Q: It looks like memory
capacity varies considerably. Why?
A: It depends on the chosen recording interval, and,
for the Polar S-710, which features are turned “on” and “off.” Scott Harvel has compiled this chart showing
all the possibilities:
|
FEATURE
|
Recording Rate
|
|
Altitude
|
Speed
|
Cadence
|
Power
|
5 s
|
15 s
|
60 s
|
|
On
|
On
|
On
|
On
|
4 h 57 min
|
14 h 53 min
|
59 h 34 min
|
|
On
|
On
|
On
|
OFF
|
8 h 56 min
|
26 h 48 min
|
99 h 59 min
|
|
On
|
On
|
OFF
|
On
|
5 h 35 min
|
16 h 45 min
|
67 h 01 min
|
|
On
|
On
|
OFF
|
OFF
|
11 h 10 min
|
33 h 31 min
|
99 h 59 min
|
|
On
|
OFF
|
OFF
|
OFF
|
14 h 53 min
|
44 h 41 min
|
99 h 59 min
|
|
OFF
|
On
|
On
|
On
|
5 h 35 min
|
16 h 45 min
|
67 h 02 min
|
|
OFF
|
On
|
On
|
OFF
|
11 h 10 min
|
33 h 31 min
|
99 h 59 min
|
|
OFF
|
On
|
OFF
|
On
|
6 h 23 min
|
19 h 09 min
|
76 h 37 min
|
|
OFF
|
On
|
OFF
|
OFF
|
14 h 53 min
|
44 h 41 min
|
99 h 59 min
|
|
OFF
|
OFF
|
OFF
|
OFF
|
44 h 42 min
|
99 h 59 min
|
99 h 59 min
|
|
|
total one file
|
99 h 59 min
|
|
total all files
|
520 h 00 min
|
Q: If the data is
sampled every 60 seconds, what good is that?
After all, it seems to change so quickly.
A: (Alan and Jean-Joseph Coté,
Jason Yanota, Andrew Coggan, Hunter Allen, Tom Compton, and Dave Wendt) On longer rides, it is likely sufficient to
record data with relatively infrequency, whereas on the track, every second may
not be often enough. Still, 60 seconds seems
too long to be of use, at least for Polar units, which record current values at
the end of each interval, rather than an average over its duration. They do so since they are designed as
multisport devices, and the longer recording intervals are more appropriate to
other data they collect (HR, temperature, altitude), which does not change as
rapidly as power.
There
are three different measures of the ‘data stream’ which must be distinguished:
Signal
rate – the
number of times torque is measured in a given period. The strain gauges in the PowerTap hub do this
60 times per second (Hertz), while the sampling rate for the SRM is 200 Hz. The Ergomo measures torque 72-144 times second,
i.e., every few degrees, which is why it or a future model has the potential to
provide data on variations in torque within a single revolution.
Display
(or refresh) interval – the
length of time between each update of the readout (display).
Recording
interval – the
length of time between each record of elapsed time, distance, speed, power, and
cadence that is stored in memory for downloading.
The
Polar S-720i and S-710i group power data by crank revolution, and consider each
crank revolution to be indivisible with respect to the actual power reading. Values saved in display memory are an average over the last few crank revolutions. The number of crank revolutions isn’t always
the same, i.e., when you pedal really slowly, it doesn’t average over a really
long time, but over the recently completed revolutions (in a specific time window)
where the instantaneous samples are grouped by revolution. At higher cadences, more revolutions are
included in the calculation. Since the
update is faster than 5 seconds, the instantaneous values during a given
revolution may be included in more than one successive reading. It isn’t a straight average, i.e., the total
work over the interval divided by its duration, but rather an average that is
weighted toward the more recent revolutions.
The details of this calculation are proprietary, but the bottom line is
that viewing it as a 5-second average is a pretty good approximation.
The
values stored in memory that is downloaded
are not averages of some interval, but rather, the current numbers as displayed
on the monitor which are captured and stored once every 5, 15, or 60
seconds. Since the update interval is
faster than the recording interval, a high value may be displayed (and then
saved as the maximum value on the S-720 “FILE”), even though it doesn’t occur
at the point when the value gets stored for downloading, so the true maximum
value might not appear in the downloaded data.
Again, the 5 second interval is really the only useful one for recording
wattage.
Other
than the Polar units, power-measuring devices display current power as an
average over some short time period, which leads to a problem known as the
“precession effect.” That is, unless you
are pedaling at a rate where one or several revolutions are exactly completed in each averaging
interval, an extra quarter-revolution can occur, and that partial turn of the
crank may be either a power stroke or a dead-center (and perhaps the opposite
for the next sampling period), which will produce a less consistent reading,
especially for short intervals; the maximum power value captured in the PowerTap’s
display memory, for example, is significantly affected, since it is the highest
average value achieved over just 1.26 seconds.
Thus, averaging over one (or just a few) crank revolutions would reduce
variability in the current power display, track power more nearly as a rider
senses it, and result in more accurate maximum values for instantaneous
power. Recorded power values could, and
perhaps should, still be based on time.
For
current power, the PowerTap Standard displays only the power calculated every
1.26 seconds, and when set to record every 2.52 seconds, discards values
calculated at 1.26 seconds, i.e., it records every other value without
averaging. The Pro model, on the other
hand, can display average over the last 1.26, 2.52, 5.04, 10.08, or 30.24
seconds for the current power value, but like the Standard, it records the
instantaneous value at the selected recording interval, so for instance, when
at the 10.08 second recording interval, every 8th value is stored, and the
other 7 are discarded. Some have noted that
displayed memory is often a couple Watts higher than what is downloaded. In fact, the “raw,” recorded data represents
is the most accurate and unaltered information, coming directly from the hub. The reason the display is slightly off is that
it uses lower-precision arithmetic, rounds improperly, or computes running
averages using a method that is prone to accumulated errors or truncation. These corners are cut because memory and CPU
computing power are at a premium.
The
SRM averages torque during each pedal revolution, then multiplies the result by
the average angular velocity (cadence) during the revolution, then makes
calculations according to the specified interval:
0.1 second – all completed revolutions are averaged,
if a revolution hasn’t been completed then the previous data is sampled again.
1 second – all completed revolutions in the previous
second are averaged. One revolution will
be sampled in the first sample, two revs will be used in the second sample,
etc.
10 second – all completed revolutions are averaged; at
90 rpm this would mean the average of the previous 15 pedal revolutions.
Instantaneous power is estimated using the torque
analysis function, which samples torque at 200 Hz, and in this way, SRM claims
there are no artifacts in its power calculations, however, this is only an
estimation of instantaneous power, because we don’t know instantaneous crank
speed, and speed variations, though slight, do occur while pedaling. The crank torque and angular velocity that
are combined to calculate power aren’t necessarily time-aligned properly, which
can be an issue if cadence is changing rapidly.
As
previously mentioned, the Ergomo takes 72-144 measurements per second
(depending on cadence), averages them each second, and temporarily saves the
result. Each 1-second average in turn is
averaged every 5 seconds, and then this number is recorded for download. For example, 300 W, 300 W, 300 W, 305 W, and
310 W will be averaged by the computer and recorded as 303 W. The Ergomo display is updated every second
from a rolling average of the last 8 rpm, so a new number appears in the
display each second as the rolling average is kicking out the last number.
Although both the Ergomo and SRM measure the torque, or
twisting force, generated by the rider’s leg(s), the Ergomo measures it at the
bottom bracket, whereas with the SRM, it is measured between the chainrings and
the right crank arm. When you push down
with the left pedal, that torque is transmitted through the bottom bracket
spindle, to the spider, and then to the chainrings. When you push down with your right leg,
however, the torque is transmitted only through the spider to the chainrings –
none is transferred to the bottom bracket spindle, hence, the Ergomo measures
the power output of the left leg only (and then doubles it), whereas the SRM
measures the power output of both legs.
(If you use your right leg to help lift your left back to the top of the
stroke – and many of us do – then there is some torque applied to the bottom
bracket spindle. This is, however,
considerably smaller than the torque generated during the downstroke with
either leg, furthermore, it is in the opposite direction.) Some therefore claim that an imbalance between
left and right leg strength (due perhaps to an injury or even just occurring
normally) renders the Ergomo inaccurate, even as it may give consistent and
repeatable values, however, this has yet to be demonstrated.
Q: It sounds like
each ride produces a lot of data. How to
make sense of it all?
A: Software is included with each system, and there
are also several aftermarket packages with enhanced capabilities, including http://cyclingpeakssoftware.com, http://www.crosstrak.com, and http://www.powercoach.ch (comes in
versions for both Mac and PC). Lastly, http://analyticcycling.com has some
useful analysis features.
Q: I just had my first ride with my new power
meter. Is it normal for current power to
fluctuate so much?
A: First-time power meter users are almost invariably
surprised at how “jumpy” the current power display is, and question the
readout’s reliability. This is a result
of having become accustomed to the heart rate monitor (HRM) as a gauge of
intensity, and being fooled into thinking that the energy requirements of outdoor
cycling are relatively steady by the delayed response heart rate to changes in
intensity, an effect that is accentuated by the smoothing algorithm
incorporated in the HRM’s firmware. Although
some of the variability in power is due to instrument artifact (the “precession
effect,” as just discussed), the energy demands of road cycling do indeed fluctuate
very rapidly and widely (sometimes referred to as the “stochastic” nature of
on-road power output), something that can easily be verified by comparing power
data collected outdoors against that obtained from most any indoor trainer. Using your power meter’s interval feature, if
it has one, or setting it to smooth (average) readout data over a period of
time such as 30 seconds can help to ‘settle’ the display.
Q: My friend says he can average 275
Watts for 30 minutes. Is that any good?
A: It all depends.
Power is somewhat ‘personal,’ such that three riders traveling the same flat
section of road together, at the same speed, side-by-side (not drafting each
other), might each have considerably different power outputs, so absolute Watts
do not necessarily provide a valid basis of comparison. This is even more outstandingly true going
uphill, where the force you must overcome is determined largely (75%+ for an 80
kg bike/rider putting out 300 W on grades over 5%) by weight. For instance, if Dan’s mass is 70 kg and he
averages 315 W on a particular climb, while Felicia is 49 kg and maintains “only”
270 W on the same hill, she will drop him easily, since she is putting out 5.5
W/kg, while he generates only 4.5 W/kg.
On flat terrain, by contrast, the main resistance (80%+) is from air, so
speed is a question of Watts per square meter of effective frontal area (CDA), which determines air drag.
The
amount of good data on power-generating capabilities for cyclists across
different skill levels, disciplines, and gender is limited, and of course,
statistics don’t determine the outcome of a race; if they did, we could just
set up trainers at the starting line, run a few tests, and proclaim the
winner! Still, it may be useful to gauge
your own power against others, or those with whom you hope to compete, and if
so, be sure to “normalize” (divide by) body mass or frontal area. For timed events, such as a 40 km time trial,
you can get a reasonable idea of the power needed to achieve a certain time for
yourself by using an online model like those available at http://www.kreuzotter.de/english/espeed.htm,
http://analyticcycling.com, or http://www.machinehead-software.co.uk.
For
his World Hour Record in 1996, Chris Boardman averaged an estimated 442 W, while
Miguel Indurain needed about 510 W when he broke the same standard in 1994
(both about 6.5 W/kg), and an analysis of Lance Armstong’s time of 38 minutes 1
second in climbing L’Alpe d’Huez during the 2001 Tour de France produced an
estimate of 6.5 W/kg, which came at the end of a 209 km long stage featuring
two prior “hors categorie” (beyond category) climbs. In setting a new women’s record of 54 minutes
2 seconds at the 2002 Mt. Washington (NH) Hillclimb, Geneviève Jeanson averaged
an estimated 278 W (5.56 W/kg). At the
other end of the power-duration relationship, the best male match sprinters have
hit 23 W/kg, females ~20 W/kg, however, comparable and even higher values than
these have (somewhat surprisingly) been recorded by non-cyclists, such as
weightlifters, hockey players, etc.
Q: What in the
heck is this ‘CDA’ you refer
to?
A: It’s the product of your aerodynamic drag
coefficient, CD, and
frontal area A. Let’s start with A, which is simply the area of the
profile a rider presents to the air they move through. Stand directly in front of a rider, look at
his or her outline, and you’ll see that a smaller rider has a smaller frontal
area, while larger rider has more area to push through the air, and therefore
must put out more power for a particular speed.
CD is a measure of how ‘streamlined’
you are, i.e., how smoothly air flows around your body/bike without swirling
behind you. Imagine two riders of
exactly the same size and position, where one is using a Cervélo, an aero
helmet, shoe covers, etc., while the other has a standard round-tubed bicycle,
a Pneumo helmet, and no shoe covers (both wheelsets have 42 spokes, but the
first has 58 mm deep-section rims, while the second has standard box-section
rims). Although both riders present the
same frontal area, the former will have a lower CD, encounter less aerodynamic drag, and go faster at a
given power output.
The
product of these two components is CDA, or effective frontal area, and it is
most accurately determined in a wind tunnel, but it may be possible to measure
it with a power meter, on a flat course in calm air. As a rough rule of thumb, an 0.005 m2
reduction in CDA = 0.5 seconds/kilometer = 0.1 lbs
difference in drag at 30 mph = 5 W.
Q: How can I test my
progress when training by power?
A: (Eddie Monnier) Keep in mind that testing does not necessarily
guarantee racing results, rather, it allows for periodic evaluation of your condition
and training program. If you do this
often enough, you will have a bad test from time to time, so it’s important not
to get too hung up on any one result.
There
are several test protocols to determine what might be termed ‘functional threshold
power,’ including the critical power and
maximal aerobic
power (MAP) tests (the latter is usually administered indoors). The method proposed in the training guide
referenced further on, however, is not a lab test, but a functional test of
average power for a 40 to 60 minute time trial which is used for determining
training levels since it integrates the underlying physiological mechanisms of endurance
exercise: maximal VO2, lactate threshold, and efficiency, thereby giving a
sort of “bottom line” measure of fitness.
You
may also want to test your power-generating capacity at various durations,
depending on race objectives and personal development needs; for instance, a
criterium specialist will be more interested in maximal and average power over
a 200-meter sprint than a climber, who will tend to focus more on average power
on a particular climb. In this regard, something
like Power Profiling
can be helpful.
Q: What are “normalized” power,
intensity factor, and training stress score?
A: Created by Andrew Coggan, Ph.D., a noted exercise
physiologist, this is obtained via an algorithm that adjusts for variations in
power with reference to lactate threshold and other physiological responses.
Suppose
you race a 1 hour criterium, where you are frequently sprinting out of corners,
covering breaks, etc., and you race to your limit, such that there are very few
if any slack periods. Average power with
coasting time will nonetheless be considerably lower than a flat 1 hour time
trial where you paced steadily and had nothing left at the end, yet you feel
just as stressed physically. The normalizing
algorithm adjusts for variations in power, such that the resulting normalized
power value will be very close to what you would have achieved in a TT of
equivalent duration. In short, it is
meant to more accurately reflect the actual metabolic strain that the body
incurs, rather than the average stress load imposed.
Here’s how it is calculated: first, a rolling 30-second
average (mean of the last 30 seconds) is applied to the wattage values from the
downloaded workout file, since the body does not respond instantaneously to rapid
changes in exercise intensity, rather, most physiological responses follow a
predictable time course with a half-life of approximately 30 seconds. Next, each of the values obtained from this
is raised to the 4th power, just as blood lactate response has been shown to
closely fit the plot of y = x4,
where y = blood lactate and x = power output; indeed, many critical
physiological responses (e.g., glycogen utilization, lactate production, stress
hormone levels) are similarly related to exercise intensity in a curvilinear,
rather than linear relationship. Finally,
all these values are averaged, and the 4th root is taken.
If
that all seems a bit complex, just think of adjusted power as the equivalent
power you would have achieved if the race course had been perfectly flat and
the pace perfectly steady, with no variations.
Two
other metrics of workout performance, intensity factor (IF) and training stress
score (TSS), are derived from normalized power, though space considerations
preclude further discussion; the previously-mentioned CyclingPeaks Software
includes all three of these features and has a nice explanatory article as
well, at http://www.cyclingpeakssoftware.com/defined.html,
while there is a free on-line calculator at http://www.virtusphysica.com/htmlspecialedition2003.htm.
Q: How does
altitude affect power output?
A: The effects of altitude on VO2
uptake (and hence aerobic power) are highly individual, so it is difficult to
predict to what extent any one person will be affected, although as a general
rule it has been shown that elite athletes, as compared to normal individuals,
have a greater decline in VO2max under conditions of reduced ambient
pO2 (partial oxygen pressure).
This is caused by their higher cardiac output, which results in a
decreased mean transit time for the erythrocytes (red blood cells) within the
pulmonary capillary, and thus less time for equilibration between alveolar air
and blood in the pulmonary capillary.
These equations from Bassett et al.1 were generated from 4 groups of highly trained or
elite runners, so they are population-specific to that group, but can be used
to estimate aerobic power at a given altitude as a percentage y of what is normally available at sea
level, where x = elevation above sea level
in km:
acclimatized
athletes (several weeks at altitude): y =
-1.12x2 - 1.90x + 99.9 (R2 = 0.973)
non-acclimatized
athletes (1-7 days at altitude): y =
0.178x3 - 1.43x2 - 4.07x + 100 (R2 =
0.974)
whereas Peronnet et al.2 found y = -0.003x3 + 0.0081x2
- 0.0381x + 1. Here is a table derived from these equations:
|
ELEVATION
|
AVAILABLE
AEROBIC POWER
|
|
(feet above sea level)
|
Bassett et al.1
|
Peronnet et al.2
|
|
acclimatized
|
non-acclimatized
|
|
|
0
|
99.9%
|
100.0%
|
99.9%
|
|
1,000
|
99.2%
|
98.6%
|
98.8%
|
|
2,000
|
98.3%
|
97.0%
|
97.8%
|
|
3,000
|
97.2%
|
95.2%
|
96.8%
|
|
4,000
|
95.9%
|
93.2%
|
95.8%
|
|
5,000
|
94.4%
|
91.1%
|
94.7%
|
|
6,000
|
92.7%
|
88.9%
|
93.5%
|
|
7,000
|
90.7%
|
86.5%
|
92.2%
|
|
8,000
|
88.6%
|
84.2%
|
90.7%
|
|
9,000
|
86.3%
|
81.7%
|
88.9%
|
|
10,000
|
83.7%
|
79.3%
|
86.7%
|
|
11,000
|
80.9%
|
77.0%
|
84.3%
|
|
12,000
|
78.0%
|
74.7%
|
81.4%
|
|
13,000
|
74.8%
|
72.5%
|
78.0%
|
|
14,000
|
71.4%
|
70.4%
|
74.2%
|
1Bassett, D.R. Jr., C.R.
Kyle, L. Passfield, J.P. Broker, and E.R. Burke. Comparing cycling world hour records,
1967-1996: modeling with empirical data.
Medicine and Science in Sports and
Exercise 31:1665-76, 1999.
2Peronnet, F., P. Bouissou, H.
Perrault, and J Ricci. A comparison of
cyclists’ time records according to altitude and materials used. Canadian
Journal of Sport Science 14(2):93-8, June 1989.
Thanks to David
Bassett, Jr., Ph.D., for his contributions to this section.
Q: Where can I get
more information on training by power?
A: Wattage forum member Andrew Coggan has created a
‘schema’ of Power-based
training levels (backup sites #1
and #2), which
Charles Howe has included in Part 1 of The
Road Cyclist’s Guide to Training by Power, at http://pdqcleveland.org/power.htm.
Another article by Coggan is Racing and Training With a
Power Meter (backup sites #1 and #2), while a number of
power-related articles can be found at the PowerTap
(archived), SRM (plus more at The Bike Age),
and Cycling Peak
Software web sites. Additionally, Robert Chung and Amit Ghosh have personal web sites with many good articles,
as do http://analyticcycling.com and http://bicyclewattage.com.
Finally, the Wattage Forum (http://topica.com/lists/wattage) can
provide much help and advice from that list’s many members; supplemental list archives
are at http://users.icubed.com/~mayhew/wattage
as a StuffIt file, while the StuffIt Expander utility needed to open this file is
at http://stuffit.com.
Q: What about
power-based training indoors?
A: (Charles Howe, Andrew Coggan, and Bruce Sargent) Stationary trainers can offer an important
form of supplementary training, not merely over the winter months or when the
weather is foul, but even during good weather when a controlled, structured
workout is desired. Stand-alone trainers such as the SRM and Velotron
appear to be well-calibrated, as is most any good lab ergometer, such as those
manufactured by Lode. In evaluating the
wattage readout accuracy claims of bike-stand models, however, it is important
to realize that just because a device has a digital display (such as the
Computrainer, Cateye CS-1000, and various Tacx models) does not mean it
accurately or reliably reports power output – and precision in administering
the exercise load is one of the most important benefits of power-based
training. Indeed, of the various models
that claim to be calibrated for power, only the Velodyne (http://www.velodynesports.com)
appears to be consistently accurate and precise (see http://tinyurl.com/36j5v), while
realistically replicating the actual demands of cycling on the road. It achieves this with a feedback control
system that measures and adjusts the resistance of an electrically-controlled
brake as well as a 10 kg flywheel that simulates the “inertial” forces
encountered during actual riding. When
properly calibrated, it faithfully reproduces the experimentally validated
speed-to-power equation (by software design), although it assumes a set value
for frontal area which may or may not correspond to that of the individual
riding it. Additionally, it will
simulate drafting (though the magnitude of the simulated reduction in air
resistance is unknown), but not headwinds or tailwinds. In ergometer mode, it will hold wattage
constant down to ~5 mph, i.e., power will not vary as your speed changes at
levels over 100 Watts. The ability to
maintain constant power in “erg” mode is a bit speed-dependent in the other
direction too, i.e., rolling resistance accounts a large enough fraction of the
total power demand that it is hard to get the power down when wheel speed is too high.
Overall,
an on-bike power meter in combination with most any bike-stand load generator
is the most affordable, flexible, and accurate arrangement for precision power-based
training indoors; those with a flywheel adequately heavy to simulate inertial
forces (designated with an “*” below) are recommended. Except for the Velodyne, an accurate power
meter is the only way to know the energy requirement of bike-stand models,
which renders the load generator’s readout superfluous. (Note: the accuracy of the Polar S-720i/710i
on indoor trainers has frequently been observed to be highly unreliable.)
Stand-alone trainers:
Cardgirus
– http://www.cardgirus.com/english.htm
(See also Andy
Birko’s review)
Cyclesimulator
– http://www.cyclesimulator.com
Kettler
Ergo Racer – http://www.worldclasscycles.com/kettler_ergo_racer.htm or http://www.kettler.net
Lode
ergometers – http://www.lode.nl/en/index.html
PowerTap
– http://www.power-tap.com SRM – http://www.srm.de/trainerclothing.html
Velotron
– http://www.velotron.com/index.asp
Bike stands:
1up
USA CPR A-2000 – http://www.1upusa.com/bike_trainer.html
*Bike Technologies Advanced
Training System – http://www.biketechnologies.com/products/ats/ats.html and http://www.usauzziesales.com/training_system_details.htm
Blackburn
Fluid and Mag Trainers – http://www.blackburndesign.com/2004/trainers_04.htm
Cateye
CS-1000 – http://www.cateye.com/products/accessories/CS_1000.html
*Chaindriver
– http://www.chaindriver.com Cycleops – http://cycleops.com
Kinetic
– http://kurtkinetic.com (*custom
version: http://analyticcycling.com/ThingsWeSell.html)
Kreitler
– http://www.kreitler.com Tacx – http://ww.tacx.nl
Power
requirements of numerous bike-stand models have been charted at http://www.geocities.com/almost_fast/trainerpower,
while Robert Wells has posted a careful evaluation of the Tacx Flow at http://home.comcast.net/~rwwells/Tacx/DOEFlow.html.
Q: What does accuracy matter anyway, so long as a unit’s
readout is consistent?
A: (Andrew Coggan) Accuracy is important if:
1.
you wish to compare your performance over the long term with different devices;
2.
you wish to compare your performance to others;
3.
you want to use a self-assessment tool such as use my “Power Profiling” tables,
or
4. you
wish to do any modeling, e.g., to predict your time on a new course.
It
is true, however, that precision (reproducibility) is probably more important
than accuracy – but that includes across various power outputs, pedaling
speeds, etc., as well as across brands of powermeter. Care must be exercised when comparing data
collected using different power meters, even if they have all been carefully
calibrated. In my own case, for example,
the slight improvement in power I’ve seen at various longer durations recently,
as compared to previous years, can potentially be accounted for entirely by my
switch to SRM from PowerTap, since the former measures power “upstream” of the
chain, whereas the PowerTap measures it “downstream,” or after the chain. On the other hand, my power at 5 seconds is
down significantly, as are peak values at 10, 15, and 30 seconds, but 1 minute
isn’t, which makes me believe that it is real, and not an artifact of the
change in systems. Without careful
assessment of the data collected with each powermeter, however, an incorrect
conclusion about whether a certain type of training is or isn’t working might
be reached.
Q: Can a power meter
be used as an aid in dieting?
A: Since they accurately measure energy output, power
meters can be used to estimate
metabolic energy expenditure in kilocalories (simply “calories” in common
usage.) Most models give the total work
for a ride in kilojoules (kJ), but if not, average power output for the ride
can be converted to kJ when multiplied by ride duration in hours (decimal form)
and a factor of 3.6. For instance, if
you averaged 142 Watts for 1 hour 22 minutes, that’s 142 W ´ 1.37 hr ´ 3.6 = 699 kJ.
Since the body is ~20-25% thermodynamically efficient, this roughly
cancels out the unit conversion factor (4.184 kJ = 1 kcal), and the work
accomplished in kJ during a ride is pretty near equal to kcal burned by the
body. Unfortunately, efficiency varies
during a ride, increasing directly with intensity, and it must be determined in
a lab, but here are factors for converting kJ to kcal over a range of values
for efficiency:
If you are 25% efficient, kJ × 0.96 = kcal, and 87.1 W
are produced by each liter of oxygen uptake
for 24% efficiency,
kJ × 1.00 = kcal, and 83.6 W are produced by each L of O2
uptake
for 23% efficiency,
kJ × 1.04 = kcal, and 80.1 W are produced by each L of O2
uptake
for 22% efficiency,
kJ × 1.09 = kcal, and 76.6 W are produced by each L of O2
uptake
for 21% efficiency,
kJ × 1.14 = kcal, and 73.2 W are produced by each L of O2
uptake
for 20% efficiency,
kJ × 1.20 = kcal, and 69.7 W are produced by each L of O2
uptake
The OwnCal
feature of Polar HRMs only estimates calories
expended, based on averages derived from large samples, and thus can vary
widely by individual, as
the manufacturer itself admits.
Q:
I’ve heard that temperature really affects the accuracy of the SRM and
PowerTap. True?
A: (Andrew Coggan, Chris Cleeland, Jesse Bartholomew, and
Andy Birko) A recent
study found that both read higher in colder air than warmer (8° C, or 36 F)
as compared to the lab (70° F), but this was because the investigators tested
them without re-zeroing. In other words,
they deliberately disregarded the manufacturer’s recommendations for use, and
the error should therefore be viewed as a worst-case scenario due to improper
operation. If you reset the zero at the
same temperature at which you collect data, then accuracy will be unaffected.
The
PowerTap autozeros when coasting (i.e., whenever there is zero torque applied),
however, if there is an offset of more than ±8 in-lbs, the unit will require the user to re-zero. This usually occurs due to a large
temperature change, so to obtain the most accurate data, you shouldn’t just
look for non-zero watt values while coasting, you should look for non-zero
torque values.
This
page from a software company shows that proper engineering can
detect strain in the presence of thermal-induced stress.
Q: How do I calibrate my power meter?
A: (Andrew Coggan)
Neither the Polar S-710 nor the PowerTap require calibration after
initial set-up. Calibration of the SRM
via slope adjustment can be performed by the user, as described in the Owner’s
Manual at http://www.srm.de/index.php?la=3&lb=3&lang=ger
(click on “Troubleshooting,” then “Calibration check”), and a more complete
calibration procedure is now available un the U. S. as well.
Technically,
the PowerTap cannot be user-calibrated, but its accuracy can be checked using a
simple test that is similar to the SRM calibration check. First, check that the transmission icon is
on, and if not, give the rear wheel a spin.
Then, enter the torque mode by holding the “Select” button down for 2
seconds or longer (the “WATTS” designation will disappear from the top
line.) Apply the rear brake sufficiently
to lock up the rear wheel. Now, measure
torque as follows: with the cranks exactly horizontal (right crank at 3
o’clock), hang a known weight of at least 50 lbs from the right crank, or
simply stand on it – hence the name ‘stomp test’! Measured torque =
(weight in lbs) × (crank length in mm) × (1
in/25.4 mm) × (cog teeth/chainring teeth). For a 159 lb rider standing on a 175 mm
crank, with the chain on the 39 tooth ring and the 23 tooth cog, 159 lbs × 175
mm ×
1 in/25.4 mm × 23/39
= 646 in-lbs. Compare this to the displayed value by
calculating % error as (measured torque - displayed torque)/measured torque.
Q: Can I race with a
power meter?
A: Sure! The
most obvious application is time trialing, where it is invaluable for pacing,
particularly in the initial stages of the race, and even for pursuit events on
the track, as well as short (<10 minute), prologue-type events on flat
terrain. Although criteriums allow fewer
situations where power data can be conveniently accessed during the race, it
can be used in road races to judge effort when off the front, in a breakaway,
or bridging up, and when seeking the “sweet spot” in a paceline or echelon. Even if not useful during the race, a power meter can be used as a “black box” (ride
data recorder), allowing informa-tion to be analyzed afterward to quantify the
demands of the race, and training programs to be tailored accordingly. Still, some who train using a power meter
choose to race without it for psychological reasons, and ultimately, its use in
competition is a matter of personal preference, like an HRM.
Q: How does the
PowerTap calculate cadence without a sensor?
A: (Andrew Coggan)
This “virtual cadence” feature estimates crank rpm based on the time
from one peak in torque to the next as your legs pump up and down. Such peaks occur very frequently (e.g., every
333.3 milliseconds at 90 rpm) and have to be identified “on the fly,” so any
slight variation in either when pushed down the hardest or when the computer
thinks you pushed down the hardest will therefore have a significant effect. Depending on how/how fast you pedal, the
cadence values can therefore be quite erratic, even though the power
measurements are still accurate.
Q: Can I use the PowerTap
just as a computer, without the hub?
A: (Andrew Coggan) An undocumented function of the original (grey
case) PowerTap Standard, this is now explained in the owner’s manual for the PowerTap
Pro. Anyway, in the normal (not
interval) mode, scroll to current cadence (it should be all dashes if the
cranks are not turning). Hold down the “select”
(right-hand) button for about 3 seconds until it says “OFF” on the top line
where Watts are normally displayed. Speed,
heart rate, and distance will now be shown, but not cadence or Watts. Return to normal mode by reversing the
process (you will have to cycle back to current cadence, since as soon as you
let up on the right button during the above procedure, the computer jumps to
average cadence).
You need to mount a magnet on the rear wheel and ensure it passes very close to
the sensor (5 mm or so). According to a PowerTap,
the receiver on the bike may be more “particular” about magnet strength and location
than your average cycling computer, which may be why this function was left
undocumented.
Q: The drive-side bearing cone in both my
PowerTap Pro and Standard hubs wore out after less than 3,000 miles – it makes
a crunching sound and does not turn very smoothly. Any suggestions?
A: This is a clear deficiency in materials that should
be corrected by the manufacturer. Until
they do so, try using item SH-3AO9803, a right cone for Shimano Dura-Ace rear
freehub FH-7700, available from Loose
Screws for $12.20 each. Grind or machine the narrow end of the cone
down a few millimeters since it is too long, and file the inboardmost ‘step’
off the aluminum spacer that comes with the PT hub, but once you do, it all works
fine, and since there is a rubber lip seal on the cone, the hub will be
double-sealed. There are other Shimano
cones that may fit better, but I don’t know which ones for sure, and this one is
the best quality.
Note that the non-drive side bearings are sealed, and must be serviced by the manufacturer.
Q: What can I do to improve the waterproofness of my PowerTap
hub?
A: (Chris Cleeland and Lindsay Edwards) Get some tune-up grease, also known as dielectric
grease (or heat sink grease in the electronics world, although that tends to
have thermal conductivity properties as well as being dielectric) from the
nearest auto parts store (or “auto spares” as they say in the U. K.) This is the stuff that’s made for the inside
of spark plug wire boots to ensure that they can be removed, but won’t conduct
electricity. Squeeze a liberal portion
of this on to your finger, then smear it all over the leaf contacts both on the
cradle and the nubs on the back of the CPU.
This will keep water and moisture out of the contacts, but maintain the connection.
The
other issue is water in the hub itself, which happens to me less often, perhaps
1/3 of the time I ride in rain (though I have yet to be caught in an all-out
downpour). It also happens in heavy fog
occasionally. A simple overnight period
where you take the cover off is enough to dry it out and get things working
again. I’d suggest using tune-up grease
here, too. It’s a little thicker than Pedro’s
syngrease, doesn’t break down in heat as much, and if it does, it won’t affect
electrical connections. Here’s a link
describing Permatex’s product and to a place selling it online:
http://www.permatex.com/products/prodidx.asp?automotive=yes&f_call=get_item&item_no=22058
http://shop.store.yahoo.com/autoaccessconnect/digr.html
Finally,
apply some silicone sealant around all the joints of the receiver, paying
special attention the point where the cable enters the body of the receiver.
Q: I want to build custom wheel from a PowerTap hub,
but I’m not sure how to spec it.
A: Critical dimensions for both old and new versions
are given below, and you also need the effective rim diameter (ERD). Armed with these parameters, you can
determine spoke length using one of the on-line calculators at http://www.sheldonbrown.com/wheelbuild.html#length. The following table provides specifications
for selected rims (more ERDs can be found at the above link as well).
|
OLD PowerTap Hub (130 mm over-locknut distance)
Mfd.
before 2001 by Tune; painted matte silver finish.
Center-to-flange width,
left side: 36.2 mm
Center-to-flange width, right side: 16.7 mm
Diameter
through spoke holes, left side: 78.0 mm
Diameter through spoke holes, right side: 66.0 mm
Spoke hole diameter: 2.4 mm
|
|
NEW PowerTap Hub (130 mm O.L.D.)
Introduced late 2001 by Graber; shiny polished
silver finish.
Center-to-flange width, left side: 32.7 mm
Center-to-flange width,
right side: 16.7 mm
Diameter through spoke
holes, left side: 78.0 mm
Diameter through spoke
holes, right side: 66.0 mm
Spoke hole diameter: 2.4 mm
|
PowerTap SL Hub (130 mm O.L.D.)
Introduced late 2004 by Saris/CycleOps; carbon fiber
center section.
Center-to-flange width,
left side: XX.X mm
Center-to-flange width,
right side: XX.X mm
Diameter through spoke
holes, left side: XX.X mm
Diameter through spoke
holes, right side: XX.X mm
Spoke hole diameter: 2.4 mm
|
|
RIM MODEL
|
TYPE
|
PROFILE
DEPTH
(mm)
|
MASS
(g)
|
ERD
(mm)
|
|
CAMPAGNOLO Sydney (32° only)
|
C
|
30
|
553
|
581
|
|
MAVIC Open Pro CD
|
C
|
18
|
439*
|
602
|
|
SUN ME14A
|
C
|
20
|
421
|
601
|
|
SUN Venus
|
C
|
25
|
440
|
592
|
|
VELOCITY Aerohead
|
C
|
21
|
405
|
598
|
|
VELOCITY Deep V
|
C
|
30
|
520
|
582
|
|
VELOCITY Pro Elite
|
T
|
30
|
500
|
582
|
|
ZIPP 415
|
C
|
38
|
415
|
567
|
|
ZIPP 280
|
T
|
38
|
280
|
569
|
|
ZIPP 505
|
C
|
58
|
568*
|
529
|
|
ZIPP 360
|
T
|
58
|
360
|
530
|
|
*Actual mass; all others
are manufacturer’s claims.
ERD – effective rim
diameter C – clincher T – tubular.
For 28 spokes, use
2-cross, for 32, 3-cross (all rims available in both drillings unless
noted). As a general rule, round the
calculated spoke length down if using brass nipples, up for alloy nipples.
|
|
|
|
|
|
|
The
CH Aero wheel cover, available from Excel Sports,
essentially converts the PowerTap wheel to a disc, although the hub opening on
the left-side cover must be cut larger to ~73 mm diameter; a carbon fiber
version is available by calling the manufacturer at (800) 227-6751. To make a home-made disc/wheel cover, see these
instructions
from Warren Beauchamp and Bob
Schwartz, as well as an additional note from Ken
Lehner. Note: wheel covers will
become illegal under when the U. S. Cycling Federation adopts UCI bicycle
regulations on January 1, 2007.
Q: What about a PowerTap
hub with 135 mm over-locknut distance?
A: (Rick Moll and Jesse Bartholomew) The hubs are the same with except for axle length
and spacing; a 5 mm spacer is added to the left (non-drive) side, so the right
side flange is shifted 2.5 mm away from the hub center, while the left side
flange is shifted 2.5 mm towards it, therefore:
Center-to-flange
width, left side = 32.7 mm - 2.5 mm = 30.2 mm
Center-to-flange
width, right side = 16.7 mm + 2.5 mm = 19.2 mm
Q:
How can build up a Power Tap hub as a fixed-gear wheel?
A: Check out this article: http://www.smartcyclinginc.com/helparticle.html
Q:
How can I mount the PowerTap harness on my stem?
A: (Chris Mayhew)
You can do this by crossing the zip ties so that they exit one side of
the mount but cross over and enter the other side. A cleaner method, however, is to tightly wrap
electrical tape around the stem and the lower part of the mount, behind the ‘ears.’
Be careful how much tape you use; too
much will cause a poor fit between the harness and PT. With both methods it’s best to put a very
small piece of pipe insulation under the harness to fill in the gaps.
An
off-the-shelf mount can be purchased at http://www.cingcycling.com.
Q: My bike has a
Campagnolo derailleur, but the PT has a Shimano 9-speed freehub. What to do?
A: (Brian Smith and Eddie Monnier) Quoting Sheldon Brown, “For reasons that are not
quite clear, 9-speed hubs/cassettes seem to work pretty well with the opposite
brand of 9-speed derailleur/shifter,” the operative words being “pretty well,”
so results may vary, but many report doing so without any problem (avoid using
a Campagnolo chain on Shimano cogs, however).
An
excellent cassette to convert the PowerTap for use with Campy 10 is available
from Wheels Manufacturing (see http://www.wheelsmfg.com/4.html),
but the one from American Classic is not
recommended (see “Important Notes” at http://www.amclassic.com/Cassettes_Conv.html
– “The following wheels and hubs are incompatible: Shimano pre-built paired
spoke wheelsets, and ALL Powertap hubs.”)
It seems that the spacers on this model, and on the cassette from Miche
as well, are fixed so that the smallest cog (e.g., the 11) just barely seats
onto the PowerTap freehub body, making it vulnerable to “spinning” on the
freehub body. With the Wheels cassette
you will get a few spacers that are wafer thin, so you can fine tune how much
the smallest sinks into the body. This
gives a more positive fit so that you shouldn’t have any problems.
Q: Which of the two
types of PowerTap pickups should I use?
A: (Jesse Bartholomew)
All PowerTap hubs made by Tune (matte silver-grey hub body) and some CycleOps
PowerTap hubs are designed to be used with a receiver that mounts 7" from
the hub for optimum signal transmission; these have a serial number of 27383 or
less. In a successful attempt to limit
data drops, we “tuned” the hub and receiver, resulting in a new receiver that
needs to be mounted closer (3-5") to the hub; these have a sticker
indicating how to mount them on one side, a CycleOps brand sticker the other
side, and serial number greater than 27383.
Part
of the tuning was to desensitize the receiver a bit, and because the new PowerTap
SL hub transmits through carbon “windows” in the hub shell, the signal is
weaker, and the receiver won’t pick it up consistently, so we’ve gone back to
the original 7" style receiver, but we recommend mounting it no more than 3"
from the hub to maximize consistent data transmission. I know that’s terribly confusing, but the
short version is that the only combination that won’t work together is the SL
with the 3-5", current receiver. So
if you do upgrade to the SL you’ll need to use the SL model receiver with
whatever other hub you are using for training.
Q: I have a Mac G3
Powerbook without serial ports and want to run the Polar software with IR
interface. How can I do this?
A: (Bill Pence) Use the Keyspan High Speed Serial to USB
adapter and the serial version of the IR receiver from Polar. You need to be running Virtual PC 5.0 and
Windows 2000. The Mac OS needs to be 9.2,
as 9.1 does not seem to work too well with VPC, nor does OSX (OS 10). I’ve run the Polar PPP under VPC 3.0, which
worked fine, but the IR adapter didn’t work.
I found that I needed to upgrade my device driver to Keyspan V 1.9.
Finally, I needed to plug the Keyspan Serial Port
adapter into a USB port on the back of my G3 – for some reason it was not happy
plugging into the spare USB port on the keyboard. I have not been able to make the USB IR
interface work with this setup, but serial port works just fine the way I have
it set up now with my S-710.
After
that it is a matter of setting all of the dialog boxes correctly. With the IR adapter plugged into Port 2 of the
adapter, the Keyspan Control panel it will advise you of the devices attached. Click Advanced Settings for more detail. Pull down the menu in the dialog box to Port
#2. It should read something like
P#2USA28X02. This is specific to the
Keyspan device, and identifies port number 2 (where you plugged the adapter). If funny things have been happening, you may
reset the port here. Mine is set to receive
FIFO of 16 and a buffer of 64, both default values. Also, make sure interrupt endpoints is set. I do not know what effect it has, and am not
anxious to find out.
Leaving
Keyspan and launching VPC, once Windows has booted up, look in the menu bar on
the Mac side of the house (hold down the Apple key and the Mac menu bar
appears) and pull down “Edit Windows 2000 Settings’” which brings up the
settings list. Click on COM 1. A dialog box will appear to the right of the
window with various radio buttons. Click
“Mac Serial Port.” P#2USA28X02 should
appear below it; this is your Keyspan Port with the IR adapter on it. Check “Non-Modem device” on the next line
below, and the COM 1 port is now mapped to the Keyspan port that the IR Adapter
connects to.
One
more step. Launch the Polar Software
(new versions are best). Pull down the “Options-Preferences”
menu. A dialog box will appear labeled “Software
Preference,” and click on the “Hardware” tab. In the top of the box is a section devoted to
the Polar S-series HR monitor. Set the
pull down menu to COM 1. Click the “Options”
button. I have mine set to USB Autocheck
and Keep HR in Connect Mode. I don’t think
it matters unless you check “Use Windows Internal IR” port, which would be very
bad.
Set
the 710 in front of the IR receiver and click the connect button. It should connect and work just like running
on a Windows PC, and that should be all there is to it. I do not believe the USB IR adapter will work
with Virtual PC. I tried. A lot. VPC
is not happy sharing USB devices with the Mac OS.
Q: (Brian
McLaughlin) Hold on there! I have been using OSX with VPC 6, using the
USB Keyspan converter model USA-19QW. It
all works fine with PowerTap software and CyclingPeaks software. The driver CD that I have is 1.2 for OS X,
1.9 for 8.6-9.x, and it works fine as long as I make sure it is recognized in
the set-up of VPC before I try to download data. I use COM 2.
A: Thanks!
Q: When starting VPC 6
on my iBook (OSX), it takes forever – so much so I have never actually opened
it. My iBook has 128 MB – and I have a
suspicion that VPC 6 needs 256 MB. Does
anyone know?
A: (Anne Grofvert, Chris Bartholomew, and Jeff
Lawson) 192 MB of RAM is what Microsoft
specs for 6.1, but to run VPC you really need to allocate much more RAM than
recommended. I have 756 MB and have
allocated almost 400 MB to VPC to get it running smoothly. When you exit VPC be sure to “save all and
quit” to preserve your settings, so that VPC it doesn’t take forever to load the
next time.
The
beauty of the new Panther operating system is that it allocates memory to the
programs being used, so VPC does not affect the functioning of the machine when
you are using other applications. VPC
does not seem to run on a G4 either, again, due to lack of memory capacity. It just sits there and does not load.
Q: My Polar S-710 has
been difficult to install. Any
suggestions beyond what’s in the owner’s manual?
A: Wattage Forum member Robert Chung has devoted a
page to this at his web site: http://tinyurl.com/ijav,
and there is a video at the Polar web site http://polarusa.com/consumer/powerkit/installvideo.asp
(Tom
Anhalt) The angle of the chain across
the sensor and whether or not the sensor module is parallel to the chain do not
matter; all that counts is to position the module so the chain is no farther
than 30 mm, in all usable gear combinations, from an approximately 1” square
area centered on the “middle” mark on the module. If this requirement is met, and if the
cadence sensor is properly positioned (which depends on the particular magnet
you use), you’ll get consistent readings, otherwise, the chain vibration signal
will be weak and the signal processor will tend to “lock on” to signal noise, causing
erroneous readings.
Some
comments on the Polar installation video:
1. why
mark the center of the chainstay? This
is the first thing shown, but it’s not used for anything. The location of the module on the chainstay
is driven solely by the placement of the magnet on the crank, and then placing
the module so that the cadence sensor lines up.
2. the routing
of the speed sensor wire just begs for it to get snagged and ripped out. There are much better techniques for routing
and securing this wire to the derailleur that will minimize this threat.
3. it is a
mistake to make the vertical spacing measurement 5-10 mm in the small-small
combo. It’s the wrong end of the range from
which to make this critical measurement, since the chain will be much farther away
than 30 mm even in the small chainring-large cog combo. I run a pretty “normal” gear setup (53/39, 12-25
cogs), and if I try to run the 39 x13 (which I don’t because of the cross-chaining),
the chain actually rubs on the sensor.
4. there is no mention of making sure the chain passes
over the sensor in all gear combinations, a significant omission.
Finally,
to protect the sensor module, I first tried some mylar, but that didn’t last
long. The best thing to do is to grab a
couple of black zip ties and wrap them around the module right over the top of
the magnetic frequency sensor (that’s where the chain will be pulled down). This way, the chain will rub on the “sacrificial”
zip ties instead of the top of the module.
Q: I’ve had problems
with the cadence magnet that came with my Polar power kit. It just doesn’t want to stay in place on the crank
arm.
A: (David Bilenkey and Tom Anhalt) The magnets supplied with the kit work poorly,
if at all – don’t bother with them. Instead,
get a 1/2 or 3/8” diameter “rare earth” magnet, such as from Radio Shack, Lee Valley Tools, or National Imports.
These
are small disks, 1/8” – 1/16” thick, and should cost less than $2. If your pedal spindles have any ferrous
content (“stainless steel” may or may not), just drop one on the backside of
the spindle. Align the magnet in the
best location to make the little cadence light blink. No tape required; it’s strong enough not to
fall off, but not strong enough to pull the chain over against the crank.
To
remove the magnet, remove the pedal and slide the magnet sideways to get a grip
on it and peel it off. If your pedals
have titanium spindles, or non-ferrous stainless steel, simply place a piece of
electrical tape across over the magnets so they won’t fall off during bumps.
You
might also try gluing the rare earth magnet to the center of one side of a ½”
diameter ceramic magnet (making sure you match a south pole to north, or vice
versa) and then glue this “stack” to the backside of your pedal spindle. This should eliminate any problems with chain
interference.
Q:
As an aside, how does the Polar power module ‘know’ the free length of the
chain?
A: (Jean-Joseph Cote)
Since cadence, wheel speed, and chain speed are measured, there’s enough
information to calculate the number of chainring and sprocket teeth, and from
there, the diagonal length of the vibrating section of the chain can be obtained
(this is in the patent). Polar chose not
to display the gear numbers, presumably due to limitations of the display size.
Q: Is it true that
downloading drains the battery in the PowerTap Standard computer?
A: It will if you leave it in the download
cradle. Remove it after downloading,
replace it in the handlebar mount, and let it “fall asleep.” Then, remove it and do a “clr all.” It is now in its most efficient mode. When the battery starts to get low, HR
function seems be the first thing to go, becoming unreliable, with many “data
drops.”
Q: I'm running
XP Pro on a Pentium 4 CPU, and when trying to install the PowerTap Link
software (vers. 1.02), I get the same crash.
In the “Quick Access” Screen, whenever I click on
rider management, I get the following run-time error
Runtime Error
'-2147024769(8007007f)'
Method '~' of object '~' failed
A: (Rick Sladkey) It sounds like your data access components did
not get properly updated by the link installer.
You might try manually installing the latest Microsoft Data Access
Components (MDAC) 2.8:
http://www.microsoft.com/downloads/details.aspx?FamilyID=6c050fe3-c795-4b7d-b037-185d0506396c&DisplayLang=en
Q: I need a converter
so I can download the PowerTap to a USB port, rather than a serial port for
which it was designed.
A: Try item the adapter Keyspan High Speed USB-Serial Adapter
USA-19HS (recommended by PowerTap; formerly USA-19QW), at http://keyspan.com/products/usb/usa19hs/,
item GUC 232A from iogear at http://www.iogear.com/products/product.php?Item=GUC232A,
or Belkin item USB-A/DB9M at http://www.officedepot.com/shop/catalog/sku.asp?ID=913114&LEVEL=SK. There are also http://sewelldirect.com/USBtoSerial.asp?kid=-691449310&match_type=search
and http://www.tigerdirect.com/applications/searchtools/item-Details.asp?EdpNo=542934&sku=B131-5002
Q: Help! My PowerTap Standard stops downloading after
only 250 records!
A: (David Easter) First, some background: during a download, the
firmware in the PowerTap CPU transmits data in blocks of 256 records. Each record contains data from a single sample
(once every 1.26 seconds or longer, depending on the recording interval). Each block is terminated with a calculated
“checksum,” i.e., a consistency value designed to detect any errors that might
be introduced on the serial link between the PowerTap and the PC. As the PC receives each block of data, it
calculates the check value from the received data and compares it with the value
transmitted by the PowerTap. If it
matches, the download continues, if not, you get the download error.
For
reasons nobody seems to understand, the PowerTap sometimes generates bad check sequences.
That’s why the download goes to ~250
every time and quits. The data isn’t
being corrupted on the serial line but the PC thinks it has, and bails out
after the first block. I’ve had this
problem most recently after fighting with what turned out to be a bad receiver.
Once I got a good one, did a “clr all,”
went for a ride, and then got a “250” download error. Curiously, it got to 250 records, thought it had
succeeded, but generated a file containing only the header record. The next day, I removed and reinserted the
battery in the PowerTap CPU, and all has been well since. I’ve also seen variants of the problem with a
CPU that stopped receiving power data and also started showing screwy data
while in magnet mode.
One
theory is that the CPU doesn’t do proper range checking on data coming from the
receiver, and allows noisy data from interference, or a bad receiver, to
corrupt the internal memory in such a way as to break the download protocol, however,
data in the CPU still seems to be good. The
trick is to get it out. A download
program that simply ignores the check values might work in some cases, though I
haven’t tried it, but in other cases, some of the data has obviously been
corrupted and the CPU probably can’t generate the normal download stream anyway,
regardless of what the PC would do with the check value.
I
have created a web site at http://www.david256.com/power/ptrescue/
to rescue data trapped by this problem. Note: this recovery process works only
with a (grey) PowerTap Standard CPU; the newer PowerTap Pro CPU (yellow) is
different, and this process won’t work with it.
Q: I’m having
problems importing a PowerTap file that was e-mailed to me. I’m getting a message, “error with .csv
file.”
A: (Brian McLaughlin, Craig Upton, & Robert Chung) The workaround is to have the sender
compress the files. Have the sender
WinZip or StuffIt the file. These
compression softwares will “protect” the file, so that when the receiver’s MS Outlook
handles it, nothing is done to the file that is enclosed in its compression
shell. When you receive it, download the
attachment, put it on your desktop, un-Zip or un-StuffIt. It should work fine. This also happens with Entourage e-mail
software, as well as Hotmail, which apparently uses some compression of its own
to alter the file.
Comma
separated value (.csv) files are actually plain text files where the fields are
separated by commas. For reasons unknown
the PowerTap software requires not only that the values be separated by commas
but also that the spacing be exact, which sort of defeats the philosophy of
CSV.
For
odd historical reasons, e-mail programs were often allowed to treat text mail
differently than other kinds of data streams. The foregoing CSV problem is akin to the CR/LF
annoyances that used to occur when sending text mail to and from Unix systems. In some sense, the blame is equally shared:
e-mail programs for modifying files, and the PT software, for marrying the
inefficiency of text files with the inflexibility of binary files.
Q: I forgot to make
sure I had zeroed out the torque on my PowerTap before my last ride, and so all
the values for power are off. How do I
now go back through the file and correct the error?
A: (Rick Sladkey and Rick Moll) You
need to subtract the unzeroed torque value recorded when you were coasting from
each value of measured torque, and then recalculate power from torque and
speed. Find a section where you know you
were coasting and actual torque was zero (i.e., when power and cadence were
zero) and pick a representative torque value, then compute actual torque:
(1) torqueactual = torquemeasured - torquecoasting, then re-compute power as
(2) poweractual (Watts) = 1746 × torqueactual (N-m) × speed (km/h)/wheel circumference (mm)
where
wheel circumference is the same as in setup (about 2093 for a 700C × 23 mm tire). Chris Mayhew has posted a spreadsheet at http://users.icubed.com/~mayhew/mayhew.xls
to perform these calculations.
Alternatively, equation 2 can be divided by
(3) powermeasured = 1746 × torquemeasured × speed/wheel circumference
and
both sides multiplied by powermeasured to give
(4) poweractual = powermeasured × (torqueactual / torquemeasured)
which,
if used in a spreadsheet, must be protected against division by zero.
Since
torqueactual / torquemeasured is not a constant, neither is poweractual / powermeasured, however, equation 4 demonstrates that power is
proportional to torque.
Q: What should
smoothing percentage be set at?
A: (Andrew Coggan and Rick Sladkey) It depends on what you’re looking for. There are times when you might wish to apply
really gross smoothing, e.g., to better detect any overall downward trend in
power during a very long ride. On the
other hand, there are times when you don’t want to smooth the data at all, such
as when trying to capture the details of a 500 meter race on the track. The same logic applies to data recording
frequency . . . during very long rides it is probably sufficient to record data
relatively infrequently, whereas on the track, even 1 second intervals may not
be frequent enough. Properly designed
hardware and software should give the user maximum flexibility with regards to
these issues.
To
calculate the equivalent rolling average for a given smoothing level, multiply
the duration of the ride or interval in seconds by the selected smoothing percentage. For instance, 1% smoothing for a 1 hour ride
would be calculated as 3600 seconds × 0.01 = 36 seconds, however, if the ride
(or an interval within a ride) was 30 minutes (0.5 hours) long, then smoothing
to 1% would be equivalent only to an 18 second rolling average, whereas 2 hours
1% smoothed would be the equivalent of a 72 second average.
It
should be noted that the smoothing which the PowerTap Link software (versions
1.04 and lower) gives is nothing like a true rolling average, rather, it
smoothes a curve by taking fewer points and then simply connecting them with sinusoidal
curves. This is so poor both mathematically
and visually that 1% and 2% are the only useful settings.
Q: My PowerTap keeps
‘cutting out’ – both power and heart rate will plummet, then come back up. This shows up as a lot of zeroes in the
downloaded file. I have checked the location
and orientation of the receiver on the chainstay, and both the hub batteries as
well as the CPU batteries are fine. All
contacts (battery and CPU/harness) are clean and uncorroded, and I used only
electrical tape (no zip ties) on the harness wire. What next?
A: This is commonly known as a case of the “data
drops,” and you have taken the first steps to correct it, but if they don’t
work, perhaps the firmware in the PowerTap CPU needs upgrading. You can download the latest from http://velo-fit.com/articles/power%20tap_200.zip,
or contact
Jesse
Bartholomew, PowerTap Product Manager
jesse@cycleops.com;
1-800-783-7257, ext. 159
Q:
How can I open a PowerTap database file in Microsoft Access?
A: You need to know the
password, which is “link.”
