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The final key to my meager cycling
accomplishments was using the most economical
training technology at the time, a heart rate
monitor (HRM). This device was one of the
easiest ways to provide training feedback and
act as a backbone to my training program.
Training with a HRM is still probably the most
economical way to structure a training plan, but
with the entry of several different
manufacturers into the power-measuring arena,
training with power is becoming a tempting
alternative.
There exists some uncertainty and confusion for
consumers when it comes to picking which power
meter to use. For a few weeks in early
2003, I had the opportunity to simultaneously
equip my ride with three power measuring
systems:
SRM,
Power Tap (PT), and
Polar. I will review these systems
using the three fundamental design variables of
price, performance, and durability - but first,
let’s cover some power basics.
Power
What is power? Power is defined as the
amount of work done during a given period of
time and has the metric units of watts
(horsepower in English units).
For cycling, power data is fundamental in
determining trends in overall performance due to
changes in equipment, positioning, and more
importantly training methods. The power
produced by a cyclist is used to overcome
aerodynamic, inertial, rolling resistance,
gravitational, and miscellaneous drivetrain/bearing
forces. Cycling speed is fundamentally
bound by how much power one can produce.
Work isn’t necessarily sitting at your desk,
slamming down a couple cups of coffee and making
yourself look busy while mumbling about
red Swingline staplers. Work in this
context is a force applied over a distance and
has the units of energy (Newton-meter or
Joules). Everyone does work when they lift
a box, lift up their kids, or pedal their bike.
For rotating systems, power can be derived by
multiplying torque (force applied at a distance)
by the angular velocity (rpm or radians/sec).
A torque applied without any resulting motion
means that no work has been done, thus, zero
power is produced. This means that if you are
stomping on the pedals but you are not moving,
you will get real tired, won’t go anywhere, and
subsequently will be putting out zero watts.
There needs to be motion of the system in order
for work to be done – remember, a force needs to
act over, or through, a distance (the foot needs
to sweep out an arc).
There are many parts that rotate on a bicycle
due to an external torque (e.g, pedals, crank,
bottom bracket, chain, and rear hub).
These spots are prime locations for measuring
power on a bicycle system.
Measuring Power
The three power systems evaluated all measure
the same fundamental parameters of interest
(torque and angular velocity) but they use
different methods. The primary reason for
this can probably be attributed to different
design goals, or just plain wanting to do things
a bit differently. SRM was the first of
the three systems to enter the portable cycling
power measurement business, distantly followed
by Tune/Graber and its Power Tap product.
The Power Tap folks decided to measure power a
bit differently than SRM and acquired a US
patent in the process. Polar followed
a similar path, in terms of measuring power in
an alternative method, but the system had its
own unique design goal. According to one
of the Polar power product developers, Alan
Cote, the system looks like it does today
because they wanted to be able to measure power
without removing or replacing any components on
the bicycle. Cote achieved this
objective and in the process named Polar as the
assignee of the U.S.
patent rights. The latest entrant into
the power market is the Ergomo system that
measures power at the bottom bracket. This
system is not included in this review.
Each company achieves power measurement through
different methods. SRM uses strain gage
technology in a crank based application. Power
Tap uses strain gage technology in a hub based
application. Polar uses magnetic induction
in a chain based application.
SRM
Instrumenting a mechanical structure allows one
to convert the phenomenon of interest into an
electrical signal that can be subsequently
analyzed, mathematically manipulated, and then
displayed to the user.
Strain gages do just that in the SRM power
measuring device.
The strain gages that SRM uses are nothing more
than a piece of foil embedded in a plastic
carrier. The resistance of the foil
element changes depending on how much it is
stretched/strained. The strain gages
unique characteristic of changing resistance
under strain is what allows the mechanical
deflections that naturally occur in the
structure to be converted into an electrical
voltage signal.
Voltage = Current x Resistance or V = I x R
SRM has designed a flexure system (purposely
micro-flexible elements) in the spider of the
crank that will deflect/stretch as a result of a
load applied at the pedal. With the strain
gages applied to the surface of these flexures,
the small deflections can be converted into a
voltage. Through careful calibration
(applying known loads in a known orientation and
in a controlled environment while monitoring the
resulting voltage) it is possible to convert the
measured strain into a torque value. It is
also interesting to note that SRM does a dynamic
calibration at the factory at a variety of
torques and cadences to ensure linearity across
the entire usage spectrum.
There are several fundamental sources of error
that must be mentioned when using strain gages.
These include temperature effects, strain field
assumptions, and orientation of the
gages/transverse sensitivity. Since the
underlying structure will deform with a change
in temperature, the strain gages will also sense
these changes. Therefore, there may be a
drift in the zero torque point depending on how
warm or cool it is outside. Through clever
design and increasing the total number of gages,
it is possible to auto-correct these effects and
minimize their impact on the final magnitude of
the measured value. No system is perfect
at accomplishing zero temperature sensitivity
and this is perhaps one of the reasons why SRM
recommends that the zero point be determined
prior to every ride.
Strain gages occupy space on the flexible
element being measured and will, therefore,
reflect an average strain over the occupied
area. If the strain field being measured
is constant over the area covered by the gage,
or if the gage is infinitesimally small (not
likely!) there is no error associated with the
measurement. These type of errors can be
addressed by clever gage design (smaller gages
that occupy less space), or clever flexure
design (making sure that strain field is
constant where the gage is placed – this is done
by manipulating flexure geometry).
Typically, the errors associated with
non-linearity in the strain field are small and
are, more than likely, not fully addressed by
any of the SRM units.
Individual strain gage elements are designed
to be sensitive in only one direction.
Transverse sensitivity is affected by how the
gages are oriented and the inherent properties
of the gage itself. Ideally, the gages
should be oriented along the same direction as a
known principle strain axis. Any deviation
from this orientation will mean that loads
applied in an off-axis direction (for a crank
arm this off-axis direction would be any
component of the pedal force applied in a
direction parallel to the bottom bracket axis –
these off axis loads are common and significant
when riding out of the saddle) will cause a
change in voltage by the strain gage. The
off-axis loads should not be included in the
power calculation since they do not produce any
work (there is no associated motion with this
force – i.e, no work is done). Improper
installation, poor gage layout design, or poor
gage selection will increase the error
associated with transverse sensitivity. A
perfect transducer would have zero transverse
sensitivity, but this is not practically
achievable. Typical load transducers can
routinely achieve fractions of a percent of
error in this category.
The positioning and number of strain gages
used to measure the applied torque will affect
the accuracy of the measurement.
Additional gages will boost the signal to noise
ratio, lower the transverse sensitivity and
allow for automatic temperature compensation.
This is the fundamental reasoning behind the
different price points of the SRM offerings –
the higher priced models use more gages and have
better accuracy. For example, the SRM
amateur model uses 2 strain gages and has a
quoted accuracy of +/- 5%. The Pro Model,
which was used for this review, uses 4 gages and
has a quoted accuracy of +/- 2%. The
Science version of the SRM uses 8 gages and
claims to have +/- 0.5% accuracy.
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