With increasing demand for efficient and clean modes of transportation, e-bikes have recently seen a large increase in popularity. As a design challenge, the mechanical engineering department wanted to create a bespoke bike designed around the electric drive. The bike had to meet UK e-bike regulations: have a maximum power output of the motor had to be under 250W, have an assistance capability of up to 15.5mph and feature pedal-assisted motor control. Additionally, a department-based challenge required a design that did not employ a hub motor.
The project was split into four separate groups: the frame, steering, gearbox & motor, and the battery. The sub-assemblies decided to focus on designing a commuter urban bike targeted for an average male rider.
The steel frame features a custom geometry to not only meet the anthropometric constraints but also provides the needed strength to support the full assembly. Certain design features such as sliding dropouts and flat-faced tubing were added to better integrate other sub-assemblies.
The steering system of the e-bike has been designed to fit together with the frame to allow users to navigate the direction of the bike with ease. It also aids in the braking of the bike as well as help support the user’s weight and provide stability.
The motor and gearbox were designed specifically to integrate with the frame. This sub-assembly was the most strictly regulated by the UK EAPC legislation. With these limits in mind and more given in the project brief, a robust and maintainable solution was created.
The battery pack was designed to supply power to run the motor and other electrical components. The pack utilises 18650 Li-ion batteries and is rated at 292 Wh. A battery management system was employed to ensure that the batteries are operating safely and to maintain the battery health.
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DMT01A Electric Bike Frame
With rapid urbanization, the need for faster yet environmentally friendly transportation is increasing. Electric bikes are one of solutions to this problem and as a result, have seen a near exponential rise in popularity. They share the convenience of conventional bikes while also having the power to provide for longer and more comfortable journeys that take less effort.
This year, the Mechanical Engineering Department offered a group of third-year undergraduate students to design and manufacture an electrical bike from the ground up. This sub-group focused on designing the frame of the e-bike, a critical component responsible for providing strength while also being a canvas for any other attached components.
The frame features a bespoke geometry, designed around human anthropometrics and targeted towards urban commuting. In the most basic form, the frame is dual triangular in structure with steel tubes, silver brazed together. The cold-drawn, low-carbon steel tubes provides a high tensile strength, fatigue resistance and transition temperature. Each tube is butted for added thermal resistance near braze joints while saving material elsewhere.
Being the canvas for the overall project, key design parameters had to be considered. To be compatible with disc brake calipers while having a chain-tensioning mechanism, custom sliding rear dropouts were designed. The three-piece design allows the entire rear wheel assembly to move horizontally without conflicting with other parts. Furthermore, motor and battery integration were achieved through a square-sectioned seat tube and downtube for increased ease of brazing. The design was iterated and validated through extensive FEA analysis.
The electric bike uses an integrated electric motor to assist the propulsion of the bicycle. The steering mechanism of the electric bike includes several functions: the effortless steering, braking of the bicycle, and the weight support of the bike through the front wheel. Even though the steerer seems simple in terms of functionality, it requires a fine degree of precision.
The steerer can be split into a few main parts – the handlebar, the bearing-steering assembly in the headtube section, and the main fork, which consists of a crown and two fork blades. The handlebar is made of an aluminum tube, bent at the ends to provide comfort for the user. In the bearing-steering section, it consists of spacers, cups and steering tube. The spacers and cups constrain the bearings to prevent excessive movements, while the steering tube provides a clearance fit for the bearings.
The strongest part of the steerer is the fork blades as it needs to withstand the maximum braking force. The fork blades are made of aluminum
rods with holes bored at the end. Thru axles will be placed through these holes, and a wheel will be mounted on the thru axle. Disc brakes are the preferred braking mechanism, and a flat brake mount will be installed onto the fork blade. To attach the fork blades and the steering tube securely to the crown, the connections will be welded together.
Electrically Assisted Pedal Cycle (EAPC) reduces the CO2 emissions caused by unnecessary traffic, while providing us with a sustainable and healthy mode of transport. Recent advancements in technology have allowed electronic components to become cheaper and more compact, making EAPCs a viable alternative to driving for short journeys.
The team aimed to design, make, and test a drive transmission and motor control prototype for use in a larger EAPC assembly. The final solution is an elegant one, making use of spur gears and sprockets to step down the motor RPM and to increase the torque; this allows the motor to provide the necessary torque with greater efficiency. The design utilises an Arduino microcontroller to adjust the motor assistance based on the torque exerted by the cyclist, measured through a NCTE torque sensing bottom bracket. The assembly will be welded to the frame's seat tube and will take advantage of a standard bike transmission system. The final design has an overall weight of 4.85 kg and a cost of £2,100.83.
The electric-bike battery pack is part of a wider super-project to make a pedal-assisted electric bike (e-bike).
The battery pack was designed to supply the necessary power to run a motor and other electrical components. Cylindrical 18650 lithium-ion batteries were used to create a 292 Wh pack which power a 24V, 220W motor. The e-bike is a city/hybrid bike and thus the battery pack was designed to meet a requirement of IP64 water and dust resistance.
An effective passive thermal management system was the major focus of the design of the battery pack. This was achieved with Aluminium casings with wavy internal structures that hold batteries, conduct heat and are convectively cooled by the airflow during bike movement. Several safety and assembly considerations were made during the design process. This includes the incorporation of a battery management system (BMS), to ensure that the batteries are operating safely and to maintain battery health. To validate the mechanical integrity of this design, comprehensive stress analysis calculations were carried out with both hand calculations as well as FEA to validate our findings. The battery pack is expected to undertake 7G acceleration and maintain >90% state of health after 300 cycles.
Various testing procedures were developed to verify the mechanical and electrical aspects of our design. These entail mechanical integrity, cell heat generation, water and dust resistance and impact test through the shaker machine was planned.
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