gbg

Well I just put my Powertap on my MTB and went for a quick 1h 20m shake down ride. The thing is awsome, Since I am a guy I didn't read any instructions just fired it up and took off.
It seemed to read pretty low but steadily increased to around 160 WATTS, but I couldn't seem to get much higher even when I pushed alot harder. I thought wow 200 watts must be
hard to get to. So I stopped and looked at the display and noticed it was AVERAGE WATTS. I immediatley set it to real time WATTS and took off again. Now I was consistantly riding at
220-250 at about 17mph. In 80 minutes at a pretty decent pace, the Powertap indicated I burned 800Kj which relates to 800 calories or about 500-600 cals per hour.

The training with power literature also states TDF riders on a 5 HOUR stage burn about 4000 cals. Thats about 800 cals/hours (24mph+ avg??).
So as I suspected getting over 700 cals per hour would be very hard. And these 1300-2500+ cals per hour estimates are pure fantasy.

I was surprised how fast the WATTS rise even on a moderate climb. I went under a bridge and pushed it up the "on ramp" to the trail and hit 1034 WATTS. This
was in the saddle not near a maximum effort, but pushing 250+lbs up a hill does take some WATTAGE. I will have to try some maximum effort sprints just for fun.
I was really impresssed with the speed of the updates on the read out.

So 500-700 cals per hour seems to be about right (for pretty hard effort).

Wogster

One problem in your calculations, 800Kj isn't 800 calories, it's 191 calories. That means your looking at 143 calories/hr, not 500-600. Yeah 700, 800 these numbers are going to be extremely hard to get to, without spending the majority of your time in training, as most top seed professional riders do.

I think a lot of times when we get calorie readouts on GPS units and bike computers, the rather generous estimates are part of the marketing ploy for these devices. Think about it, you sell a bike computer for $25, add in a calorie display (about 5 minutes worth of programming) and you can sell that unit for $75! Now in order to make that computer worth the extra $50, you want the estimates to be rather generous, because you don't want Tony Ten-ton to go out for his minute long ride to McPukes and back, to look at the display and get a .2 flashing back, now if the machine flashes back 200 or so, then Tony feels good, even though the break at McPukes added on about 20 times even that number.

The Powertap on the other hand is a training tool used by athletes in training, it doesn't have to lie, so it gives you the actual amount of energy at the hub, and when your losing weight knowing that you need to burn about 32,000 Kj for a kilogram of weight loss, is disheartening, but probably accurate. You see the bicycle is just too energy efficient.

ochizon

For what it is worth, I have been working on weight loss for the last 2 and a half months.

Eating has been releatively strict, but VERY consistent. Anyway, here are numbers that I have noticed over the last 4 weeks:

Week 1 - 75 miles ridden, 2lb weight loss
week 2 - 35 miles ridden, 1lb weight loss
week3 - 145 miles ridden, 4lb weight loss
week4 - 70 miles ridden, 2lb weight loss

now, this is far from scientific, but a pattern seems to be emerging. When I am consistent on a strict diet, I lose weight, apparently, at a rate of roughly 1lb per 35miles.

Im not too sure of the relevance to this conversation, but figured I would share.

BTW, im currently 275lbs, to give you an idea of the load.

ochizon

question: is that output only the torque that the crank sees? If so, wouldnt there my more to the "calorie" story than that? You leg muscles are consuming those calories to pump the pedals, but what about the energy your heart is consuming from pumping, or your diaphram from pumping your lungs, or your core from supporting your upper body? Are these included?

I think the calorie thing is very complicated.

dscheidt

Humans are about 20 to 25% efficient, most people are in the 22 to 24% range (including top athletes. This number doesn't change much with training, just the ability to make more power.). 24% gets used a lot, because it makes numbers that are easier to work with. If you know how much power you're putting out, it's fairly straightforward to go from that to calories expended by the rider. The trick is to get to know how much you're putting out. power meters are expensive!

So his 800 kJ at the wheel (we'll assume a perfectly efficient drive train) is about 3300 kJ out of the human. 3300 kJ is 788 kilo-calories (the sort we count).

Tour de France riders on the long flat stages, averaging about 250W, which for six hours works out to about 5700 calories. In time trials (which are much shorter), they average about 350 W. I don't have numbers for the climbing stages, but I bet they're very high for the winners.

gbg

Bingo!!! Even though 800Kj is about 191 calories, since we are 25% efficient (3/4 of our work goes into heat ,respiration etc), this nice little fluke in human biology makes 1Kj = approx 1cal.
So 800Kj is about 800cals.

gbg

Well I would suggest your figures imply you are losing a lot less than 1lb/ 35 miles.
The only way this would be true is if you were on a 100% calorie neutral diet.
I assume if you did not bike you would still lose some weight, lets say 50% of what you have.
Therefore you would be at 1lb/70 miles. if you avg 15mph (for argument sake) so 3500cal/4.66hrs or 750 cals/hour (and I would think this is high).

The really dissapointing thing I noticed was how fast WATTS drop off at 16,15,14 MPH. I only rode once (raining today)
but I think at 14mph I was around 100 WATTS. So this would mean I would be under 200cals/hr.
And if I were lighter it would even be less.

dscheidt

aerodynamic drag is the big factor. In general, it goes up with the cube of speed. There's a reason most non-racing riding is done at a fairly narrow range of speeds, with 20 mph being the top end of that. At speeds up to about 10 mph, aerodynamic drag is low, and is out weighed by the weight and rolling resistance of the bike. it takes something on the order of 50 to 75 watts to cruise at 10 mph on most bikes. At above 10 mph, air resistance becomes a factor, and going faster than 15 mph or so is real work, and feels like it.

ochizon

This is absolutely true, but why I find the numbers so strange is linear nature of the mileage/weight loss curve. If I were in a calorie deficit of 3500 on the week with out riding, I would lose 1lb a week. If, as you say, I am burning 3500cal/70miles, then at 35 miles on a week it would be 1.5lbs lost, at 70miles it would be 2lbs, at 140miles it would be 3lbs, at 210 it would 4lbs, etc. But it doesnt seem to work out that way.

Again, its far from scientific data, but all I really mean to imply is that this is far more complicated than what HR meters, power meters or rules of thumb can work out.

DogBoy

I find this interesting conversation. I've been using my Garmin to estimate caloric deficit and it has been fairly accurate in predicting weight loss.

My Daily metabolic rate is somewhere around 2100 calories and so is my diet. Garmin tells me I burn about 1300 kcal/hr. This is average between climing/flat/windy/calm rides, and my avg HR is typically around 155. I consume about 220 kcal/hr in food/drink while on the bike in addition to my diet plan. So now I take a look at a typical week. 1.5 hr ride on Monday, 2.25 hr ride on Wed, 1 hr ride on Thurs and 6 hr ride on Saturday.

My net kcal (a/t Garmin) is 1080 per hour, and my weekly total is about 9-10 hours. That's 9720 to 10800 cal deficit per week. If I assume 3500 kcal/lb, I should be losing 2.7 to 3 lb per week.

I've been doing this for 11 weeks. My total weight loss to date? 33 lbs.

socalrider

Those are very high numbers.. 16cal per minute is possible at 90% of MAX HR or above..

You are burning 18cal per minute at around 80-85% of max hr.. That is too high..

dscheidt

That's the caloric burn from roughly 300 W power output. That's not really impossible for an hour or so, for someone that's in decent shape. Over six hours, no way, outside pro riders.

gbg

I agree. DogBoy stated "My net kcal (a/t Garmin) is 1080 per hour" would mean you are burning more calories riding than your average TDF rider in a TDF stage.

So you are saying 100% of your weight loss is from cycling??
I would suggest as before, more than 50% of your weight loss is from your diet and the other is from cycling.

This does drastically reduce the amount lost due to cycling, but if you look at it that cycling is doubling your weight loss
thats still pretty good. Cutting your cycling weight loss in half reduces your 1080/hr to 540/hr which I think is much more reasonable.

mtalinm

I put my ideal weight instead of my actual weight -about 100# less - into gamin and now it gives me what seem to be accurate calorie burns. I say they are accurate because. I have been tracking what i eat carefully and the difference between intake, basal metabolic burn, and exercise burn matches my actual weight loss almost exactly.

gbg

And what does the Garmin state as cals/hr?

mtalinm

The low 700s, or about 50 per mile. At my weight (280) it seems about right at least when calculating from those tables at the top of the thread.

gbg

Actually I found this caculator to be pretty close to my Powertap data (granted I only did 1 ride).
But I think I will see how it fares with more rides/data.

https://bikecalculator.com/veloUS.html

DogBoy

Not really saying anything other than the discussion surprised me since my weight loss was tracking so closely to what the Garmin was reporting. I think it probably makes sense what you are all saying, I just think its odd that the diet alone was keeping my weight steady, but when I started cycling, my weight loss corresponded to the numbers the Garmin was putting out. Is Garmin trying to estimate the post-workout increase in metabolic rate also? Again, not trying to say Garmin is accurate, just that the math worked for my situation.

gbg

Some things I noticed when you see your power output.

"FOR ME" (I know some points seem obvious)


1) Preceived effort is 95% dependant on cadence.

I seem to have a cadence (about 90) that I am comfortable with.
If I am travelling at 27kph (About 16.9mph) at 220-250 WATTS and I hit a slight uphill (I mean slight 1 degree maybe) I will maintain 27kph
while my power goes to 300-320. When I hit the flat and slight downhill after I will maintain 27kph and my power will be 160-180.
While 160-320 is a 100% difference it only feels like 10-15%. For argument sake if my max power is 1600, 160 is 10% so that might be why
it feels like 10% harder and not 100% harder.

2) If my cadence is at 90 and my power is 180, and my speed is 24kph, increasing my cadence to get to 27kph seems
WAY harder than grabing 1 more gear and maintaing 90 rpm.

3) Trying to maintain 32kph instead of 27kph is WAY HARDER than I would like it to be.
But I think part of this is due to my 11-34 cassette , the jumps in ratio are just too big. I think
I am going to get an 11-28 and see if this hits some better ratios so the jumps aren't so
drastic.

4) The second 33 miler of the day is WAY harder maintaining 220-250 WATTS then the first.

5) It is easy to FEEL you are working as hard when your power is down 10-20%. In my case you
definitley have to feel like you are pushing a bit to maintain 220-250. Even the slightest easing up
in the legs can take me down under 200 (but like I say it feels almost like 220-250).

Dr. Banzai

Again, this calculator spits out 1117 kcal for me riding in a 10kph head wind for 40kms at 30kph. Following ALL the above it seems high.

I'm desperately working hard to train right, eat right and drops some weight. Fat and excess muscle. Tired of being a Clyde. But to have a functional life with friends and wife I have to monitor intake and burn with a finely tuned eye. As we all do.

I think I'll just use this calculator and just roll with it. Worry about my intake and cycling and just deal with it.

pacificaslim

For those of you who've done the math, do you burn a different amount of calories per mile at different power outputs? Or is it constant and the only difference being, obviously, the amount of calories used per unit of time (1 hour for example) because more miles could be ridden in the time period.

mtalinm

I don't know for sure, but if you look at the original post you will see a chart of calories burned at various speeds. They certainly do differ per hour, but they are fairly constant per mile except at very high speed.

This matches my intuition, and because I don't ride at very high speeds I find it more useful to use calories per mile instead of calories per hour.

dscheidt

You do. As speed increases, more energy is spent fighting aerodynamic drag. In general power consumption to over come aerodynamic drag increases with cube of speed. (It's much more complicated than that, but that's close enough for us). Most other energy costs are fixed or increase rougly linearly with speed (rolling resistance, energy lost to bearing friction), and are totally overwhelmed by air resitance by about 15 mph. So it it takes 100 W to go 15 mph, it will take something close to 600 to go 30. 100 W for an hour will burn roughly 350 calories, and get you 15 miles, for 23 calories/mile/ To get 15 miles at 30, you only need to ride half an hour, but you're still burning 300 Whr, about 1050 calories, about 70 calories/mile. (and if you can do that, there's a pro team looking for you...) Going from 15 to 20 wouldn't be as dramatic, but it would be noticeable.

mtalinm

Sounds like we agree...speed makes a difference in per-time calculations of calories burned, but not really for per-mile unless you're going really fast

Which is why I use 50/mile

socalrider

If you are averaging 20mph then that means you are burning 16.67 cal / minute.. Still a little too high for just 20mph average speed.. I would figure for most recreational cyclist who average 15mph or 4min / mile - 30 to 40 cal / mile would be a more appropriate range.