Design front wheel will house a motor and two

Design of an Electric Drivetrain in a
Spoke-Less Bicycle Wheel

Rajdeep Mukherjee1,
Nayan Arora2

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1, 2 Department
of Mechanical Engineering, University of Petroleum & Energy Studies

 

Abstract:
The most interesting and exciting thing about electric bikes is that, they
bridge gap between human-powered bicycles and motor-powered vehicles. The
redesigning of the electric bicycle could be effectively done by changing the
position of the drivetrain considering the dynamics, the feasibility and the
aesthetic values of the present design. With the aim of modifying the existing
trend in the electric bicycles and to improve its ergonomics and aesthetics, a
new model needs to be fabricated. Since the front wheel is not an active member
of the drivetrain of a bicycle, hence it can be used to implement a new
electric drivetrain. This study focuses on front spoke-less wheel that will
house a motor and two idler pulleys in a triangular formation. The idlers will
fulfill the purpose of force balancing and stability of the friction drive.  The rear transmission will be left unchanged
leaving the option of changing the front wheel to the user. Friction-drive will
be chosen as the means of transmission, due to fewer parts and sufficient power
transmission capabilities. The front wheel will house a motor and two idler
pulleys with an angle of 120° between them to counter balance the forces. The
idea of this setup is primarily to provide the rider with the buffer power that
can take him along difficult terrains and inclines

Keywords:
Spoke-less, Idler pulleys, Triangular formation, Friction drive, Electric
Drivetrain

I.
INTRODUCTION

An
electric bicycle uses the same designs and components as any other bicycle, but
also includes an electric motor that supplements the power available to the
rider. The motor is powered by a rechargeable battery which provides a
convenient, and less strenuous cycling experience, making electric bicycles the
world’s most accessible way to travel.

The
electric bicycles in market today have their drivetrain installed in the
bicycle frame itself. Also, the cost of an electric bicycle poses a major
drawback. But, this gap in the present electric bicycle can be sorted out by a
mechanism that would allow the rider to convert any traditional bicycle into an
electric one. With this aim a new model needs to be fabricated. This project
aspires to modify the existing trend in the electric bicycles, to improve the
ergonomics and aesthetics of the bicycle by modifying the front wheel of the
bicycle.

Since
the front wheel is not an active member of the drivetrain of a bicycle, hence
it can be used to implement a new electric drivetrain. The front wheel will
house a motor and two idler pulleys in a triangular formation. The idlers will
fulfill the purpose of force balancing and stability of the friction
drive.  The rear transmission has been
left unchanged leaving the option of changing the front wheel to the end user.

Friction
drive was chosen as the means of transmission as it has less number of parts
and sufficient power transmission capabilities required in our drive train.

The
front wheel will house a motor and two idler pulleys with an angle of 120°
between them to counter balance the forces. The power from a 250W motor will be
directly transmitted to the inner rim of the front wheel. The speed will be
controlled by a throttle integrated in the existing handle bar of the bicycle.

II.
DESIGN OF FRAME

RIM

IDLER

MOTOR

120o

While designing the frame, the main aim was to make it
strong enough to withstand static loads and impact loads from the road shocks.
The other design considerations kept in mind were to make a compact &
durable drivetrain with low Centre of Gravity. Low centre of gravity ensures
better stability to the vehicle. Also, the entire drivetrain had to fit inside
the front wheel. A simple frame was designed by placing the motor on the lower
part of the rim and two idlers in a triangular formation. The final frame comprises
of linkages which add rigidity to the frame and provides ample space for
placing all the drivetrain components. Further provisions were added so that
the wheel could be attached to the front fork of the bicycle. The design of
frame and assembly of the vehicle was done on SolidWorks 2016.

 

III.
ANALYSIS

IV.
MATERIAL SELECTION

Material selection ensures that the
desired weight, strength and safety of the component is achieved at minimum
cost. To make an optimal material selection extensive study of materials on the
basis of mechanical properties and availability was done. Yield strength, ultimate
tensile strength, density, bending stiffness and welding requirements were the
key parameters for the final selection. Aluminium of Grade 6061 was chosen as
the material for fabrication of the frame.

Tensile strength

420 MPa

Yield strength

350 MPa

Modulus of elasticity

205 GPa

Shear modulus (typical for steel)

80 GPa

Hardness, Brinell

121

 

V.
TORQUE CALCULATIONS FOR MOTOR SELECTION

Consider,

Mass of rider = 75 kg

Radius of Crank = 10 cm
(Rc)

Radius of Pedal = 15 cm (RP)

Radius of Rear Tire = 30
cm (RT)

Radius of Rear Sprocket =
3 cm (RS)

T= Tension

?= Torque

µ= Coefficient of
friction

F= Force due to friction

?f= Torque due to
friction

At constant speeds, ??=0

? =0 = Rc*(T) – RP*(W)

T= (0.15)*(75*9.81)/
(0.1)

T= 1103.61 N

At rear sprocket,

? = (Rs) T= 0.03*1103.61

? = 33.108 Nm

 

(Max Torque applied by
rider which is enough to make bicycle move)

Now, for static friction,

 

 

µ= 0.8

Therefore, F= µ
*(mTotal)*g     

(say mTotal = 100Kg)

F= 0.8*100*9.81

F= 784.8 N

?f= 235.44 Nm

For rolling friction,

µ= 0.05

F= 49.05 N

?f= 14.715 Nm

? (Rear) = ? (Tension) –
?(Friction)

=18.39 Nm

 

VI.
TRANSMISSION BASED ON V-BELT DRIVE

Friction
drive was chosen as the means of transmission as it has less number of parts
and sufficient power transmission capabilities required in our drive train. It
is also easy to fabricate, hence it was the best system for this model.

The
rim has a V-shaped cross section which meshes with the pulleys. The surface is coated
with rubber, which increases the contact and thus the friction between the
surfaces.

VII. SPECIFIC COMPONENTS USED IN THE
WHEEL ASSEMBLY

·        
Motor: A 250 W motor producing 21 Nm of
torque

 

·        
Batteries: Lithium Ion batteries.

·        
Wheel rim: A double walled cycle rim was
purchased.

 

·        
Solid Tyre: A solid tyre made from butyl
rubber was purchased.

Figure 9.  Solid Tyre

 

2.6 Fabrication Process

The fabrication of the model was
completed in following phases:

·        
Machining
of the idlers and the motor groove.

Figure
10.  Machining on the Motor Rim and the
Idlers

·        
Welding
of the Mild Steel frame according to the wheel rim.

·        
Devising
a mechanism for the assembly of the wheel.

Figure
11.  Machining on the Motor Rim and the
Idlers

·        
Assembly
of the wheel.

Figure
12.  Structure of the Wheel

·        
Assembly
of the wheel with the cycle.

Figure
13.  The completed cycle

·        
Testing
the cycle without the electric motor, to determine the maximum resistance due
to the friction drive.

VII. CONCLUSION

The aim of this study was
to reduce the existing anomalies in the electric bicycle design including the
ergonomics and economics of the bicycle. Front wheel was selected to implement
the electric drivetrain because it is not the part of the manual drivetrain of
the bicycle. The frame consisted of a 120° setup supporting the motor and two
idler pulleys. Al 6061 was selected as the material of construction to conform
to the values of strength and hardness. The starting torque was calculated to
be 18 Nm thus, 250W motor used which could produce 21Nm torque. For the
transmission of power from motor to wheel, friction drive was selected due to
its optimum transmission characteristics and less number of moving parts
reducing the maintenance costs. The motor was machined in V-shaped cross section
to implement the friction-drive which increased the contact area and thus avoided
slippage.

XI. REFERENCES

 

Jobst Brandt (1981).
The Bicycle Wheel 3rd Edition. California: Avocet, INC.

 

Heinen, E., van Wee,
B., Maat, K. (2010). Commuting by Bicycle: An Overview of the Literature.,
Transport Reviews, Vol. 30, No. 1, 59–96, January 2010.

 

Mordkovich, Yevgeniy,
Mordkovich, Boris. (2015). The Complete Electric Bike Buyers Guide. NY: EVELO,
INC.

 

GeoOrbital Wheel.
(n.d.). Retrieved from GeoOrbital website: https://www.geoo.com/pages/how-it-works.

 

AZO Materials. (n.d.).
Retrieved from: http://www.azom.com/article.aspx?ArticleID=3328.

 

Batavus (2011).
Characteristics electric bicycle, 2011.

 

Song, M., Park, S.,
Alamgir, F.M., Cho, J., Liu, M. (2011). Nanostructured electrodes for
lithium-ion and lithium-air batteries: the latest developments, challenges, and
perspectives. Materials Science and Engineering: R: Reports, 72, 203-252.

 

Lew, E. Paul (1995).
U.S. Patent No. 5,419,619. Indianapolis: U.S.

 

Deepak, A., Siva
Sankar, R., Vinoth Kumar, S., Hariharan, V. (2016). Design, Fabrication and
Analysis of Spoke less Wheel. Middle-East Journal of Scientific Research 24
(S2): 64-70, 2016. doi: 10.5829/idosi.mejsr.2016.24.S2.118.