Building wheels for Cass’ Surly Krampus has called several considerations to mind. The Rabbit Hole rims supplied by Surly for this build are designed with 64 spoke holes, to be built with 32 spoke wheels The spoke holes are patterned in pairs so that the the wheel can be built symmetrically as a “normal” wheel, using holes on alternating sides of the rim. The wheel can also be built asymmetrically, to a single row of holes on one side of the rim. This is the required method if building a 29″ wheelset for a Pugsley, whose frame is also offset. But the Krampus has a normal rear spacing and doesn’t require any wheelbuilding tricks. Why consider building to the rim asymmetrically?
On typical rear wheels for road and mountain bikes, the drive-side flange is spaced toward the centerline of the wheel to accommodate the growing number of cogs in modern drivetrains. Technically, the width of the freehub has not grown since the leap from 7-8 speed systems, as modern 9 and 10 speed cassettes simply use narrower cogs and chains. The disparity between the location of the left and right flange from “center” creates mismatched bracing angles (the drive side spokes are more vertical), and unbalanced tension (drive spokes require almost twice the tension). Worth noting is these perceived imperfections are not a problem for most people in the real world– these are mostly theoretical considerations. As long as the wheel is designed for the task at hand with appropriate materials and is properly built, few wheels cause problems for their users. The Surly Rabbit Hole rims used in this build present an interesting question. If the wheel is built to one side of the rim, asymmetrically, the spoke angles are then well balanced and the tension is even. But then, is there anything wrong with a wheel whose spokes are biased to one side of the rim?
Building the wheel in the obvious mode by alternating spoke holes provides a positive bracing at the rim, as spokes connect to either side and support the structure of the rim. The second option is to build to one side of the rim only, favoring the row of spokes on the non-drive side to balance the tension in the wheel and reduce the chance of spoke failure. For reference, a “normal” wheel is also shown below.
Normal rear wheel
This is how a normal wheel would look, built to a rim such as a Velocity P35 with spoke holes down the centerline. This rim is notable as one of the wider 29″ rim on the market and would be a suitable platform for both the 3.0″ Knard tire, and standard width 29″ tires. Note, the drive-side spokes are more vertical than the non-drive side and thus, under higher tension. This is how most multi-speed wheels look; the red line is the centerline of the frame:
Normal wheel build: Phil Wood disc hub to Velocity P35 rim
Bracing angle: L-6.5°, R-4°
Tension distribution: L-62%, R-100%
Assymmetric wheel build with Rabbit Hole rim
The wheelbuilder would suggest that balanced angles and spoke tension are ideal. Traditonally, only drive-side spokes were prone to failure as drivetrain forces put greater strain on this side of the wheel. WIth the proliferation of disc brakes, the opposing forces on the opposite side of the wheel are known to cause spoke breakage in relative frequency, especially as these spokes are relatively undertensioned. From the perspective of the wheelbuilder, with the option to build to an asymmetrical rim, building to the row of holes on the non-drive side would be best. This is how an asymmetric build would look, offset 5mm to the non-drive side:
Assymmetrical build, Phil Wood disc hub to Rabbit Hole
Bracing angles: L-5.4°, R-4.8°
Tension distribution: L-91%, R-100%
Symmetric wheel build with Rabbit Hole rim
The rider, the mountain biker, would look at a rim with spokes to one side and would wonder if the rim could withstand the forces of riding, as if it would fold over itself sideways. It may even seem an unfounded concern as the rim is still comprised of 700g of aluminum, but it would be prudent to ask such a question. This wheel is built to alternating sides of the rim, and spoke holes are offset 5mm from center. Note, drive-side tension is proportionally greater than in the “normal” wheel, but within an acceptable range.
Symmetrical build, Phil Wood disc hub to Rabbit Hole rim
Bracing angles: L-5.3°, R-2.9°
Tension distribution: L-55%, R-100%
The crux of this discussion is whether the rim, a singlewall (sorta doublewall) hoop with cutouts can effectively act as a rigid structure. If the rim were made of solid steel, we would readily opt for the ideal spoke angles of the asymmetrical build. But, can the Rabbit Hole support the load of a full-sized man with a bikepacking load, who sometimes rides like an Ogre or a Troll? Prudence says only “maybe”, but I have been riding offset wheels for a year, including the lightweight singlewall Marge Lite rim. The construction of these rims is similar, yet the offset of the Marge Lite is more than twice as great as the Rabbit Hole. On my Pugsley, my front wheel is built symmetrical to a dynamo hub, while the rear is laced to one side of the rim only. I haven’t had any problems over many months of touring on Marge Lite rims. Rabbit Holes should be fine.
Both approaches to building a wheel with Rabbit Hole rims would be fine, and there is no clear winner. Which would you choose?
Feel free to justify your vote in the comment form below.