Rear hub motor keeps breaking spokes

[rick_p]

Important note; I picked up this wheel second hand knowing it had a couple of broken spokes at the time I purchased it.

At first I thought it was just a case of lack of maintenance because some of the spokes were loose, so it was logical to me that loose spokes would rub against the hole in the hub and eventually cut through the spoke, or cause it to flex back and forth until it snaps. However, when I replaced the broken spokes, I did so with factory original replacement spokes, properly trued the wheel, and made sure none of the spokes were loose.

I thought for sure this would resolve any issues, it was easy to true the wheel, it didn’t give me any trouble, but not the case, it not only broke more spokes within a hundred miles, one of the broken spokes was a new one, and they all break at the same location every time. So, something else has to, or had to be wrong.

What I did next was to completely disassemble the wheel so I could get a really good look at things and check the condition of the rim, I was concerned that maybe the rim was way out of whack, and thus causing undo stress on the spokes to pull it into true, but no, it’s flat and round within a 64th of an inch with no spokes. I do see a little wear on a few spokes at the location where others had snapped, but nothing extreme. I also see a little wear at a couple of the holes in the hub, but again, nothing severe.

I’m kind of perplexed and hence the reason for the post, I’d love to hear what others think. The only idea I have left is maybe it was poorly or incorrectly assembled, and maybe the issue will resolve itself when I lace and true it correctly.

Here’s are a few pictures, which might help with diagnosis.

 

[rick_p]

Sorry for the long delay, between the holidays and the bad weather we've been having, I didn't get a chance to fully test the work I performed until recently. I'm calling this resolved.

I've put about 150 miles on the bike since that evening and haven't broken a spoke yet. So, I'm pleased with the outcome and very grateful to those who took the time to respond to the post.

The short version of the story is, every comment and suggestion everyone made were all correct, but since I followed all of the advice, it will be difficult to say whether a single thing or a combination of all the things fixed it. I'm going with the latter. If you're interested in the full story, read on.

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As mentioned in an earlier post I met up with a few bike mechanics, one of which builds custom wheels as one of the services his shop provides. The mechanic who builds custom wheels said the wheel was laced properly, but also said that although none of the spokes were excessively tight or loose, there was too much variation in tension from one spoke to another on the same side of the hub. When I asked about the excess space between the elbow and the flange of the hub, he said it was not desirable and might be adding to the problem, but if the tension were even across all the spokes on each side of the hub, it might not matter.

Well, I trust the opinions of the bike builders/mechanics on this site as much I as I do this guy, so I decided to shim the spokes. The shop didn't have spacers to use on the head side, and I wanted to get this done so I ran to the hardware store to see what I could find. The closest thing I could do to shimming the head until the elbow was touching the flange was to put very small washers on the elbow side, which essentially has the same effect. See the picture below.



The next step was to get the tension correct, since I was unable to get the factory recommended specifications, I used this post as a guideline: https://www.endless-sphere.com/forums/viewtopic.php?t=109845

The shop had a Park Tool; TM-1 Spoke Tension Meter and I used Park Tool's online tool to convert the meter's readings to KGF: https://www.parktool.com/en-us/wta

Because the spokes tend to be tighter on the drive side due to wheel offset, the drive side was fairly easy to get into the middle of the recommended range of tension, but to keep the rim centered the tension on the spokes on the brake side were at the low end of the recommended range. I decided against re-lacing the wheel with all elbows out on the drive side to solve this issue, and instead I just brought all the spokes on the drive side up towards the upper end of the range for that side, which got me a little above the low end on the brake side. I spent like 2 hours truing the wheel in order to get the tension on all spokes on each side within a few KGF of each other, but it seems to have paid off.

 

[tomjasz]

Bad build, period.

 

[Chalo]

Spoke head washers go under the head, not the elbow. Those will probably get squeezed out of place when they see some use.

Cross-2 lacing is usually bad for hub motors. The spokes kink where they enter the nipple, and then after a while they break at the root of the thread.

 

[rick_p]

I thought my post would help others with the same or similar problem, but I guess I was wrong. I would delete the whole damn thread but I don't see a way to do that so instead I'll just say... Hey, if you're breaking spokes, don't do what I did, because even though it's working for me, apparently it's not the right way.

 

[Chalo]

I'm not convinced that unsupported elbows were the critical factor in your spoke breakages. Most likely it was insufficient spoke tension (for the spoke gauge) and low material quality in the original spokes.

I find that even deep spoke elbows in thin steel flanges (like coaster brakes) are reliable as long as the spokes were adequately stress relieved and seated at the time of building. So I wouldn't worry too much about your wheel, even if the washers get loose and wander around.

 

[ZeroEm]

by rick_p » Jan 15 2023 12:42am

I thought my post would help others with the same or similar problem, but I guess I was wrong. I would delete the whole damn thread but I don't see a way to do that so instead I'll just say... Hey, if you're breaking spokes, don't do what I did, because even though it's working for me, apparently it's not the right way.

Always testing, use it until it quits. Data is always useful. You may not have any issues. As Chalo stated, keep the spokes tight.

 

[rick_p]

Spoke head washers go under the head, not the elbow. Those will probably get squeezed out of place when they see some use.

Cross-2 lacing is usually bad for hub motors. The spokes kink where they enter the nipple, and then after a while they break at the root of the thread.

I’m not sure how many miles constitutes some use, but we’re approaching 200 miles and not one washer appears to have budged yet. I wanted to use proper washers but they were not available. If failure occurs I will order the correct washers. I’m also not sure if the wheel being 28” opposed to 26” makes a difference as far as cross-2 lacing is concerned but that’s how it was laced from the factory and all of the previous breakages were at the elbows.

Future readers, I must emphasize that just because these methods worked for me this time doesn’t mean they are best practice. The points being made why they are not best practices are valid and you may not have the same success.

 

[rick_p]

I'm not convinced that unsupported elbows were the critical factor in your spoke breakages. Most likely it was insufficient spoke tension (for the spoke gauge) and low material quality in the original spokes.

I find that even deep spoke elbows in thin steel flanges (like coaster brakes) are reliable as long as the spokes were adequately stress relieved and seated at the time of building. So I wouldn't worry too much about your wheel, even if the washers get loose and wander around.

I completely agree, hence the reason I said there’s no way of knowing exactly which thing fixed the problem, but the one thing everyone (all the experienced wheel builders) agreed on was improper tension was the most likely cause. So, the one thing future readers seeking to solve the same problem should take away from this thread is, get the tension right at all costs.

 

[ZeroEm]

Wheel building threads are a guide to build a reliable wheel. One thing is not a deal breaker. The objective is to eliminate any weak areas to minimize maintenance. If your happy with your build great. If you get long life then it works for you. Everyone stresses there ebikes differently. My last wheel build has over 5000 miles had to tighten up the spokes at 500 miles. Planning on changing out the wheel with a smaller one with a different motor. So my build lasted for as long as it was needed. That all that matters.