How does Strava calculate calories?

[vickster]

Yeah, agreed, it's all approximate.

But when I was commuting before retiring, my morning journey took anything from 32 minutes to 45 minutes, depending partly on traffic but largely on whether I was in a mood to push myself or not. 50% difference in time equates to considerably more than 50% difference in power, and still more than 50% difference in calories per mile, once you're into the speed regime where wind resistance dominates?

I don’t really care that much how many calories I may or may not have burnt

 

[si_c]

I don’t really care that much how many calories I may or may not have burnt

I do. A lower number means I have to moderate my cake intake. I don't like that.

 

[vickster]

I do. A lower number means I have to moderate my cake intake. I don't like that.

Makes no real difference to me, I’m overweight regardless

 

[si_c]

Makes no real difference to me, I’m overweight regardless

Same here, but I have aspirations towards being as skinny as I was at eighteen.

 

[vickster]

Same here, but I have aspirations towards being as skinny as I was at eighteen.

I’m probably lighter now than I was at 18!

 

[swansonj]

I don’t really care that much how many calories I may or may not have burnt

No problem- it's just that, as a former and still part-time physicist, I just can't resist these challenges😋

Per mile, the difference is small - high effort for short time vs low effort for longer time

That works if speed is roughly proportional to power, which is broadly true at low speeds. But as soon as the speed is high enough for wind resistance to be significant, then even more so when it becomes dominant, i suggest that we're into a regime where speed is not proportional to power, and it ceases to be true - you spend more calories beating the wind than you gain by the shorter time.

 

[roubaixtuesday]

No problem- it's just that, as a former and still part-time physicist, I just can't resist these challenges😋

That works if speed is roughly proportional to power, which is broadly true at low speeds. But as soon as the speed is high enough for wind resistance to be significant, then even more so when it becomes dominant, i suggest that we're into a regime where speed is not proportional to power, and it ceases to be true - you spend more calories beating the wind than you gain by the shorter time.

For speeds where wind resistance dominates (maybe 15mph), resistance is proprtional to speed squared, and energy (calories) to resistance* distance.

So you'd expect the number of calories to be proportional to the square of speed for a given distance ie quadrupling from 15mph to 30mph (should you be able to maintain 30mph!).

 

[si_c]

For speeds where wind resistance dominates (maybe 15mph), resistance is proprtional to speed squared, and energy (calories) to resistance* distance.

So you'd expect the number of calories to be proportional to the square of speed for a given distance ie quadrupling from 15mph to 30mph (should you be able to maintain 30mph!).

Almost, calorific burn for any given distance can be calculated as (effort to overcome resistance) * time. So for a doubling of speed you might see a quadruple of effort, but a half the time, so calorific burn to be around double.

 

[roubaixtuesday]

Almost, calorific burn for any given distance can be calculated as (effort to overcome resistance) * time. So for a doubling of speed you might see a quadruple of effort, but a half the time, so calorific burn to be around double.

That's not actually correct.

Air resistance (force, units Newtons) is proportional to velocity squared (drag coefficient multiplied by fluid density multiplied by velocity squared)
Power (units Watts) is force multiplied by velocity, proportional to velocity cubed
Calorific burn, energy (Work, units of Joules or Newtonmetres) is force multiplied by distance, so is proportional to velocity squared

All assuming that only air resistance is important.

 

[PK99]

For speeds where wind resistance dominates (maybe 15mph), resistance is proprtional to speed squared, and energy (calories) to resistance* distance.

So you'd expect the number of calories to be proportional to the square of speed for a given distance ie quadrupling from 15mph to 30mph (should you be able to maintain 30mph!).

Try punching some numbers in here : http://bikecalculator.com/

using the standard parameters:

100W gives 15.04mph. 3.99mins per mile =23 calories
200W...……...19.74mph 3.04mins...………….=35 calories

 

[Dogtrousers]

None of this really explains the OP's conundrum of a set distance with similar times returning very different figures.
Like the OP I'd have expected Strava's simplistic calculations to be quite consistent.

Maybe one of your rides had one of those inexplicable 800mph "blips" in it or something?

@KneesUp We need more than two data points! Was one of your two an outlier? (As things stand they are both outliers!)

 

[Milkfloat]

None of this really explains the OP's conundrum of a set distance with similar times returning very different figures.

How very dare you, my dartboard explains it all . Seriously, Strava's figures on pretty much anything are educated guesses at best.

 

[roubaixtuesday]

Try punching some numbers in here : http://bikecalculator.com/

using the standard parameters:

100W gives 15.04mph. 3.99mins per mile =23 calories
200W...……...19.74mph 3.04mins...………….=35 calories

They give roughly the same ratios. The relationship I quoted is purely for air resistance and is exactly that used in the calculator, see here http://bikecalculator.com/what.html

They also take into account rolling resistance, hence there is some discrepancy

 

[si_c]

That's not actually correct.

Air resistance (force, units Newtons) is proportional to velocity squared (drag coefficient multiplied by fluid density multiplied by velocity squared)
Power (units Watts) is force multiplied by velocity, proportional to velocity cubed
Calorific burn, energy (Work, units of Joules or Newtonmetres) is force multiplied by distance, so is proportional to velocity squared

All assuming that only air resistance is important.

My understanding is your description of air resistance is correct but in your original post you said that calorific burn would quadruple for a doubling of speed. That was incorrect.

For example, if you travel 20miles at 10mph and burn 320 calories you would expect to burn 1300 if that was the case, but in reality it works out around 750ish according to bike calculator.

 

[roubaixtuesday]

My understanding is your description of air resistance is correct but in your original post you said that calorific burn would quadruple for a doubling of speed. That was incorrect.

For example, if you travel 20miles at 10mph and burn 320 calories you would expect to burn 1300 if that was the case, but in reality it works out around 750ish according to bike calculator.

No, it is exactly correct, assuming air resistance dominates as I said. Read the formulae, it's just algebra.

At 10mph air resistance will not dominate, rolling resistance will.

On the bike calc numbers, using defaults in there (I suspect you may have used kmh).

20 miles at 10mph= 834 calories, 29 W
20 miles at 20 mph = 1182 calories, 82 W
20 miles at 40 mph = 2577 calories, 358W
20 miles at 80 mph = 8147 calories, 2263W
20 miles at 160 mph = 30433 calories, 16907W

Note how the increase in calories approaches the square as velocity increases, and the increase in watts approaches the cube. It's almost exact at the ludicrously high final step.

The rolling resistance from bike calc is a greater proportion at sensible cycling speeds than I would have guessed, but the principles remain.