Why did my spokes break twice at the same climb?

I'm trying to figure out why my spokes would have broken during the same exact climb twice. I am using 11/12 gauge spokes on a dirtbike rim. Tension is checked before each ride. I would say that I tension very tight.

I've beat this rim to death, over 20 rides before I broke 6 spokes yesterday. Everything from, jumps, climbing lots of stairs, running over curbs at high speeds, nothing seemed to break the spokes. Spokes were all broken at the elbows all in the same location.

I replaced the 6 spokes yesterday and today checked tension a few times during the ride and everything was tight. I even beat the bike hard to test my shock today. Then I decided to do the same exact climb that broke my spokes the previous day. I could not believe I heard the same loud BANG as yesterday, and noticed 5 spokes (maybe 6 as one spoke looked to have pulled out of the nipple) were broken at the elbows all right next to each other.

I'm just confused and was looking for any insight into why they would break at this same hill when I never had any problems anywhere else. I wasn't even going very fast as you can see in the videos. The only thing special about this climb is that it is a steep little hill with maybe a 6 inch curb at the bottom and when the tire hits that 6" curb the spokes pop.

What is also interesting is that I broke 6 yesterday and heard the pop I didn't realize I broke the spokes, then I actually climbed it again maybe twice more with probably all 6 spokes broke and the wheel didn't fall apart.

The first time I climbed the hill today I heard the POP and noticed 5 spokes broken.

What confuses me is why is it breaking only at this exact climb, and why after breaking 6 spokes am I able to do the same climb without the wheel completely falling apart? I would think if it was able to break the first time when the wheel was fully built, after losing 6 spokes that it would surely break completely.

Could I possibly have my spokes tensioned too tight?

Bike is a torque raptor bike, with a Downhill rear bike shock that has 10" of rear travel.

First video when I broke 6 spokes, you can hear the POP.

[youtube]jWUstvMx4h0[/youtube]

Another angle of the climb, this was right after I actually broke the 6 spokes and am driving with 6 broken spokes without knowing they are broken.

[youtube]3p6zVsTRNPY[/youtube]

My non-professional opinion is that your spokes are failing due to shear, not tension. You state that you have them tight, real tight. Then you beat the wheel with jumps, stairs, drops, etc. That's all shock tension loading.

When you climb the big hill, you're loading the spokes in shear, with the force being concentrated at the weakest point at the elbow. Because it is a big hill, you are applying full power and thus the highest possible shear loading at the elbow. As opposed to the abuse you gave the spokes with jumps and stuff, which was tension loading (exactly what spokes are designed to be strong for), your hill climb was killer shear loading, where spokes aren't as strong.

mlt34 said:

My non-professional opinion is that your spokes are failing due to shear, not tension. You state that you have them tight, real tight. Then you beat the wheel with jumps, stairs, drops, etc. That's all shock tension loading.

When you climb the big hill, you're loading the spokes in shear, with the force being concentrated at the weakest point at the elbow. Because it is a big hill, you are applying full power and thus the highest possible shear loading at the elbow. As opposed to the abuse you gave the spokes with jumps and stuff, which was tension loading (exactly what spokes are designed to be strong for), your hill climb was killer shear loading, where spokes aren't as strong.

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That's is interesting. If it is true that the tension on the spokes is different from the motor pulling the wheel then what I think may happen is I have the bike in full throttle with the tension on the spokes very high from the power of the motor.

Since the bike is lifted so high up as I climb the hill it has all the weight on the rear. Then as my rear tire hits the corner of that curb it is a sudden jolt or shock to the spokes from shear or the forces that the motor is putting on the spokes, especially with all my weight on the rear wheel, this probably instantly stops the tire from spinning and this causes the spokes to break at the elbows.

Think of it like spinning the rear wheel of your ebike with it off the ground and then instantly stopping the rear wheel from spinning by jamming something in it. This sudden stop or jolt while the motor is spinning at full speed would probably increase stress on the spokes by a lot. And since the elbows are taking more of the force, that is where they break.

What still confuses me is why would I be able to climb that same hill again with 6 spokes broken and not break a lot more of them. That still puzzles me.

What do you think I need to go with thicker spokes?

You need better spokes, and IMO, run them tight, but not too tight. I didn't look to see if that's a hardtail bike, but I have not broken a spoke in years, using bikes with rear shocks.

I wonder if the rim is out of round enough that the spokes had too be ultra tight in one specific location to true it and every time that heavy shock hits them they snap. Are the same spoke locations breaking? Whilst the wheel is strong enough to support loading with only the other " slightly more loose" spokes to allow continued use?

When you are checking tension, do you check all the spokes? Are they all very even in tension?

I would say that the spokes are not able to stretch, so they break. You either have them so tight that they are close to yield OR some are uneven in tension, creating a spike in tension on the other spokes that is beyond their yield.

What kind of 11/12 spokes are you running?

standfast said:

I wonder if the rim is out of round enough that the spokes had too be ultra tight in one specific location to true it and every time that heavy shock hits them they snap. Are the same spoke locations breaking? Whilst the wheel is strong enough to support loading with only the other " slightly more loose" spokes to allow continued use?

Click to expand...

This is interesting because, yes the wheel is not exactly round, it also has a pretty bad hop when riding. I just noticed this and it wasn't like this.

I would say its as much as 5-8 mm from low to high point on the rim.

I wondered about this also. I also think that maybe I over tightened the spokes and that is the reason why the wheel is not round anymore.

The other thing is, I didn't check for uniform tightness, some were much more tight then others, I did this by constantly checking the tension and just kept tightening. None were loose, they were either tight or extremely tight.

I am also wondering if the spokes are too tight as I once blew 9 spokes out of the bike when over tightening them as you can see in this post.

http://endless-sphere.com/forums/viewtopic.php?f=3&t=56012

The other interesting thing is that after the spokes break with the initial hit and I lose 6 spokes, I am able to climb and hit the same curb with 6 less spokes and none break. You would think that once you break 6 spokes on the first hit, that successive hits would easily break the rest, but that's not happening. It is like the spokes fare better when the are not tight. Is this a sign that I over tightened them?

So I have a rim that is not exactly round and it is really noticeable as I can see much more threads on the spoke in certain areas where the rim is obviously a high point.

What can I do about making the rim round? These 11/12 gauge sapim spokes don't seem strong enough to true radially as the wheel I have is a prowheel aluminum motorcycle rim. What do you think I should do?

you definitely would want an even & correct tensioned whee per the spoke gauge etc used
in order for the wheel to be strong and reliable..
uneven, loose, or over tension will result in spoke failures as you notice..
its best to bring the wheel up to tension evenly, do radial true first, then lateral true..
also you can true by loosening opposite instead of over tightening..
to fix your wheel or see if the rim is damaged unlace it and lay on flat surface..
inspect for roundness as well..
if it bent out of shape when unlaced then it should be replaced..
(or maybe beat on if your desperate)
good luck..

i also agree with the load & spoke stressing mentioned..
max loaded torque of that motor on incline would def be a lot a stress on an unstable wheel build..

Offroader said:

standfast said:

I wonder if the rim is out of round enough that the spokes had too be ultra tight in one specific location to true it and every time that heavy shock hits them they snap. Are the same spoke locations breaking? Whilst the wheel is strong enough to support loading with only the other " slightly more loose" spokes to allow continued use?

Click to expand...

This is interesting because, yes the wheel is not exactly round, it also has a pretty bad hop when riding. I just noticed this and it wasn't like this.

I would say its as much as 5-8 mm from low to high point on the rim.

I wondered about this also. I also think that maybe I over tightened the spokes and that is the reason why the wheel is not round anymore.

The other thing is, I didn't check for uniform tightness, some were much more tight then others, I did this by constantly checking the tension and just kept tightening. None were loose, they were either tight or extremely tight.

I am also wondering if the spokes are too tight as I once blew 9 spokes out of the bike when over tightening them as you can see in this post.

http://endless-sphere.com/forums/viewtopic.php?f=3&t=56012

The other interesting thing is that after the spokes break with the initial hit and I lose 6 spokes, I am able to climb and hit the same curb with 6 less spokes and none break. You would think that once you break 6 spokes on the first hit, that successive hits would easily break the rest, but that's not happening. It is like the spokes fare better when the are not tight. Is this a sign that I over tightened them?

So I have a rim that is not exactly round and it is really noticeable as I can see much more threads on the spoke in certain areas where the rim is obviously a high point.

What can I do about making the rim round? These 11/12 gauge sapim spokes don't seem strong enough to true radially as the wheel I have is a prowheel aluminum motorcycle rim. What do you think I should do?

Click to expand...

mlt34 said:

My non-professional opinion is that your spokes are failing due to shear, not tension. You state that you have them tight, real tight. Then you beat the wheel with jumps, stairs, drops, etc. That's all shock tension loading.

When you climb the big hill, you're loading the spokes in shear, with the force being concentrated at the weakest point at the elbow. Because it is a big hill, you are applying full power and thus the highest possible shear loading at the elbow. As opposed to the abuse you gave the spokes with jumps and stuff, which was tension loading (exactly what spokes are designed to be strong for), your hill climb was killer shear loading, where spokes aren't as strong.

Click to expand...

Truing for lateral trueness is the easy part, hop can be a real pain in the butt. Even one overtightened spoke can make it nearly impossible to suss out a round wheel. That probably contributed to this large degree of hop you're noticing now.

I agree that overtightened spokes probably played a role in your spoke failures. Spokes are designed to have a little bit of 'bounce' in them, and when they are overtightened then they have no room left to stretch.

If you're breaking spokes that thick, it's because the spokes have a problem. Needlessly thick spokes like yours will give you various kinds of trouble, but not spoke breakage.

Breakage at the elbow is characteristic of fatigue cracks precipitating from localized stresses that have not been adequately relieved. But with log-thick spokes the stress levels are low enough that even this should be an infrequent problem. My best guess is that your spokes are poorly manufactured, made from substandard material, or both.

Use 14ga or 14-15ga DT Swiss or Sapim spokes, get them tightened to at least 100kgf each, stress-relieve them thoroughly, and use washers under the spoke heads to support them in oversized hub holes. If you do this, you should have no further problems.

mlt34 said:

I agree that overtightened spokes probably played a role in your spoke failures. Spokes are designed to have a little bit of 'bounce' in them, and when they are overtightened then they have no room left to stretch.

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Um, no. Spokes are springs. Up to the point where they yield (which is tighter than most rims can withstand), you apply more tension and they elastically stretch more in direct proportion. There is no possible way the OP's spokes could have been "overtightened", whatever that means. Compared to the center of a 14-15ga spoke, a .120" spoke has about four times as much cross-sectional area. You could tighten it to 500kgf (and destroy your rim) without overstressing its material. Unless the material is defective or unsuitable, of course, which I expect is the OP's case.

Chalo said:

Um, no. There is no possible way the OP's spokes could have been "overtightened", whatever that means.

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I'm not certain that you understand. Spokes can quite certainly be overtightened. There is an optimal tension for every spoke in every wheel. That's why tools exist to measure the tension. If a nipple is wound to tight, loading the spoke in too much tension, it is by definition overtightened. A skilled wheel builder will tension each spoke to the correct amount. Many people just trying to do a quick truing job simply spin nipples until things look straight, often resulting in over and undertightened spokes.

If you try to overtighten a spoke, you'll either break the rim, the hub flange, or tear the head off the nipple. The spoke would be the last thing to break.

We need a closeup of the spokes where they broke to get a better idea of how they failed.

I'd suspect it's a combo of factors.

Your spokes are too thick. Spokes are springs. They are designed to share the load around the entire wheel. when you hit a bump like a curb corner, they stretch to absorb the impact, spreading the load over all 36 spokes. If the spokes are too thick, they can't properly stretch and won't be able to share the load with the other spokes. The localized stress on the spoke will be too high, and you'll end up with stress failures. 14g is about right for under 5000kw. Or a single butted 13/14

Your wheel build had issues. Out of round wheel will overstress the spokes and cause fatigue failures. Its much worse if you combine with too thick of spokes that can't share the load, and a third factor, poor quality spokes that just can't take it.

From the video, it looks like you're riding with that suspension topped out. And worse, it looked like it didn't even move when you hit that curb. If the suspension is too stiff, it can't do it's job, and the point of impact has to take all the force of the hit. That will make things even worse if you combine it with a wheel that is out of round. You should have around 25% sag for road riding, around 35% for off road

I think it was a combination of problems at work here.

First, my spokes were either tight or extremely tight. I gave up on keeping the wheel true, so I just kept cranking down on any spokes that were loose. I also constantly went around the rim checking for any loose spokes and probably kept over tightening them.

The rim is not radially true anymore, I am not sure how I can fix this on a dirt bike wheel, this may have caused some more uneven tension.

What leads me pointing towards the spokes were too tight is that after breaking 6 spokes, I was able to do the same climb without breaking any additional.

The spokes all broke at the elbows in a loud pop. This was the same thing that happened when I over tightened the 13 gauge spokes and 9 of them just exploded at the same time.

Now when I tension them I am backing off on any that are tight and trying to get a uniform tension, it is surprising how some of them will get really tight. I would imagine I had some of them super tight.

Equal spoke tension is decently important on normal wheels, but high power Ebikes and wheels that get abused a lot need well tensioned spokes. This is a great tool to use to check your spoke tension when building a wheel or repairing a fairly untrue wheel. http://m.parktool.com/product/spoke-tension-meter-tm-1

mlt34 said:

Equal spoke tension is decently important on normal wheels, but high power Ebikes and wheels that get abused a lot need well tensioned spokes. This is a great tool to use to check your spoke tension when building a wheel or repairing a fairly untrue wheel. http://m.parktool.com/product/spoke-tension-meter-tm-1

Click to expand...

Thanks, but will that fit on spokes that are 130mm long?

I don't recall what the minimum length is. I don't own one, I always borrowed one since I only build a few wheels per year, but if I recall correctly the minimum was somewhere around 100 mm, so I think you should be fine.

I just wanted to add that I'll be keeping my spokes not as tight and more uniform. I am constantly checking them, even when I am riding I drop the bike on the side and quickly check spoke tension. I currently have about 4 13 gauage spokes on the rear right now so I want to wait until I get more 11/12 gauge before I try and break them again. Hopefully with less tension and more uniform tension they won't break.


Also, I really beat my bike hard. First, I'm running a heavy 110 lbs ebike and I hit curbs at very high speeds. I would say that most people would never subject their rims to as much abuse as I do. Hitting a corner of a curb doing 20 + MPH is a lot of stress on the rim. Any bicycle that tried to do that would instantly pinch flat the tube, you would need a dirt bike tire to take those hits.

Even with most of my aggressive riding, the rims and spokes hold up. But as soon as I start riding up stairs and hitting large holes that have sharp edges, the problems start. This is a lot of force and especially at the speeds I'm hitting them at. When climbing up stairs, I'll come at those stairs sometimes with a lot of speed to keep my momentum going. Riding down stairs isn't really a problem.

My point is that if most people were abusing their bikes like I do, there would be a lot more issues in my opinion.

"Doctor, it hurts when I do this!"

"Well, don't do that."

So I think the problem my spokes were breaking was most likely due to actually way too high and possibly uneven tension. Both of which I did too a high degree. I should have known over tensioning spokes is bad as I once snapped 9 13 gauage spokes at the same time by over tensioning the wheel during a build.

I've since lowered the tension of the spokes and check them often for uniform tension. I also have a few 13 gauge spokes mixed in with the 11/12 gauge spokes and they are holding up fine for weeks now.

I haven't been killing the rim either as hard, but I still do a lot of jumps and riding down stairs, although it seems some of the hard hits I did that broke a lot of spokes I have mostly avoided. Maybe I will do more testing when I get a new rim and spokes, just incase I break some. I am sure that I won't be able to break any.

The reason that makes me certain that it was over tensioning of the spokes is because after a broke a few spokes, I continued to do what broke the spokes in the first place and no more broke even though I had 4 or 5 broken spokes. By breaking these spokes it actually reduced tension in the wheel and made for a stronger build.

You would think that if it was too low tension or hitting the curb too hard that broke 4 spokes, it would absolutely break more during initial hits after the 4 or 5 spokes have broke.

Good spokes usually will pull through the rim, or tear the heads off their nipples, before they break. So to suggest the spoke tension was too high (which may in fact have been true) is to ignore the real reason your spokes broke: They were lousy. Most no-name spokes from China are terrible, even if they look alright.

Can you post a picture of your lace pattern?

This is an old picture but you can see the lace pattern, typical hubmotor lacing.

The other thing is I'm using a motorcycle wheel and the spokes have large steal nipples where I use a 5mm large motorcycle spoke wrench. This makes it very easy to torque down those spokes, and the motorcycle wheel won't break either.

This is unlike a bicycle wheel with those tiny spoke wrenches, and tiny brass nipples that round if you put any torque on them.

I also bent my $600 dollar cromotor recently. The stator is rubbing the magnets and I can't find out what bent. I ordered a new cromotor and I will not be abusing this new motor as it sucks to spend $600 dollars on a motor and warp bend something to make it rub inside. I don't know if I bent the axle or side covers. I took it apart and you can't see what is bent because the tolerances are so small.

the OP is using high quality Sapim spokes..
is there a better high quality spoke than sapim or DT/swiss that you recommend??

Chalo said:

Good spokes usually will pull through the rim, or tear the heads off their nipples, before they break. So to suggest the spoke tension was too high (which may in fact have been true) is to ignore the real reason your spokes broke: They were lousy. Most no-name spokes from China are terrible, even if they look alright.

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efMX Trials Electric Freeride said:

the OP is using high quality Sapim spokes..
is there a better high quality spoke than sapim or DT/swiss that you recommend??

Click to expand...

Sapim will do the job. In fact I have never had a Sapim spoke break on any of my bikes (with spokes as small as 14-17ga).

It sounds like there is a problem with something else about the build.

If the breakages are at the threaded end, there are two likely causes: Poor alignment between the spokes and their nipples, or else badly cut threads.

If the breakages are at the elbows, as they are most of the time, there are also two likely causes for that. First is that the spokes have not been adequately stress-relieved. That's easy to do with thin spokes (squeezing parallel pairs like Jobst Brandt, or prying crossed pairs like Sheldon Brown), but can be difficult or even impossible to do with overthick spokes. The objective is to briefly load the spokes to approximately twice their normal operating tension, to cause the elbows to yield and then relax when the spoke tension returns to normal.

The second likely cause of breakage at the elbows is inadequate support at the hub flange. If the flange is steel and is thin or has sharp edges on the holes, it can indent the spokes at the elbow and precipitate cracking. The antidote for this is spoke washers that better support the elbows.