Spoke Breakage. Defect or Explainable?
 

This is mostly a curiosity question but I still have a front wheel that has these spokes.

Last month I had the displeasure of having several spokes break on a century ride.

The full story is here:

http://backroom.hardsdisk.net/rivervalley.html

The vintage Nashbar (Maruishi) touring bike was new to me and had one broken spoke when I got it. I also found an odd replacement spoke on the drive side, after the ride, when I rebuilt the wheel with all new spokes.

I weigh about 195. This 27", 40 spoke wheel, should have easily carried my weight and a few accessories.

All 5 spokes that broke on the ride, broke right at the place where the head (button) meets the spoke, before the bend (leaving an invasion force of 5 tiny flying saucers along the road somewhere). Spokes broke on both sides of the wheel, 2 on the drive side.

The original stainless spokes had an abrupt, sharp change where the spoke meets the head (left two, in picture). The new replacement Wheelsmith spokes have a more gradual curving transition at the head (right one, in picture). It seems to me that this is a better way to manufacture the spoke, with less of a stress riser.

I've had a couple of spokes break on other old bikes in the past but I recall them breaking at the bend, which I thought was the usual place for them to fatigue. Is it unusual for a spoke to break at the head?

Did these spokes break due to a defect that was just manifesting itself before I got the bike or was it just likely due to improper tensioning, made worse as more spokes started to break?

I only checked the tension by feel when I initially tuned the bike up and replaced the broken spoke it had when I got it. It seemed fine but the tension may have been tweaked by whoever installed the 1st replacement spoke before I got the bike.

I'd just like to get an educated guess as to why these spokes failed and if I need to worry about the front wheel, which has been fine so far but obviously hasn't had/doesn't get the stresses the rear wheel has.

http://backroom.hardsdisk.net/images/nashspokes.jpg

you can clearly see the break following a line from where it exits the hub to the rim. Poor spokes indeed.

since the bike already had a broken spoke, I suspect the spokes were all stressed to their breaking point, or near to it. This is almost always caused by too little tension and the cyclical stress caused as the wheel rotates. If you get the wheel respoked and the tension checkedw ith a tensiometer (and probably re-tensioned after a couple hundred kms) then you will likely have no problems for years.

The reason the front spokes have not broken is because front wheels see much less load than rear wheels, and therefore need less tension to hold up.

For a number of reasons, the normal point of breakage is at the elbow.

Breaking at the head like this is very unusual, and may be the result of a manufacturing defect, but it could also be because the hub flange was too thick putting excess stress on the head.

If it's a manufacturing defect in five of 40 spokes on the same wheel, then it would be safe to assume that large quantities of spokes share the problem. Check for reports of spoke problems of bikes made that year. As I said it might be the hub flange, and the tipoff would be spokes sitting unusually close to the flange face on their way to the rim, or you could remove one and check the fit.

BTW- spoke defects aren't all that rare, some time back, I bought two GV hybrid bikes to keep in Cozumel, MX. All was well for 2 years and wen I went down the third year all four wheels had large numbers of broken spokes. I suspected vandalism and thought nothing more of it. On my next trip I brought a handful of replacement spokes, only to find that 100% of the spokes were as brittle as spaghetti out of the box. Turns out they were made of the wrong grade of stainless steel, and it was highly vulnerable to the Chlorine ion in salt.

While granting that defective spokes do exist, and can cause problems, I think your problem is spoke tension. All the spokes need to be replaced, and the wheel tensioned properly.

The rear wheel definitely needs to be rebuilt, no matter the cause. But tension doesn't explain the mode of failure. Breakage at the head is virtually unheard of, and wouldn't be caused by improper tension. Properly cold headed the head, is as strong as the rest of the spoke, so 5 identical failures there is significant.

Breakage at the head is not unheard of...

It's caused by excessive stress induced prematurely during the mushroom process - i.e., that stage at which the end of the spoke is hammered to create the mushroom head. Maybe the spoke didn't get the rigid support it needed while receiving a direct line non-deviating blow.

Usually it'll break before or while on the way into the bag/box - i.e., headless spokes will be in the bag or box with the little heads floating around.

Undertensioning of the spokes in a complete wheel will quickly finish off the rest that have already been fatiqued at that poiint.

Cold forming is usually a pretty reliable process - but then again even that process is at the mercy of the humans applying it and monitoring it for QA purposes.


=8-)

That's what I was thinking too - that's an unusual place for a spoke to break. My question is: "How old is the wheel?" 27" says to me that it could likely be 25 years old. Why'd it wait this long to break?

And add...

Stress could also be induced by applying too much force against the head during the elbow forming bend. I've seen headless spokes and heads in boxes/bags from CN, SL, Union back in the days, never DT, Wheelsmith, Echelon, Asahi or other "top" line brands though.

=8-)

Like chains where plates are pushed to the end of the pin, often parts are primed to break because of a weak area, but need that little bit of extra provocation to make it happen. 25 years sitting in a garage is nothing compared to 1 day of riding. The concept of that last straw breaking the camel's back is very apt in the real world.

It sounds like it may have been a manufacturing defect of spokes. If so, it's likely that there is enough separation in the manufacturing of wheels that the front would not have used spokes with the same defect. Assuming you've checked and adjusted the tension on the front wheel, I'd just ride it. Consider rebuilding only if it starts showing signs of the same problem.

Assuming that it's an older wheelset it's possible that it largely sat unused but I'd think unlikely. So I'm shooting for undertensioned at some point that weakened a lot of the spokes. That and maybe something in the hub design that put a higher degree of loading on the head than normal.

Spokes with defects in manufacturing should/would show up their problems early and not after many years and I's assume more than a few miles of use.

But since we haven't heard about the wheel's history all this is just conjecture.

You raised another possbility when looking at the hub design...

For example, lacing a 7.0mm elbow spoke in today's standard alloy hub results in an angular lean and rest of the head in the hole when the wheel is finished and under tension. That of course results in a much higher load on one side of the flat saucer profile mushrooms common to el-cheapo spokes which doesn't occur as much in tight fitting and flush and well seated heads of 6.1mm/6.2mm elbows. That quick non-gradual transition point from mushroom to wire on the el-cheapo spoke looks pretty inviting as a failure point in such a situation.

=8-)

When I'm lacing a wheel if the spoke can't line up with the nipple while still having some free angular play I start looking to see why. The only time it was an issue it was clearly a case of the elbows not being bent quite far enough for the head in spokes and the "fix" was simply be put a little pressure on the spoke near the flange to increase the angle of the elbow for these spokes. Either from good luck or good planning (yeah, right... :D) I've avoided some of the cases of head bind that I've seen written about recently. There was another thread recently that discussed a topic very similar to this one about much the same thing with spokes being too tight or loose in the flange hole.

If nothing else threads like this sure do help to educate us into taking NOTHING for granted and to check into the most innocent looking factors and conditions when it comes to something like a new wheelbuild.

Thanks for all of the quick replies.

My own instinct is that it was a manufacturing defect that probably just needed a little push to emerge. Maybe one head was already cracked, lasted for a while and when it went, the change in tension set more up to go. It might be interesting take a pair of pliers and see how many more heads are ready to pop off.

Used bikes are a gamble and you're right, without knowing the history, you can only look at the evidence and use your experience, which I don't have much of, in this area.

I did several searches on the combined terms; nashbar, maruishi, 1985, spokes, "broken spokes", defective, etc. and came up with nothing (now the search will bring you here). I've made a note of the problem on my page about this bike, with a link to this thread. It may save somebody some trouble or provide a bargaining chip when buying a used '85 Nashbar bike. Unfortunately, these spokes have no distinctive way of identifying them other than looking very generic.

I had the bike back on the road a few days ago, for a 20 mile ride with the respoked, properly tensioned rear wheel and there have been no problems with it. It's a nice stable ride but for my own peace of mind, I guess I'd better rebuild the front wheel as well. I'm hoping to do some more longer rides with this bike and I don't want to be preoccupied with potential problems when I should be just enjoying the ride.