Questions about power calculations

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Questions about power calculations

Postby silkishuge » Wed Mar 17, 2010 1:39 pm

Hi,

I decided to log in two of more recent timed climbs up two hills on Cycle2max to determine how much power I was exerting.

There are two hills.

Hill number 1: 4.0 km distance, 150m elevation. Power calculated was 296 W, time 9min 36s (myself = 66kg, bike + accessories = 11-12 kg, ~2-3 kg backpack)

Hill number 2: 2.94 km distance, 150m elevation, Power calculated was 255 W, time 9min 6s (myself = 66kg, bike + accessories = 11-12 kg, ~2-3 kg backpack)

First question: should the gradient of the hill affect how much power I am able to put out?

Second question: how reliable are these equations and what is the basis? Empirical? They don't look very theoretical

I don't normally fuss too much about power and just ride as hard as I can on any given day but just thought it would be interesting to know how much power my chopstick legs are able to put out. Looking at those numbers though, it seems tiny compared to pros. :oops: :oops:

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by BNA » Wed Mar 17, 2010 1:51 pm

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Re: Questions about power calculations

Postby DanielS » Wed Mar 17, 2010 1:51 pm

First question: In my experience, sort of. I find very steep hills are harder to maintain a steady effort on.

Second question: I think the equations are pretty reliable, they're more reliable the steeper the hill is. If you look at the equation that c2m uses, there are two components: an air resistance component and a gravity component. For riding up a hill, the air resistance component is generally low, and its the less accurate of the two. Its a theoretical equation (just sum of energies), with some empirical values thrown in for the air resistance component.

And don't compare yourself to the pros, ~300W for a 66kg rider for 10 mins ain't bad at all!
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Re: Questions about power calculations

Postby twizzle » Wed Mar 17, 2010 2:05 pm

If you can stay in your 'comfortable' cadence, then gradient doesn't matter too much - although (obviously) the higher the average speed, the more aerodynamics starts to play a role.

FWIW, my powertap showed a recent climb at just over 400W but Hillclimbr (spin off from cycle2max) showed ~360W when I plugged the number in.

At the end of the day, the actual power doesn't matter too much, it's the repeatability of the climb - if you are faster, then you are fitter. Power becomes useful when you use it for targeted training - and for that, you really need a power meter.
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Re: Questions about power calculations

Postby silkishuge » Wed Mar 17, 2010 2:50 pm

Thanks for the advice. I have only timed myself the last two weeks and it usually takes me around the same amount of time each time I do it with +25s on my slowest climb so far, but I was using a cyclocross bike with 30mm tyres and 90 psi rated tyres because of the rain.

twizzle, I don't really do repeats because it just a commute to and from work. I don't really train much anymore since I left Melbourne. But wow, 400W, that sounds like the kind of power a pro rider would accomplish. Thats great!! :D
Thanks for the encouragement Daniel. I might check and see how much I have improved in 6 months time using this same route each day. I might cut down the time from experience of the climb as well.

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Re: Questions about power calculations

Postby toolonglegs » Wed Mar 17, 2010 4:00 pm

I think the calc are pretty accurate IF you weights and measurements are near perfect...if they are not then the numbers get skewed pretty quick.
I tried them before I got a power tap...and I have some perfect climbs here for working out ftp power...ie:constant 5% plus and 20-30 minutes long.It over calculated for me by a fair bit.
Xavi Tondo did 24 minutes at 403w for his winning stage in Paris Nice last week...that was starting at 175km's into the stage after 5 days racing at about 67kilos...there is us and THEM!.
If you have power reading software you can check it out here.
http://www.tamalpais.es/wp-content/uplo ... -tondo.csv
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Re: Questions about power calculations

Postby twizzle » Wed Mar 17, 2010 6:07 pm

silkishuge wrote:Thanks for the advice. I have only timed myself the last two weeks and it usually takes me around the same amount of time each time I do it with +25s on my slowest climb so far, but I was using a cyclocross bike with 30mm tyres and 90 psi rated tyres because of the rain.

twizzle, I don't really do repeats because it just a commute to and from work. I don't really train much anymore since I left Melbourne. But wow, 400W, that sounds like the kind of power a pro rider would accomplish. Thats great!! :D
Thanks for the encouragement Daniel. I might check and see how much I have improved in 6 months time using this same route each day. I might cut down the time from experience of the climb as well.

J


Lol, I meant 'repeatability' as in being able to repeat the test at a later time. But yes, I usually do repeats... which means a little bit slower. My lactate threshold is ~ 360W, or 3.49 W-per-kg which is an unimpressive number compared to elite pro's who are up closer to 7 W-per-kg. But... I have good power-to-frontal-area, so it's not all wasted. I compete with people who can leave me for dead up hills but struggle in lower grades for criteriums.

But... in another 11 days the summer race season will be over and I can spend the winter losing the 10kg I've been talking about for the last year, which should put me at 3.9W-per-kg even before I start working on the power. :)
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Re: Questions about power calculations

Postby toolonglegs » Wed Mar 17, 2010 7:44 pm

You still at 103kgs then Twizzle...the day I do a 20 minute at 4 watts per kilo will be a very happy day!...but it is coming in about 5kgs or less...the day I have an ftp of 4watts per kilo I will be extremely happy!.
On the pro side...there would be very very few pros with an ftp of 7wpkg....maybe 1...most are not much over 6....pass the stomach staples :lol:
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Re: Questions about power calculations

Postby twizzle » Wed Mar 17, 2010 7:49 pm

Yep - and four events to go before the end of the season. Then I get serious about weight loss instead of maintaining fitness.
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Re: Questions about power calculations

Postby silkishuge » Thu Mar 18, 2010 8:35 am

Getting quite technical. I have no idea what my lactate threshold is and I guess for myself and riding, I don't find that important for me to monitor..... yet :wink:

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Re: Questions about power calculations

Postby Alex Simmons/RST » Thu Mar 18, 2010 4:50 pm

toolonglegs wrote:On the pro side...there would be very very few pros with an ftp of 7wpkg....maybe 1...most are not much over 6....pass the stomach staples :lol:

Actually there would be none. To do so would require the extremely unlikely combination of:

- Gross Metabolic Efficiency of 25%
- A VO2 max of 90ml/kg/min
- An ability to ride for an hour at 90% of VO2max

twizzle wrote:My lactate threshold is ~ 360W, or 3.49 W-per-kg

Lactate threshold is a measure of blood lactate concentrations, which can/is done in different ways (some better than others), and not a measure of power.

Power output at LT can typically be sustained for several hours and is ~10-15% less than Functional Threshold Power (power one could maximally sustain for ~ 1 hour).
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Re: Questions about power calculations

Postby Ant. » Thu Mar 18, 2010 5:53 pm

toolonglegs wrote:You still at 103kgs then Twizzle...the day I do a 20 minute at 4 watts per kilo will be a very happy day!...but it is coming in about 5kgs or less...the day I have an ftp of 4watts per kilo I will be extremely happy!.
On the pro side...there would be very very few pros with an ftp of 7wpkg....maybe 1...most are not much over 6....pass the stomach staples :lol:


twizzle wrote:Yep - and four events to go before the end of the season. Then I get serious about weight loss instead of maintaining fitness.

:lol:
Tsk tsk, you pair :lol:

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You *can* lose weight without losing sustainable power. I'm 2kg lighter than I was 2 weeks ago and my 20min power in that time has gone up too. n=1 is fine if that 1 is you :D (though I actually know 2 others that dropped 1kg per week which didn't hurt their threshold. One guy actually dropped 4kg in the month leading up to nationals :shock: )
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Re: Questions about power calculations

Postby toolonglegs » Thu Mar 18, 2010 6:06 pm

I have lost 8-9 kilos since Jan 28th...I reckon my 20 minute power is now about 375 but I am not going to be testing it for a couple of weeks as I want to win on the next two Saturdays :D .I did 365w for 16km on Monday...at the end of a hard four days.
And on the plus side I am not even close to best form yet :D ...hehe...my ftp was set at 335w on WKO+ just 2 weeks ago :lol: .
Last edited by toolonglegs on Thu Mar 18, 2010 6:07 pm, edited 1 time in total.
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Re: Questions about power calculations

Postby twizzle » Thu Mar 18, 2010 6:06 pm

I said 'closer to 7W-per-kg', because I'd see numbers quoting around 6.6 - 6.7 W for Lance...

Alex Simmons/RST wrote:
twizzle wrote:My lactate threshold is ~ 360W, or 3.49 W-per-kg


Lactate threshold is a measure of blood lactate concentrations, which can/is done in different ways (some better than others), and not a measure of power.

Power output at LT can typically be sustained for several hours and is ~10-15% less than Functional Threshold Power (power one could maximally sustain for ~ 1 hour).


It was easier than saying "My FTP is at ~360W" and having to explain FTP.

But Coggan uses the terms Lactate Threshold and Threshold Power interchangeably here, and his Threshold Power is the one hour value... I took it to mean that FTP is LT power without the lactate having been formally measured.

Where is the 10 - 15% difference between LT & FTP mentioned? I've never seen that documented anywhere.
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Re: Questions about power calculations

Postby Alex Simmons/RST » Thu Mar 18, 2010 8:10 pm

twizzle wrote:I said 'closer to 7W-per-kg', because I'd see numbers quoting around 6.6 - 6.7 W for Lance...

Not for an hour.
twizzle wrote:But Coggan uses the terms Lactate Threshold and Threshold Power interchangeably here, and his Threshold Power is the one hour value... I took it to mean that FTP is LT power without the lactate having been formally measured.

Where is the 10 - 15% difference between LT & FTP mentioned? I've never seen that documented anywhere.

he uses them interchangeably in the sense of what they mean conceptually and the physiological adaptations that occur/ are induced.

It's mostly confused terminology. what you are referring as LT, is probably more likely OBLA (onset of blood lactate accumulation) or MLSS (maximal lactate steady state) or sometimes referred to as IAT ("individual anaerobic threshold"). LT is an initial inflection/rise in the BL - typically 1mmol/L above resting levels or ~ 2.x / 2.5 mmol/L. this is significantly below MLSS/OBLA or the BL levels experienced when riding at TT effort levels. This is pretty standard ex phys definition.

Unfortunately BL levels are something that many have attempted to "redefine" in various ways. But in essence, they are all very closely related - power improvement at one level, is very closely correlated with power improvement in the others - IOW they all have similar ability to predict performance.

here is some info on what Andy says about LT (he mentions that LT most likely occurs somewhere down in low level 3 in Coggan power level terms):
http://groups.google.com/group/wattage/ ... c163b409c7
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Re: Questions about power calculations

Postby toolonglegs » Thu Mar 18, 2010 8:51 pm

Twiz,we would make a good team for my Saturday race :D ...67km on flat windy course...2 tanks off the front :lol: ....everyone else in my club is a wippet and the talk of this course which is always windy seems to scare them!.
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Re: Questions about power calculations

Postby twizzle » Fri Mar 19, 2010 12:31 pm

I wish my next race was the same. :( 42km of hills, including some 10% grade stuff.


Soooo hungry. :( 351km so far this week.
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Re: Questions about power calculations

Postby twizzle » Fri Mar 19, 2010 1:05 pm

Alex Simmons/RST wrote:... It's mostly confused terminology. what you are referring as LT, is probably more likely OBLA...


Thanks for the headache. :evil: :P

Can't read the article until I join the wattage group... but seriously, you would think they have the terminology worked out!

O.K. - from now in I'll just say I've done do 2x20 at 373w... least ambiguous statement I can think of.
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Re: Questions about power calculations

Postby Alex Simmons/RST » Fri Mar 19, 2010 5:01 pm

twizzle wrote:Can't read the article until I join the wattage group... but seriously, you would think they have the terminology worked out!

it happens. e.g. most people, when asked about Critical Power, think of the bastardised terminology that Joe Friel uses rather than the accepted ex phys definition.

that's why power is so good as a measure of performance - it's unambiguous because it's a measure of what you can actually do.
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