Nipple hole beds are complex and there is still much to learn.
Few parts of rim design are less addressed and more important than nipple bed design. We consider the nipple bed to be rim material supporting nipples as well as transitions from sidewalls of the rim to this bed. Only one shape is fully supportive of spoke tension, and it is extremely uncommon, one we created and it can be easily demonstrated why it is so correct. Historically, the search for solutions to this problem led to the invention of eyelets, double eyelets, nipple washers and more. Fig 1 shows a patent for the first aluminum rim that proposed all three current methods of nipple hole reinforcement. Rims at this time were made from rolled aluminum tubing, then joined, drilled and reinforced.
Flat spoke bed axially sectioned
Flat spoke bed laterally sectioned
When viewed from the side, there is no initial fore and aft contact between nipple and rim. As the nipple and rim bed yield, contact shifts from two point to very small surface contact. Stress remains very high. Additionally, this spoke has a tendency to rock fore and aft if loads other than pure tension are applied. The result is a feeling of low wheel stiffness as nipple are allowed one degree of freedom
Convex spoke bed axially sectioned
A commonly proposed solution is a convex spoke bed, this changed the orientation of the degree of freedom but does nothing to reduce or eliminate it. It could be argued this increases the flexy feel as the direction of pivot is now axial.
Convex spoke bed laterally sectioned
Here is the lateral sectioned view of the convex spoke bed. The contact is readily noticeable and behaves as a loose rocking joint between these critical components, all under enormous loads.
The most elegant solution
Now imagine a sphere with a hole drilled in it, that aims at the center of the sphere. The interface of a nipple in this hole will be a plane. The most elegant solution is with axial and lateral radii of the nipple bed nearly equal. This simply requires a convex nipple bed using a radius equal to the diameter of the bed. As an example, a 17mm deep 700C rim would use a radius of about 300mm, effectively spherical in form and, therefore making a substantially planar edge to the nipple hole. When these holes are drilled at compound angles of 15° or more, we still see substantial stress equilibrium around the periphery of the nipple hole. Initial values are 95% lower while post local yielding we remain at 65% lower stresses.
Here is a spherical spoke bed applied to a rim section, the curvature left to right is the same radius as the fore and aft radius. Tested compared to a flat nipple bed, this feature offers a 10% increase in lateral wheel stiffness. The wheel enjoys a 10% gain by simply arranging a full contact nipple seat.