REVIEW: Soma Sandworm Fatbike with Rohloff SPEEDHUB and Gates Carbon Drive Belt

Soma Fabrications called us recently and told us they were designing their first fatbike frame named the Sandworm; they wanted us to test out the prototype and give it the Rohloff SPEEDHUB 500/14 internal gear hub and Gates Carbon Drive belt drive treatment that we are known for. We were glad to hear they had decided to expand into fatbikes with this frame and potentially bring the fun of fatbiking to more people by filling a hole in the market for an affordable Rohloff/belt drive frame. Once we received the frame, we began mapping out a build kit suitable for this project.

Soma is based just across the bay from us in South San Francisco, and they make affordable steel bikes for touring, cargo, mountain, track, cyclocross, and road frames. This prototype Sandworm features Rohloff SPEEDHUB-compatible 135mm axle spacing. Like most early fatbike designs, the rear end is offset 17.5mm to allow for a traditional 135mm wide MTB hub and still provide enough chain clearance not to rub on the big tires. The left stays are shaped such that they bend in towards the drive side, and the hub is laced into a rim with offset spoke holes to keep the rim and tire centered. The offset of the rim combined with the offset frame usually cancel each other out on an asymmetric derailleur hub and allow for a dish-less wheel build. But, when you start with a symmetric Rohloff hub, you end up adding dish to the wheel.

The Sandworm frame also includes sliding dropouts to adjust belt (or chain) tension, with overlapping halves that separate to fit a Gates Carbon Belt Drive into the rear triangle and then sandwich back together with the sliding insert and securing bolts.  Keeping with current fat bike frame trends, the production Sandworms will most likely have 170mm-spaced, symmetric rear ends for wider modern fatbike specific hubs like the new 170mm Rohloff SPEEDHUB XL. Although the design is not finalized, the geometry on the production version of the Sandworm should be largely unchanged from this prototype.

Adding to its versatility, the Sandworm frame also fits 29x3” tires and rims for a speedy 29er trail bike build and is convertible to bikepacking mode with Soma’s steel Battleaxe fork, which has plenty of mounts for racks to carry gear.

We decided to build this bike up in trail mode, so we installed an inverted Carver Trans-Fat fork to handle suspension duties. The Carver fork was one of the first fatbike-compatible suspension options on the market before the mass-produced Rockshox Bluto fork was made available. Its inverted design offer excellent tire clearance, since there is no arch near the top of the tire. The fork has 100mm of suspension and makes the bike much more suited to rocky, rutted trails, which more and more people have begun riding on fatbikes in recent years. We also installed the X Fusion Hilo SL dropper post, which is one of relatively few posts available in 27.2mm size, to aid in the bike's trail riding prowess.

The wheel build consisted of Surly’s Marge Lite rims, Carver’s fatbike front hub, and a Rohloff SPEEDHUB in the rear. Both wheels were laced up with Sapim Race spokes and brass nipples. The 65mm wide Marge Lite rims are situated in the middle of Surly’s wide rim line, wider than the 50mm Rabbit Holes yet smaller than the 85mm Rolling Darryls and 100mm Clown Shoes. They provide the right amount of traction and flotation to ride on moderate amounts of sand, snow, and mud and mate well with 3.7-4" wide tires. When the wheels were trued and tensioned, we mounted them both with Surly Knard 26x3.8 tires.

The Rohloff SPEEDHUB works well for mountain biking and is especially well-suited for fatbiking, the two riding disciplines that this bike blends nicely. Since derailleurs can easily be broken or damaged on mountain trails when rocks and roots catch on them, the SPEEDHUB adds to the durability of any mountain bike by eliminating derailleurs from the equation altogether. In addition, the SPEEDHUB's sealed shifting system protects shifting components from the deep sand, mud, dirt and other contaminants that fatbikes are often ridden through. With hardened steel gearwheels and a 526 percent gear range, the SPEEDHUB adds versatility and dependability for two very demanding forms of riding conditions. With the addition of the Gates Carbon Drive belt, which has a usable lifespan at least twice as long as a conventional chain and does not require lubrication, this is one of the most low maintenance drivetrains available.

To finish out this build, we added an array of components that are both comfortable and offer solid performance. Salsa's Bend 2 handlebars provide a comfortable amount of sweep, which mimic a rider's natural hand position. Magura MT2 hydraulic disc brakes provide ample stopping power for mountain trails and the Race Face Turbine crankset is light and strong.  An SQ Lab 611 Active saddle features a flexible shell that allows the saddle to tilt slightly from side to side for a more efficient pedal stroke while also allowing the saddle to give a little on bumps. Ergon GP1 grips provide a large palm rest for hand comfort on long rides.

A Fatbike at a very affordable price!  We can't wait to see the final version when it hits the market. If you’re interested in a trail-oriented fatbike of your own, keep the Soma Sandworm in mind and contact us for an estimate.

Build Details:

• Frame: Soma Sandworm

• Fork: Carver Trans-Fat Suspension fork

• Headset: Tange Seiki Zero Stack 1 1/8"

• Stem: Thomson X4

• Handlebar: Salsa Bend 2

• Shifter: Rohloff

• Grips: Ergon GP1

• Seat post: X Fusion

• Saddle: SQ Lab 611 Active

• Seat Clamp: Soma Silver

• Front Hub: Carver

• Rear hub: Rohloff SPEEDHUB 500/14

• Spokes: Sapim Race

• Nipples: Sapim Brass

• Rims: Surly Marge Lite

• Tires: Surly Knard 26x3.8”

• Cranks: Race Face Turbine

• Pedals: Shimano XTR SPD

• Bottom Bracket: Race Face

• Chain ring: Gates Carbon Drive

• Rear Sprocket: Gates Carbon Drive

• Chain/Belt: Gates Carbon Drive

• Brakes & Levers: Magura MT2