Brother Cycles Big Bro Cobalt Blue - Frames

Brother Cycles Big Bro is a steel frameset for those who take bikepacking seriously and want to build a bike that can handle considerably more than smooth gravel roads. It is made for long, loaded rides, rough forest roads, and trips where the bike needs to keep working long after the asphalt ends. Big Bro has also completed some of the world’s toughest bikepacking races, including the Tour Divide from Canada to Mexico and the Silk Road Mountain Race in Kyrgyzstan.

The great thing about Big Bro is how many different ways it can be built. The sliding rear dropouts let you run either gears or single speed, while Boost spacing with 12 × 148 mm at the rear and 15 × 110 mm at the front provides room for really large tires. You can build it with 29 × 2.6" tires or switch to 27.5 × 3.0" for even more volume and grip. From experience, we can point out that the maximum 3" tire volume requires a single chainring at the front – with a double chainring, the chain may rub against the tire.

The frame is built from butted 4130 chromoly, and both the frame and fork are ED-treated for better corrosion protection. The included steel fork is designed for the same geometry as a 100 mm suspension fork, allowing Big Bro to be built either fully rigid or with front suspension. Internal dropper post routing and plenty of mounting points for bottles, bags, and additional luggage make the frame easy to adapt to how far you plan to ride and how much you need to carry.

Cobalt Blue is the 2024 version’s bright blue paint and gives Big Bro a considerably more colorful look than many traditional expedition and MTB frames. The construction, however, is just as uncomplicated: large tires, a steel frame, Boost hubs, and the option to build it with gears or single speed.

Big Bro is not a gravel frame with slightly more tire clearance. It is a true mountain bike for bikepacking, long-distance touring, and riding in the woods. Choose it if you want to build a 29er or 27.5+ that can be light and stripped down for a day on the trails, then fully loaded for a longer trip the next.

Quick Specs

• Model: Brother Cycles Big Bro
• Type: Steel frameset for mountain biking, bikepacking, and long-distance touring
• Color: Cobalt Blue
• Frame: 4130 chromoly, butted main triangle, ED-treated
• Fork: 4130 chromoly, tapered unicrown, ED-treated
• Sizes: Small (16"), Medium (18"), Large (20"), and X-Large (22")
• Wheels: 29" or 27.5+
• Maximum tire width: 29 × 2.6" or 27.5 × 3.0", depending on the drivetrain
• Rear axle: Boost 12 × 148 mm thru-axle, included
• Front axle: Boost 15 × 110 mm thru-axle, included
• Rear dropouts: Sliding dropouts for geared or single speed setups
• Brakes: 51 mm IS mount, maximum 180 mm brake rotor
• Headset: ZS44/28.6 upper / EC44/40 lower, not included
• Seatpost: 30.9 mm
• Seatpost clamp: 33.1 mm, included
• Dropper post: Internal cable routing
• Bottom bracket shell: 73 mm, ISO threaded
• Front derailleur clamp: 34.9 mm high direct-mount, adapter sold separately
• Maximum chainring: 38T single or 44/30T double
• Bottle cage mounts: 3 sets on all sizes
• Frame weight: Approximately 2.60 kg in size Medium
• Fork weight: Approximately 1.37 kg with a 300 mm uncut steerer tube
• Certification: Tested to ISO 4210 MTB standard