Sole vehicle platform design engineer · Three-person micromobility startup · Aug 2022 to Aug 2024

From a linked performance model to every part's safety factor

Company, brand and product withheld under NDA; figures are simplified or masked.

Fig.: 1
Title: The chassis load cases in the design calculators, with their load factors
Kind: Workbook, drawn in perspective; load factors shown, forces masked
Source: The project's design calculators
Note: Load factors as in the file; max shock and max acceleration, and every force, masked. Along the foot, the twelve calculators.

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.: 2
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.

Problem

There was no earlier vehicle to measure, and every geometry change moved every load. The design needed answers at two levels: what the vehicle can do with a given layout, and what each part has to carry. Worked by hand, each iteration meant redoing both.

Approach

A performance model of three linked sheets, where any change to the vehicle, the drive layout or the use case runs through to every output (Fig. 2). Beside it, twelve design calculators in the order the loads travel (Fig. 3), each stage's results becoming the next stage's inputs: the load cases at the suspension pickups, then each part's stresses, its fatigue life and its safety factors. Hand calculation where a closed form exists, FEA where it does not, and one register that names the source of every check.

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.: 3
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.

What I did

Built the performance model as three linked sheets: an interface for the vehicle specs, the drive layout and the use case (Fig. 4); the vehicle data, with masses and CG by assembly, sprung and unsprung; and the vehicle dynamics, from the weight distribution to the weight transfer, split into its unsprung, geometric and elastic parts.
Set it to answer the design questions directly: maximum cornering speed, minimum stopping distance, required motor power, achievable top speed and gradient capability for front-, rear-, four- or all-wheel drive, with a g-g diagram.
Built the design calculators for the loads into the structure and the part checks: the load cases at every suspension pickup, steering effort, wheel loads, shaft bending and torsion, splines, bearings and a fatigue life in kilometres.
Added combined cases, braking and cornering at once and accelerating and cornering at once, so parts were sized against combined loads (Figs. 1 and 5).
Carried the load cases into FEA for the complex parts and into one register for every part.

Vehicle specs

Vehicle weight configuration

Occupant height

Occupant weight

Luggage weight

Empty vehicle weight

Total vehicle weight

Powertrain and drivetrain

Drive layoutFWD

FWD · RWD · 4WD · AWD

Drive typeChain

chain drive · belt drive

Powertrain optionoption 1

Front transmission

Rear transmission

Environment

Road surfaceAsphalt

asphalt · concrete · gravel · rough paved

Road inclinationUphill

WindHeadwind

Use cases

Max cornering speed

Corner radius

Min stopping distance

Initial speed

Required motor power

Desired speed

Slope

WindHeadwind

Achievable top speed

Slope

WindHeadwind

Achievable gradient

Speed

WindHeadwind

Calculations

Resistances

Air drag

Rolling resistance

Gradient resistance

Total resistance

At the wheels

Required power

Required torque

Wheel RPM

Motor

Motor power

Motor torque

Motor RPM

Output

Max cornering speed

km/h

Min stopping distance

Required motor power

Achievable top speed

RPM-limited

Power-limited

Achievable gradient

degrees

g-g diagram

Envelope

Fig.: 4
Title: The interface: vehicle specs, use cases, calculations, output
Kind: Interface recreated, values masked
Source: The project's performance model

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.: 5
Title: Chassis load cases: seven cases, four corners, 156 values
Kind: Workbook extract, load factors shown, forces masked
Source: The project's design calculators

Result

One register: 48 parts in four systems, each with its load case, stresses, material, safety factors and the source of the check (Fig. 6).
Every FEA load case traceable to the platform's dynamics.

Register excerpt: 8 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
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

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
Caliper adaptor · Brakes
Load case
Max braking, 0.7 g
Loading
Tension
Material
AL 7075 T6
Source
FEA
Forces Stresses Safety factors
Drive shaft · Drivetrain
Load case
Max acceleration
Loading
Torsion
Material
AL 7075 T6
Source
Bending+Torque
Forces Stresses Safety factors
Hub splines · Suspension
Load case
Max braking, 0.7 g
Loading
Shear
Material
AL 7075 T6
Source
Spline
Forces Stresses Safety factors

Fig.: 6
Title: The register: 8 of 77 checks on 48 parts
Kind: Workbook extract, values masked
Source: The project's design calculators