Rear hub motor keeps breaking spokes

rick_p

100 W
Joined
Feb 16, 2021
Messages
260
Location
Los Angeles
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.
A1E86DEC-95F6-4A88-AF51-9767DC2B5839.jpeg
D41FF618-D290-41AD-A142-EDB0BE28546D.jpeg
 
Do you know what thickness the spokes are? They may be too thick. Thinner spokes have more spring to them and will actually be more resistant to breaking (up to a point). Proper spoke tension is important too, Too loose or too tight can cause problems.

The lacing pattern in important too, and which way the spoke ends pass through the holes.

Post a picture of the entire wheel.
 
Could be spoke quality issue.
DT Swiss or Wheel Smith brands are known to be reliable.
A smoother radius on the hub spoke hole might be helpful.
Spoke head to bend dimension might be smaller then the thickness of the hub flange.

These have .3mm extra spoke diameter at the head
https://www.sapim.be/spokes/butted/strong

Copied from Sapim brochure ...
Our standard spokes have a bending length of 2,8 mm. This is optimal for at least 90% of the hubs in the market.
But sometimes the standard bendings are wrong. Steel hubs need shorter bend. You can adjust this with our spoke washer.

Paint is missing from hub flange and also from spoke suggesting hard contact between the two.
Measure hub flange thickness.
Measure spoke between head and bend.
Is there enough clearance ?

Spoke head identifier
http://www.mrrabbit.net/docs/spokeheads/main.html
 
If it's an OEM hubmotor wheel, especially a "kit", it's really common for them to use spokes that are too thick (12g commonly) for the rim they used. The usual result I see is the rim then cracks around the nipple holes (sometimes just a barely visible (in direct sunlight) fracture, sometimes obvious deformation/etc), which then allows the spoke to loosen, which may result in breakage of that spoke over time (at the elbow usually). Sometimes it's other spokes that break instead. Retensioning (and replacement of any broken spokes) won't help because at that point the rim is broken and cannot maintain tension on any of the damaged nipple holes. :(

This post
https://endless-sphere.com/forums/viewtopic.php?f=51&t=117575&p=1731018&hilit=spoke%2A+rim%2A+crack%2A#p1731018
shows one such rim failure; if you look thru my posts for
spoke* rim* crack*
you'll find a number of pictures of such things to compare to (others have posted some too).
https://endless-sphere.com/forums/search.php?keywords=%2Bspoke*+%2Brim*+%2Bcrack*+&terms=all&author=amberwolf&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=-1&t=0&submit=Search
 
fechter said:
Do you know what thickness the spokes are? They may be too thick. Thinner spokes have more spring to them and will actually be more resistant to breaking (up to a point). Proper spoke tension is important too, Too loose or too tight can cause problems.

The lacing pattern in important too, and which way the spoke ends pass through the holes.

Post a picture of the entire wheel.

The spokes are 2.6mm and are factory original. To my surprise they do not stick to a magnet, so I guess they are aluminum. I've laced a lot of wheels and I can assure you that the lacing pattern was correct from the factory, and I laced it correctly for sure, so posting a picture of the whole wheel wouldn't have given you any hints to the problem.

I was also thinking that the spoke tension might have been a factor because when I made the repair the first time I didn’t re-lace the whole wheel, and I noticed that some spokes were a little tighter than others after I trued it, but none seemed excessively tight or loose so I didn’t pursue making them all even tension at the time, but when it broke more spokes I decided to disassemble and inspect everything very closely.

When I dissembled the wheel, I checked all the spokes for wear, checked all the holes for wear or sharp edges, and checked the rim for roundness and flatness, I found nothing of any significance, at all. In fact, the one picture I posted that shows a wear spot near a hole on the hub, that happened from someone riding a long time with a broken spoke, and it was the only mark on the whole hub, and the few spokes that looked like they had wear at the elbow, under magnification I only found paint removal, barely any wear at all, and those might have been loose. So I replaced any spoke that showed any wear at all, laced up the wheel properly and attained very even tension on all spokes. I rode the bike to work twice (approximately 50 miles total) and noticed I had a broken spoke when I arrived home the second day. Bummer!
 
PaPaSteve said:
Could be spoke quality issue.
I agree, especially after noticing the spokes are not steel and broke another spoke after rebuilding the wheel properly. The wheel is Pedego brand and the replacement spokes I used were factory replacements, but I think they are also available in stainless steel, which I would have bought had known these were not painted steel.

PaPaSteve said:
DT Swiss or Wheel Smith brands are known to be reliable.
Duly Noted, but I might try factory original stainless spokes before going to a third party spoke.

PaPaSteve said:
A smoother radius on the hub spoke hole might be helpful.
I don’t think this was a factor, there was no indication of damage from the hub to the spokes.

PaPaSteve said:
Spoke head to bend dimension might be smaller than the thickness of the hub flange.
This has been checked, it’s perfect.

PaPaSteve said:
Paint is missing from hub flange and also from spoke suggesting hard contact between the two.
Only on one hole, which I think happened after a spoke had broken, and only a few spokes out of thirty-six had missing paint, which may have been caused by riding with loose spokes.

PaPaSteve said:
Measure hub flange thickness. Measure spoke between head and bend. Is there enough clearance ?
Checked, we’re good here.

I’m really leaning towards the problem being the spokes are not strong enough for the job. I’m going to order the stainless steel spokes before pursuing anything else. The motor is quiet and has good torque, it’s worth the effort.
 
amberwolf said:
If it's an OEM hubmotor wheel, especially a "kit", it's really common for them to use spokes that are too thick (12g commonly) for the rim they used. The usual result I see is the rim then cracks around the nipple holes (sometimes just a barely visible (in direct sunlight) fracture, sometimes obvious deformation/etc), which then allows the spoke to loosen, which may result in breakage of that spoke over time (at the elbow usually). Sometimes it's other spokes that break instead. Retensioning (and replacement of any broken spokes) won't help because at that point the rim is broken and cannot maintain tension on any of the damaged nipple holes. :(

This post
https://endless-sphere.com/forums/viewtopic.php?f=51&t=117575&p=1731018&hilit=spoke%2A+rim%2A+crack%2A#p1731018
shows one such rim failure; if you look thru my posts for spoke* rim* crack* you'll find a number of pictures of such things to compare to (others have posted some too).
https://endless-sphere.com/forums/search.php?keywords=%2Bspoke*+%2Brim*+%2Bcrack*+&terms=all&author=amberwolf&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=-1&t=0&submit=Search

It’s not a kit motor, and although Pedego isn’t known for outstanding engineering, I think they make an honest attempt to put out a good product. I think they just underestimated the strength of non-steel spokes and messed up here. I’m hoping that the stainless steel spokes will be significantly stronger.
 
It just occurred to me that these spokes might be painted stainless steel :shock: Is that possible? I can't imagine anyone painting over stainless for the look, that seems absurd, but on the other hand, I have never heard of aluminum spokes either. I guess I can wire brush a spoke to see if I can determine if it's aluminum or not.
 
Just my option. The spoke failure does not look to be weak spokes. They all broke at the head. This is a fatigue issue from the bend flexing. Several things can cause this. When rebuilding the wheel make sure the bend/head has proper placement and does not need to be shimmed. Well built wheel last for 10k + miles. Then their is, could have been a bad batch of spokes.

people paint spokes all the time.
 
ZeroEm said:
Just my option. The spoke failure does not look to be weak spokes. They all broke at the head. This is a fatigue issue from the bend flexing. Several things can cause this. When rebuilding the wheel make sure the bend/head has proper placement and does not need to be shimmed. Well built wheel last for 10k + miles. Then their is, could have been a bad batch of spokes.

people paint spokes all the time.

I double checked the bend/head placement and fitment, it seems perfect and doesn't need to be shimmed, I think we can rule that out. And one of the broken spokes is a new one, so that is too different batches of spokes that have broken, which kind of rules out the bad batch theory.

What I find most interesting is this particular comment:
They all broke at the head. This is a fatigue issue from the bend flexing.
What I mean by "interesting" is, that sounds very logical, but had you not stated it as fact, I would not have suspected that flexing occurring at right the bend location is creating the fatigue and point of failure. And given that I'm somewhat sure the bend/head has proper placement and fitment, I'm now starting to suspect that maybe the tension is too low on all the spokes, maybe they are flexing because electric motors exert a lot more torque than human legs do, and require the spokes to be a little tighter than regular wheels. I'm not using anything special to gauge tension, I can feel it with the spoke wrench, and then I use the pinch test at the crisscross, and I am comparing it to other similar hub motor wheels. I don't want to over-tighten them though either.
 
rick_p said:
I've laced a lot of wheels

How many properly? :lol:

rick_p said:
To my surprise they do not stick to a magnet, so I guess they are aluminum.

rick_p said:
It just occurred to me that these spokes might be painted stainless steel :shock: Is that possible? I can't imagine anyone painting over stainless for the look, that seems absurd, but on the other hand, I have never heard of aluminum spokes either. I guess I can wire brush a spoke to see if I can determine if it's aluminum or not.

This is not meant to offend, but given these statements I'd say there is almost 0% chance you are tensioning the spokes properly. I'd start here if you want to get closer to a properly built wheel.
 
The spokes are crap, like most too-fat spokes and those that come included with hub motors. They're junk for selling rather than for using.

My advice is to get good quality 14ga (2.0mm) spokes in the right length, plus washers for both the heads and the nipples, and rebuild the wheel correctly (including stress relief). You could also have a professional do it for you.

Reputable spokes include DT Swiss, Sapim, Wheelsmith, Pillar, and Union/Marwi. They don't include no-name chinesium.
 
rick_p said:
What I mean by "interesting" is, that sounds very logical, but had you not stated it as fact, I would not have suspected that flexing occurring at right the bend location is creating the fatigue and point of failure. And given that I'm somewhat sure the bend/head has proper placement and fitment, I'm now starting to suspect that maybe the tension is too low on all the spokes, maybe they are flexing because electric motors exert a lot more torque than human legs do, and require the spokes to be a little tighter than regular wheels.

no.

proper tension is proper tension; if you tighten them more than proper tension for the specific spokes and specific rim then they are overtensioned.

once that happens the rim is overstressed and the nipple holes begin to fail

you may not see it as an obvoius failure, but it happens; they deform, crack, etc., and then the tension is lost

when the tension is lost the spokes are loose

when the spokes are loose they flex at the elbow

when they flex at the elbow they break

if spokes are too thick for the rim (oem hubmotor wheels) this whole process is started just by tensioning the spokes "properly" for those spoke thicknesses, which make them overtensioned for the rim.

after taht, see previous steps.


no guarantee this is the specific cause of any particular wheel failure without sufficient documentation of the specific failures of that wheel for us to see, but i've seen it so often i'd bet it is the case here as well
 
Bicycle Wheel building has been around over a hundred years. Why companies don't do it right is about money. A well built wheel is not cheap.

Bet most here when buying a motor laced in a wheel don't expect it to last. But some last quite a while so best not to over look the money savings. When and if they do fail then we just put the motor in a good rim with good spokes, build it right or have it done for you then your done with it.

The expert here is Chalo and Amberwolf stresses wheels more than most here so they know.
Have only built three wheels myself and don't have any special tools out side of a spoke wrenches. All still rolling with no issues. A lot of material on wheel building. One thing that is seen all the time is big spokes failing.
by rick_p » Dec 08 2022 3:43pm

ZeroEm wrote: ↑Dec 08 2022 12:35pm
Just my option. The spoke failure does not look to be weak spokes. They all broke at the head. This is a fatigue issue from the bend flexing. Several things can cause this. When rebuilding the wheel make sure the bend/head has proper placement and does not need to be shimmed. Well built wheel last for 10k + miles. Then their is, could have been a bad batch of spokes.
people paint spokes all the time.

I double checked the bend/head placement and fitment, it seems perfect and doesn't need to be shimmed, I think we can rule that out. And one of the broken spokes is a new one, so that is too different batches of spokes that have broken, which kind of rules out the bad batch theory.

What I find most interesting is this particular comment:
They all broke at the head. This is a fatigue issue from the bend flexing.
What I mean by "interesting" is, that sounds very logical, but had you not stated it as fact, I would not have suspected that flexing occurring at right the bend location is creating the fatigue and point of failure. And given that I'm somewhat sure the bend/head has proper placement and fitment, I'm now starting to suspect that maybe the tension is too low on all the spokes, maybe they are flexing because electric motors exert a lot more torque than human legs do, and require the spokes to be a little tighter than regular wheels. I'm not using anything special to gauge tension, I can feel it with the spoke wrench, and then I use the pinch test at the crisscross, and I am comparing it to other similar hub motor wheels. I don't want to over-tighten them though either.

Here is a new thread, lets see how this goes. First Hub Wheel Build
 
ZeroEm said:
Bicycle Wheel building has been around over a hundred years. Why companies don't do it right is about money. A well built wheel is not cheap.
My own experience is that there is way too much dish/offset on some rear hub motors relative to the flange spacing. For example, my first rear hub had 42mm flange spacing with 8mm offset (13g @ 2X) and this gives a tension ratio of 29:13 which is ridiculous (imo). In this situation, its almost impossible to get any kind of tension on the non-drive side without putting too much tension on the drive side.
I corrected the situation by changing from 8 speed to 6 speed freewheel and reducing the dish/offset by over 4mm, and fitting lighter 14g spokes on the non drive side.
 
Sturmey said:
My own experience is that there is way too much dish/offset on some rear hub motors relative to the flange spacing.
[...]
I corrected the situation by changing from 8 speed to 6 speed freewheel and reducing the dish/offset by over 4mm, and fitting lighter 14g spokes on the non drive side.

In that situation, I run all the spokes outside the flange on the flatter side, and all inside the flange on the more conical side. That's good for 3.5-4mm of offset in the right direction.
 
Chalo said:
Sturmey said:
My own experience is that there is way too much dish/offset on some rear hub motors relative to the flange spacing.
[...]
I corrected the situation by changing from 8 speed to 6 speed freewheel and reducing the dish/offset by over 4mm, and fitting lighter 14g spokes on the non drive side.

In that situation, I run all the spokes outside the flange on the flatter side, and all inside the flange on the more conical side. That's good for 3.5-4mm of offset in the right direction.
I can see the above works well with single cross. But my motor was a 250W geared 122mm diameter with 2X cross spoke pattern (700c) with the inside cross very close to hub. It looks that will be very untidy in this case with one spoke pushing heavily on the other spoke close to the hole. Will it work in this case with two crosses ?
 
Sturmey said:
Chalo said:
I run all the spokes outside the flange on the flatter side, and all inside the flange on the more conical side. That's good for 3.5-4mm of offset in the right direction.
I can see the above works well with single cross. But my motor was a 250W geared 122mm diameter with 2X cross spoke pattern (700c) with the inside cross very close to hub. It looks that will be very untidy in this case with one spoke pushing heavily on the other spoke close to the hole. Will it work in this case with two crosses ?

No, you have to use cross-1 lacing or radial lacing to do it. But there's no reason to use 2 or more crosses when dealing with a hub as large as a hub motor. It doesn't help anything and it predisposes the spokes to enter the rim at a bad angle.
 
After reading the recent posts here and a couple of other threads, I see now that my past experience with building regular bicycle wheels isn’t directly applicable for wheels with hub motors because there are considerations that are not always applicable to regular wheels.

I’m meeting up with a few bike mechanics on Monday evening at a shop I frequent and donate time, one of which builds custom wheels as one of the services his shop provides, so I’ll be getting the opportunity to get their opinions on this situation too. Between then and now, I’ll post more pictures so you can see things like pattern, angle of the spokes, amount of dish, condition of holes in the rim, nipples, etc.
 
rick_p said:
More pictures coming soon.
hub.jpgspoke-fitment.jpg

Looks like they could've used spoke washers, or laced all elbows-out, to get a tighter fit. Not sure if that could cause the problem you're having.

It would help if you can post good, clear, well lit (direct sunlight is best) close-up pictures of the rim, all around it, where each of the spoke nipples are, along with a full picture of the whole wheel from the side. This will help us see if there are any distortions/etc at the nipple holes, as those will loosen spokes, and generally grow worse with time. A distortion or crack does not have to be large enough to see to be enough to loosen a spoke a bit.
 
rick_p said:
More pictures coming soon.
hub.jpgspoke-fitment.jpg
Just my opinion but the spokes look too long at the j bend and are probably too hard and strong to bend and wrap themselves nice and tight around the flange like lighter 14g spokes do when they bed in after prestressing. (I do this using leather gloves to tighten/stress opposite parallel pairs).
Interestingly, I once bought cheap 13g galvanise spokes to rebuild a troublesome rear motor wheel (with heavy rider) and they gave great service. None of them ever broke over a long life (>10000 miles) and even a second rim change.(v brakes). The spokes were much softer/supple than their equivalent harder stainless steel but they bedded in after prestressing and nicely wrapped around the flange leaving no gaps. I am inclined to think that gaps at the j bends allow movement and movement causes fatigue and eventual breakage.
 
amberwolf had a good eye and spotted it right off, must be all the wheel lacing he's done!
Sturmey
Just my opinion but the spokes look too long at the j bend and are probably too hard and strong to bend and wrap themselves nice and tight around the flange like lighter 14g spokes do when they bed in after prestressing.

Was not going to post, this is a good picture that shows this issue (don't remember seeing another).
spoke-fitment.png
Just wanted to expand on the issue for anyone that may not understand. Talking about it is one thing but a picture!

If you look at the top little red arrow, looks like a crack in the spoke, may not be.

As amberwolf stated you would want to shim the spoke until it touches the flange/hub. Even is it gets to tight you would bend the spoke around the flange to point at the spoke hole. (stress relieving) The hub should support the elbow so it does not flex as you ride. Would fix this first and see if the issue goes away, if not keep trouble shooting!
 
Back
Top