# Power/speed estmates



## Rider07 (Feb 25, 2007)

I do not have a power meter on my bike. Thinking about getting one. I have a spin bike at work with a power meter. Not sure how accurate it is so looking for some numbers people see to give me an idea. Spin bike is a Schwann AC Pro Carbon Blue, so a newer one. I had an older version of this bike but could never get the power calibrated-I could hold 400+ watts for an hour. Pretty sure no chance of that for me. I have a job that limits my riding to a few times a month in the summer but I can spin almost every day. 
Anyway, flat and no wind, how many watts to go 20mph? 22 mph? 25mph? What kind of numbers are you seeing?

I am 6 foot, 190 pounds.

I have looked at a few estimaters online. The numbers seem off compared to numbers I have been given by people I ride with. That brings up another question, how accurate are power meters? Do they vary even with the same brand and model?
What are power meter options that are accurate besides rear hub?


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## tom_h (May 6, 2008)

"How many watts to go XX mph" is a bit like "how long is a piece of string". Speeds above 20 mph, aero drag starts to dominate, and Watts required scales as speed^3 (speed raised to 3rd power). Eg, to go 10% faster (1.1x) requires 1.1^3 = 33% more power (!).

The 2 most significant factors affecting your aero losses are:
- body position on bike.
- clothing (very snug, no flapping, & no 'ruffles' is best).

Nevertheless, my typical #s -- which only apply to me -- are about 250 watts to maintain 25 mph under ideal conditions.

re power meters: you may not be the best candidate or best bang for the buck, to spend $800-1000. What are your goals? "interesting" numbers to look at, or training to race, or ??

BTW, Strava can give fairly accurate #s on power +/-10% *IF* : 
- relatively steady uphill slope of at least 4%.
- be scrupulously honest when entering your body weight while wearing your normal cycling clothing & shoes.
- accurately enter bike weight with all its normal "accessories" (water bottles, seat bag, etc).


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## OldZaskar (Jul 1, 2009)

I have found gym/spin bikes to be comically inaccurate. I am Cancellara at the gym... on the road, not so much. 

Strava is much closer, but also over-reports - as tom_h said, in the 10% range. I've never seen it underreport for me. 

Power meters should all report the same value. They are tested for accuracy by their manufacturers and by third party reviewers/testers. According to what I've read, the major players are within a percentage point of being accurate/each other.


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## Rider07 (Feb 25, 2007)

Interesting info, thanks.
There is an Italian TV channel called Bike HD. They have a show where a few ex-pros will do a ride hooked up to Polar computers with watts. Everything is displayed on the screen as they do a climb. Amazing to see their numbers compared to mine. If I am climbing 7 degrees at 7 mph my heart rate is 160-165. These guys were chatting away up the hill at 7-8 mph around 140 bpm. 
Anyway, I think about getting a power meter every few years. I am used to climbing based on my heart rate so may just stick with this. I enjoy climbing the most-not good at it- and adding another bit of info may not help in the end.


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## OldZaskar (Jul 1, 2009)

I've had a power meter for 5+ years... I still focus more on heart rate climbing. The power meter is very useful (lots of head on here debating this), but if I had to choose between the two, I'd take the heart rate monitor over the power meter.


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## Srode (Aug 19, 2012)

Watts vs speed depends so much on position on the bike, tires (and chain lube of course..  )etc but a ballpark number for me at 20mph is about 220 watts in the drops on flat ground no wind or drafting.


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## pedalbiker (Nov 23, 2014)

tom_h said:


> Nevertheless, my typical #s -- which only apply to me -- are about 250 watts to maintain 25 mph under ideal conditions.


Are you like 5 feet, 100 lbs? Or does ideal include a 15 mph tailwind?

Otherwise I call no friggin' way.


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## pedalbiker (Nov 23, 2014)

Srode said:


> but a ballpark number for me at 20mph is about 220 watts in the drops on flat ground no wind or drafting.


Yes, about this give or take 10-15 watts for a typical someone 150 lbs + that isn't sitting up like a sailboat.


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## cobra_kai (Jul 22, 2014)

pedalbiker said:


> Are you like 5 feet, 100 lbs? Or does ideal include a 15 mph tailwind?
> 
> Otherwise I call no friggin' way.


+1. If ~220 watts is reasonable for 20 mph on a flat, which I agree with, then you would need close to 400 watts for 25 mph everything else equal. Physics is a cruel mistress.


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## tom_h (May 6, 2008)

pedalbiker said:


> Are you like 5 feet, 100 lbs? Or does ideal include a 15 mph tailwind?
> Otherwise I call no friggin' way.


I'm 5'10" and 160-165 lbs. 

Overall, my CdA is very low.

When I'm in a "race" position, in the drops with elbows bent -- my back is nearly horizontal, and my arms & legs don't stick out.
All my kits are "race cut" -- essentially skin tight, and minimal wrinkles.

Of course, I'm not doing entire 100 mile rides in that position, but I will when it's my turn to pull @ the front.

While aero drag is biggest power loss component, tires & tubes power loss should not be overlooked. Eg, 

Pair of good all around tires: Conti GP4000S-II = 25.8 watts @ 18 mph and *36 watts @ 25 mph*.
Vs
Pair of crap tires: Conti Gatorskin = 40.4 watts @ 18 mph and *56 watts @ 25 mph*.
*20 watts or more *can be saved by using good quality rubber.

(reference: Road Bike Tires Rolling Resistance Reviews 
Their test data is @ 18 mph, and loss/Crr scales linearly w/ speed and normal force.)

PS: Don't forget to shave those legs ;-) There was a Specialized "Win Tunnel" video clip where they claimed something like 10 watts saved vs a furry "sasquatch" look!


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## oct3 (Oct 2, 2012)

Rider07 said:


> flat and no wind, how many watts to go 20mph? 22 mph? 25mph?


A calculator can provide answers easily to this specific question for an 'average' rider (believable values for CdA, Crr can be easily looked up).

For everything else you need a power meter or other piece of equipment that can record every relevant piece of information that must be input in a calculator to get an accurate reply.

The latter may be hard or impossible as changes in the conditions that affect drag (wind speed and direction, surface roughness, temperature, etc), including fast and slow changes, are generally not supported by your run of the mill power calculator (you'd have to 'guess' the right average)


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## pedalbiker (Nov 23, 2014)

tom_h said:


> I'm 5'10" and 160-165 lbs.
> 
> Overall, my CdA is very low.
> 
> ...


Right. So you're not out on your own doing 25 mph at 250 watts. Some of the world's most slippery time trialists aren't doing that. So your cda isn't actually that low.

I'm almost the same size and I've done 300w for over two hours and only averaged 23. In a race concentrating on aero it'd be closer to 330+ to average 25 mph. 

You're just not averaging 25 mph on 250w at your size. Even your "maintaining" assertion is obviously not true for anything more than seconds. If it were, you'd be riding off the front of p12 fields all the time. 

Physics don't play around. Results are easy to see.


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## pedalbiker (Nov 23, 2014)

.....


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## pedalbiker (Nov 23, 2014)

tom_h said:


> While aero drag is biggest power loss component, tires & tubes power loss should not be overlooked. Eg,
> 
> Pair of good all around tires: Conti GP4000S-II = 25.8 watts @ 18 mph and *36 watts @ 25 mph*.
> Vs
> ...


That site doesn't use latex tubes. You should be always be using latex tubes if you want to go as fast as possible (or possibly tubeless).


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## Alex_Simmons/RST (Jan 12, 2008)

This graph by Robert Chung will help answer your question, i.e. it depends mostly on your drag coefficient and size.


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