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Calculating Calories???(2 posts)

Calculating Calories???Gios Rider
Nov 17, 2002 4:47 PM
Does anyone know of a formula for calculating Calories expended based upon duration of ride and averager heartrate during ride? Thanks.
No formula to do what you wantKerry
Nov 17, 2002 5:45 PM
Your heart rate is your heart rate. If you and I do the same ride, and we weigh the same/have the same aerodynamics, then we probably expended the same power at the pedals, unless one of us is very inefficient or has bad/superior equipment. However, we may not have the same heart rates nor the same metabolic efficiency. HRs are much more variable between people than metabolic efficiency, so HR is the lowest quality predictor of caloric expenditure. Best predictor is speed, wind, body mass, and hill grade. The equation is as follows:

calories/hr = [Vg*W(.0053 + %G/100) + .0083(Va^3)]*7.2

where Vg is ground speed, Va is speed through the air (includes head/tail winds), W is bike + rider weight in lbs., and %G is grade in per cent. The factors listed here (0.0053 for friction + rolling resistance and 0.0083 for aerodynamic drag) are obviously not absolute. They will vary with efficiency of the tires and drive train, and with the aerodynamics of the bike + rider combination. Power to overcome friction and gravity is proportional only to rider weight and ground speed. Power to overcome wind drag is proportional to the cube of the air speed. For reference, 1 hp = 2700 calories (because of human metabolic efficiency of 24%); 1 calorie = 0.276 watts; 1 hp = 746 watts. Here, all calories are kg-calories, or "food calories."

Recognize that true air speed is a bit difficult to estimate in anything but calm conditions. You have to take into consideration wind angle (and the change in drag coefficient that comes with it) and the fact that the air speed near the ground is not the same as it is at head level. A simple head wind faced by a cyclist can be considered to be roughly 1/3 of the "posted" wind speed from a weather report (that wind speed is taken 30 feet above ground and away from any buildings, trees, etc.) By definition, the wind speed right at the ground is zero, so a bike sees a zone where the speed is heading toward nothing. Because of this, you should always be feeling a head wind and the more aero your position, the faster you will go. That doesn't prevent a lot of us from sitting up and relaxing a bit when we get pushed by the Giant Hand, but we would be faster if we stayed on the drops. No one has a model that can adequately represent these variables. For this reason, aerodynamic testing outdoors must be done in calm conditions.