Author Topic: Watts your whatage?  (Read 4396 times)

SoreTween

  • Most of me survived the Pennine Bridleway.
Watts your whatage?
« on: March 12, 2017, 09:19:46 pm »
Is there an accepted means of estimating what watts are needed to achieve a given effort?  Say for example (and there's no reason for this, just a random ficticios example :-X) I wanted to estimate the watts required to propel a 70kg person riding a 15kg bike up 200 meters vertical in 1.2km on gravel (not ingleby incline though mmmmkay) at 3kmh with n inches gear. Is there an estimate method for that sort of question?
2019 targets: TINAT 160 rough
There is only one infinite resource in this universe; human stupidity.

LittleWheelsandBig

  • Whimsy Rider
Re: Watts your whatage?
« Reply #1 on: March 12, 2017, 09:36:33 pm »
http://www.bikecalculator.com might be a good place to start.
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Karla

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Re: Watts your whatage?
« Reply #2 on: March 12, 2017, 10:04:48 pm »
Uses of power you're going to have to factor in are:

1) Power used to accelerate your bike
2) Power used to overcome gravity
3) Power used to overcome air resistance
4) Power used to overcome rolling resistance

Whether (1) is significant will depend on how constant a speed you're riding at.  See e.g. here for the equations.   For a rough estimate, I suspect you're best calculating the power for a constant speed and then treating it as an underestimate
(2) is a simple calculation and will be the majority of the power you exert.  Force = mass * gravity = 85kg * 9.8 N/kg = 850 N (allowing a bit extra for your clothes ;) ).  Work done = force * distance = 850 * 200 vertical metres = 170,000 J.  Divide by your time in seconds to get the power.
(3) will dominate at high speeds but at the speed you're likely to be riding up a 1 in 6 gravel incline, it will be negligible. 
(4) will depend n your coefficient of rolling resistance.  Google around for an estimate, or find someone with a power meter to test it out. 

For a rule of thumb calculation, work out the power to overcome gravity and then add a bit to account for acceleration and rolling resistance. 

Kim

  • Timelord
Re: Watts your whatage?
« Reply #3 on: March 12, 2017, 10:11:11 pm »
And remember that in the real world air resistance is composed of wind, as well as bike's groundspeed.  The difficulty in estimating that, combined with the other factors, means that it's easiest (if prohibitively expensive) to just measure the power.

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SoreTween

  • Most of me survived the Pennine Bridleway.
Re: Watts your whatage?
« Reply #4 on: March 13, 2017, 09:31:35 am »
Thanks, bikecalculator suggests 3kph would be unwise but at a sustainable wattage I should still be moving. Just.
2019 targets: TINAT 160 rough
There is only one infinite resource in this universe; human stupidity.

mattc

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Re: Watts your whatage?
« Reply #5 on: March 13, 2017, 11:18:04 am »
I would have thought rolling resistance on gravel* is way higher than tarmac. But as Farty suggested, in this scecnario overcoming gravity feels like it would still be the vast majority of power required.


*I haven't googled - are there estimates out there for such things?
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LEE

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Re: Watts your whatage?
« Reply #6 on: March 14, 2017, 12:43:51 pm »
I'd think the only way to put numbers around such variables is to create your own data and extrapolate external influences from real data.

Strava does a good job of estimating Watts (if you don't have a power meter). 

Maths will tell you how many Watts are needed to lift a weight a vertical distance.  The remainder is friction, rolling-resistance ..etc.  You can plug that number into other calculations.
Some people say I'm self-obsessed but that's enough about them.

Karla

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Re: Watts your whatage?
« Reply #7 on: March 14, 2017, 12:48:56 pm »
Strava does a good job of estimating Watts (if you don't have a power meter). 

What's your evidence for saying this?

I'd say it does a uselessly awful job at estimating power.  If you really want the pretty numbers, you might as well roll some dice.

Re: Watts your whatage?
« Reply #8 on: March 14, 2017, 02:43:10 pm »
Strava does a good job of estimating Watts (if you don't have a power meter). 

What's your evidence for saying this?

I'd say it does uselessly awful job at estimating power.  If you really want the pretty numbers, you might as well roll some dice.

+1

The estimated figures that Strava shows for segments differ from reality (i.e. the readings from the various power meters I have) so much they're pretty much useless.
"Yes please" said Squirrel "biscuits are our favourite things."

Re: Watts your whatage?
« Reply #9 on: March 14, 2017, 03:19:33 pm »
(2) is a simple calculation and will be the majority of the power you exert.  Force = mass * gravity = 85kg * 9.8 N/kg = 850 N (allowing a bit extra for your clothes ;) ).  Work done = force * distance = 850 * 200 vertical metres = 170,000 J.  Divide by your time in seconds to get the power.

And to finish this off...

1.2km at 3kph is 1440 seconds giving:-

170,000J / 1440s =~ 118W

That's just the power required to lift that weight (~85kg) up 200m in 24 minutes.

Playing around with the bike calculator linked above gives 10W adequate for 3kph on the bar ends (e.g. not particularly aero) on MTB tyres so that accounts for air resistance and rolling resistance along (I assume) tarmac.

So the only other unknown is the rolling resistance difference between tarmac and gravel. Adding another 10W for that (pulling a figure out of thin air) you've got ~140W.

Plugging those figures (75kg, 15kg, 16%, 140W, MTB on offroad tires) into http://www.kreuzotter.de/english/espeed.htm (another calculator) gives 3.4kph.
"Yes please" said Squirrel "biscuits are our favourite things."

Re: Watts your whatage?
« Reply #10 on: March 14, 2017, 04:02:51 pm »
Here is the Strava data for the E2/25 where Dowsett set what used to be comp record.

https://www.strava.com/segments/820299?hl=en-GB

His actual power data shows that he averaged 409w which sounds about right given his pedigree and what he said afterwards.   The estimated power for the riders ranked 3rd and 4th is just closer to 600w.    I think not.

Samuel D

Re: Watts your whatage?
« Reply #11 on: March 14, 2017, 04:11:40 pm »
Regarding rolling resistance on gravel (and grass for that matter), this document (PDF) may be of interest (though the graph on page 7 is clearly mislabelled).

As mattc suggested, the rolling resistance on gravel is much higher than on tarmac.

(On grass, it surprised me that wider tyres had lower rolling resistance. Presumably sinking into soft soil was more harmful than batting away more blades of grass.)

Karla

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Re: Watts your whatage?
« Reply #12 on: March 14, 2017, 04:17:11 pm »
Here is the Strava data for the E2/25 where Dowsett set what used to be comp record.

https://www.strava.com/segments/820299?hl=en-GB

His actual power data shows that he averaged 409w which sounds about right given his pedigree and what he said afterwards.   The estimated power for the riders ranked 3rd and 4th is just closer to 600w.    I think not.

Strava doesn't know your Crr, CdA or the wind conditions, so it guesses the first two and assumes no wind.  This is a fatal flaw as at any speed done by most people who want to know about power, air resistance will be the largest component and the wind speed will be crucial.  I've previously posted two of my 50 mile TTs, done on the same course and the same bike, one a few minutes faster than the other.  The faster activity is 130W less powerful according to Strava, because the wind was stronger that day so while my airspeed was pretty constant, my groundspeed was slow in one direction and fast in the other, which skewed their calculation severely.

SoreTween

  • Most of me survived the Pennine Bridleway.
Re: Watts your whatage?
« Reply #13 on: March 14, 2017, 09:10:45 pm »
I'm firmly in square 1 when it comes to quantifying my ability in terms of watts, I can see already it could get addictive.  I have had precisely one session in the gym at which I maintained 120W for 15 mins and then 125 20 mins.  My heart rate was far above all day sustainable, upper zone 4 then low 5, but I felt I could have repeated that effort twice more with breaks which should do next month.  I was hoping for 135 at the top of Z4, no idea why I latched on to that so I really shouldn't feel disappointed.  At 100W bikecalculator says I'll be at 2.25kph up Ingleby.  And today Deano has posted that the 2nd climb is harder :facepalm:

Its interesting playing about with bikecalculator, I must see if I can reduce that 15kg for the bike.  I hope it was a pessimistic guess!  OTOH stick another 15kg of luggage on and the power requirement goes way way up which explains why I ended up walking so frequently on the Pennine Bridleway.  As for Strava, someone on here mentioned it a while back so I looked.  Even with my pathetic efforts and few repeated routes it was pretty obviously a random number generator and no use as a guide to my sustainable power output.

2019 targets: TINAT 160 rough
There is only one infinite resource in this universe; human stupidity.

Re: Watts your whatage?
« Reply #14 on: March 14, 2017, 10:51:52 pm »
16% (200m climb in 1.2km) is generally more than most people can cope with regardless of the gears they have available.

Even with a 28x30 bottom gear I've often found it faster (and generally easier) to just walk (~5kph uphill).
"Yes please" said Squirrel "biscuits are our favourite things."

Re: Watts your whatage?
« Reply #15 on: March 15, 2017, 06:46:01 am »
My feelings are that counting Watts is an input process, not an output . By this, I mean that if you use power, it is about using what you are producing to provide a specific training effort, or to maintain an achievable level over a course. Once the base line has been measured ( maximum minute power or the - for me more flawed- ftp) then it's about using your zones or %. Obviously this needs to be regularly re- assessed.
Looking back at figures for a ride, and especially " questimates" such as Strava, doesn't allow to adjust your effort during the ride, second to second, which is what, in my view, riding with power is about.

Watts your whatage?
« Reply #16 on: March 05, 2018, 10:10:04 pm »
Help me here please: on a Wattbike yesterday I output 155w for an hour and averaged 20.7mph and tonight a friend of mine outputted 222w and averaged 20.3mph on his Wattbike. How? Why? Is it the cadence? My cadence was around 88rpm. I’ll find out the other bloke’s. It was 85.

I am still working out how power affect everyone else as well as me.

pdm

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Re: Watts your whatage?
« Reply #17 on: March 05, 2018, 10:43:07 pm »
From my brief experiences with a Wattbike and comparing with other users, the "speed" seems to be an estimated derived from the power developed by the rider fudged by a factor dependent on sex and weight.
I suspect it tries to guestimate from these parameters a body size and shape and a rolling resistance to give an overall estimate of resistance to travel on a track and thence a speed dependent on that.
I don't think it is particularly accurate....  :P

Watts your whatage?
« Reply #18 on: March 06, 2018, 07:33:02 am »
Must be! I bet it is entirely possible that you could have the entire room full of them all giving different reading with all the settings at the same settings. Pointless! The bike itself doesn’t know jack shit who you are.

Re: Watts your whatage?
« Reply #19 on: March 06, 2018, 07:47:14 am »
Must be! I bet it is entirely possible that you could have the entire room full of them all giving different reading with all the settings at the same settings. Pointless! The bike itself doesn’t know jack shit who you are.

They're supposed to be very accurate (+/- 2% I think) for power readings.

Derived stats such as "speed" should be taken with a large bag of salt.

Also remember that two rides can easily have the same average power but different speeds. You'll go "further" with a smoother application of power (this is generally the goal of TTers who ride to a power meter) and "less far" if your power profile is spikey or lumpy.
"Yes please" said Squirrel "biscuits are our favourite things."

T42

  • Tea tank
Re: Watts your whatage?
« Reply #20 on: March 06, 2018, 09:25:14 am »
I forget where I picked this up, but it's what I use on my GPXes. Judging by the preferred units it's American but the important bits are in metric.

Ideally, you would apply this in segments to allow for different road surfaces, changing wind-speed & -direction, and shelter from wind due to buildings, vegetation and slope.

Kinda complex:

Terms

    Frl - Force, in newtons, caused by rolling resistance
    Prr - Power, in watts, to overcome Frl
    Crr -  coefficient of rolling resistance - typically 0.004 but can be as high as
           0.008 for bad asphalt or as low as 0.001 for a wooden track.
    g - acceleration due to gravity - 9.8 m/s2
    Wkg - mass of the ride plus bicycle in kg
    Vmps - Veloicty in meters/sec

Formulas

    Frl = Wkg x  g x  Crr
    Prr = Frl x Vmps

Example

Take a rider and bike combined weight of 165 lbs (75 kg) traveling at traveling at 20
mph ( 8.92 meters per second), using Crr of 0.004 and with g being 9.8 meters/sec/sec.
The force would be:

    Frl = 75kg x 9.8 m/s2 x 0.004 = 2.94 newtons.
    Prl = 8.92 m/s x 2.94 newtons = 26 watts

Since the power is proportional to speed, the same rider traveling at 5 mph would
require 6.5 watts to overcome rolling resistance.

Air and Wind Resistance:
Terms

    Fw - Force on rider and bicycle due to wind drag
    Cw - drag coefficient, typically 0.5
    Rho - air density in kg/m .  Depends on temperature and  barometric pressure.
          Some typical values are sea level: 1.226, 1500m: 1.056 and 3000m: 0.905
    Vmps - Speed in meters/sec
    A - effective frontal area of the rider and bicycle in m^2.  Typical value is 0.5.

Formulae

    Fw =  1/2 x A x Cw x Rho x Vmps^2
    Pw = Fw x Vmps

Example

Take a rider and bike combined weight of 165 lbs (75 kg) traveling at traveling at 20
mph ( 8.92 meters per second), with no headwind, using Cw of 0.5, Rho of 1.226 and
front area of 0.5. The force due to wind drag would be:

    Fw = 1/2 x 0.5 x 0.5 x 1.226 x 8.92 x 8.92 = 12.19 newtons
    Pw = 12.19 newtons  x 8.92 m/s  = 108 watts.

If you at traveling at 5 mph, instead of 20 mph then:

    Pw = (1/2 x 0.5 x 0.5 x 1.226 x 2.23 x 2.23) x 2.23 = 1.7 watts

Gravity:
Terms

    Fsl - Force in newtons due to the pull of the rider and bicycle down the slope
    Psl - Power in watts required to overcome the force of Fsl
    Wkg - Combined weight of the rider and bicycle in kg
    g - Acceleration due to gravity, 9.8 m/s^2
    GradHill - gradient of the hill, in decimal, the ratio of the rise to the
    horizontal run.

Formulas

    Fsl = Wkg x g x GradHill
    Psl = Fsl x Vmps

Example

Take a rider and bike combined weight of 165 lbs (75 kg) traveling at traveling at 5
mph ( 2.23 meters per second), climbing a hill with a grade of 12% (GradHill = 0.12).
The force due to gravity would be:

    Fsl = 75 x 9.8 x 0.12 = 88.2 newtons
    Psl = 88.2 x 2.23 = 196 watts.

Combined Forces
Formula

    Total Power = Prl + Pw + Psl   or Total Power = (Frl + Fw + Fsl) x Vmps

I've dusted all those old bottles and set them up straight.

Re: Watts your whatage?
« Reply #21 on: March 06, 2018, 11:25:40 am »
http://www.kreuzotter.de/english/espeed.htm bundles all of that into a handy webpage.
"Yes please" said Squirrel "biscuits are our favourite things."

T42

  • Tea tank
Re: Watts your whatage?
« Reply #22 on: March 06, 2018, 02:18:59 pm »
Yes, but it only applies to short distances.
I've dusted all those old bottles and set them up straight.

Karla

  • car(e) free
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Re: Watts your whatage?
« Reply #23 on: March 06, 2018, 02:43:02 pm »
Eh?  I wasn't aware the laws of physics changed when you rode a long way?

CrazyEnglishTriathlete

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Re: Watts your whatage?
« Reply #24 on: May 10, 2018, 10:02:40 am »
http://www.bikecalculator.com might be a good place to start.

I put my Alpe d'Huez data into this and it came up with a value pretty close to what I was expecting for the power I could sustain for an hour or so (compared with other medical tests rather than power meters).  I'd recommend this.
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