Spoke fractured at end of thread

Same again here

A problem that has arisen with a front Shimano 7850 Dura-Ace Clincher wheel and a rear
RS20 wheel; spoke fatigue breakage after around 2000 miles of riding.

The first point to be made in this article is that the 7850 wheels ( or Carbon 1380s as they are also known) tested by RCUK were pre-production samples, so production examples may not be prone to the problem.

That production 7850 wheels may be better is entirely possible since the problem is one that could be solved with little difficulty. In both cases it is one of spoke breakage at the nipple or, rather, where the threaded section of spoke enters the threaded section of nipple.

Such breakages are invariably the result of poor alignment of the nipple with the spoke. Conventional rim eyelets work with spoke nipples to allow the nipple a large degree of free movement, ensuring that each one is in precise alignment with its spoke. If, however, the nipple is rigidly aligned by the width and direction of the hole it sits in, the spoke may take a slight kink as it leaves en route for the hub or, in some cases, the rim.

Should the kink happen to coincide with the threaded section of the spoke, the V of the thread acts as the perfect stress-rising notch. Successive cycles of increased and decreased tension in the spoke as the wheel rolls and the rim is compressed against the ground will flex the spoke slightly. The notch starts a crack, which sooner or later works its way though the spoke.

Interestingly, the 7850 spoke broke while the wheel was sitting in the back of the tester’s car following a time trial; the RS20 spoke broke during a ride. Close examination of both indicates that the nipple sits in a close-fitting hole with little possibility of self-alignment to alleviate the problem. The obvious cure would be the remodeling of the spoke hole to permit correct nipple alignment.

RCUK would not expect this to afflict the 7850 SL Clincher wheel for use with tubeless tyres, since both ends of the spoke are devoid of a thread, the nipple threading directly in to the rim to provide tension adjustment. Without a thread to start a crack, they should be far less susceptible to this problem even if the nipples are not optimally aligned.