"Most bicycles have a free-wheel which
allows the wheel to rotate even when the
pedals and chain are stationary. In our
front-wheel drive design we are not using
a standard bike chain and cannot use an
off-the-shelf bicycle free-wheel." says Alex
Selwa, a member of the student engineering
team, explaining why an overrunning clutch
solution was required.
After careful research, the team contacted
Formsprag Clutch in Warren, Michigan to
request an overrunning clutch solution. As
part of Altra Industrial Motion, Formsprag has
access to the product catalogue of its sister
company, Stieber Clutch. It donated a Stieber
CSK25 PP overrunning clutch to the AeroVelo
team as part of its Student Support Program.
The CSK clutch was a clear choice
to accommodate the size and load
requirements of the unique hub. "The front
hub design involved fitting many different
components into a small and efficient
package. The clutch needed to be as small
as possible in order to fit inside the sprocket
and stay within the width requirements of
the hub," explained Selwa. "The clutch is
press-fit onto the hub body and then pressed
inside of the sprocket. It is responsible for
driving the wheels at speeds up to 145km/h
or 90 mph."
Stieber's model CSK25 PP is a bearingsupported sprag-type clutch, delivered
grease-lubricated and protected against
dust particles as small as 0.3 mm. It provides
instant locking/engagement when the clutch
shifts from overrunning to engagement, with
a maximum overrunning speed of 5,000 RPM
and a 77.5 lb. ft (105 Nm) torque capacity.
The sprag-type backstop keeps the shaft
from turning backwards with keyways on
both the inner and outer races that prevent
the bearing from rotating on the shaft. All
CSK clutches feature Formchrome® sprags
with chromium-infused steel that produces
an ultra-hard surface to maximize wear
resistance, which results in extra-long
clutch life.
With the help of Stieber Clutch,
Team AeroVelo's Eta speed bike
reached a top speed of
139.45 km/h (86.65 mph) at
the 2015 World HumanPowered Speed Challenge,
breaking the world record
for the third time that week.
"Our goal with Eta was not simply to
break a speed record. Our goal was
to reach such unbelievable speeds
that it gets people to think twice
about preconceived limits of what
is possible," stated Todd Reichert,
cofounder of AeroVelo, Inc. "We want
to give a huge thanks to all of our
sponsors, volunteers and supporters,
who have put in their time and money
to help us pursue a dream and take
on a true engineering adventure."