Hunt-man

I have a Powertap laced up with CX-Ray spokes. I had a shop build it up. Started breaking spokes. Had them do it again with new CX-rays / nipples and rim. Now I'm breaking spokes again. They all seem to break at the bend by the head. Why?

I really don't want to let the same shop "re-do" this wheel a third time.

Just pissed cause I got stranded 45 miles from home today. Tweaked the spokes but it was still hitting the frame.

Al1943

This is usually caused by insufficient spoke tension. Low tension causes excess repeated flexing and the spokes usually fail in the "J" bend at the hub flange.

HillRider

That's it in a nutshell. Inadequate spoke tension = fatigue failure, usually at the j-bend but sometimes at the threads.

Hunt-man

Hmmm. Builder used a Park spoke gauge. It was done by a local shop. After 200 miles he checked the tension again. I witnessed the second tension check, on all spokes. About 500 miles or less since last checked.

Anyone happy with these spokes long term?

I'm 175lbs and not all that fast / strong and ride on the road. I do climb a lot of hills.

AEO

CX-rays use a steel that is less ductile than DT aerolites, which is why they are cheaper.

Ingleside

CX-Rays can be nice spokes, but if you really don't want any problems there isn't anything wrong with some DT double butted spokes. Sure, they're nothing special but they last.

Hunt-man

At this point I just want something that will stop breaking. Deciding if I just have the spoke replaced or rebuild. Leaning toward spoke replacement but I know where that got me the first time, a rebuild and new spokes....

If I rebuild I'll try the DT's double butts.

reptilezs

wheelsmith spokes are good too. a builder can use a tension guage all day but he or she needs to know what they are looking at and interpret the reading

davidad

A friend who is heavy and strong cracked two Open Pro rims built on a Power tap hub. I rebuilt the second one. We decided to get him a stronger rim because powertaps don't come in 36 hole.
Then we had problems with the spokes on the non-drive side breaking at the nipples. Because of the extra dish with that hub I found that tensioning the drive side to 130kg I could get the tension high enough on the left to stop the spoke breakage.
It sounds like the builder is not putting enough tension in the spokes. That is the biggest cause of breakage.

AEO

depending on the rim, spoke tension above 120kgf can cause the spoke to pull through and crack the rim when running over some bumps and dips.

mrrabbit 

Poor tensioning...

...that wheel builder you are using needs to do their homework big time. My guess is that they are not used to using the tensionmeter properly with non-round spokes. Get the front wheel at least into the 100-103 kgf range and the rear drive side up around 110 kgf...anything less is begging for trouble...the Park manual describes exactly how to use the tool on non-round spokes.

=8-)

Hunt-man

Well I sent Sapim an email and they had the US Rep get in touch with me. I've sent pictures and discussed it. They are looking into it. I don't think spoke tension is the issue but I may be wrong. The initial thought is the CX-rays would like a washer at the spoke head. They suggest the spoke should be in contact with the hub flange, after the spoke bend. That is not the case now.

So we are waiting for the factory folks to come back to work at the end of July. I'll let ya know what they think. I just hope we can get it fixed.

Thanks for the help.

FBinNY 

Spokes break at the elbows for the simple reason that stainless is roughly 80% as strong in shear (across) as it is in tension. That means the moment your spoke makes the 90° bend to enter the hub flange it looses 20% of it's strength at that point. And being the weakest point, it shouldn't be a surprise that that's where it breaks. Every once in a while I'm surprised by a spoke that breaks elsewhere.

That's the original rational for butted spokes. Why carry the weight of a 2mm section the entire length when it's weakest point is only as strong as a 1.8mm section anyway? Also as someone else pointed the spoke is free to flex over it's entire length, but constrained at the hole, so the constant flexing works the spoke at the bend eventually work hardening it to where it's brittle and fails. To see this effect, bend a paper clip back and forth in one place, after a while it'll stiffen than a bend or two later snap.

Now that's the science, but you have spokes breaking and most other folks don't. I hate to say it, but tension gauge notwithstandiung, I suspect it's the builder, and not the spokes. If you're willing to give up the hub for a while, I suggest you let someone with a solid reputation re-build the wheel, using the spokes that he recommends. If you don't know anyone, here's a shop that builds dozens of wheels per week, at reasonable cost and fast turnaround and has a long history of satisfied clients.

operator

There is NO reason to build a powertap on anything but double butted spokes. We've built a million of those ptaps onto open pros/dt 1.1's with dt straight gauge/double butted spokes with no problems.

Besides the low tension issue they'll break at the head if the fit is loose as well. Which is why spoke washers are made. I can't confirm if the powertaps hub need those as we never build with cx-rays.

FBinNY 

It's possible that the flange is thinner than average and these spokes want washers to pull the bend closer, but it's still the builder's fault and not the spokes. The proper selection of materials, and knowledge of things like whether particular spoke and flange combinations are OK or need a washer, are as much part of the builders service that you pay for as delivering a true wheel.

Hunt-man

Thanks for the info. What should I do? Sapim seems willing to give me new spokes to rebuild. Very nice of them. Should I rebuild with washers, CX-rays, Powertap and Kinlin XR-300's?

I was ready to rebuild with DT / Open Pro or something else. I just want it to work and maybe even use it to train for Cyclocross, never wanted to put the stress of cyclocross on this current build.

I know others have great success building durable training wheels with powertaps, even lightweight carbon rimed wheels. Mine shouldn't suck this bad.

I'll try to post some pictures.

Hunt-man

You can see that the spokes don't come close to filling the holes in the powertap.
IMG_1929..jpg

IMG_1932..jpg

This shows the gaps between the spoke body and the powertap flange:



This is the front wheel. I'm very nervous that only the spoke heads are holding the spoke on. I've broken spokes before on this wheel:
IMG_1937..jpg

Here is where the spoke broke and where they have been breaking:

FBinNY 

I can't say washers or not, without seeing the hub and spokes, but if a washer under the head will bring the spoke in so it sits closer to the flange, then do that. Also, consider DB spokes. Possibly Sapim will let you pay for the upgrade. It's counter intuitive but butted spokes generally build more durable wheels.

The rim is less of a factor, as long as it meets reasonable quality standards it should be fine. Go with your instincts and find yourself another builder, you want one with the experience and judgment to put together the right package of rim, spoke, and technique to build a wheel that'll last. This one had two shots and came up short both times.

______________

PS- after seeing the pictures, I suggest that on the next build you put the heads in and spokes out on that front wheel. It isn't that the heads are the only thing holding it together, that's true either way. It's that the spoke coming off the outside will touch the rim of the flange getting some support beyond the elbow, and thereby reduce flexing there.

As to the rear, if the flange is counter sunk alternatingly inside and out, the heads belong on the non-countersunk side, with the bend in the countersink. If all the holes are countersunk on the same side it's a crappy design, but washers under the heads (or half of them) will compensate for it. Lastly, and this is a who do you believe thing that probably barely makes any difference, but I build rear wheels with the pulling spoke to the outside of the flange because they generally get better support. (about half the builders will vehemently disagree with this analysis, and have an unrelated but equally valid reason to do it the other way, so get a quarter and toss it)

Hunt-man

Thanks so much for you thoughts. I think the DB's are where I'm heading. It is interesting how strong the CX-rays "test" at on their web site. The seem to be the strongest or most durable spoke they sell.

Click on the yellow arrow:
https://www.sapim.be/index.php?st=pro...40&detail=aero

Tunnelrat81

This might be an unnecessary question, since the break you show is closer to the flange...But have you dropped the chain off of the top of your cassette a time or two? I see some damage on the leading (heads in) spokes a small distance up from the bend.

Proper tuning and periodic attention will eliminate this. "Chain damaged" spokes are compromised spokes, and will be more likely to fail.

-Jeremy

Hunt-man

Good catch on the photos. I had the wheel rebuilt and during the rebuild they found a bad bearing and replaced it. Well, you have to send the wheel in for a bearing replacement and it didn't work. So it went in to Saris for repair at the shops expense. When the wheel came back, the marks on the spokes were there. I don't know that they did it or if it happened at the shop but I think it happened at Saris. It was an inside spoke that broke, not one that was slightly damaged.

Tunnelrat81

Probably no way of proving that it was their doing, but if it was my wheel, I'd be contacting them about replacing those spokes too. Occasionally I've seen "plier" marks on straight pull spokes from a builder having trouble getting the tension high enough without the spokes spinning, but there shouldn't be any reason for them to return your wheel with visibly damaged spokes that close to the flange. Perhaps they tossed it onto a test bike and didn't adjust the limit screw. That's their mistake and personally I wouldn't be happy with it. The spoke on the left side of the flange in your picture #1 even looks like it's got a slight kink in it. No bueno if it wasn't there when you gave them the wheel.

-Jeremy

Hunt-man

No matter what happens, all new spokes will be in there. If it was just one spoke, replace it. Repeated spokes breaking, something is way wrong and that is where I sit now.

Tunnelrat81

mrrabbit 

I was thinking about this some more...I still think tension is the primary issue....however from the photo...using 2.3/2.0 would be a workable option....it's something I'd consider. Snugs up the hole a little more...hugs the flange a little more.

Do have a question though..

Are those elbows 6.1/6.2/6.3mm elbows...or 7.0mm elbows? Measuring from the outside of the mushroom to the outside of the shank will answer. The photo seems to suggest that you ended up with something more than 6.3mm...which is not good. Of course online photos without a caliper is not as good as having on hand.

Just a thought...

=8-)