Date post: | 12-Sep-2014 |
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Symptum CyclesTechnical Design Concept Presentation
Michael Allan, Mitchell Angus & Vladimir Borisov
Problem
Current limitations– Conventional bicycles have pre-selected
gear ratios• The rider is prevented from performing at
optimal efficiency– Conventional systems are prone to the
elements• Chain slipping• Damage and wear caused by dirt and debris• Costly and time consuming repairs
Proposed Solution
Create an internal gearbox with a continuously variable transmission Eliminate derailleur Protect the shifting mechanism from the elements Smoother and more reliable shifting
Design requirements & constraints
Performance Requirements Shifting
Shifting accuracy at or above conventional shifting Power transmission
Within a competitive range 85-90% (Gordon)
Physical Constraints Weight
Less then 10 pounds Geometric
Length ≤ 12 inches Height ≤ 8 inches Width ≤ 6 inches
(Benitez,2002)
Considered designs
Toric drive Fluid needed Weight
Planetary Gears Weight Bulkiness Mechanical complexity
(Loewenthal,1983)
Belt Drive CVTUses one or a combination of variable sheaves to allow the belt to
move radially changing the input to output gear ratios
– Advantages• Mechanical simplicity• Cheap production cost• Commonly used design• Eliminates shift “jerking”
– Increased shifting efficiency
– Disadvantages• Potential for losses in power transmission
– Belt slip
(Stuhler EP 047299A1)
First prototype suggested
Based off of a 1970’s patent. The design consisted of two variable sheaves connected by a belt, and moved
by a spring to change gear ratios.
(Chao USP 3837234)
Design complications
Mechanical complexity Expensive and intricate parts Difficult to predict shifting
Relies on spring tension to select ratios
Selected DesignA variable sheave is positioned on a movable control arm and attached with a friction drive belt to a fixed pulley. The motion of the control arm causes the belt to move radically changing the gear ratio
Advantages Linear shifting
Mechanical simplicity Lower production costs
Increased shifting accuracy Simplified control function
Disadvantages Gear range limitations
θ
Design evaluation
Power transmission test Single most important indicator of success Shifting efficiency Slip conditions
Shifter force profile Actuator specification
Pedal attachment
Variable Sheave
sheave
Shifter bar motion
Testing set up
Friction rollers
Force sensor
Next step
Actuator design Rider input control
Frame integration Geometrical reconstruction Frame integrity
The Future
Production of diversified friction drive CVT bicycle models
Rider specific customizations Frame dimensions Gear ratio ranges Special Features
Handle bars Seats