oxygen can come into contact with the metal.

Since there's rust penetrating in to where the faces of the axle were machined from the main part of the axle, I'd say the crack had started before the failure. That being said, oxidation expands surface area, so corrosion getting in to that part of the axle would have created outward forces that could have accelerated the failure (like Ice expanding in cracked pavement).

NeedForSpeed: I'm really glad you're OK. In this case, since the cracking started at the base of the point where the flats were machined, I don't agree with Chalo's comment (I'm not trying to start a flame-war here) that torque arms would have prevented this. A torque arm placed right against that face where the cable exits still wouldn't have prevented that particular fracture (IMHO). In any case, I'm glad you're OK, and please don't let this turn you off to eBikes.

EDIT: I was writing this as Chalo was posting his follow-up comment. My comments are only pertaining to his original post in this thread.

Chalo said:

Corrosion could have been the cause of the initial crack, but then so could have poor workmanship (e.g. sharp inside corner, flawed surface finish).

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I agree with this part of Chalo's follow-up. I think that point where the flats and the rest of the axle met up was the weak point. Again, just my humble opinion.

EDIT AGAIN:
I guess what I'm trying to say is that based on where the break happened, even with 2 or 4 torque arms, the break would have happened eventually. The TAs may have slowed it down a little, but I think it was inevitable based on where the break happened.

I think you are missing Chalo's point. His point was that the torque should not be carried by the small dimensions of a flattened axle. He'd agree that an added torque arm would not help. His point is that the Heinzmann design distributes the torque through material of a larger dimension resulting in smaller localized stresses and a more robust and reliable design.

Or more directly, he's saying the flattened axle design is garbage.

Ohhhh OK. I get it now. Sorry for being slow on that and adding noise to the thread.

Is it the pictures, or did that axle have rust almost to the middle? It looks like is was cracking for a while from the pictures.

What a luck that this happened at low speed.

When a axle brakes, it often is because the nut has been tightened to much and then the capability to transfer torque suffers.
Or with other words: for lowering the stress on the axle you should tight the nut just A LITTLE BIT and the clamping dropouts will do the rest (of course given that the alignment is ok and there is no play in the dropouts).
If there is some play (you often can hear a "click" when switching between regen / accelerating), people often think the big nut needs to be tightened more, but this is not good. Better reduce the play by adding small metal sheets (available in 0,05mm or less) so the clamping mechanism function properly.

I am not saying you did something wrong, i just wanted to bring this up as it is really important.

Well f__k! I have a TC4080 on a 130 pound, non-suspension bike with vertical steel dropouts, running two Grin torque arms. I do all my braking, down to 12 mph where the regen drops out, regen only, up to -2000 watts, for 17K miles now. I figured I was golden with the two torque arms. This has put a little brown in my shorts, for sure. :-(

madin88 said:

When a axle brakes, it often is because the nut has been tightened to much and then the capability to transfer torque suffers.
Or with other words: for lowering the stress on the axle you should tight the nut just A LITTLE BIT and the clamping dropouts will do the rest (of course given that the alignment is ok and there is no play in the dropouts).
If there is some play (you often can hear a "click" when switching between regen / accelerating), people often think the big nut needs to be tightened more, but this is not good. Better reduce the play by adding small metal sheets (available in 0,05mm or less) so the clamping mechanism function properly.

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Agree you should not overtighten.

However, undertightening (so you get the axle moving back and forth slightly every regen/accel cycle) can be worse.

1) You are loosening the nut a little every time, so you have to keep tightening it daily.
2) Every time it hits its limits it wears the contact point a little more. Wear it enough and it will cam out.

The right solution is a non-axle-based torque arm, like the old EMS system used to use (and like Justin's hub uses.) But absent that, you have to find the tightening torque that prevents motion but also doesn't overstress the axle.

There are several factors involved here. One is corrosion. It looks pretty obvious there was significant corrosion going on. If the axle was never exposed to water, it may have survived longer, maybe much longer. Another factor is high stress. Balancing the forces on both sides of the motor would help as Luke pointed out. High regen levels also increase the stresses. If the stress is high enough for the metal to exceed the elastic range, it will fail quite quickly. Intrinsically shitty design is another factor, as Chalo points out. Using flats on an axle might be OK for a 250w motor, but even at 750w they tend to spin out.

So how to avoid this? Avoid all of the above or mitigate them.

I avoid riding in the rain, but somebody who commutes may not have this option. Paint, oil or something to inhibit corrosion around the area would help.

Balance the torque on both sides of the axle as much as possible (two torque arms).

Use the proper torque to tighten the nuts. Too much or not enough are both bad.

Limit how much regen or thrust is used. Ideally we'd want to not have a limit other than where the tire skids, but hammering back and forth between throttle and regen seems to be particularly destructive. Take any piece of metal and bend it back and forth a bunch of times and see what happens.

The best thing is to avoid this kind of design all together. If the axle was sufficiently large in cross section it would work OK. The more power you run, the bigger it needs to be.

I don't see any torque arm pics. I missing something.?

999zip999 said:

I don't see any torque arm pics. I missing something.?

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He wasn't using torque arms per se. But the axle never spun out either.

So I zoomed into the dropouts in his picture, and it seems to me that the steel there is 5/16" thick or even thicker. These aren't your typical dropouts from what I can see. You might think of them as having integral torque arms. It's anybody's guess how hard he torques down the axle nuts.

BTW, the "almost died" in the title reminds me of this bit by Ron White.


[youtube]QC0yfREBxWw[/youtube]

NeedForSpeed said:

45pmh.

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I had to convert to get an idea of speed
45mph = 72kph
On a bicycle, with bicycle parts.

Glad nothing happened to you.

markz said:

NeedForSpeed said:

45pmh.

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I had to convert to get an idea of speed
45mph = 72kph
On a bicycle, with bicycle parts.

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Like he said, he got this instead of getting a motorcycle. The way I see it, he basically built a low powered motorcycle.

Thanks for all the replies, interesting stuff, I hadn’t considered rust as potential the culprit, or a contributor.

Yea that’s an all steel frame and pretty thick, so didn’t think torque arms were even necessary.

And yes all bicycle components, aside from the all steel frame, but they’re all pretty heavy duty downhill MTB parts, which I figured could handle the stress okay for the most part (eg rock shox double dual crown fork, Magura brakes)

Thinking about things some more, I’m realizing the only thing that really freaks me out about building a DIY high speed bike is the axle torque on a hub motor. I’m now considering a mid drive motor instead, Potentially a MARS 0913. My hub motor days are behind me.

You might want to look at TangentDave's motor.

https://endless-sphere.com/forums/viewtopic.php?f=28&t=70099

I don't wanna sound like a dick but this is what happens when you aren't doing checks/maintenance. The rust was clearly visble outside the axle and if I saw it on my bike's axle, I'd be very suspicious.

Fair enough. My defense is ignorance. Hadn’t considered that would ever be an issue.

SirLongAss said:

I don't wanna sound like a dick but this is what happens when you aren't doing checks/maintenance. The rust was clearly visble outside the axle and if I saw it on my bike's axle, I'd be very suspicious.

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I don't think that's true at all. I work on all kinds of rusty bikes with all kinds of high mileage, and rust has not caused them to break. This is the consequence of bad, lazy, cheapskate design and poor manufacturing, and nothing else.

Rust makes fasteners stick, sometime permanently. It's ugly, and it can abrade moving parts. But rust doesn't get inside a metal part unless that part is cracked. This part cracked because it sucked, not because it had rust on it.