Brother Cycles Big Bro Cobalt Blue - Frameset

Brother Cycles Big Bro is a steel frameset for anyone who takes bikepacking seriously and wants to build a bike that can handle much more than smooth gravel roads. It’s made for long trips with luggage, rough forest roads, and journeys where the bike needs to keep working long after the asphalt ends. The 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 the Big Bro is how many different ways you can build it. The sliding rear dropouts let you run it geared or single speed, and Boost spacing, with 12 × 148 mm at the rear and 15 × 110 mm at the front, leaves room for really big tires. You can build it with 29 × 2.6" tires or switch to 27.5 × 3.0" if you want even more volume and grip. From experience, we can advise that maximum tire volume of 3" requires a single chainring up front - with a double chainring, the chain can rub against the tire.

The frame is made from double-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, so the Big Bro can be built either fully rigid or with front suspension. Internal routing for a dropper post and plenty of mounting points for bottles, bags, and extra cargo 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 job, giving the Big Bro a much more colorful look than many traditional expedition and MTB frames. The construction, however, is just as straightforward: big tires, a steel frame, Boost hubs, and the option to build it geared or single speed.

The Big Bro isn't a gravel frame with a little more tire clearance. It's a proper mountain bike for bikepacking, touring and trail riding. Choose it if you want to build a 29er or 27.5+ that can be light and stripped down for a trail ride one day and fully loaded for a longer trip the next.

Quick Specs

• Model: Brother Cycles Big Bro
• Type: Steel frameset for mountain biking, bikepacking and touring
• Color: Cobalt Blue
• Frame: 4130 chromoly, double-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 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
• Brakes: 51 mm IS mount, maximum 180 mm rotor
• Headsets: ZS44/28.6 upper / EC44/40 lower, not included
• Seatpost: 30.9 mm
• Seat 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 an uncut 300 mm steerer tube
• Certification: Tested to the ISO 4210 MTB standard