Brother Cycles Big Bro 2025 Stealth Black - Frames

Brother Cycles Big Bro is a steel frameset for anyone who takes bikepacking seriously and wants to build a bike capable of handling much more than smooth gravel roads. It is made for long trips with luggage, rough forest roads, and journeys where the bike needs to keep working long after the asphalt ends. Big Bro has also proven itself in some of the world's toughest bikepacking races, including the Tour Divide and Silk Road Mountain Race.

For 2025, Brother has significantly updated the frame. The main triangle is now made from heat-treated, double-butted 4130 chromoly, and the sliding rear dropouts have a new design with rails for improved strength and stability. At the same time, Big Bro has gained a UDH derailleur hanger with SRAM Full Mount support, top-tube mounts for a top tube bag, and more options for mounting front racks.

The great thing about Big Bro is how many different ways it can be built. The sliding 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 leaves room for really large tires. You can build it with 29 × 2.6" or switch to 27.5 × 3.0" if you want 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 included steel fork is designed for the same frame geometry as a 100 mm suspension fork, so Big Bro can be built either fully rigid or with front suspension. The frame also has internal cable routing for a dropper post and plenty of mounting points for bottles, bags, and racks. The fork legs have three mounting points per side for additional bottle cages or cargo cages, for example. It is a frame that lets you actually pack gear onto the bike without having to improvise every mounting point.

Big Bro is not a gravel frame with slightly more tire clearance. It is a mountain bike for bikepacking and long-distance touring that can also be built in a stripped-down setup for forest trails and technical riding. Choose it if you want to build a 29er or 27.5+ that works just as well with lightweight bikepacking gear as it does fully loaded for a longer trip – while still retaining the option to run single speed.

Quick Specs

• Model: Brother Cycles Big Bro 2025
• Type: Steel frameset for mountain biking, bikepacking, and long-distance touring
• Color: Stealth Black
• Frame: Heat-treated 4130 chromoly, double-butted main triangle, ED-treated
• Fork: 4130 chromoly, ED-treated, designed for geometry with a 100 mm suspension fork
• 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
• Derailleur hanger: SRAM UDH, compatible with Full Mount
• Brakes: 51 mm IS mount, maximum 180 mm 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, top-pull or high direct-mount with separate adapter
• Maximum chainring: 38T single, 44/30T double, or 48/36/26T triple
• Bottle cage mounts: 3 sets on all sizes
• Fork mounts: Three mounting points per fork leg for bottle cages or cargo cages
• Other mounts: Top tube bag, bags, and racks
• 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