A four-wheel micromobility vehicle under 50 kg, from vision to prototype design

From the founder's vision to a concept, and from the concept to a manufacturable prototype design, covering sketches, part design, assembly and the bill of materials.

Fig.: 1
Title: The platform, side and plan
Kind: Drawing, simplified, not to scale
Source: Drawn for this site
Note: Complete vehicle under 50 kg. Outline simplified from the design; only the overall width is marked.

Role: Sole vehicle platform design engineer
Team: Three-person micromobility startup
Period: Aug 2022 to Aug 2024
Mass: Under 50 kg complete, summed from the bill of materials
Width: No wider than 95 cm
Status: First prototype in production
Tools: SolidWorks, ANSYS, Lotus Suspension Analysis, Adams Car, Excel
Note: Company, brand and product withheld under NDA; figures are simplified or masked.

On this page: 7 figures

Fig. 1 The platform
Fig. 2 The corner, forces

Register excerpt: 5 checks, with load case, loading, material, the source of each check, and masked values
System	Part	Load case	Loading	Material	Source
Suspension	Wishbone tubes	Braking + cornering, 0.55 g + 0.55 g	Bending and axial force	AL 6061 T6	FEA
Suspension	Wishbone tubes	Max heave, 2 g	Bending	AL 6061 T6	Bending+Torque
Suspension	Damper mount	Max heave, 3 g, stiffest damper	Tension	AL 6061 T6	FEA
Suspension	Steering knuckle	Max braking, 0.7 g	Tension	AL 7075 T6	FEA
Suspension	Knuckle bearing	Braking + cornering, 0.55 g + 0.55 g	Bearing reaction	Bearing	Bearing Loads

Wishbone tubes · Suspension
Load case
Braking + cornering, 0.55 g + 0.55 g
Loading
Bending and axial force
Material
AL 6061 T6
Source
FEA
Forces Stresses Safety factors
Wishbone tubes · Suspension
Load case
Max heave, 2 g
Loading
Bending
Material
AL 6061 T6
Source
Bending+Torque
Forces Stresses Safety factors
Damper mount · Suspension
Load case
Max heave, 3 g, stiffest damper
Loading
Tension
Material
AL 6061 T6
Source
FEA
Forces Stresses Safety factors
Steering knuckle · Suspension
Load case
Max braking, 0.7 g
Loading
Tension
Material
AL 7075 T6
Source
FEA
Forces Stresses Safety factors
Knuckle bearing · Suspension
Load case
Braking + cornering, 0.55 g + 0.55 g
Loading
Bearing reaction
Material
Bearing
Source
Bearing Loads
Forces Stresses Safety factors

Fig. 3 The corner's checks

Register excerpt: 3 checks, with load case, loading, material, the source of each check, and masked values
System	Part	Load case	Loading	Material	Source
Brakes	Caliper adaptor	Max braking, 0.7 g	Tension	AL 7075 T6	FEA
Drivetrain	Drive shaft	Max acceleration	Torsion	AL 7075 T6	Bending+Torque
Suspension	Hub splines	Max braking, 0.7 g	Shear	AL 7075 T6	Spline

Fig. 4 Brake and drive checks

User Interface: inputs
Vehicle specs 5 f
Cornering case
Stopping case
Power case 1 f
Top-speed case 1 f
Gradient case
Vehicle Data
Basic vehicle data 6 f
Masses and CG, by assembly 55 f
Powertrain and drivetrain settings 19 f
Vehicle Dynamics
Vehicle data, as used 23 f
Scenario input 1 f
Vehicle Dynamics, continued
Static weight distribution 5 f
Lateral weight transfer 10 f
Longitudinal weight transfer 5 f
Load on a slope 8 f
Vehicle Dynamics, continued
Tyre forces and limits 20 f
Braking 2 f
Cornering 1 f
Static rollover angles 3 f
Vehicle Dynamics, continued
Motor power 13 f
Top speed 13 f
Gradient 12 f
User Interface: calculations
Motor power 28 f
Top speed 6 f
Gradient 6 f
User Interface: outputs
Max cornering speed 1 f
Min stopping distance 1 f
Required motor power 15 f
Achievable top speed 15 f
Achievable gradient 13 f
Fig. 5 The performance model
1. Vehicle level
Tyre Data
Lateral force against slip angle and load for the candidate tyres
Weight Transfer
Masses, track, wheelbase, CG and roll centres to the vertical, longitudinal and lateral force at each wheel, per case
Corner
Lateral acceleration from speed and corner radius
Drivetrain
Air, rolling and gradient resistance, power and ratios for front, rear and four-wheel drive
F at each wheel, per case
2. Loads into the structure
Chassis Loadcases
Seven cases, four corners: the forces at the wishbone and damper pickups
Steering Torque
Castor, trail, kingpin inclination and scrub radius to the steering forces and effort
Spokes
Wheel loads in cornering and combined cases
forces at the pickups
3. Part checks
Bending+Torque
Shaft bending and torsion, principal stresses and safety factors by material
Spline
Shear and compressive stress in splined shafts and hubs
Bearing Loads
Static and dynamic bearing reactions against the catalogue ratings
Fatigue
Alternating and mean stress, surface, loading and size factors; life in kilometres from the S-N data and the load encounters per kilometre, with the Goodman, Gerber and Soderberg mean-stress criteria
FEA
Outside the workbook: 26 of the register's checks, with load cases from Chassis Loadcases
stresses, life, safety factors
4. Register
Loadcase - FOS
77 checks on 48 parts in four systems: load case, loading, stresses, material, three safety factors and the source of each check
Fig. 6 The calculators

Chassis load cases: forces at the suspension pickups for each case and corner, values masked
Load case	Load factor	Pickup loaded	Held fixed	FL			FR			RL			RR
Load case	Load factor	Pickup loaded	Held fixed	F_z	F_y	F_x	F_z	F_y	F_x	F_z	F_y	F_x	F_z	F_y	F_x
Max shock		damper	wishbone, fore and aft
Max heave	2 g	wishbone, fore	damper
Max heave	2 g	wishbone, aft	damper
Max acceleration		wishbone, fore	damper
Max acceleration		wishbone, aft	damper

Held fixed: the wishbone, fore and aft, when the damper pickup is loaded; the damper in every other case.

Fig. 7 Chassis load cases

The brief

I took the founder's brief, a sporty four-wheeler narrow enough for a bicycle lane, light and cheap to make at small volume, and before any detailed design benchmarked comparable vehicles of 100 to 200 kg, mapped the architecture and drivetrain options and checked the packaging of each layout, so no concept failed late. I put the trade-offs of weight, cost, complexity and performance to the founder in business terms.

Fig. 1 is where that ended: four wheels inside 95 cm, and under 50 kg complete by the bill of materials, less than half the mass of the lightest vehicle benchmarked. The structure the suspension mounts to was another engineer's work; I aligned my mounts and load paths with it.

One corner, two load paths

I designed the corner as a hybrid of the wishbone and McPherson layouts. The upright is fixed to a single lower wishbone, which sets the camber and takes the cornering and braking loads; the damper sits on a triangular mount where the two meet, on joints free to rotate, so it carries axial force only and an off-the-shelf bicycle damper does the job. The tie rod matches the wishbone's length and pivot axis, so bump steer is zero by geometry.

Fig. 2 is that corner as a free-body diagram: the side and fore-aft forces at the contact patch end in the wishbone's pivots, and the damper takes its share of the vertical load along its own axis. Fig. 3 is how it was signed off: the register's checks on the wishbone tubes, the damper mount, the steering knuckle and its bearing, each with its load case, the kind of loading, the material and the source of the check.

Fig.: 2
Title: The corner as a free-body diagram
Kind: Schematic, principle only, not to scale
Source: Drawn for this site
Note: The damper sits on joints free to rotate, so it carries axial force only, no side load and no bending. The tie rod has the wishbone's length and its inboard joint on the wishbone's pivot axis: bump steer zero by geometry. The moment balance shows the vertical load only.

Register excerpt: 5 checks, with load case, loading, material, the source of each check, and masked values
System	Part	Load case	Loading	Material	Source
Suspension	Wishbone tubes	Braking + cornering, 0.55 g + 0.55 g	Bending and axial force	AL 6061 T6	FEA
Suspension	Wishbone tubes	Max heave, 2 g	Bending	AL 6061 T6	Bending+Torque
Suspension	Damper mount	Max heave, 3 g, stiffest damper	Tension	AL 6061 T6	FEA
Suspension	Steering knuckle	Max braking, 0.7 g	Tension	AL 7075 T6	FEA
Suspension	Knuckle bearing	Braking + cornering, 0.55 g + 0.55 g	Bearing reaction	Bearing	Bearing Loads

Wishbone tubes · Suspension
Load case
Braking + cornering, 0.55 g + 0.55 g
Loading
Bending and axial force
Material
AL 6061 T6
Source
FEA
Forces Stresses Safety factors
Wishbone tubes · Suspension
Load case
Max heave, 2 g
Loading
Bending
Material
AL 6061 T6
Source
Bending+Torque
Forces Stresses Safety factors
Damper mount · Suspension
Load case
Max heave, 3 g, stiffest damper
Loading
Tension
Material
AL 6061 T6
Source
FEA
Forces Stresses Safety factors
Steering knuckle · Suspension
Load case
Max braking, 0.7 g
Loading
Tension
Material
AL 7075 T6
Source
FEA
Forces Stresses Safety factors
Knuckle bearing · Suspension
Load case
Braking + cornering, 0.55 g + 0.55 g
Loading
Bearing reaction
Material
Bearing
Source
Bearing Loads
Forces Stresses Safety factors

Fig.: 3
Title: The corner in the register: 5 of 77 checks
Kind: Workbook extract, values masked
Source: The project's design calculators

More depth: the motion ratio, the camber gain and why the corner is built this way

Designed for manufacture

I redrew the suspension mounts, which the packaging study had left as complex 3D forms, as laser-cut and bent sheet metal with every pickup where it was, and checked them again in FEA against the same load cases. I carried the change into the drawings and the bill of materials and sourced the machining and fabrication suppliers; the mounts cost much less to make.

Steering, brakes and drivetrain

I designed the clean-sheet steering knuckle and joints the new geometry needed, with the rotor, hub, caliper, disc adaptors and drive axle of the same corner, and checked each for stress and fatigue against its own load cases. Fig. 4 is three of those checks from the register: the caliper adaptor, the drive shaft and the hub splines, each signed off by FEA or a hand calculation.

Register excerpt: 3 checks, with load case, loading, material, the source of each check, and masked values
System	Part	Load case	Loading	Material	Source
Brakes	Caliper adaptor	Max braking, 0.7 g	Tension	AL 7075 T6	FEA
Drivetrain	Drive shaft	Max acceleration	Torsion	AL 7075 T6	Bending+Torque
Suspension	Hub splines	Max braking, 0.7 g	Shear	AL 7075 T6	Spline

Fig.: 4
Title: Brakes and drivetrain in the register: 3 of 77 checks
Kind: Workbook extract, values masked
Source: The project's design calculators

The belt drivetrain of the motor-assisted drive serves front, rear or four-wheel drive from one layout; I routed it with the belt supplier for the hardest case, the multi-bend four-wheel-drive path, and chose the bearings from SKF's catalogue against life and friction calculations.

Two calculation tools

I built two calculation tools: a linked performance model (Fig. 5) for what the vehicle can do with a given layout, and twelve design calculators (Fig. 6) for what each part has to carry.

formulas: 288
linked sheets: 3
calculation blocks: 30
dependencies between blocks: 68
of them across sheets: 24
drive layouts: 4
use cases: 5

User Interface: inputs

Vehicle Data

Vehicle Dynamics

User Interface: calculations

User Interface: outputs

User Interface: inputs

Vehicle specs 5 f
Cornering case
Stopping case
Power case 1 f
Top-speed case 1 f
Gradient case

Vehicle Data

Basic vehicle data 6 f
Masses and CG, by assembly 55 f
Powertrain and drivetrain settings 19 f

Vehicle Dynamics

Vehicle data, as used 23 f
Scenario input 1 f

Vehicle Dynamics, continued

Static weight distribution 5 f
Lateral weight transfer 10 f
Longitudinal weight transfer 5 f
Load on a slope 8 f

Vehicle Dynamics, continued

Tyre forces and limits 20 f
Braking 2 f
Cornering 1 f
Static rollover angles 3 f

Vehicle Dynamics, continued

Motor power 13 f
Top speed 13 f
Gradient 12 f

User Interface: calculations

Motor power 28 f
Top speed 6 f
Gradient 6 f

User Interface: outputs

Max cornering speed 1 f
Min stopping distance 1 f
Required motor power 15 f
Achievable top speed 15 f
Achievable gradient 13 f

Fig.: 5
Title: The performance model's calculation graph
Kind: Model graph, from the formulas
Source: The project's performance model
Note: Blocks are the sheets' own sections; a number is the formulas in that block; a line is a dependency, thicker for more references, in the accent when it crosses sheets. 71 of 288 formulas cross a sheet: change any input and every output updates.

1. Vehicle level

Tyre Data

Lateral force against slip angle and load for the candidate tyres

Weight Transfer

Masses, track, wheelbase, CG and roll centres to the vertical, longitudinal and lateral force at each wheel, per case

Corner

Lateral acceleration from speed and corner radius

Drivetrain

Air, rolling and gradient resistance, power and ratios for front, rear and four-wheel drive

F at each wheel, per case

2. Loads into the structure

Chassis Loadcases

Seven cases, four corners: the forces at the wishbone and damper pickups

Steering Torque

Castor, trail, kingpin inclination and scrub radius to the steering forces and effort

Spokes

Wheel loads in cornering and combined cases

forces at the pickups

3. Part checks

Bending+Torque

Shaft bending and torsion, principal stresses and safety factors by material

Spline

Shear and compressive stress in splined shafts and hubs

Bearing Loads

Static and dynamic bearing reactions against the catalogue ratings

Fatigue

Alternating and mean stress, surface, loading and size factors; life in kilometres from the S-N data and the load encounters per kilometre, with the Goodman, Gerber and Soderberg mean-stress criteria

FEA

Outside the workbook: 26 of the register's checks, with load cases from Chassis Loadcases

stresses, life, safety factors

4. Register

Loadcase - FOS

77 checks on 48 parts in four systems: load case, loading, stresses, material, three safety factors and the source of each check

Fig.: 6
Title: The design calculators: twelve sheets in the order the loads travel
Kind: Calculator map, values masked
Source: The project's design calculators
Note: One arrow per stage, labelled with what it carries: each stage's results are the next stage's inputs. Dashed: outside the workbook.

Chassis load cases: forces at the suspension pickups for each case and corner, values masked
Load case	Load factor	Pickup loaded	Held fixed	FL			FR			RL			RR
Load case	Load factor	Pickup loaded	Held fixed	F_z	F_y	F_x	F_z	F_y	F_x	F_z	F_y	F_x	F_z	F_y	F_x
Max shock		damper	wishbone, fore and aft
Max heave	2 g	wishbone, fore	damper
Max heave	2 g	wishbone, aft	damper
Max acceleration		wishbone, fore	damper
Max acceleration		wishbone, aft	damper
Max braking	0.7 g	wishbone, fore	damper
Max braking	0.7 g	wishbone, aft	damper
Max cornering	0.9 g	wishbone, fore	damper
Max cornering	0.9 g	wishbone, aft	damper
Acceleration + cornering	0.55 g + 0.55 g	wishbone, fore	damper
Acceleration + cornering	0.55 g + 0.55 g	wishbone, aft	damper
Braking + cornering	0.55 g + 0.55 g	wishbone, fore	damper
Braking + cornering	0.55 g + 0.55 g	wishbone, aft	damper

Held fixed: the wishbone, fore and aft, when the damper pickup is loaded; the damper in every other case.

Fig.: 7
Title: Chassis load cases: seven cases, four corners, 156 values
Kind: Workbook extract, load factors shown, forces masked
Source: The project's design calculators

The performance model (Fig. 5) answers the design questions directly: choose a drive layout, chain or belt, a road surface, a slope and a wind, and its three linked sheets return the maximum cornering speed, stopping distance, required motor power, top speed, gradient capability and a g-g diagram.

The twelve design calculators (Fig. 6) carry the loads in the order they travel, from tyre data and weight transfer to the forces at each pickup, then to each part's stresses, fatigue life and safety factors. Each stage's results are the next stage's inputs, and the register at the end names the source of every check. Fig. 7 is the hand-off from the vehicle to the parts: seven load cases, from maximum shock to braking while cornering, as forces at the wishbone and damper pickups of all four wheels.

More depth: what each calculator computes, the combined cases and the whole register, 2 more figures

Release package

I released production-level 2D drawings and a bill of materials with the system structure, part numbers, quantities and the cumulative weight of every part; summed, it gives the mass in the title block.