This guide explains how to convert an old mountain bike (MTB) frame into a functional cargo bike. The process involves several key steps:

Step 1: Preparing the MTB Frame

  • Select an MTB frame size based on the rider’s physical dimensions.
  • Cut the head tube, down tube, and remove V-brake mounting bolts and rack eyelets.
  • Use an angle grinder or half-round file to grind and clean the frame after cutting.

Step 2: Geometry Design

  • Use stainless steel tubes (ST37-3, 1.5mm thickness) for the frame to balance stiffness and weight.
  • Choose a 1″ 1/8 threadless headset (EC34) requiring a 34mm internal diameter tube.
  • Determine loading bed dimensions: 520mm length (for carrying kids/goods) and 530mm width (using three spaced tubes).
  • Set fork trail to 57mm for stability and responsiveness. Use CAD or online calculators to determine head angles.

Step 3: Tube Notching

  • Use a CNC bending machine for precise angles, or manually cut and bend rectangular profiles using the formula: D = a * tan(α/2).
  • Notch tubes with a rotary hole saw based on CAD specifications.

Step 4: Frame Jig and Welding

  • Construct a welding jig to maintain precise angles (horizontal, loading bed bends, head tube).
  • Weld tubes in stages, first securing positions with spot welds, then completing the frame.
  • Calculate rear drop (vertical distance between bottom bracket and rear axle) using: D = 230mm – crank length (typically 55-60mm for 26″ frames).

Step 5: Disk Brakes

  • Mount disc calipers using IS standard (51mm bolt spacing).
  • Position rear caliper eyelets on 78.1mm and 39.9mm radii circumferences centered on the wheel axle.
  • Use laser-cut or manually fabricated stainless steel plates for caliper supports.

Step 6: Steering and Kickstand

  • Use a single rigid arm steering rod (16×1.5mm stainless steel) with a 1.2:1 handlebar-to-wheel steering ratio for responsiveness.
  • Mount the rod using M8 female joint heads and hexagonal cap screws.
  • Design a kickstand with 5° ground inclination and 20-30mm leg height to lift the front wheel for stable parking.

Step 7: Painting and Assembly

  • Apply powder coating for durability.
  • Assemble all components and test the bike for functionality.
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