Bachelor Thesis · Silver Oak College of Engineering & Technology · Gujarat Technological University · 2020–21 Engineering Analysis Update · 2026
A regenerative braking system designed as a modular attachment for conventional bicycle front wheels. The system replaces conventional friction brake shoes with two small DC dynamos pressed against the inner wall of a drum housing rigidly mounted to the wheel. Friction at the contact serves two purposes simultaneously: it decelerates the wheel (braking) and drives the dynamo shafts (generation). The recovered electrical energy is stored in a battery for auxiliary use.
This repository contains the full CAD design, working principle, and first-principles engineering analysis — including free body diagrams, force calculations, stress analysis, and a documented design correction identified through analytical validation.
Tools: Fusion 360 · Mechanical Design · Kinematic Mechanism Design · Hand Calculations
In conventional bicycle braking, all kinetic energy is dissipated as heat through friction between brake pads and the rim. For daily commuting, this represents a continuous and unrecovered energy loss. This project explores whether braking energy can be partially recovered using a compact, retrofittable mechanism — without modifying the bicycle frame or drivetrain.
The system is built around a drum-brake-inspired scissor mechanism.
- At rest / riding: The drum housing rotates freely with the wheel. Dynamos are held away from the housing wall — no contact, no drag.
- Brake applied: Rider pulls the brake lever → cable pulls the sliding plate upward → scissor linkage forces both dynamo arms outward symmetrically.
- Contact & generation: Dynamo rollers press against the inner surface of the rotating drum → friction drives the dynamo shafts → electrical energy is generated.
- Braking: The same contact friction that drives the dynamos decelerates the wheel — braking and energy generation happen in the same physical event.
- Storage: Generated electricity is stored in a 12V lead-acid battery for auxiliary use (lighting, mobile charging).
| Parameter | Symbol | Value |
|---|---|---|
| System mass (rider + bike) | m | 95 kg |
| Tire circumference | C_tire | 1170 mm |
| Tire radius | R_tire | 186.2 mm |
| Drum housing inner diameter | D_drum | 134 mm |
| Drum radius | R_drum | 67 mm |
| Dynamo roller diameter | D_roller | 50 mm |
| Dynamo roller radius | R_roller | 25 mm |
| Gear ratio (drum / roller) | GR | 2.68 : 1 |
| Scissor arm length (pivot–pivot) | L_arm | 100 mm |
| Scissor arm angle (rest) | θ | 105.1° |
| Scissor arm cross-section (original) | — | 20 × 4 mm steel |
| Scissor arm cross-section (corrected) | — | 20 × 6 mm steel |
| Main axle diameter | d_axle | 15 mm |
| Number of dynamos | — | 2 |
| Friction coefficient (steel) | μ | 0.5 |
| Dynamo efficiency (estimated) | η | 50 % |
The 2.68:1 speed step-up from drum to dynamo roller is an inherent result of the radius ratio — the dynamos spin approximately 2.68 times faster than the wheel without any additional gearing.
A complete first-principles analysis requires identifying all forces acting on each component. The mechanism is first abstracted into its geometric kinematic representation, and then the forces at the drum–roller contact interface are resolved for both rollers.
The scissor linkage (red) crosses at the central pivot block on the axle. When the brake cable pulls upward via the green link and blue guiding rod, the red arms force both rollers (Roller 1 and Roller 2) radially outward against the drum's inner wall. The drum rotates clockwise (ω), and both rollers — by the internal-contact rule — also rotate in the same direction.
For each roller (both at lower-left and lower-right contact positions):
- F_N (normal force on roller): 395.6 N pointing radially inward (toward drum center). This is the drum wall pushing back on the roller.
- F_arm (force from scissor arm): 395.6 N pointing radially outward — balances F_N for radial equilibrium of the roller.
- F_f (friction on roller): 197.8 N tangent to the drum surface, in the direction of drum surface motion at that contact (friction drags the roller, driving its rotation).
For the drum (Newton's 3rd Law reactions at each contact):
- F_N on drum: 395.6 N radially outward at each contact (roller pushing drum wall).
- F_f on drum: 197.8 N tangent to drum surface, opposing drum rotation — this is the braking action.
Same contact, dual purpose: the friction at the drum–roller interface simultaneously drives the dynamo (energy generation) and brakes the wheel (deceleration). Both rollers rotate in the same direction (CW) as the drum — consistent with the geometry of internal contact (a small wheel inside a larger ring).
KE = ½ × m × v²
| Speed (km/h) | Speed (m/s) | Kinetic Energy (J) |
|---|---|---|
| 10 | 2.78 | 367 |
| 15 | 4.17 | 826 |
| 20 | 5.56 | 1,468 |
| 25 | 6.94 | 2,288 |
At 20 km/h (typical urban cycling speed), 1,468 J of kinetic energy is available per braking event.
For comfortable deceleration a = 1.5 m/s²:
F_brake = m × a = 95 × 1.5 = 142.5 N
τ_wheel = F_brake × R_tire = 142.5 × 0.1862 = 26.5 N·m
Stopping distance: s = v² / (2a) = 10.3 m
Braking time: t = v / a = 3.7 s
The braking torque is generated by friction between the two dynamo rollers and the inner drum wall. Each contact shares half the load.
F_friction = τ_wheel / (2 × R_drum) = 26.5 / (2 × 0.067) = 197.8 N per contact
F_normal = F_friction / μ = 197.8 / 0.5 = 395.6 N per contact
The scissor linkage must deliver 395.6 N of outward normal force at each dynamo contact — this is the critical design requirement for the mechanism.
Gear ratio: GR = D_drum / D_roller = 134 / 50 = 2.68 : 1
Wheel speed: ω_wheel = v / R_tire = 5.56 / 0.1862 = 29.86 rad/s = 285 RPM
Dynamo speed: ω_dynamo = 2.68 × 285 = 764 RPM
This speed is within the efficient operating range of small DC motors used as generators.
Contact velocity: v_contact = ω_wheel × R_drum = 2.00 m/s
Mechanical power: P_mech = F_f × v_contact × 2 dynamos = 791.2 W
Electrical power: P_elec = η × P_mech = 0.5 × 791.2 = 395.6 W
Energy per event: E_recovered = P_elec × t × 0.5 (avg.) = 732 J
System efficiency: η_system = E_recovered / KE ≈ 20–25 %
Real-world meaning: Over a 10 km commute with ~30 braking events, ~5.2 Wh recovered — sufficient to charge a phone by 15–20% per ride. Modest, but compares well for a purely mechanical contact-based recovery system.
The scissor arm carries the full normal force as a bending load. With the pivot at the centre of the arm, the maximum bending moment occurs at the pivot:
M_max = F_arm × (L / 2) = 395.6 × 0.050 = 19.78 N·m
Original 4 mm cross-section:
I = b·h³/12 = 0.020 × (0.004)³ / 12 = 1.067 × 10⁻¹⁰ m⁴
σ = M·c/I = 19.78 × 0.002 / 1.067×10⁻¹⁰ = 371 MPa
Safety factor = σ_yield / σ_max = 250 / 371 = 0.67 → ARM YIELDS ✗
Design correction → 6 mm cross-section:
I_new = 0.020 × (0.006)³ / 12 = 3.60 × 10⁻¹⁰ m⁴
σ_new = 19.78 × 0.003 / 3.60×10⁻¹⁰ = 165 MPa
Safety factor = 250 / 165 = 1.52 → SAFE ✓
This redesign was identified analytically before any physical prototype was built — demonstrating the value of first-principles validation in early-stage design.
The main axle (Ø15 mm steel) carries the full braking torque:
τ_shear = 16·T / (π·d³) = 16 × 26.5 / (π × 0.015³) = 40 MPa
Safety factor = τ_yield / τ_shear = 145 / 40 = 3.6 → SAFE ✓
| Parameter | Value | Status |
|---|---|---|
| Kinetic energy at 20 km/h | 1,468 J | — |
| Required braking torque | 26.5 N·m | — |
| Contact force per dynamo | 395.6 N | — |
| Dynamo speed at 20 km/h | 764 RPM | — |
| Peak electrical output | ~396 W | — |
| System recovery efficiency | 20–25 % | Realistic for contact-based recovery |
| Scissor arm — original 4 mm | 370 MPa, SF = 0.67 | REDESIGNED → 6 mm |
| Scissor arm — corrected 6 mm | 165 MPa, SF = 1.52 | ✓ Safe |
| Main axle (Ø15 mm) | 40 MPa, SF = 3.6 | ✓ Safe |
| Component | Material | Function |
|---|---|---|
| Drum housing | Steel | Rigidly attached to wheel — rotates with the wheel, acts as contact surface for dynamos |
| Scissor arms (×2) | Steel (20×6 mm) | Lever arms — transfer brake cable pull into radial dynamo engagement |
| Dynamo (×2) | Vega DC motor | Converts rotational contact energy into electrical energy |
| Central pivot block | Steel | Mechanism axis — supports scissor link intersection |
| Main axle | Steel (Ø15 mm) | Mounts the full assembly to the front fork dropouts |
| Battery | Lead-acid 12V | Stores generated electrical energy for auxiliary use |
Two design configurations are provided as STEP files, compatible with all major CAD packages.
/cad
├── wheel_assembly_internal_hub.STEP
└── wheel_assembly_external_attachment.STEP
Version 1 — Internal Hub: Drum housing replaces the standard wheel hub. RBS mechanism integrated within the wheel alongside the spoke structure. Compact, but requires a custom-built wheel.
Version 2 — External Attachment: Drum housing mounts externally onto a standard hub. RBS mechanism attaches to the side face of the wheel. Fully retrofittable onto any conventional bicycle.
/cad → STEP files for both design configurations
/renders → CAD renders (full assembly, exploded, section, redesign)
/docs/fbd → Free body diagrams and mechanism sketches
README.md → This document
LICENSE
| Type | Bachelor Thesis (Final Year Project) |
| Institution | Silver Oak College of Engineering & Technology |
| University | Gujarat Technological University, Ahmedabad, India |
| Original Period | December 2020 – April 2021 |
| Engineering Analysis Update | 2026 |
| Team Size | 4 members (original thesis); analysis & redesign by individual contributor |
| Software | Fusion 360 |
| Note | Project completed as CAD and theoretical study due to COVID-19 campus access restrictions. The 2026 update adds the full first-principles analysis, free body diagrams, stress validation, and the documented arm redesign. |
- First-principles mechanical analysis: force balance, free body diagrams, bending and shear stress calculations
- Kinematic mechanism design: scissor linkage with symmetric force transmission
- Design validation: identifying a structural failure analytically and correcting the design before prototyping
- CAD modelling: complete assembly in Fusion 360 with moving components, exploded views, and section views
- Energy systems thinking: integration of braking, generation, and storage in a single mechanism
- Engineering communication: clean documentation of problem → physics → analysis → design decision