GiuliaG

Hi
I'm new to this forum and to cycling. I started using an ic6 bike at my gym and it show all sorts of intriguing data, eh Watts and 'Levels', and I was wondering how they translate to real life measures, like elevation. Does anyone have an inch bike and know what Levels are? Also, how does the resistance translate into real life cycling? Am I still carrying my weight on a stationary bike? Thank you to anyone who will be so kind as to satisfy my nerdy curiosity

PeteHski

The power you produce (Watts) on the stationary bike will transfer pretty well onto the road. Resistance on the road will vary with many factors including wind, road surface, gradient, tyres, aero, weight and gearing.

BTinNYC 

You can loosely correlate your watts/kg to the real world. For instance, lots of us use a maximum output over 20 minutes to measure cycling strength. This metric is called FTP (Functional Threshold Power, and it's just a good metric, not a holy predictor). My FTP is just under 3 W/Kg. @PeteHski is stronger than that. Grand Tour riders might be in 9-10 W/Kg (Animals!!!).

If your gym bike will give you data for the entire ride, you can do an FTP test on the gym bike, I would do 10 minutes of warm-up, 20 minutes of Maximum Effort, and 10 minutes of warm down. Get the average Watts for the 20 minutes, multiply by .95 and that's a rough FTP. Divide that by your weight in Kg to see how you stack up.

If the gym bike levels are difficulty in pedaling, find the level that you get the most watts over 20 minutes. For me, that's a pedal RPM around 90. For most folks that cadence for max power over 20 minutes is between 80-110 RPM.

GiuliaG

Thank you, BTinNYC, that's a great tip!

GiuliaG

Thank you, PeteHski, so on a stationery bike I am still working to carry my 50 kg weight? It's strange, cause it doesn't ask for my weight when I start using it...which made me wonder also how it calculates my kcals consumed, as those also depend on my weight...(not that I look at kcals, for that matter...)

PeteHski

It’s estimating your calories from power over time. Your weight doesn’t really matter for that. Weight only affects your virtual speed, which will be garbage anyway on a gym bike.

rm -rf

Indoors, on a "smart trainer", the watts come from variable pedaling resistance built into the bike.
Outside, watts usage include 1. wind resistance, 2. elevation gain, 3. drivetrain losses.

Watts from:
Wind resistance goes up by the cube of the speed. For example, using an online bike speed calculator, with the same rider, 100w is 15 mph, 210 watts is just 20 mph. More than double the watts to add 33% more speed! (Even 1 or 2 mph increase is a significant amount of increased work.)
Elevation gain is quite proportional to the climbing speed for a rider -- 20% faster climbing is 20% more watts (ignoring the low speed's wind resistance changes, and power train changes). And climbing watts is proportional to the rider+bike weights at the same speeds
Drivetrain losses are fairly linear.

In summary, food calories consumed by exercise can be fairly accurately estimated from recorded watts and the exercise elapsed time. Calories have been estimated from heart rates and/or road speeds, but those numbers vary depending on conditions, so the accuracy is much lower.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Calories calculated: (The "nerdy curiosity" section, my favorite.)
It's actually kilocalories, but we just say calories most of the time when talking about food energy. It can be fairly accurately estimated from the ride's watts x ride time.

Humans aren't very efficient. Approximately 20% to 24% of the food energy goes to actual work. The rest of the calories burned becomes heat! No wonder we need cooling!

The formulas:
kilojoules = kilowatts * time in seconds
kcals of useful work pedaling = kilojoules *0.24 (a units conversion multiplier)
kcals including heat losses = work kcals / 0.24 approx (the 20% to 24% human efficiency number) We want to know how much food energy was consumed by the exercise, including the heat lost.

Note that the "human efficiency factor of 20-24% kind of cancels out the typical 0.24 kcals multiplier.


~~~~~~~~~~~~~
An example: I did a ride, recorded on Strava. It reports:
Time 1:09:23
Avg power 99 w
kjoules 413
calories 509

Checking the math:
joules: 99w* (69minutes*60seconds per minute) = 409,860. Divide by 1000, kjoules=409.8 Strava rounded off the displayed average watts to a whole number, that's why it's slightly different.
Using 22% useful work: calories burned = kjoules *0.24 / 0.22 = 450 food calories.
That's 13% lower than Strava's estimate. It looks like they use 19.5% as the human efficiency number. Huh, interesting. Or are they adding in some kind of baseline "sitting on the couch" calorie usage? It's not clear.

Anyway, whether it's 509 calories or 460 calories or 530 calories, that gives a good idea of how few calories are burned in real life pedaling. Humans on bikes are very efficient!

~~~~
But, the same ride uploaded to ridewithgps.com shows higher numbers:
kjoules = 434
calories=664. Seems high. I wonder how rwgps calculated this.

GiuliaG

oh it's a bit counterintuitive for me, as I would have thought the more I weigh, the more kcals I burn? But that is obviously a misconception! Thank you, PeteHski!

GiuliaG

Wow, thank you for the extensive reply and for the example, rm-rf! Kcals are all clear!
So elevation gain...is it possible to work out elevation gain from wats, time and speed? For example, I did 1 hour at 100w, at 23 Km/hour....That is really the question I wanted to ask and that my instructor says there's no answer to...mmmh I don't believe him

rm -rf

"1 hour at 100w" and weight and grade is all you need to calculate a reasonably good estimate of climbing speeds. It would be interesting to use the 100w, grade of 0%, and your weight+bike weight to calculate the km/hour speed. Will it match up or not?

This bike speed watts calculator is reasonably accurate. It has two columns, so you can compare different settings.
metric: Bike Calculator
usa: Bike Calculator

Plug in weight, grade%, distance to ride, etc. I rarely use most of the entries -- I just enter weight, grade%, and sometimes hoods vs drops for aero improvements.
Then I just plug in different watts to see the results for each watt choice.

It calculates speed, elapsed time for the entered distance (which I usually just ignore -- I'm just looking at Speed on the results), and calories.
Elapsed time in the real world will be longer -- climbs have variable grades that can never be as fast as a steady grade.
Ha, calculated weight loss -- but I usually offset my burned calories with food after the ride...

GiuliaG

Ah but I have speed but no grade%: the grade% is what I would like to work out!
Offsetting the burnt kcals with food is what we cycle for, right

zacster

I use RWGPS as my main tracker and I can calculate the calories in a spreadsheet that matches their calculated value exactly. They use 25% as the human efficiency number, but when I plug your numbers in I get a much lower number for calories, 394. kjoules should always roughly equal calories as the .239 and 25 mostly cancel out. Something isn't right there.

B20×(B21+B22÷60)×60÷1000×.239006÷.25
Where
B20 = avg watts
B21 = Minutes
B22 = Seconds

The calc is a little convoluted as written only because I originally wrote it just for minutes, but then included the seconds for accuracy. It is Watts * (minutes + seconds/60) then times 60 again to get to seconds to get to joules. Divide by 1000 to get kJoules, then apply conversion to Calories and factor in human pedaling efficiency.
413 kJoules is right, but the Strava efficiency looks to be about 20%, where the RWGPS numbers you get use 15%. Is that number part of a setting somewhere that it could be so different? I don't remember that, I probably wouldn't have known what it was anyway as I've been using it for years an only recently looked into the calc.

Is your power accurate? Are you using a power meter or is it estimating power based on HR? When I ride my commuter bike without the power meter it uses HR to estimate calories and power and it is way off. In order for it to be closer to accurate you need VO2 Max.

rm -rf

I occasionally glanced at ride calories, but I don't pay attention to them.
I have a Stages left crank meter, that both matches up reasonably well with my Kickr's watts, and with online bike calculators for watts on steady climbs.

Todays' ride on rwgps: 1172 kjoules, 1479 calories. Still high calories.

Browsing my profile, I see this note, and two links:~~~
I'm 70, with the corresponding lower heart rates. I skimmed the calories vs heartrate paper (which is from 2005), and noticed a formula that takes age into account. perhaps that's why? I'm not sure why old riders would burn more calories for the same watts output -- generally less efficient in the food calorie conversion?
That's all I know.

rm -rf

Use the calculator, enter your watts and your weight and bike weight. Leave everything else as default.
What speed does it calculate?
Now plug in different grade percentages to see what grade is near your target speed (23 km/sec) from the bike trainer data.The calculated speed will either be above or below the target 23 km/sec, so adjust the next grade percentage up or down. You'll get close enough after a few tries.

~~
That's an unusual calculation. More often, a cyclist wants to know what effort to use on a specific hill. That depends on the grade and the length of the hill -- short climbs at high watts, very long climbs at steady state low watts.

PeteHski

The speed indicated on your gym bike will just be a generic estimate on a flat road for the power you are producing. Put your power and weight details into Bike Calculator as suggested and it will translate into an approx speed for any slope you input. I find it quite accurate.

PeteHski

The more you weigh, the slower you climb for a given power output. But your calorie burn is related to power over time.

RChung

Hmmm. There are methods to estimate total elevation gain from power, time, and speed. You do need to know total weight and some assumptions about the tires you're using and the contribution of aerodynamic drag but the calculations work (surprisingly to me) pretty well. Pretty damn well. I call this "virtual elevation" but other people have other names for this method.

GiuliaG

Thank you!!

GiuliaG

Ok it make sense, thank you! When I do squats etc, my heart rate took will need my weight in order to calculate the kcals, but on a bike I have watts so weight doesn-t matter!

GiuliaG

This is a great forum with a great bunch of people

zacster

For this reason you do burn more calories the more you weigh, if you are riding a target distance at a target power on a trainer in something like Zwift. In the real world, wind and road resistance can kick in too and vary the results.

PeteHski

Yes, simply because it takes longer to complete the target distance. But if you are on a gym bike then you are most likely to be riding for a fixed length of time eg 1 hour, rather than a virtual distance. So your weight won't matter. Weight only matters when you start riding outdoors over a specific distance and elevation. Power and duration are what determine your energy burn in all cases. On a gym bike, virtual distance, speed and elevation don't matter.