Thursday, July 1, 2010

Breaking Spokes on Tandem

In past posts I have written considerable amount about bicycle wheels. I am a proponent of high spoke tensions and do believe if you build the wheel correctly (stress relieve etc) the spokes should never break.

So imagine my surprise when I had two broken spokes on the drive side of tandem wheel (36 spoke 30mm deep rim). Did I put too much tension into the spokes? The answer is evidently yes, for the spokes used. For some reason I didn't use DT, wheelsmith or Sapim spokes, etc, but some other stainless db spokes I happened to have. My thinking must have been, "well if stress relieve correctly they will be fine."

Pic above: Broke spoke, Unbroke bad "poor" spoke, Proper Wheelsmith spoke (notice the nice filleted transition into the head). [what is a little concerning is I noticed some DT spokes have the same sharp edge...]

The spoke head was formed quite poorly, and that is what broke, more like "popped" off. The 'elbow' certainly didn't break. There is quite a sharp edge transition from the spoke wire into the head. This stress concentration spot is where a crack must have formed.
As expected for spokes that are going to break, they started breaking well under ~ a year of riding. Under the 'magic' 1e6 cycles.

I replaced all the drive spokes last week. Last night a non-drive side spoke broke, so it appears as though I will be replacing those spokes this weekend.

2014 comments and review:  As far as I can remember, I re-did the wheel and it has been perfect since.  the Tandem is now sold and we have a new lighter tandem (by tsunami ).  The new wheels for the new tandem are h plus son 43 mm deep rim with 24 spoke rear (2to1 pattern).  Its been great as well !  I used "rim bed washer" (custom made out of aluminum, yes that was lot of hacksaw and file work) to spread out the high spoke force.  The deep rim is nice for heat sink and for convection of heat during heavy braking. 


aaron said...

i really enjoyed your blog! your approach to finding and solving bike problems is refreshing.

john said...

thanks ! I have a lot more to do, as I find the time..

erik said...

I had exactly the same thing happen on a tandem wheel which I didn't build. I would have replaced all the drive-side spokes but we were on a tour and unable to get enough replacements (doh). Ultimately, we replaced the wheel with much tougher one.

The spoke breakage was one of the more lively parts of the trip. Every broken spoke led to an unexpected meeting or adventure. Luckily, none were ever more uncomfortable than an hour passed on the side of the road as I spliced in a new spoke and trued the wheel.

John Schmidt said...

I obviously need to set up some sort of notification for myself, Poor Erik made this comment in August, and I only saw it now !

John Schmidt said...
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