MAIN FUNCTIONAL REQUIREMENT: Provide a bicycle rider with a means of selecting three different gear ratios in the bicycle's drive train
DESIGN PARAMETER: 3 speed hub (this is one possible design parameter)
GEOMETRY/STRUCTURE AND PARTS:
EXPLANATION OF HOW IT WORKS/ IS USED:
Three speed hubs use a planetary gear system to achieve a gear ratio between the sprocket and the wheel. In this gear system the sun gear is fixed in place while the planet carier is rotated. This causes the planet gears to rotate which in turn causes the ring gear to rotate.
The rider selects a speed using a shifter. The shifter pulls on a cable which is connected to the cable lever. This causes the cable lever to rotate.
The cable lever pushes on a pin that goes through the axle. This pin is connected to the planetary gear system and forces it to move side to side when the lever is pulled.
When the planetary gear system shifts from side to side the ring gear and the planet carier are engaged or dissengaged from the wheel and the chain. This is what changes the gear ratio.
The gear ratio between the planet carrier and ring gear can be calculated based on the number of teeth in the sun and planet gears:
WHERE YOU CAN FIND THREE SPEED HUBS:
Three speed hubs are mostly found on bikes built before the 1970s
Modern internal gear hubs are based on this design but use several
planetary gear systems chained together to acheive a greater number of speeds.
Internal gear hubs - http://www.sheldonbrown.com/seven_speed.html
Planetary gear trains - http://www.cs.cmu.edu/People/rapidproto/mechanisms/chpt7.html#HDR123
How a 14 speed hub works - http://www.rohloff.de/english/products/speedhub/speedwork.html
General Info | Syllabus