How accurate are online calculators for power/speed?



dot

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Apr 28, 2003
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I don't have a PM and don't want to buy it since I'm keeping my tools simple and cheap but I wonder how much power per kg I need to keep 40 km/h on a road bike with non-aero wheels in the drops? I see there aresome online calculators, but I think they might be pretty off.

I have a Tacx Satori trainer, there are power curves available for it, both from Tacx and users with PMs, at the moment, according to them my 20 min power is only 3.2 W/kg after a month of riding (and used to be closer to 4.2 but it's only an estimate). I've been completely off the bike for 1.5 years though kept maintaining fitness with some running and gym work.
 

Mr645

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Jun 22, 2013
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At least with Strava, it's accurate to within +/- 75% or so.
Today was a perfect example. Heading North, into a 20+ mph headwind plus guests took over 300 watts to pull the pace line at 18mph, while heading back with the tailwind, 23mph was in the 170-180 range. Only the strongest 10 or 12 out of 40 riders rotated into the front. On the way back the group split as some riders were moving at better then 26

Strava would have no way to know what the conditions are and would have reported that the 23-24 speeds heading back required more power then 17-18 into the wind.
 

dkrenik

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Dec 5, 2003
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Originally Posted by dot
I don't have a PM and don't want to buy it since I'm keeping my tools simple and cheap but I wonder how much power per kg I need to keep 40 km/h on a road bike with non-aero wheels in the drops? I see there aresome online calculators, but I think they might be pretty off.
You mention that you're basically doing this for recreational purposes. Pick the on-line calculator you like best, make appropriate assumptions, and it should be accurate enough for your pruposes.
Originally Posted by dot
I have a Tacx Satori trainer, there are power curves available for it, both from Tacx and users with PMs, at the moment, according to them my 20 min power is only 3.2 W/kg after a month of riding (and used to be closer to 4.2 but it's only an estimate). I've been completely off the bike for 1.5 years though kept maintaining fitness with some running and gym work.
I wouldn't bother with power curves in your situation. Just use speed as a proxy for power on your trainer. As long as you're improving, that's what it's all about - right?
 

AyeYo

Active Member
Mar 21, 2014
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Originally Posted by Mr645
At least with Strava, it's accurate to within +/- 75% or so.
Today was a perfect example. Heading North, into a 20+ mph headwind plus guests took over 300 watts to pull the pace line at 18mph, while heading back with the tailwind, 23mph was in the 170-180 range. Only the strongest 10 or 12 out of 40 riders rotated into the front. On the way back the group split as some riders were moving at better then 26

Strava would have no way to know what the conditions are and would have reported that the 23-24 speeds heading back required more power then 17-18 into the wind.
Just to play devil's advocate... if you're doing a roughly round trip or up/back ride like that, wouldn't the errors basically cancel out? It's going to grossly underestimate power going into the wind and grossly overestimate power going with the wind. Average power output for the ride should be more or less unaffected though, right? That's how I read Strava anyway (plus a few grains of salt).
 

tomw1974

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Jan 10, 2011
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Originally Posted by dot
I don't have a PM and don't want to buy it since I'm keeping my tools simple and cheap but I wonder how much power per kg I need to keep 40 km/h on a road bike with non-aero wheels in the drops? I see there aresome online calculators, but I think they might be pretty off.

I have a Tacx Satori trainer, there are power curves available for it, both from Tacx and users with PMs, at the moment, according to them my 20 min power is only 3.2 W/kg after a month of riding (and used to be closer to 4.2 but it's only an estimate). I've been completely off the bike for 1.5 years though kept maintaining fitness with some running and gym work.

The starting question isn't even really valid since w/kg does not limit speed on flat ground. It's going to be determined more by your absolute power and your aerodynamic coefficient than your w/kg.

Trainers vary depending on the air pressure in your tires and the amount of press-on force between the tire and the roller. Comparisons are only accurate if those two variables (along with humidity and environmental factors) are all equal.

Originally Posted by AyeYo

Just to play devil's advocate... if you're doing a roughly round trip or up/back ride like that, wouldn't the errors basically cancel out? It's going to grossly underestimate power going into the wind and grossly overestimate power going with the wind. Average power output for the ride should be more or less unaffected though, right? That's how I read Strava anyway (plus a few grains of salt).

As with anything else, the big question is "what are you doing with the data?" Average power over an entire ride, which is the only time this balancing your describe could occur, is not really meaningful by itself (the other factors have multiple chapters of books dedicated to them). That average that's guessed at by Strava (or most other sites) includes warmup, cooldown, stopping for red lights, drafting, coasting for a few seconds while you take a drink, and a whole host of other factors that will never be accounted for.

If you ask any time trialist whether winds balance out, the answer will pretty much always be "never." It's always slower going into the wind than you will ever make up when you turn around and have a tail wind. An inaccurate average of the two won't be an accurate average of the entire thing.
 

maydog

Well-Known Member
Feb 5, 2010
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I don't have a power meter, but I have used http://bikecalculator.com/

Assuming ideal conditions, I think it gets close. My poor mans power meter is a stopwatch and a steep hill. The likes of STRAVA should be pretty close when it comes to inclines as long as the elevation data is correct.
 

Alex Simmons

Active Member
Mar 12, 2006
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Originally Posted by maydog
I don't have a power meter, but I have used http://bikecalculator.com/

Assuming ideal conditions, I think it gets close. My poor mans power meter is a stopwatch and a steep hill. The likes of STRAVA should be pretty close when it comes to inclines as long as the elevation data is correct.
Yes, assuming ideal conditions, i.e. you know the wind as well....

http://alex-cycle.blogspot.com.au/2013/07/windbags.html
 

dot

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Apr 28, 2003
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tomw1974

The starting question isn't even really valid since w/kg does not limit speed on flat ground. It's going to be determined more by your absolute power and your aerodynamic coefficient than your w/kg.

Trainers vary depending on the air pressure in your tires and the amount of press-on force between the tire and the roller. Comparisons are only accurate if those two variables (along with humidity and environmental factors) are all equal.
That's my issue, I am not able to cringe myself into a decent aero shape since I have wide shoulders and wide upper body and also when I was away from cycling I built even bigger and wider upper body :- (think Meatball Friedman but wider) and I'm not gonna let it go.

There is also another point: I won't be able to test myself on my ultimately flat course until mid-summer, that's why I'm asking about numbers - I want to set targets and make some predictions :)

As for the trainer: my trainer is quite consistent since I have made tests with MTB and a thick trainer specific Conti tyre and on a cross bike with a road clincher. Readings for my CTS tests basically concur.