Estimating power on the cheap and "big" cyclists

TheGardenGnome

If one were a morbidly obese cyclist and they wanted to compare effort-wise to other cyclists, how would one do it? I'm curious to know if I'm putting in at least as much effort in my commuting as someone else, even though they whizz by me- is there a way to take BMI into account?

My ego rocketed when I found that I was the top 10 in my weight category on Strava (my local area), but it plummeted just as quickly when I discovered only 10 people were in that weight category

super_davo

Weigh yourself and your bike as accurately as possible, and enter into Strava.
Find a long steady uphill segment on Strava and ride it, look at your estimated power.
Obviously very easy to distort e.g wind, drafting, road surface, position etc. but about as good as you'll get on a road.
If you have a turbo trainer, much easier, just look up the power curve and work out based on speed.

Alex_Simmons/RST

If you are morbidly obese, comparing performance with others isn't the issue and attempting to emulate effort levels of those leaner and fitter carries significant risk. Focus instead on your own health improvement.

ck101

super_davo wrote:

Weigh yourself and your bike as accurately as possible, and enter into Strava.
Find a long steady uphill segment on Strava and ride it, look at your estimated power.
Obviously very easy to distort e.g wind, drafting, road surface, position etc. but about as good as you'll get on a road.
If you have a turbo trainer, much easier, just look up the power curve and work out based on speed.

Don’t mean to disagree but I have never seen any reference to weighing the bike and combining with body weight to calculate power, I just input my body weight. Is there something on the Strava site that mentions this?

cgfw201

Ck101 wrote:

super_davo wrote:

Weigh yourself and your bike as accurately as possible, and enter into Strava.
Find a long steady uphill segment on Strava and ride it, look at your estimated power.
Obviously very easy to distort e.g wind, drafting, road surface, position etc. but about as good as you'll get on a road.
If you have a turbo trainer, much easier, just look up the power curve and work out based on speed.

Don’t mean to disagree but I have never seen any reference to weighing the bike and combining with body weight to calculate power, I just input my body weight. Is there something on the Strava site that mentions this?

Enter bike weight in the 'My Gear' section.
Enter human weight in the 'My Profile' section.

Ride up a hill, ideally with constant gradient, longer the better. Not something too steep you have to get in and out the saddle.

Strava will then calculate estimated power for you based on how much power would be needed to get you up the hill in the time you did it.

It's a guess but not usually too far off the mark, all other things considered.

Alex_Simmons/RST

Ck101 wrote:

super_davo wrote:

Weigh yourself and your bike as accurately as possible, and enter into Strava.
Find a long steady uphill segment on Strava and ride it, look at your estimated power.
Obviously very easy to distort e.g wind, drafting, road surface, position etc. but about as good as you'll get on a road.
If you have a turbo trainer, much easier, just look up the power curve and work out based on speed.

Don’t mean to disagree but I have never seen any reference to weighing the bike and combining with body weight to calculate power, I just input my body weight. Is there something on the Strava site that mentions this?

Estimating the energy required to move an object up a change in altitude requires knowing its total mass and the change in altitude. Estimating the power also requires knowing how long it took.

Of the other energy demand factors, energy to overcomes rolling resistance also requires knowing total mass and the coefficient of rolling resistance

The energy demand for changes in kinetic energy also requires knowing the total mass and velocity changes (it also requires knowing the wheel's moment of inertia but these are tiny). Changes in KE for climbs will not be much of a factor, except perhaps if you finish with a somewhat different velocity than you began with.

Then there is the energy required to overcome air resistance, but that is not mass dependent. Rather you need to know the objects coefficient of drag, frontal area, air density, wind velocity relative to the ground in the direction the object is travelling and the velocity of the object.

If you'd like to know more, perhaps read here:
http://www.aerocoach.com.au/math-model/

And to make some calculations for steady state power - speed relationships, you can play with the calculators here:
http://www.aerocoach.com.au/power-from-speed/
http://www.aerocoach.com.au/speed-from-power/

Alex_Simmons/RST

Now it get's quite tricky to dynamically estimate power when all of the inputs are varying.

Steeper hill climbs tend to be simpler because:

i. the altitude change, duration and mass are relatively easy to measure, and

ii. the change in gravitational potential energy dominates the total energy demand (accounting for up to 90% of total energy demand) and and so errors in the other energy demand factor assumptions are diminished, and

iii. it's estimating average power only for the entire duration of the climb, not the instantaneous power value at every moment along the way

ck101

Thanks for that, I never bothered with the bike specific stuff in Strava and wouldn’t have thought it was being used to calculate power, duh.

Good to know, thanks.

zefs

Try this website: http://bikecalculator.com/what.html

Alex_Simmons/RST

zefs wrote:

Try this website: http://bikecalculator.com/what.html

Problem with that site are large assumptions are being made about aerodynamics and rolling resistance.

On flatter terrain aero drag accounts for 85+% of energy demand, so small errors in that result in large errors in estimated outputs.

zefs

It's better than strava which doesn't calculate wind at all. On this site you can do better calculations if you add the headwind value. On today's ride Strava estimates 160watt for my lap/effort, bikecalculator gives 260watt for the same lap, with a 5km/h headwind value which I added. There was 10-15km/h actual wind based on weather forecast but I only had headwind on one side of the lap then tailwind.

Alex_Simmons/RST

zefs wrote:

It's better than strava which doesn't calculate wind at all. On this site you can do better calculations if you add the headwind value. On today's ride Strava estimates 160watt for my lap/effort, bikecalculator gives 260watt for the same lap, with a 5km/h headwind value which I added. There was 10-15km/h actual wind based on weather forecast but I only had headwind on one side of the lap then tailwind.

And neither will be particularly accurate.

For a start you can't just halve the wind value because it was there half the time or as a head/tailwind. The maths of the physics just doesn't work like that.

Wind values reported by weather stations are made with sensors at a standardised height of 10m above ground and in open plane conditions. The wind actually experienced at rider level will typically be somewhat less due to the surface layer velocity gradient. In an open plane condition like the wind meter, then at rider height the wind will be approximately half the reported wind velocity. Then you have ground surface features that will be variable, other vehicles and so on that means the actual wind on a rider varies quite a bit.

What value for CdA does it use? Do you really think CdA is the same for everyone? Because that's what this site is assuming.

It also assumes a drivetrain efficiency of 95%. That's a pretty awful drivetrain!

zefs

I don't disagree but if it calculates more watts it can't be worse
This could be more accurate https://www.gribble.org/cycling/power_v_speed.html

Alex_Simmons/RST

That's nice but it doesn't account for wind. That's assumed to not exist.

Even the most imperceptible wind differences (e.g. +/- 1km/h, which is not even noticeable air movement) can make a 10% difference in the power required to ride at 35km/h.

Reality is if really you want to know your power output it requires a quality power meter.

N0bodyOfTheGoat

Just in case you missed the PSAs in the deals thread yesterday, 4iiii Precision 105 left crank power meters are £341 at Cycle Republic (where in theory you could only get 10% BC discount at London stores) and Merlin (where you can get some Topcashback, plus a chunk of reward points that you could redeem for future % discounts).
~£325 instead of £379 is relatively cheap.

Halfords seem to be running a series on one day only deals, yesterday was turbo trainers (again in the deals thread), you could have bought an Elite Direto for ~£540 with BC discount, which has a well regarded power meter for turbo training only. Deal has finished, now ~£607.