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David Butcher: PPPM Efficiency

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Occasionally I am asked to explain how the PPPM can be more efficient than a "bicycle-in-a-training-stand" pedal generator. The explanation can be found in the answers to these questions. Ask yourself:

  1. Do you believe a fixie (fixed gear, single speed bike) is more efficient than a derailleur bike, at a constant speed?
  2. Do you believe a disc wheel is more efficient than a spoked wheel, especially at higher speeds?
  3. Do you believe higher tire pressure increases efficiency?
  4. Do you know how many bearings are turning on your bicycle while you are pedaling it, and are you aware that each bearing wastes a little bit of power?
  5. Do you know the seals on bicycle hub bearings (if you have them) can create a small but constant drag?

If you answered "yes" to all these questions (and that is how I would answer) you have just answered the question of how the Pedal Powered Prime Mover will be more efficient than a bicycle in a training stand.

The PPPM is a fixie, with a disc wheel (spinning at cadence speed, not road speed, which makes it even MORE efficient), and the "tire" is wood - comparable to enormous tire pressure (perhaps infinite - it does not "give" at all). There are only six bearings turning before you encounter the drive roller; two in the bottom bracket, and four in the pedals (a derailleur bicycle will have four more bearings, plus chain-to-sprocket losses). There are no hub seals to create drag. The PPPM does not even have the losses in a chain, which every bike has, even the single-speed ones.

Unless you are converting a very high-end, single-speed bike with a disc wheel and rock-hard tires, you will be operating at lower efficiency. Even then, the bike will lose energy in the chain drive (and possibly hub bearing seals) while the PPPM will not.

Some energy is lost on the PPPM where the drive wheel on the generator meets the flywheel, but even this loss is smaller on the PPPM than on many bicycle conversions because one of the two surfaces on the PPPM is wood, and no flexing occurs on that surface.

Add up all of these differences, and you have substantially better efficiency on the PPPM. As this hard-working team has discovered, even a carefully planned bicycle design is only one half as efficient as the PPPM (50 Watt sustained output vs. 100+ Watt.)

If you are researching different designs, and you need to have a rough measurement of efficiency, write the designer and ask them what their heart rate (pulse rate) is during a 30 minute workout at a sustained 100 Watt pace. In May of 2008, at 55 years old, my pulse is stable at around 105-110 20 minutes into a 100 Watt workout. Ask them to take the measurement as they are working out, as heart rates drop quickly after exercise stops. Of course everyone is different and you can't read too much into this, but if they are in decent shape (ask then their physical condition as well) and they are close to my size (150 Lbs/68 Kilos) and they tell you it's 135, their system will be much less efficient than mine, and if they tell you it's 85, they are "stretching the truth" and you should be suspicious.

For more details on PPPM efficiency and the physical effort needed to generate power at many different levels, see the PPPM Science page.

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