Bicycle Frame Manufacturing Process

The bicycle frame is the skeleton of the bicycle, connecting the front and rear wheels, seat, and drivetrain. It needs to balance strength, stiffness, lightweight, and comfort. The frame manufacturing process directly affects the bicycles performance, weight, and cost, and is applicable to various types such as road bikes, mountain bikes, and folding bikes.

1. Design and Preparation

• Frame Design: Use CAD or SolidWorks for 3D modeling, optimize geometric structure and tube distribution, and consider riding type (e.g., racing, off-road). Finite Element Analysis (FEA) is used to verify strength and stiffness.
• Material Preparation:
  • Steel/Aluminum Alloy: Purchase fixed-length tubes or plates, and cut them to the required length.
  • Carbon Fiber: Prepare prepreg (carbon fiber cloth + resin), which needs to be stored refrigerated.
  • Titanium Alloy: Purchase high-precision tubes and check the surface quality.

2. Tube Forming

• Steel/Aluminum Alloy:
  • Extrusion Molding: Extrude metal ingots into tubes, controlling the wall thickness (0.8-2.0mm).
  • Drawing: Form special-shaped tubes (such as double-butted tubes with variable wall thickness) through cold drawing to improve strength and reduce weight.
  • Hydroforming: Used for complex special-shaped tubes, high-pressure water (approximately 100 MPa) inside the mold makes the tube fit the mold.
• Carbon Fiber:
  • Layup Molding: Lay up prepreg at specific angles (0°/45°/90°), controlling the fiber direction to optimize stiffness.
  • Mold Pressing: Heat and cure in a mold (120-180°C, pressure 5-10 bar) to form tubes or the entire frame.
• Titanium Alloy: Mostly uses seamless tubes, which require precise cutting and bending.

3. Welding and Connection

• Steel:
  • Tungsten Inert Gas (TIG) Welding: Commonly used, the weld is uniform and suitable for thin-walled tubes.
  • Brazing: Used for chromoly steel, the temperature is lower (approximately 900°C), reducing the heat-affected zone.
• Aluminum Alloy:
  • TIG or MIG Welding: It is necessary to control the welding temperature (approximately 600°C) to avoid strength reduction due to overheating.
  • Post-Treatment: Heat treatment (T6, approximately 500°C quenching + 180°C aging) restores material strength.
• Carbon Fiber:
  • Bonding: Use epoxy resin to bond tubes and joints (such as aluminum alloy or carbon fiber joints).
  • Monocoque Molding: High-end frames are formed using an integral mold, without welding.
• Titanium Alloy:
  • TIG Welding: Must be operated under inert gas protection to avoid oxidation.
  • Fixture Positioning: Ensure weld accuracy (deviation <0.2mm).
• Connectors: Such as the bottom bracket and head tube, require high-precision machining (tolerance ±0.1mm) to fit standard accessories.

4. Hole Machining and Finishing

• Hole Machining:
  • The frame needs to be machined with threaded holes (for water bottle cages, brake installation) and through holes (for cable routing).
  • CNC Machining: CNC machine tools are used for high-precision drilling and milling, with a hole position tolerance of ±0.05mm.
  • Manual Drilling: Used for small batches or custom frames, with lower efficiency.
• Finishing:
  • Milling/Grinding: Remove burrs to ensure smooth joints.
  • Tapping: Machine threads for the bottom bracket and water bottle cage, requiring high-precision taps.

5. Surface Treatment

• Steel:
  • Shot Blasting/Sandblasting: Remove the oxide layer, surface roughness Ra 3.2-6.3μm.
  • Electroplating or Painting: Anti-rust treatment, commonly used epoxy primer + polyurethane topcoat, coating thickness 50-100μm.
• Aluminum Alloy:
  • Anodizing: Forms an oxide film (10-20μm) to improve corrosion resistance and aesthetics.
  • Painting: Optional matte or glossy coating to enhance appearance.
• Carbon Fiber:
  • Clear Coat: Protects the fiber layer, thickness approximately 30-50μm, maintaining the carbon fiber texture.
  • Decals: Increase brand identity, requiring wear and weather resistance.
• Titanium Alloy:
  • Brushed Finish: Forms a uniform texture, no additional coating is required, maintaining the metallic texture.

6. Quality Inspection

• Dimensional Inspection: Use a coordinate measuring machine to verify tube length, angle, and hole position accuracy (deviation <±0.2mm).
• Strength Test: Simulate riding load (approximately 1000N) to check frame deformation and fatigue life.
• Surface Quality: Check weld seams, coating uniformity, and absence of cracks and bubbles.
• Assembly Test: Install standard accessories (such as front fork, crank) to verify compatibility.