September 22, 2017 4 Comments
Nipples aim from their rim holes in order to align with spoke angle. The goal is to have no bend in the spoke as it enters the nipple because threading there is a weak spot. Spokes can fracture in thread roots after many loading cycles. Nipple and rim shape determine the amount of spoke angle that can be accommodated. The angle is a product of hub and rim size, spoke number, and crossing pattern.
Today, rims are frequently smaller (650 instead of 700, 20” for folding, cargo, and e-bikes, etc.). Hubs can be larger (generator, internal gear, e-motors, etc.). These combinations produce spoke angles that are more concerning.
A perfect spoke angle is 90deg with no bend in the spoke. Side angles generated by hub width and dishing are rarely below 80 and most nipple-rim combo’s handle them. The angles we need to address are in the rim plane and a function of rim and hub size.
Grin offers a great spoke calculator, among many valuable resources. It will determine the spoke-rim angle and, incidentally, works with paired spoking (another topic). Nipples can easily aim for entries of 80-90 degrees. Less than 75 may be accommodated. Below 70 is beyond most components and requires special attention. Solutions:
1/ Kink the spoke with a plier or wrench so it enters the rim at the nipple’s angle, a slow but effective process.
2/ Drill holes larger so nipples are freer to pivot, not an option or advisable on many rims.
3/ Reduce the lacing cross number to make an angle closer to 90. Despite sub-optimal torque geometry, many builders are doing it. As Grin says, “In spite of the popular wisdom not to use radial lacing on drive wheels, empirical experience has been that this isn't really an issue with the large hubs in small rims.” The burgeoning e-bike scene cannot be slowed down even though appropriate rims are not available.
4/ Rims could address this issue, for example, with a bulge at each nipple. Motorcycles figured this out 100 years ago. Drill the rim to accommodate the required angle. Here is a solution with optimal spoke angle and torque transmission. Cycling will certainly figure this out soon.
Anticipate spoke angles and plan accordingly. An engineered solution to the situation requires initiative from rim makers. Let’s hope it is sooner than later so I can stop envying motorcycle wheels!
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