Electric Cargo Trike Frame for Last-Mile Delivery

The problem

Last-mile delivery in Indian cities needs a vehicle that carries a heavy payload in narrow lanes, is stable when cornering laden and can be repaired locally. Cargo trikes built from bicycle parts often tip or crack at the cargo bed joint. A designed frame, with stability checked from the centre of gravity, is safer and lasts longer.

Objectives

• Compare front-loader and rear-loader trike layouts for stability and packaging.
• Model the chosen frame and cargo bed in Weldments.
• Calculate laden centre of gravity and static tipping angle.
• Check the frame under payload, bump and cornering load cases.
• Produce a fabrication drawing and cut list.

What you need

Key concepts

Static tipping angle

The lateral tilt at which the centre of gravity passes over the line joining two wheel contact points. A larger angle means more margin against roll-over when cornering.

Tadpole and delta layouts

Tadpole has two wheels at the front, delta has two at the rear. The layout changes where the payload sits relative to the two-wheel axle and so changes stability.

Payload-to-kerb mass ratio

Payload divided by vehicle mass. Cargo vehicles carry a large ratio, so the laden centre of gravity is dominated by where the cargo sits.

Lateral load transfer

In a turn, load moves to the outer wheels in proportion to centre of gravity height and lateral acceleration divided by track. It sets the cornering load case.

Step-by-step guide

1. 1

   Define the delivery requirement

   Set example targets: 200 kg payload in a 600 × 800 × 600 mm box, 80 kg rider, a 48 V battery of about 15 kg, a rear hub or mid-drive motor, overall width under 1000 mm for narrow lanes and a laden tipping angle target well above the angle reached in a typical turn. Choose a steel tube or ERW section and record material properties. Write the delivery duty, such as daily distance and stops.

   Checkpoint: A requirement sheet with payload, box size, width limit and stability target.

2. 2

   Sketch two layouts

   In two configurations of a layout part, sketch a front-loader (cargo box between two front wheels) and a rear-loader (box over a two-wheel rear axle). Link wheelbase, track and box position to Global Variables. Place simple blocks for rider, battery, motor and box with assigned masses so Mass Properties gives a meaningful centre of gravity for each layout. Record turning circle and loading height for each layout in the same sketch so the comparison includes more than stability.

   Checkpoint: Both layout configurations rebuild and show the loaded centre of gravity position.

3. 3

   Calculate tipping stability

   For each layout, read the laden and unladen centre of gravity from Mass Properties. In Excel, compute the distance from the centre of gravity to the tipping line joining the front and rear contact points on one side, then the static tipping angle as the arctangent of that distance over centre of gravity height. Also compute the lateral acceleration at which inner wheels unload.

   Checkpoint: A stability table for both layouts, laden and unladen, with a chosen layout justified by the numbers.

4. 4

   Model the main frame

   For the chosen layout, draw a 3D Sketch skeleton for the main spine, rider section, steering head and the axle or steering mount. Place Structural Members, for example 40 × 40 × 2 mm box tube for the spine and round tubes for the rider section. Use groups and Trim/Extend at joints and add Gussets at the steering head and the spine-to-bed joint, which usually carries the highest bending.

   Checkpoint: The frame is built with no overlapping bodies and the cut list is complete.

5. 5

   Model the cargo bed

   Add the cargo bed as a weldment frame of angle or box sections with cross members at an example 300 mm pitch. Model a floor panel as a sheet body or separate sheet metal part bolted to the bed. Add tie-down points and the box mounting holes with the Hole Wizard. Check the bed height so the box can be loaded by hand at a comfortable height.

   Checkpoint: Bed and box fit together in the assembly, and the loading height is recorded in the report.

6. 6

   Check packaging

   Assemble frame, wheels, steering, motor, battery, rider block and cargo box. Run Interference Detection and check steering lock, pedal and foot clearances, and battery removal access. Measure turning circle from the steering geometry with the Measure tool or a sketch. Update Mass Properties with the real frame mass and recompute the tipping angle for the final design. Check that the cargo box can be removed or replaced, since delivery fleets often swap box types.

   Checkpoint: Interference Detection passes and the final tipping angle still meets the target.

7. 7

   Set up and run load cases

   Create a Static beam study. Restrain the axle mounts with Fixed Hinge and Roller/Slider restraints. Apply the payload as a distributed force over the bed cross members, rider weight at the seat and battery at its mount. Run cases for static payload, a 2.5 g bump and a cornering case with an example 0.4 g lateral force at the centre of gravity reacted at the wheels.

   Checkpoint: Reaction forces balance the applied loads and each case has a recorded minimum factor of safety.

8. 8

   Hand-check and iterate

   Hand-calculate the bending stress at the spine-to-bed joint by treating the spine as a cantilever carrying the bed reaction, using section properties from the section property tool. Compare it with the beam result. If the factor of safety is below 2 in the bump case, increase section, add a gusset or a diagonal brace, and record mass and stress in an iteration table.

   Checkpoint: Hand and Simulation stress agree within about 15 per cent and the final design meets the target in every case.

9. 9

   Drawings and report

   Produce a frame drawing with a Weldment Cut List table, a bed drawing and bracket details with ISO 2768 general tolerances. The report includes both layouts, the stability comparison, packaging, load cases, the hand check and iteration table. State that tipping calculations are static and that a tilt-table or cornering test on a prototype is needed to confirm stability. Include the Mass Properties output so a reader can repeat the stability calculation.

   Checkpoint: A complete drawing pack and a report with stability and strength results and stated limitations.

Testing & validation

• Tipping angle calculated for both layouts, laden and unladen.
• Reaction forces balance applied loads in each study.
• Hand bending check at the critical joint agrees with Simulation.
• Interference Detection passes for the final assembly.

Expected results

A justified cargo trike layout with a stability calculation, a packaged weldment frame and bed, verified load cases and a fabrication drawing pack for an example 200 kg payload.

Take it further

• Add a tilting mechanism concept and compare stability.
• Run a fatigue study at the spine-to-bed joint.
• Build a quarter-scale model and test the tipping angle on a tilt board.

Viva and review questions

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