hello in this video I'm going to look at the sports analysis tool Strava in
particular how it calculates power even though I don't ride with a force or a
power meter so how does Strava calculate my personal power well power is work
done over time taken and work is Force Times distance so you'd need to know the
force being applied by the bike suppose the fourth the wheels apply to the road
multiplied by the distance move to get my energy use this driver doesn't have
that data I don't have a power meter or a force meter but I do write for
straddle record my GPS data from my Garmin watch so it knows the time and
the GPS location and if I have an altimeter in the watch you can also work
out the altitude or it could get that information from maps so let's see how
Stryver calculates power from that information well there are a couple of
components that I think Stryver uses to calculate power and the first is riding
uphill when you're up a hill of course you gain gravitational potential energy
you climb through a height Delta H in a time frame T and gravitational potential
energy is M times G times Delta H so I will gain some gravitational potential
energy and because power is energy over time Strava can calculate my power
output just purely get M due to the gravitational potential energy gain now
I've given driver my mass details and the mass of my bike so it knows some of
those details already let's take a look at some data then so I did a climb on my
bike and when I click on the data you can click here and export the GPX file
and that gave me this data so it gave me a timestamp like at latitude and
longitude data altitude data and some other data at heart rate it records and
I put those data into a spreadsheet and I'm just going to switch to the
spreadsheet view now and have a look at that in a bit more detail so I'm just
hidden some of those data here but you can see we've got
latitude and longitude data now the first thing I you need to do is
calculate the interval distance that is a formula so if this complex formula I
didn't write I looked it up it's a formula to calculate how far in meters
I've traveled given the latitude and longitude data and it leads to compare
the previous timestamp to the new timestamp of course to know how far I've
gone so that gives me how far I've gone but really what's interesting is the
elevation change so because it's got the altitude data in this column I've
calculated here the elevation change ie from one timeframe to the next how much
how I got and I know the amount of time elapsed so it records a timestamp here
and here I've calculate calculated the interval time so the interval time is
just one timestamp terr subtracted by the previous timestamp and then that
gives you a timestamp in days because it's Excel has the standard unit of time
as a day and then I've converted that into seconds here by multiplying the
time the fraction of a day by 24 hours by 3600 seconds in a day
so in 4.4 seconds I climbed a total of 1.2 0 meters now that means my climbing
power is going to be my elevation change multiplied by my mass about 80 kilos for
me and my bike times by G of 10 is 800 times by 1.2 meters divided by 14
seconds gives in that particular time frame and average power of about 70
watts now sometimes I may be going downhill so what I've done is the
sections where I'm going downhill colored red here I have taken the
climbing power which of course will be negative and then the climbing power up
I've just made this column only positive values of this column ie he did turns
all the negative values into zero and so it's working out my power that I'm
outputting and I'm not outputting any power when I'm going downhill so I've
added up all those later up until the time when I reached
the top of the hill and then that averaged out to a hundred and sixty
seven watts so that is just an average of my climbing up power for that whole
column let's switch back to the presentation view so you can see my
spreadsheet analysis found 167 watts now Strava for the same section of hill
that's here gets my average powered 170 watts so you can see my estimate is ever
so slightly but not much below Strava z' estimate for my power output and that's
because when i'm going uphill at a relatively slow speed i average fifteen
kilometers an hour my power output was almost exclusively from gravitational
potential energy gain but there is a small difference and that's probably due
to the energy that I would have expended in traveling along in us at a constant
speed because ride along the flat would take energy so let's do a little
analysis for ride along the flats - if I'm not going any gaining gravitational
potential energy but I still maintain a power output at the pedals where does
that power go well let's find out let's find out how much power Stryver adds on
when you're travelling on the flat I've come here to a local velodrome I'm gonna
do some laps and then we're gonna see what Strava thinks my power up it is at
a constant speed let's go so I did some laps at five kilometers an hour at
increasing intervals up to 40 kilometers an hour let's have a look at some of
those data this is what Strava has so this was my 10 kilometer hour and Strava
has estimated my average power output at 22 watts for that lap 1533 and so on so
I took various data and then here's a summary so you can see at higher speeds
Strava estimates my power has greater but it's certainly not linear well I
just did an analysis of this data in Excel and you can see a graph of speed
in kilometers an hour against power is a nice curve as a
reasonable curve like this and in fact it
Elle suggests that why either power output is about 0.16 times by the speed
in km/h squared so there's a nice formula summarizing how power power is
calculated by Strava when you're riding on the flat and here's just a
confirmation of that I've squared at the speed data and I was like first into
meters per second and a square of the speed gives a nice linear fit between
that and power output and that corresponds nicely to the drag formula
the drag formula is 1/2 CD CD is the coefficient of drag and it's a number
which depends on the bike the shape that aerodynamic position that you're in
clothing etc Rho here is the density of air a is the surface area so if you tuck
down into a narrow position a will get smaller and V is velocity which you
squared now of course Strava doesn't know the coefficient of drag of my bike
and kicks or the area so it will be an estimate based on an average riding
position so there we go in summary Strava estimates your power output
without a power meter from a combination of your climbing power and the power of
you riding it to speed and climbing power is MGH over T and speed power
seems like Stryver calculates from a formula of about 0.16 V squared where V
is your speed in kilometers an hour have you enjoyed that little analysis