'warning' catastrophic failure on luna eclipse chainwheel

[joebreeze]

This happened on my Luna Cycle Eclipse chainwheel.

Design flaw ?

If you look at the load bearing configuration of a similar device , the Shimano rotor , the load bearing curves are orientated the opposite way around to the Luna Eclipse . The LUNA CYCLE Eclipse curves are designed backwards. The curved sections on the Eclipse are being pulled on as opposed to being pushed on , hence the horrible breakage . Luckily I was not injured .

[73Eldo]

Yikes. Its even the same color as mine. I guess its good that they decided not to drill the holes in the arm with the logo on it since it looks like that is all that kept it together.

Was it a sudden bang and something is wrong or was it more of a progressive you think you felt something strange?

Looks like you use it hard but still being as thick as it is you would expect it to take some hard use. The holes certainly look like a mostly aesthetic choice that becomes a major failure point (death star anyone?) and like you have observed on that rotor possibly another aesthetic choice is actually making things worse.

[TNC]

It would also beg the question...how long have these been out and how many have been in use? OP, no mention of how you use the bike, the circumstances, etc. Have you ever struck the chainring on a rock? What kind of bike is this on? What's the device behind the chainring in the picture that looks like cog teeth, and is that a cable of some kind running through grommets? A little more detail would be nice with maybe an overall pic of the bike.

[73Eldo]

It looks like he has a full suspension frame so my guess is that the thing with the grommets and cable is is battery bag. Looks like on that frame the motor is hanging almost straight down so the motor itself is going to be taking all the hits .

I was pondering the angle thing and I'm not sure which way would be better. Does it work like spokes in a wheel where its all pulling? So you could replace them with string and they would still work? If so then the angle you would think would be correct. Looking at the rotor its doing the same thing if you think about it. You could replace the spokes with string and it would still work at that angle because the wheel is trying to pull away from the brakes.

[TNC]

Wow! 73Eldo, I think you're right. I wasn't envisioning the motor hanging that far down from the chainring and bottom bracket. That is one reason I want to hear and see what kind of bike setup this is.

On another issue, I try not to put too much stake on a low number of failures on any component. As to the "number" of failures, I wonder how common this is? I'm fairly new to the ebike arena, but I read everything I could get my eyes on for months in advance. This is the first Eclipse 42t or bigger failure I've seen mentioned. And does the term "failure" apply here? Hard to believe the OP was just pedaling along and the chainring disintegrated...unless perhaps it had been damaged to some degree in a prior incident. Regardless, we'll just have to hear more from the OP as to the details. Without that it's just all speculation. Up front, however, I'm of the opinion that the curve of the arms probably has little to do with it. I think the multiple, drilled holes would be more of an issue if anything is.

[Mike_V]

Hi Joe. Too bad that nice looking part broke
I see Per your example design comparison:
Steel brake rotor ( works every time ) Vs.
Anodized aluminum C.R. with decorative 'lightening' holes and the subsequent structures engineering debate of multiple complete stress failure possible scenarios.
However
A Steel C.R. works every time.

[73Eldo]

I'm hoping someone that is a real engineer could tell us what happened there and if the design or materials were the problem. I suspect the offset is one thing that really plays into what happened. There is likely a twisting force on those arms? You look at the rotor example and those are either very thin steel or thicker aluminum or composite products and they are always straight or have a very slight offset in the case of a centerlock style. I bet the twisting due to the offset is/was flexing each spoke just a little when it is really under load and after say 1000 high stress flexes it just goes at the weak point which is the holes. If those holes were not there maybe it would take 2000 high stress flexes?

I saw a video about some new chain link design that said with a regular chain about 90% of the load is on like the first 2-3 teeth it contacts so I would imagine that load is carried in darn near a single line down to the hub too so I bet especially with a big offset there is some flexing going on as that load is transmitted.

[Dshue]

The "point" load is on the first few links meaning that is the point where load is transferred from one component to another. The entire upper chain run is under 100% load. What the video you saw failed to point out is that the chain/chainring interface after those first few links doesn't drop to zero load. The load actually is just as high and drops off fast to zero at the end of the interface. Think of it in terms of the first set if 2 links carries 90%, the next set of 2 links carries 70%, the set after carries 50% etc. Not precisely but that's the gist of it. The way they describe it is the first few links see 90% and the rest see the remaining 10%. But they have something to sell. And so far their product can't match chain and sprocket efficiency.
It is a cantilevered arrangement so chainline could play a role as well. Chainline is always worse in the lower gears, the gears with the most torque. My knee jerk assessment is the spider work hardened over time and failed.
The motor makes x amount of torque, the gearing multiplies that, and the chain and sprockets endure the stresses. As above the chainring stresses are highest at initial loading and then drop off. Only 1/3 or so of the chainring actually sees significant loading and in lowest gear with greatest torque the chainring sees a lot of side loading, similar to the side loading on a brake rotor with a single action mechanical caliper. Rotors for the application are designed to take the flex. If they were more rigid then the same thing would likely happen.

It is strange that they decided to curve the center struts back. Aesthetically I would prefer the match the direction of other parts.

[TNC]

OP, c'mon back and give us some details. That's not stated as a challenge or argument, we really want to know how and what happened exactly and the bike setup. Lots of people use these and would like to hear the experiences of others. This thread made me curious, so I started doing some searches on the internet. And yes, while it is just the internet, I couldn't find one report of an Eclipse chainring failure. Now, I can't believe that is the case, but it might indicate that this isn't a widespread issue. This chainring appears to have been out since 2017 from the reports of usage that I could see from our search.

OP, after your thread and examining my 42T closely, I kind of would like Luna to leave out the "lightening" holes, but maybe I'm only saying that in a kind of "pants and suspenders" way of being cautious...LOL!