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  1. #101
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    Fair efficiency test (Magnic vs dynamo)

    So all the skepticism of the Magnic Light's efficiency has me wanting to devise some reasonable and repeatable test. This is what I have come up with so far. If you find issue with it, please let me know. Otherwise I'll see if I can find time to do it this weekend:

    Premise: I live at one end of a block with gentle hills on both ends and a dip in the middle.

    Assumptions: same bike and rider, minimal ground contact on launch and stop. The total rolling distance is hard to judge w/o actually performing the test but perhaps 1/4 mile.

    Test:
    #1 : start strava
    #2 : roll down hill with no brakes or pedaling
    #3 : come to a dead stop
    #4 : stop strava
    #5 : measure distance for the run

    I'd repeat this test twice (3x?) for the bike with Magnic Lights, w/o Magnic lights and dyno lights off, w/o Magnic lights but dyno lights on.

    The only issue is whether or not to use a separate front wheel or use the dynamo front wheel for the Magnic light test. I can get a pretty close setup (32c top touring on an T520 vs 32c Marathon Supreme on an A719) but I don't have access to two identical front wheels (where the hub is the only difference).

    So, would this be a worthwhile test to perform?

  2. #102
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by dhalbrook View Post
    First of all, I've stated more than once that I too would love to see numbers. And second of all, it's no secret that the Magnics produce less power. I clearly stated that in my review, and nobody's claiming otherwise. Maybe it's me but it seems you're being deliberately obstinate.

    As for whether or not other people could feel the difference, you're right, it's hard to say. The difference is pretty clear to me, but maybe others wouldn't notice it as much.
    I think it's just difficult to visualize the forces compared to a mechanical dynamo, and that makes it harder for people to accept that it's more efficient.

    There's another thing, which I haven't mentioned because I don't know, but is implied by the design and some hints on their site. This is an expensive headlight, and there are two LED's on it, not even close to the total potential output. It makes me wonder if they're doing something with the rectifier to increase efficiency, or exploiting the efficiency droop somehow, with the cost of the extra LED and circuitry involved not as significant in such an expensive light.

  3. #103
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by dhalbrook View Post
    So all the skepticism of the Magnic Light's efficiency has me wanting to devise some reasonable and repeatable test. This is what I have come up with so far. If you find issue with it, please let me know. Otherwise I'll see if I can find time to do it this weekend:

    Premise: I live at one end of a block with gentle hills on both ends and a dip in the middle.

    Assumptions: same bike and rider, minimal ground contact on launch and stop. The total rolling distance is hard to judge w/o actually performing the test but perhaps 1/4 mile.

    Test:
    #1 : start strava
    #2 : roll down hill with no brakes or pedaling
    #3 : come to a dead stop
    #4 : stop strava
    #5 : measure distance for the run

    I'd repeat this test twice (3x?) for the bike with Magnic Lights, w/o Magnic lights and dyno lights off, w/o Magnic lights but dyno lights on.

    The only issue is whether or not to use a separate front wheel or use the dynamo front wheel for the Magnic light test. I can get a pretty close setup (32c top touring on an T520 vs 32c Marathon Supreme on an A719) but I don't have access to two identical front wheels (where the hub is the only difference).

    So, would this be a worthwhile test to perform?
    I don't think that the coast-down will be precise enough to measure the small difference in drag. Also I think you need smaller time intervals and more precise distance measurements than Strava provides. If you do the test though I think you should keep the same equipment including wheels & tires for all tests.

  4. #104
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    I could also attach an ANT+ speed sensor to that bike to measure speed and distance at the wheel instead of relying on GPS.

    My concern with using the dynohub wheel for testing the Magnics is that it's not really fair to the Magnics to add the (admittedly tiny) amount of drag that the dynamo imparts. One way to account for that would be to perform four tests -- split the Magnic test into one with the dynamo wheel and one with the non-dynamo wheel.

    You're likely right in that this wouldn't be able to reliably detect any real drag difference between the two, but it would be interesting to find out and not overly onerous to perform.

  5. #105
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by dhalbrook View Post
    I could also attach an ANT+ speed sensor to that bike to measure speed and distance at the wheel instead of relying on GPS.

    My concern with using the dynohub wheel for testing the Magnics is that it's not really fair to the Magnics to add the (admittedly tiny) amount of drag that the dynamo imparts. One way to account for that would be to perform four tests -- split the Magnic test into one with the dynamo wheel and one with the non-dynamo wheel.

    You're likely right in that this wouldn't be able to reliably detect any real drag difference between the two, but it would be interesting to find out and not overly onerous to perform.
    Go for it!

    Or, a test with the dynohub wheel by itself without any draw should tell you if it the tiny extra drag is skewing the result. I'd also try to equalize weight on all tests.

  6. #106
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    Trust me, you won't see any difference. This test would prove nothing. The margin of error in this test is far greater than the drag.

  7. #107
    Senior Member Looigi's Avatar
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    You could do a test of the drag by measuring the spindown of the wheel suspended off the ground. First, measure the moment of inertia of the wheel with a torsional pendulum. Then impart spin to the wheel and measure the record the angular velocity of the wheel over time to calculate get the instantaneous deceleration at various angular velocities. Add the lights and do the same. The differences in the decelerations will give you the torque required to drive the lights at those angular velocities.

    Torque = moment of inertia x angular acceleration. Etc...
    Ride more. Fret less.

  8. #108
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    There was an email update from the kickstarter campaign with a poll asking if we wanted a flashing version of the rear light. I would want that as that is pretty standard here in the US. The Europeans don't seem to want it. He may offer both.

  9. #109
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by wphamilton View Post
    I think it's just difficult to visualize the forces compared to a mechanical dynamo, and that makes it harder for people to accept that it's more efficient.

    There's another thing, which I haven't mentioned because I don't know, but is implied by the design and some hints on their site. This is an expensive headlight, and there are two LED's on it, not even close to the total potential output. It makes me wonder if they're doing something with the rectifier to increase efficiency, or exploiting the efficiency droop somehow, with the cost of the extra LED and circuitry involved not as significant in such an expensive light.
    If there's a rectifier in the circuit, a passive one, it will have an on-state voltage drop and hence will dissippate power (rough estimate 0.5 amp * 1 volt *50% duty cycle = 0.25 watts). If the circuit contained an active rectifier (synchronous MOSFET) with a large die geometry, the on-resistance could be as low as a few dozen milliohms, and the power dissipation will be a lot less (similarly rough estimate 0.5A ^2 *.024 ohm *50% = 3 milliwatts discounting MOSFET drive losses and the cost of a drive circuit). But a quarter-watt is really hard to argue about, in terms of an effective change in efficiency.

    If possible, the best thing to do with a rectifier is eliminate it, all things being equal.

    I have trouble visualizing the Magnic's function, but it's the magnetic field interactions I'm having trouble with, not the forces and torques. Having designed a few handfuls of magnetic and electromagnetic devices, I'm usually pretty good about visualizing fields and fluxes, but not in this case.
    Last edited by Road Fan; 01-24-14 at 04:33 PM.

  10. #110
    Senior Member 01 CAt Man Do's Avatar
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    To be completely equal you would have to use another front wheel the same weight as the Dyno-wheel...BUT....to be completely equal you need to work in a "windless" environment or else it really isn't going to be equal. Even a small variation in wind would skew the results. Wind direction and speed changes almost constantly.

  11. #111
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    His Kickstarter campaign is now around $225,000 with just a few hours to go. I guess it's pretty successful. I hope he can deliver. That's going to be a lot of lights, 1100+ by my count of just the complete sets. He also had individual lights on there.
    Last edited by zacster; 01-31-14 at 01:02 PM.

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