Two Bike Computers, Two Different Readings!

Discussion in 'Cycling Equipment' started by Aztec, Sep 13, 2003.

  1. Aztec

    Aztec New Member

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    I mounted my HAC4 today, and left my old VDO on there, just to see if they gave the same readings. Nope! A fast spin by hand showed a difference of about 10-15% in speed. I set both to the same circumference wheels, yet still the difference. And sadly, the new computer reads slower. I hate to think that my already dreadful speeds were LOWER than I thought, and my distances ridden 10% shorter!

    Any ideas on how to test which one is truly accurate? And shouldn't these things ALL be accurate????
     
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  2. abv8

    abv8 New Member

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    Possibly measure out a distance of 100 m to 1 km ( maybe the length of the street u live in or a lap around the block) or you can measure out the distance by driving a set distance and monitor the trip on the car.

    Then test both computers for the trip distance they give.

    It is unlikely they are uncalibrated, bicycle computers use electronics with on board chip timing up to most likely 1 micro second or more (dont ask why!).
     
  3. rek

    rek New Member

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    My first thought would be something wrong at the magnet/sensor end. If the programmed circumferences are the same, they should read the same distance for each tick of the sensor.

    Things to consider:

    If you're using one magnet on the wheel to provide for both sensors, maybe one of them too far away from the magnet, or has old batteries (if wireless) such that it isn't picking it up 100% reliably.

    If you're using two magnets on the wheel (one for each sensor), there's the possibility that one of the two is sensitive enough to pick up its magnet, and the other one intermittently.
     
  4. Aztec

    Aztec New Member

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    Rek, I am using the same magnet and sender -- they're identical for both units/brands. I tested with the new one and came up with similar results. Bot units are very close to together, and not the issue.

    I tested them on a ride today.
    HAC4 - 12.3 miles, 14 mph, 29 max mph
    VDO - 12.75 miles, 15.76 mph, 30.54 max mph

    The recording times were off by a few seconds due to different methods for cuttoff at standstill. That effects the avg speed, but nothing else.

    I checked with a two year old car, with stock tires, and got 12.1 miles.

    I triple-checked the wheel circumference settings, and they are accurate.

    The depressing part is that this shows that I'm quite a bit slower than I thought I already was. <sigh>
     
  5. BugMan

    BugMan New Member

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    If you increase your wheel circumference setting slightly on the new computer, you will get your old speed back!:D
     
  6. Aztec

    Aztec New Member

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    Bug -- Don't think for a moment that I didn't start calculating that same thing last night!
     
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