Calculator your watts/FTP

[DaveQB]

Lots of serious comments here.
All I am interested in knowing is, approximately, what my power output is. Is my FTP near 100 watts or 400 watts. I had no idea. I think I can conclude I am in the 200's, low 200's.
I was just curious. I am not training for anything, cycling is my second sport.
Thanks.

[Xplora]

^^^ I think that's the reason behind the responses - its exactly the same as heart rates. What is the point of knowing you have a low HR or a high HR in sport? You perform maximum efforts and it gets to its max. Eureka? Not really.

[human909]

Which is still within 35% which is still a reasonable estimate under the circumstances.

Reasonable? Totally disagree. Might as well throw some numbers on a board and start throwing darts. If 35% below is reasonable, that means 35% above is also reasonable, right? therefore this example would leave me with a range anywhere between about 180watts and 320 watts. Sorry, but that is in now way going to help anyone.

Which part of "reasonable and estimate" didn't you understand? What part of the original question of obtaining a "ballpark figure" didn't you understand? And clearly the estimates HAVE helped somebody as DaveQB pointed out.

If you want to measure something you measure it. If you want to estimate something you estimate it. They are different things with different needs. For plenty of needs, +-50% estimates is completely reasonable. For most questions with a bunch of roughly defined variables and conditions then that a good estimate.

In contrast if I'm building a steel structure and my 10m beam is 0.5% out then I'm going to have issues when boys have it up in the air on a crane and are yelling down at me the bloody thing doesn't fit.

Lots of serious comments here.
All I am interested in knowing is, approximately, what my power output is. Is my FTP near 100 watts or 400 watts. I had no idea. I think I can conclude I am in the 200's, low 200's.
I was just curious. I am not training for anything, cycling is my second sport.
Thanks.

A couple people around the place are actually 'training' and a couple percent here or there might matter. Most people just pretend they are 'training' and instead are just obsessive about metrics. As you probably know many cycling enthusiasts obsess about their numbers and their performance.

Personally I just prefer to ride in the fresh air, the warm sun and smile.

[uart]

I did the analysis for elite level riders since it's a popular pastime to estimate W/kg from ascent speeds.

Here's a chart showing the error with W/kg introduced by wind:



Note how much error there is even in Light Air on the Beaufort scale, which we can't even feel.

The less elite the rider however, then the less the effect. The higher the specific output then higher the speed and the bigger factor aero drag becomes.

In my simulations with a nominal 250 watts (at the wheel) and a 6% grade I only got a power variation of +/-8% for a headwind variation of +6.5 to -11 km/hr (-ive being a tailwind). As with your simulation Alex, there is a fair amount of asymmetry between the effect of the headwind and tailwind, which is ok because a headwind is more likely to be noticed (remembering that the goal is to conduct this test when there is little noticeable wind).

BTW. I used Crr=0.5%, CdA=0.4 and air density = 1.177 kg/m^3 (T=20C and elevation = 80m) in my simulations.

[Alex Simmons/RST]

The less elite the rider however, then the less the effect. The higher the specific output then higher the speed and the bigger factor aero drag becomes.

In my simulations with a nominal 250 watts (at the wheel) and a 6% grade I only got a power variation of +/-8% for a headwind variation of +6.5 to -11 km/hr (-ive being a tailwind). As with your simulation Alex, there is a fair amount of asymmetry between the effect of the headwind and tailwind, which is ok because a headwind is more likely to be noticed (remembering that the goal is to conduct this test when there is little noticeable wind).

BTW. I used Crr=0.5%, CdA=0.4 and air density = 1.177 kg/m^3 (T=20C and elevation = 80m) in my simulations.

Yes obviously the relativities change when the various inputs are altered although the basic pattern remains. It's not quite as you put it though as it also depends on the relative change in speed, not just the absolute speed at which that change occurs, plus the it's a combination of road speed x relative air speed^2 as well (and of course the variable relative contribution to the energy demand of overcoming gravity, air resistance etc).

Note that the error in power estimate due to head/tail winds in your personal example is larger than for the example in the chart I posted. In the chart a 6km/h headwind results in a 6% increase in power.

[human909]

And the basic pattern remains to be insignificant variations on steep hills... +-6% +-8%? These are irrelevant in the scheme of things. If you are wanting or needing more precise measurements then using calculators to estimate the power is the wrong tool.

For MOST people this is a far more useful measurment tool:

[uart]

Note that the error in power estimate due to head/tail winds in your personal example is larger than for the example in the chart I posted. In the chart a 6km/h headwind results in a 6% increase in power.

Yeah I saw that Alex. In my simulation I used a larger CdA (0.4) and a lower gradient of 6%, which of course makes the aero power more pronounced.

The point I was trying to make is that even with these parameters the non elite rider should be able to get a reasonable power estimate on any day where it's close enough to "apparently" still.

Remember that the elite level rider has more concern with getting this accurate and monitoring even small gains. They are also more likely to be spending big money on their gear, and to splash out on a proper power monitor anyway. For a rider like the OP who only wants a ball park figure then I think this method is quite reasonable.

I take your point though, definitely make sure that there is as little breeze as possible (and in particular not a headwind) to keep the error as low as possible.

[Alex Simmons/RST]

The point I was trying to make is that even with these parameters the non elite rider should be able to get a reasonable power estimate on any day where it's close enough to "apparently" still.

Yep, and is pretty much why I penned this article in 2008:
http://www.bikeradar.com/fitness/articl ... ing-19175/