Power Calculator

Discussion in 'Transmission / Drivetrain' started by loquin, Mar 23, 2010.

  1. loquin

    loquin Active Member

    After reviewing the Online Power calculation utility, I liked it, but, thought it could use some improvement. I would like a calculator that would allow you to set some default values, and keep them until they are changed. To allow entry in either English or metric units. To allow slope entry in either degrees or percent.

    Then, after reviewing the on-line code, I saw that the english-to-metric conversion factor was wrong. Not by a lot, but, it was off.

    So, I wrote a replacement with those features.

    Install it with the attached windows installer, then run it (shortcuts will be installed on the desktop and the start menu) A screen shot of the main form is also attached.

    Press F1 or use the help menu at any time for online documentation.

    Usage should be pretty straight-forward; if you have questions, after checking the help, start a thread here in the Tools form.


    Attached Files:

    Last edited: Aug 2, 2012

  2. loquin

    loquin Active Member

    Drive Efficiency

    Keep in mind that the power needed to move the bike is at the AXLE/Wheel, not at the motor output shaft. If you have a belt drive, a chain drive, a gearbox, and.or a CVT (or any combination,) when calculating the maximum possible speed you can attain, you will also need to account for the losses between the motor shaft and the axle (or, in the case of a friction drive, the wheel.)
    • A well-aligned & maintained chain is about 98% efficient. As the chain gets worn, it will stretch a bit, and you get slippage on the sprockets. (The chain roller will not be contacting the sprockets exactly in the center of the 'trough.' This leads to decreased efficiency, and increased wear. Also, misalignment of the sprockets causes a reduction in efficiency. This misalignment includes multi-speed chainring-cassette sprocket alignments where the chain is deflected. In other words, the outer chainring to the inner cassette sprocket, and vice-versa, have the greatest misalignment, and the greatest losses.
    • A toothed drive belt (GEBE) is about the same as the chain drive - about 98%. A toothed, synchronous belt MUST be aligned very well, else it tends to cause increased wear, decreased efficiency, and very short belt life, however.
    • A V-Belt can be fairly efficient as well - per PlantServices.com, "V-belt drive efficiency can run as high as 95 to 98% at the time of installation. During operation, however, V-belt efficiency deteriorates as much as five percent. The efficiency of a poorly maintained V-belt may fall an additional 10%."
    • Belt CVTs (Comet/Max-Torque) with a new belt operate at about 95% efficiency. As the belt wears over time, the efficiency slowly drops, to a low of about 85%.
    • In addition to the built-in belt drive, pocket bike CVTs also have a 3:1 gearbox on the output, which will also reduce efficiency a bit. So, you're probably looking at an output which starts off at about 92% or so, then slowly drops over time to about 82%. Put in a new belt, and the efficiency should shoot back up to the 92% range again.
    • A gearbox is variable - the number and type of gears will affect the efficiency. A rule of thumb I've seen used for efficiency, is 95%-98% per reduction for spur and helical gears. Using this, an 18.75:1 Staton gearbox, for instance, with it's three gears, and 2 gear-gear meshes, might be in the 90% to 96% efficiency range. If you split the difference, and assume 93%, you probably wouldn't be too far off. Worm gears can be much less efficient, losing anywhere from 2 to 80 percent (!!!) of the input power, depending upon their reduction ratio, gear angle, and gear material. Maximum efficiency occurs when the worm gear pressure angle is 45 degrees. Reference. Because of their potential enormous efficiency range, these gearboxes must be addressed on a case-by-case basis. No rule of thumb efficiency estimate can be assumed for worm drive gearboxes.
    • There IS no official Nuvinci published efficiency figures; at their forum, they never answer the questions about efficiency - they just evade them and refer you to tests done with it by professional riders and others, and discuss how the hub 'feels' and how it is a good 'total system' choice. The only third party data I saw indicated an efficiency of a little less than 90%.
    • Surprisingly to many, a friction drive, when aligned properly, used with a slick (highly recommended,) low rolling friction tire, and under dry conditions, has quite good efficiency. In effect, you simply double the rolling friction on one tire. There is no other gearbox or drive train to reduce the power from the engine to the wheel. My estimate, under these conditions, would be in the 95% efficiency range.

    Note that when multiple power transfer methods are used sequentially in a drivetrain, the losses are multiplied together, not added.

    So, in the case of a happy time system, you have an internal gearbox and a chain drive. If perfectly aligned, you would be looking at about .965 * .98, equaling 0.946 (94.6%) drive system efficiency. If you insert a shift kit (jackshaft) between the engine and rear sprocket, the drive efficiency is now reduced by two additional chain drive losses, so the total would then be .965 * .98 * .98 *.98, equaling 0.88 (88 %.) As with a CVT approach, even though the overall efficiency if the drive system is lowered by adding the shift kit, the acceleration can improve, as you're able to keep the engine in the max power band of it's RPM range. However, the top end will be less than the theoretical maximum for the engine, as losses between the engine and the rear axle will be greater.

    Likewise, if you were using a pocket-bike CVT, with it's built-in 1-stage gearbox and chain to the rear sprocket, the three losses are the motor-pulley to gearbox pulley, gearbox, and gearbox sprocket to rear sprocket. These three separate efficiency losses on a new system would be approximately 0.95 * 0.965 * 0.98,equaling 0.898 (89.8%)
    Last edited: May 16, 2012