iBike Gen III Operating Instructions Manual page 57

HOW IS HILL SLOPE CALCULATED?
When you climb a hill your bike's movement has two components of motion: the total
distance traveled forward on the road, and the vertical distance traveled up the hill. The
"Hill Slope" is the ratio of vertical movement to forward movement, expressed as a
percentage.
Example: You travel 1000 feet forward and climb 100 feet vertically. Your hill slope
is 100/1000 x 100% = 10%
The accelerometer in the iBike measures hill slope instantaneously, much faster and more
accurately than GPS or barometric pressure methods.
HOW ACCURATE IS THE HILL SLOPE MEASUREMENT?
Your iBike uses an accelerometer that measures hill slope and bike acceleration
simultaneously. When your bike is at rest or you ride at a constant speed (in either case
there is no bike acceleration) the hill slope displayed on your screen is very accurate
(within 0.1% of the actual slope). When you are accelerating on your bike (going faster,
coasting on a downhill, or braking) the hill slope may jump around a bit because the
accelerometer not only senses hill slope but bike acceleration as well. In this circumstance
the hill slope displayed on the screen will be only approximately correct (generally within
1% of actual slope). NOTE, HOWEVER, THAT WATTS ARE CALCULATED WITH HIGH
PRECISION UNDER THESE RIDING CONDITIONS, including those where the hill slope
displayed is "off".
WHAT IS THE RELATIONSHIP BETWEEN THE AERODYNAMIC AND FRICTIONAL
DRAG COEFFICIENTS THE iBIKE MEASURES, AND CdA AND Crr?
The iBike measures aerodynamic and frictional drag coefficients in the coast down
calibration. The numbers measured and reported on the iBike screens ("aero" and "fric")
are related to, but not the same as, CdA and Crr. The value of aero can vary from 0.2 to
1.5, depending on the location of the iBike on the handlebars. The value of fric can vary
from 5.00 to 25.00, depending on the weight of the rider.
NOTE: The iBike software will report drag coefficients in CdA and Crr format. To get this
data from your iBike, you'll need to do the multiple coast-down/calibration ride, described
in these instructions. Also, see the iBike software instructions for more details.
WHAT ARE TYPICAL VALUES OF CDA AND CRR?
CdA (coefficient of aerodynamic drag) is dependent mostly on ride position, rider height,
and rider weight. The lower the CdA, the less effort that is required to overcome opposing
wind.
The time trial position results in the lowest values of CdA: numbers between 0.22 and 0.28
are typical. The specific value will depend considerably on the details of bike type, rider
position, and rider size.
The "drops" position is not as aerodynamically efficient. Values between 0.32 and 0.38 are
typical.
The "hoods" position (hands grasping the brake levers) is the most typical riding position.
CdA values of 0.34 to 0.43 are typical.
The CdA of a cyclist riding a comfort bike can be 0.45 or higher.
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