Soma Sandworm Fatbike with Rohloff SPEEDHUB 500/14, Gates Carbon Drive Belt, and Lauf Leaf Spring Fork

San Francisco-based Soma Fabrications recently sent us a production version of their first fat bike frame, the Sandworm. This is a great affordable option for a Rohloff- and belt-compatible fat bike. The frame is designed with several key features to run a Rohloff-Gates Carbon Drive belt drive train.

We built this Sandworm up as demo with a Lauf Carbonara leaf spring fork. Assembly was very straight forward. With only a few quick spins on it, the geometry feels very neutral and familiar.

Previously, we built up a prototype version for form, fit, and function validation. Throughout the assembly, we made notes on the ease of assembly and integration with the Rohloff-belt drive train.

The production version of the Sandworm evolved based on lessons learned from the prototype, while maintaining the earlier tube shape and geometry.

The prototype was built with a 135mm offset rear triangle, which worked with the original SPEEDHUB 500/14, while the production version made the change to 170mm symmetric spacing and accepts the SPEEDHUB XL.

The sliding dropouts provide the tensioning method for the belt. On the prototype version, the dropouts served a dual function, providing a split in the frame to accept the belt via an overlapping sandwich construction that was held together by the bolts that keep the sliding inserts in place.

Belts are now installed through a seatstay splitter as seen on all Soma models with sliding dropoutsand like we commonly install when doing belt-drive modifications.

The Sandworm is the fourth bike in the Soma lineup that is Rohloff and belt drive compatible with sliding dropouts and seat stay splitters.  With these options, Soma has a range of affordable Rohloff/belt drive option that covers most types of adventure riding.

• Sandworm: 26" Fat Bike
• Juice: 29'er Hardtail
• B-Side: 27.5"/650B Hardtail
• Wolverine: 700c Cross/Gravel/Adventure Road/Bikepacking

The Sandworm accepts the wider, fat bike-specific Rohloff SPEEDHUB XL, while the other three models use the standard SPEEDHUB 500/14.  All of these models use threaded bottom bracket shells for compatibility with a wide range of crank sets and ISO disc brake mounts for using our Monkey Bone with Rohloff's OEM2 axle plate.  Our demo bike is set up with the standard Rohloff Twist shifter on flat bars.

On the whole, the Soma Sandworm seems like a solid frame design with features that work well with Rohloff and Gates belt drive, so we give it a Monkey Lab stamp of approval.

Build details

• Frame: Soma Sandworm

• Fork: Lauf Carbonara

• Headset: Cane Creek 40

• Stem: Thomson X4

• Handlebar: Salsa Bend 2

• Shifter: Rohloff Twist 

• Grips: Ergon GP1

• Seat post: Thomson Elite

• Saddle: Rivet

• Front Hub: Salsa

• Rear hub: Rohloff SPEEDHUB 500/14 XL

• Spokes: Sapim Race

• Nipples: Sapim Brass

• Rims: Sun Ringle MuleFut

• Tires: Surly Nate

• Cranks: Race Face Turbine
• Pedals: A-Flat Acros

• Bottom Bracket: Race Turbine

• Chain ring: Gates Carbon Drive

• Rear Sprocket: Gates Carbon Drive

• Chain: Gates Carbon Drive

• Brakes & Levers: Avid BB7 & Speed Dial 7