1. An Electric Assist Bicycle Drive with Automatic Continuously Variable Transmission
Peter Watterson
CSIRO Materials Science and Engineering, Australia
ICEMS 2008, 19 October 2008, Wuhan, China

2. Objectives Provide electric assistance to a bicycle
human – electric hybrid with roughly equal power inputs
low CO2 transportation
... but some exercise
An automatic transmission – no gear changing
Fixed ratio chain (or belt) drive from crank to rear wheel
Hence chain drive can be shrouded and kept clean
Minimum weight
Maximum efficiency
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

3. A Prius in a Bicycle!
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

4. Electric Continuously Variable Transmission, e-CVT
Planetary 1
output
Ring gear R
MG2
planet Carrier
Sun gear C
human input S
MG1
planet gears
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

5. Planetary gear speed relationship
For planetary gear constant
Speeds obey:
Ring gear number of teeth
Sun gear number of teeth
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

6. Planetary gear torque relationship
From force and torque balance on planet gears
External torques on the gears obey:
-F/2 F
-F/2
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

7. Two hub topology rear hub front hub
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

8. One hub topology, axial field MGs
Axial magnetic field MGs can share same stack:
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

9. One hub topology, direct drive MG2
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

10. One hub topology, geared MG2
Ratio of volumes within airgaps = 3
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

11. A possible specification
Maximum continuous total electric power
200 W
Maximum continuous rider power
Predominant rider cadence
75 rpm
Range of rider cadence
60 – 90 rpm
Range of bicycle speeds
15 – 36 km/h
Associated (700 mm diameter) wheel speeds
110 – 270 rpm
Maximum regenerative braking power
400 W
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

12. MG ratings for draft rear hub design
Gear ratios: Planetary 1, 7.24; Planetary 2, 5.32.
Chain ratio 2.33 gives equal Torque/Volume for MG1 and MG2 at their most demanding operating points. Ignoring all losses:
Crank speed (rpm)
Wheel speed (rpm)
MG1
MG2
speed (rpm)
torque (Nm)
power (W)
Low cadence, high speed 60
270
-801
-1.66
139
-1437
-0.41 61
High cadence, low speed 90
110
934
-1.10
-108
-586
-5.02
308
Regenerative braking 400 W, high speed
-1955
0.49
-100
1.99
-300
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

13. Conclusions Trial rear hub e-CVT design
200 W human power and 200 W electrical power
MGs with 19 kN/m2 tangential force at air-gap – realistic.
Higher power design (e.g. for the US market)
Two hub topology may be needed for extra volume
Automatic control
No gear changing
Can base desired speed on pedalling torque and/or cadence
Can allow rider to vary level of assistance
Other applications – petrol/electric vehicles including scooters
CSIRO. An Electric Assist Bicycle Drive with Automatic CVT

14. Thank you CSIRO Materials Science and Engineering Peter Watterson
Contact Us
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CSIRO Materials Science and Engineering
Peter Watterson
Principal Research Scientist
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Peter
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Thank you