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