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Sapim CX-Ray Spoke Life Calculation

4K views 10 replies 7 participants last post by  dcgriz 
#1 · (Edited)
Sapim CX-Ray Spoke Failure Calculation

Reading about CX-Ray spokes fatiguing to breakage in about 2,000 mi. in a poorly built wheel got me thinking, and then running some calculations:

I have a Sapim catalog downloaded in 2013, showing what may be a cycles to failure of 3,500,000 cycles (equivalent to wheel revolutions) for CX-Ray spokes. If this is a correct interpretation of the data, you can understand why Sapim has since removed this info from their website. Without more info available from Sapim on the test details, let us consider that this result could describe the performance of an under-tensioned wheel.

I measured my 700c front wheel with a 25c tire mounted to have a diameter of 26-7/8". The tire is probably somewhat under-inflated and I need to pump it up before my next ride, but I don't expect the diameter to increase much at full inflation. The circumference (diameter x pi) of this wheel is therefore, 7.04' which is also the distance the wheel will travel with one revolution. This can be expressed as 7.04 ft/rev. If this wheel were under-tensioned I calculate the CX-Ray spokes in this wheel would fail in (3,500,000 rev) x (7.04 ft/rev) / (5,280 ft/mi) = 4,664 mi, plus or minus whatever tolerance there may be in the spoke fatigue test data.

Applications to real-life experience for wheels built with CX-Ray spokes:

1. These wheels can last for a long time when properly built and with adequate spoke tensions. How long? I'd love to hear how long or how many miles wheels built with CX-Ray spokes are lasting in use on the road.

2. All spokes are subject to fatigue if they are under tensioned, not just CX-Ray spokes.

3. If the spokes lose tension they will fatigue, and with sufficient wheel revolutions in this condition, the spokes will eventually fail from fatigue, or the wheel could fail from other causes. Remember to check for even spoke tensions (pluck or tensiometer) at least annually, and maybe more frequently if you are putting high mileages on your wheels.

I rewrote my post because it seemed better to me than just deleting the text!
 
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#2 ·
A "load cycle" for the purpose of determining fatigue life is not equivalent to a revolution of the wheel. It is equivalent to loading the spoke until its yield point is reached.
Normal usage does not load the spokes to their yield limit so trying to calculate mileage out of the fatigue life of the spoke is inconclusive at best.
 
#5 ·
Yeap, and these numbers have been long removed from the Sapim website and have been replaced with an abstract chart indicating relative fatigue cycles between spokes which IMO is meaningless without the specific testing parameters.
The OP is looking at an older Sapim document.
 
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