Brother Cycles Big Bro 2025 Stealth Black - Frameset

The 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, loaded rides, rough forest roads and trips where the bike needs to keep working long after the pavement ends. The Big Bro has also proven itself in some of the world’s toughest bikepacking races, including the Tour Divide and the Silk Road Mountain Race.

For 2025, Brother has given the frame a pretty substantial update. The main triangle is now made from heat-treated, butted 4130 chromoly, and the sliding rear dropouts have a new rail design for improved strength and stability. The Big Bro also gets a UDH derailleur hanger with support for SRAM Full Mount, top tube bag mounts, and more options for fitting a front rack.

The great thing about the Big Bro is how many different ways you can build it. The sliding dropouts let you run both geared and single speed, and the Boost standard, 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 tell you that maximum tire volume of 3" requires a single chainring up front - with a double chainring, the chain can hit the tire.

The included steel fork is designed for the same frame geometry as a 100 mm suspension fork, so the Big Bro can be built either fully rigid or with front suspension. The frame also has internal 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 things like extra bottle cages or cargo cages. This is a frame where you can actually pack gear on the bike without having to improvise every attachment point.

The Big Bro isn't a gravel frame with a little extra tire clearance. It's a mountain bike for bikepacking and touring that can also be built up simply 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 fully loaded for a longer trip - and still gives you the option of riding single speed.

Quick Specs

• Model: Brother Cycles Big Bro 2025
• Type: Steel mountain bike, bikepacking and touring frameset
• 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+
• Max 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
• Derailleur hanger: SRAM UDH, compatible with Full Mount
• Brakes: 51 mm IS mount, max 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, top-pull or high direct-mount with separate adapter
• Max chainring: 38T single, 44/30T double or 48/36/26T triple
• Bottle cage mounts: 3 sets on all sizes
• Fork mounts: Three mounting points on each fork leg for bottle cages or cargo cages
• Other mounts: Top tube bag, bags and rack
• 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 ISO 4210 MTB standard