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  1. #1
    blithering idiot
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    How does Edge 305 calculate calories?

    Not that it is imperative, but how accurate is the edge 305 in calculating calories burned?

    My guess is that it is calculating calories based on road biking environments. I know that the most accurate way for it to work is with the cadence and HRM attached, but the effort and energy expended is much more climbing the same grade over technical trail than it is flat tarmac on 120PSI tires.

    Anyone know the answer, or if it would matter that much anyway?

  2. #2
    trail rat
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    I e-mailed Garmin about this a year or so back. Basically they use an algorithm based on speed, distance, and grade. What you say about surface friction will effect accuracy. So will age, fitness, wind, etc. etc. etc. Not factors that they can calculate.

    There has been much discussion about this here in the past, search "calorie" in this forum and you will get substantial hits. Also the MotionBased forums have good discussions.

    Bottom line for me is I do not use the calorie calculations. You need more accurate tests with an exercise physiologist to determine the calories burned during any exercise. No exercise computer that you can carry on your person or bike will give you that. You need a lab quality instrument and a trained operator.

    Use the Garmin reading as a comparison tool, road vs mtb rides, or the same ride on different days, for example, and you will get interesting information, but absolute values are not valid.

    edit:

    I remembered that they stated the algorithm took into account age, sex, and bike weight that you enter into user settings. Still, a very general formula that misses many factors.
    Last edited by slocaus; 05-04-2008 at 09:04 AM.
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  3. #3
    Resident Bigfoot Expert
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    While my method is not too scientific, I looked around for typical values for various levels of biking based on speed and effort and determined what my various workouts *should* be burning. Based on this and looking across the board at my Garmin numbers, I have found that multiplying the Garmin's calories by 0.58 gives a somewhat more realistic value.

    Your results may vary...

  4. #4
    Full of holes
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    Quote Originally Posted by slocaus
    I e-mailed Garmin about this a year or so back. Basically they use an algorithm based on ... age, sex, and bike weight that you enter into user settings. Still, a very general formula that misses many factors.
    What he said. I sent a similar email to Garmin and got the same answer (and the confirmation that the exact algorythm is their intellectual property). My questions were about the forerunner series, and I do remember that the HR data on the 305 is not used in the calorie calculations. I remember this because I was making a choice between the 205 and 305 models at the time.

  5. #5
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    The biggest error in all of the calcs, is that they come up with the energy expended to move you and the bike, fine....

    Then they divide by the thermal efficiency of the body to get the calories expended, this one factor can vary from 20% to 25%, so that one factor can through things off by 25/20 or 25%.

  6. #6
    blithering idiot
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    I don't mind there being an "average" used for differing body types. But, it seemed like the difference in MTB and road riding was going to be a big difference.

    I've used the edge 305 now twice since I got it, once on road and once offroad. The road ride was with my 4 year old son on a trip about 5 miles, averaging about 7mph and total elevation gain of 200ft. This was without the HRM or cadence sensors and it said 221 calories burned. The offroad ride was with the HRM on for 48 minutes, 5.2 miles total elevation gain of 275 and avg speed of 6.4mph and it said 662 calories burned.

    So, in overall specs, these trips were similar. The difference was the road trip was easy and the MTB trip was in tacky, just dry enough to ride soil with Nevegals on a very rocky/rooty trail. Based on information for many websites, calories burned from 1 hour of MTB might be between 850 and 950. The edge numbers from my MTB trip might support that if I had been riding a little faster (it was kind of a goof off ride) and rode for a whole hour instead of 48 minutes.

    The version of SW on my edge is 3.20

  7. #7
    3D guy
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    Quote Originally Posted by SwissBuster
    What he said. I sent a similar email to Garmin and got the same answer (and the confirmation that the exact algorythm is their intellectual property). My questions were about the forerunner series, and I do remember that the HR data on the 305 is not used in the calorie calculations. I remember this because I was making a choice between the 205 and 305 models at the time.
    Not sure if I agree here. I know originally it was not used and Polar has a patent on a HR only method but now if I wear my HR versus not wear my HR the calories are vastly different. I would say for mountain biking the calories with the HR on are very close to what I would see from Polar with the HR off they are undershooting by a lot.

    If someone else has a 305 with up to date software give it a try, HR belt on versus HR belt off got calorie counts.
    Geoff Stahl
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  8. #8
    trail rat
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    Quote Originally Posted by gstahl
    Not sure if I agree here. I know originally it was not used and Polar has a patent on a HR only method but now if I wear my HR versus not wear my HR the calories are vastly different. I would say for mountain biking the calories with the HR on are very close to what I would see from Polar with the HR off they are undershooting by a lot.

    If someone else has a 305 with up to date software give it a try, HR belt on versus HR belt off got calorie counts.
    I can actually compare the same loop with and without HR, as soon as I get the 305 that I left at a friends house. I always ride with my HR, but forgot the strap last Saturday , so I will have that data. I'll post up tonight or tomorrow, since he is coming back to town this evening.
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