iBike Newton+ Operating Instructions Manual page 50

The accelerometer in the Newton+ measures hill slope instantaneously, much faster and
more accurately than GPS or barometric pressure methods.
HOW ACCURATE IS THE HILL SLOPE MEASUREMENT?
Your Newton+ 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 Newton+ MEASURES, AND CdA AND Crr?
The Newton+ measures aerodynamic and frictional drag coefficients in the coast down
calibration. The numbers measured and reported on the Newton+ 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 Newton+ on the handlebars. The value of fric
can vary from 5.00 to 25.00, depending on the weight of the rider.
NOTE: The Isaac software will report drag coefficients in CdA and Crr format. See the
Newton+ 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. Lower CdA means less effort 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.
Crr (coefficient of rolling resistance) measures the amount of opposing friction caused by
road surface, tire type, and bearing friction. A low value of Crr means low rolling
resistance.
On a very smooth surface, such as those found at an indoor track, Crr can be 0.003.
Asphalt roads have a typical Crr of 0.0055, and dirt roads can have Crr values of 0.008 or
higher.
Values of CdA and Crr are accurately estimated by Isaac Dial-in Wizards, or they can be
measured by the Coast Down procedure.
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