FAQ for power-based training (version 12.04)

by Charles Howe, with contributions from members of the Wattage Forum at Topica.com

 

CONTENTS

Introduction/General Concerns

1. What is power?

2. Why is training by power suddenly so important?

3. Why should I train by power?

4. Has power-based training obviated training by heart rate and by ‘feel’ (perceived exertion)?

5. What are some of the various power-measuring systems on the market?

6. How do power-measuring devices work?

7. Where can I buy a power-measuring system?

8. Which model is best?

9. Why does memory capacity vary so much?

10. What is the difference between signal rate, display interval, and recording interval?

11. How do I make use of all the workout data?

12. Why does power fluctuate so much while riding?

13. What is a ‘good’ power output?

14. What is CDA?

15. How can I test my progress with a power meter?

16. What are normalized power, intensity factor, and training stress score?

17. How does altitude affect power output?

18. Where can I learn more about training by power?

19. What about power-based training indoors?

20. Isn’t the consistency of the data what really counts, moreso than accuracy?

21. Can a power meter be used as an aid to weight loss and dieting?

 

Use and Operation

22. Does temperature affect accuracy of a strain-gauge based system (i.e., SRM and PowerTap)?

23. How do I calibrate my power meter?

24. Can I race using a power meter?

25. How does the PowerTap calculate cadence without a sensor?

26. Can I use the PT Standard model just as a computer, without the hub?

 

Solutions

27. The drive-side bearing cone in my both PT Pro and Standard hubs wore out quickly.  Any suggestions?

28. How can I make my PT hub more waterproof?

29. How do I build a custom wheel around a PT hub?

30. What about for a 135 mm hub?

31. How do I build a PT hub into a fixed gear wheel?

32. How can I mount my PT on the handlebar stem?

33. Can I use a PT Shimano freehub with a Campagnolo derailleur?

34. Which of the two PT pickups should I use?

35. How can I transfer data to a Mac Powerbook from my Polar S-710 with an IR connector?

36. Hey wait!  I’m using Virtual PC 6 with OSX, no problem.

37. How much memory is needed to run VPC 6?

38. I’m having trouble installing my Polar power meter.  Any suggestions besides the manual?

39. The cadence magnet for my Polar power meter won’t stay in place.

40. As an aside, how does the Polar power module ‘know’ the free length of the chain?

41. Does downloading drain the battery in the PT Standard computer?

42. I can’t get the PT Link software to install.

43. How can I download the PT to a USB port?

44. My PT Standard often stops downloading at 250 records.  How can I “rescue” the file?

45. I can’t open a PT file that was e-mailed to me.

46. I forgot to zero out the torque on my PT hub; how do I correct the file after downloading?

47. What should smoothing percentage be set at?

48. My PT seems to have a case of the data drops.  How do I stop them?

49. How can I open a PT database file in Microsoft Access?

 

Q: What is power?

 

A: To Henry Kissinger it was an aphrodisiac, but for our purposes, the definition comes from physics, and in particular the science of dynamics, which is a branch of mechanics.  Power is the rate of doing work or transferring energy, such that power = work/time, or P = Wt.  As relates to cycling, it is measured in international system (SI) units called Watts (W), rather than the familiar english unit of horsepower that is used as a measure of engine power (1 horsepower = 746 W).  Since work = force applied through a distance, or W = F × Δx, these two expressions can be combined and rearranged to express power as the product of force and speed, i.e., P = F × s, and this may be the best way to think of it: the speed you can maintain times the total force resisting your forward motion.  Similarly, power can be defined as pedal force (i.e., torque, which equals = [measured frequency - zero offset]/slope) × cadence, which means you can increase power by exerting more force on the pedals at a given cadence, by increasing cadence while exerting the same pedal force, or by increasing both force and cadence.

 

Here are some examples that give an appreciation for units of power:

 

A 68 kilogram (150 lb) rider traveling on an 8.6 kg (19 lb) bike at 20 mph in on flat ground in with no wind requires about 177 W.

 

56.5 W are required to raise a 20 lb dumbbell 25 in. overhead in one second.

 

 

Q: Why is power-based training so important all of a sudden?

 

A: It’s no more so than it was previously, in fact, exercise physiologists have used calibrated ergometers for years to impose precise loads on study subjects.  Rather, the introduction of affordable on-bike power measurement systems (power/speed measuring device, handlebar-mounted computer, receiver/wiring and computer mounting bracket, download interface, software) have made it possible to use power in everyday training as well as racing, then analyze the resulting workout data.  This, and their widening use among both amateur and professional riders has generated considerable “buzz.”

 

 

Q: Why should I train by power?

 

A: (Eddie Monnier and Andrew Coggan)  Because it is the objective measure of exercise intensity, and as such directly determines physiological and perceptual responses to exercise, so training by power provides immediate and quantitative feedback on the intensity of effort.  300 Watts is 300 Watts, no matter how hot, windy, or hilly it is, or what your heart rate is – though it may “feel” easier or harder, depending on various conditions.

 

Three variables to control in any training program are intensity, duration, and frequency; of these, the latter two are easy to quantify objectively – duration is measured in hours, and frequency in sessions per week (the product of the two is volume).  Intensity, on the other hand, has traditionally been measured by perceived exertion (PE) and/or heart rate (HR).  HR is reliable enough at lower (i.e., aerobic-only) intensities, but for more race-specific (i.e., shorter but more intense) training, it becomes a much less effective proxy for intensity.  Besides being subject to numerous environmental and physiological variables (e.g., temperature, humidity, hydration status, altitude, overtraining, lack of sleep, nervousness, and upward “drift” as exercise progresses), HR responds slowly to workload demands, and thus is a lagging indicator of effort.  That is, it will be lower than power and during the early part of an effort, and higher afterward.  For example, if you bound up a few flights of stairs, your heart rate will take a while to reflect the effort, and will continue to beat at an elevated rate for a while even after you have stopped climbing steps.  The shorter the duration of an effort, the less useful HR is.

 

 

Q: So power-based training has made perceived exertion and heart rate obsolete?

 

A: Not quite, but they seem to have been relegated to a definite second and distant third, respectively!  Many still cling to HR an indicator of overtraining – though declining power for a given PE is the deciding (and often first) sign of that, too.  Nonetheless, there persists some popular, if not scientific controversy as to the role of HR, with some claiming that it indicates metabolic intensity, and therefore one should train by HR, while monitoring power.  In fact, just the reverse is true; particularly during outdoor cycling, metabolic load is more accurately reflected by power, integrated with PE, the latter being more reliable than HR and incorporating more physiological variables.  Power provides an objective standard by which effort can be quantified, thereby ‘calibrating’ PE, while PE serves to modulate power output.

 

 

Q: How do I measure power – I mean, what are some of the various power-measuring systems available?

 

A: Here are the four bicycle-based systems presently available, with links to each manufacturer’s site:

 

Ergomo Sport (a torque-measuring bottom bracket available in Campagnolo square-taper or Shimano OctaLink): http://www.ergomo-usa.com and www.ergomo.de/eng_main.html

Polar S-720i or S-710i (uses a chain vibration sensor that mounts on the right chainstay): http://www.polarusa.com/consumer/productfinder/productfinder.asp or http://www.polar.fi/power_output.  User’s manuals are on-line at http://tinyurl.com/xyny and http://tinyurl.com/xys4

 

PowerTap (a torque-measuring hub that you build into a wheel): http://www.power-tap.com/

 

SRM Powercrank (a torque-measuring crank that replaces your present model): http://srm.de and http://www.thebikeage.com 

 

Note: contrary to claims, Ciclosport models do not actually measure power, rather, they only give a rough estimate based on speed, total mass (rider/equipment), and road grade, which may be accurate on steeper grades, but is useless on flat terrain, particularly in group rides or if any wind is present.

 

Finally, you don’t need a high-tech gizmosystem to figure your power.  For instance, you can use a hill with a steady grade of ~7% or more by timing yourself over a measured portion of it, and then calculate power quite accurately (so long as air was sufficiently calm) using http://analyticcycling.com.  You can even get a consistent estimate running up a constant grade or a flight of continuous steps, such as in a stadium:  Watts  =  (mass in kg ´ 9.81 ´ net elevation gain in meters)/time in seconds; kg  =  lbs ´ 0.4536.

 

 

Q: How do power-measuring devices work?

A: (Garth Rees and Charles Howe)  The various on-bike systems measure force the applied either at the crank (SRM), the rear hub (PowerTap), crank spindle (Ergomo), or chain (Polar).  (Note: a patent was granted to Shimano in November 2003 for a torque-measuring bottom bracket, U. S. Patent 6,644,135.)  The former two use strain gauges, which are fine polymer sheet, with ultra-fine wire or foil sandwiched in it, and the electrical conductivity of the metal changes as they are twisted or deformed when force is applied, due to the secure bonding to the material under test (the energy absorbed by the strain gauge is so close to nil that it can be neglected in any loss equations).  Strain gauges are fragile when not bonded, and typically no bigger than your small fingernail, often 2 × 4 mm or smaller, depending on application.  They may be in single, half-rosette (2 gauges, 90° offset), or full rosette (4 gauges, all at 90° offset, i.e., 2 opposed half-rosettes) configuration, with the last having the best accuracy of all, since it compensates best for the strains in the two major axes, resulting in good self-cancellation of any errors in the two devices.  The difference in accuracy from half to full rosette is not as great as is the implementation cost.  Here are pictures of the PowerTap hub mechanism (U. S. Patent 6,418,797) from cyclingnews.com and bike.com , showing the strain gauges in a full-rosette arrangement:

 

 

The strain gauges measure torque inside the hub, then this data is transmitted, along with wheel speed, to a seatstay-mounted receiver via digital radio frequency (RF) waves, and then by wire to a handlebar-mounted computer with a 16-bit microprocessor, where they are used to calculate instantaneous power, road speed, cadence, etc.

 

Similarly, the SRM senses torque exerted at the crankset, then multiplies it by crank rpm (cadence), measured with a crank magnet and sensor, to give power.

 

Both Polar models measure chain tension via a chainstay-mounted sensor that detects vibrational frequency; just like a guitar string, a chain vibrates faster as its tension goes up.  This is translated into an amount of force, which is then multiplied by chain speed, as measured by an optical sensor mounted on the rear derailleur, thereby giving power output: power (W) = chain tension (N) x chain velocity (m/s).

 

Finally, the Ergomo Sport uses a bottom bracket with a photointerruptor circuit actuated by two “combs,” or flat discs mounted on the bottom bracket spindle, each having numerous slots spaced evenly in a radial fashion.  Two optical sensors measure changes in the alignment of the slots, which is determined by how much the spindle twists, and hence how much torque is being exerted.  This value is then multiplied by crank rpm (cadence), which is measured by the bottom bracket unit, thereby yielding a value for power.

 

 

Q: Where can I buy a power-measuring system?

 

A: Check with your local bicycle or triathlete shop, or the manufacturers’ web sites for dealer listings.  You may also find the products in cycling catalogs and/or on the web, and many coaches are also dealers for the several systems.

 


Q: Which model is best?

 

A: Would you believe the stadium steps?!  Accurate, reliable, and the least expensive!  Kidding aside, this is an area which can excite considerable controversy (!), so no recommendations are made here; each of the four models available can be a valuable training aid if used properly, and the final choice is largely a personal one.  Indeed, Wattage forum member Robert Chung has compiled several “Rosetta Stone” files comparing power data collected simultaneously, which showed close consistency between the models tested, as did Kraig Willett’s test at Kraig Willett’s test.  Here is a comparison chart:

 

 

Ergomo Sport

Polar S-720i/710i

PowerTap Standard

PowerTap Pro

PowerTap Pro SL

SRM Professional / Amateur

Measurement location

Bottom bracket (Campagnolo or Shimano OctaLink)

Chainstay and rear derailleur

Rear hub (130 or 135 mm; 24, 28, and 32 hole drillings)

Same as Standard

Same as Standard

Crank (Shimano OctaLink or Campagnolo; 167-182 mm lengths in 2.5 mm increments)

Method

Photointerrupter circuit

Chain speed and vibration frequency

4 strain gauges

Same as Standard

Same as Standard

4 strain gauges for Pro, 2 for Amateur

Claimed accuracy

± 2%

± 10% at any one instant, but 2-5% or less on average

± 1.5%

Same as Standard

Same as Standard

± 2.5% for Pro, ± 5% for Amateur

Recording interval

Averaged values recorded every 5 sec.

Current values recorded every 5, 15, or 60 sec.

Current values recorded every 1.26 or 2.52 sec.

Current values recorded 1.26, 2.52, 5.04, 10.08, or 30.24 sec.

Same as Pro

Averaged values recorded 0.01-30 sec.

Memory capacity

11 hr.

1 workout file

4:57-76:37 hr.

Up to 99 workout files

4 or 8 hr. depending on recording interval,

1 workout file

7.5-180 hr. depending on recording interval.

1 workout file

Same as Pro

0:45-225  hr. depending on recording interval.

Numerous workout files

Calibration

By manufacturer only; accuracy can be checked via static ‘stomp test’ described below

No; but accuracy can be checked on hill of known grade

No; accuracy can be checked via static ‘stomp test’ described below

Same as Standard

Same as Standard

Slope setting is user adjustable; manufacturer calibration now available in U.S.

Mass (grams)

BB w/bolts & wires = 344 g

Computer  & mount =168 g

Sensors = 118* g

Computer = 53* g

Mount/wiring = 71* g

Hub = 579* g (w/o skewer)

Computer =39.5* g

Mount/wiring = 36* g

Same as Standard, plus slight added mass due to crank-mounted cadence sensor.

Hub = 416 g (w/o skewer)

Remaining compo-nent masses are same as Pro

Pro = 560 g

Amateur = 640 g

Computer = 120 g

Mount bracket/wire = 30 g

Advantages

1. outstanding software (CyclingPeaks) with many useful analysis tools

2. third-generation design

3. fully hard-wired system is not affected by electronic or radio interference

4. easy installation

5. rechargeable computer battery lasts 5,000 hr., is good for ~30 hr., recharges in 2-3 hr.

6. almost no limit on component choice

1. least expensive of all options

2. feature-rich software, and extra hardware features like altitude

3. allows use of any wheel or crank that you want

4. large memory capacity, stores many workouts

5. incurs the smallest weight penalty

6. not affected by temperature

7. does not require calibration

1. easiest to move from one bike to another

2. affordable and accurate

3. compact, readable, easy-to-use display

4. most hub internals (axle, freehub, and drive side bearings) are all user-serviceable without disturbing strain gauges and electronics

5. easiest to install, and easiest to remove for racing – just swap rear wheels

Same as Std., plus:

1. expanded memory (up to 180 hr.); can store only one file but can create unlimited number of intervals

2. display has time of day, and rolling average capability for power, speed, and cadence data; can be customized for these functions

3. can display “pedaling power” (excludes 0, i.e., coasting values)

4. can be used with fixed gear

5. measures actual cadence (more accurate than the Standard model’s “virtual” cadence)

6. easier operation of interval feature

7. mileage is programmable

8. faster downloading with Link software v. 1.04

Same as Pro, plus:

1. improved hub internals (4 sets of sealed cartridge bearings), but not user serviceable

2. hub is 162 g lighter than Pro or Std.

3. available in fixed gear and Cam-pagnolo freehub versions

4. improved software is also Java-based and Mac-compatible

1. excellent software

2. time-tested, reliable design

3. can display rolling average for current wattage

4. large memory capacity, can store multiple workouts

5. no limit on wheel choice

 

Drawbacks

1. large/heavy computer

2. bearings must be factory serviced ($300) every 15-20,000 mi.

3. not easily moved from bike to bike

4. cannot accept 2004 Dura-Ace cranks

5. averaged data can be accessed only by download (cannot be viewed during interval)

6. not useful on tandems

1. most difficult to set up properly

2. difficult to move from bike to bike (to the point that it will likely never happen)

3. small display is hard for some to navigate

4. the least “clean” installation (multiple sensors and cables)

5. averaged data cannot be viewed during intervals (or ‘laps’), only at the end of the ride

6. accuracy questionable on stationary trainers, possibly from harmonic vibrations effects

7. not practical on MTB, and cannot be used with fixed gear

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