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Air Temperature and Drag
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So I raced a sprint this weekend on the same course that I raced in July. I've been working hard this winter and put out about 30 watts more yesterday than the race in July (250 vs 220, VI was 1.0 in both races). Same bike, wheels, etc, very similar wind conditions, and I was 0.4 mph slower yesterday than in July.

The only difference I can think of is temperature. The race this summer was in the low-mid 80s and humid. It was mid 60s yesterday. Does temperature make that big of a difference?? Enough so that I can put out 10%+ more power but still be 2% slower? My power is measured at the hub so any loss of power through the drive train shouldn't be a factor.
Last edited by: gd28: Mar 20, 17 5:35
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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gd28 wrote:
So I raced a sprint this weekend on the same course that I raced in July. I've been working hard this winter and put out about 30 watts more yesterday than the race in July (250 vs 220, VI was 1.0 in both races). Same bike, wheels, etc, very similar wind conditions, and I was 0.4 mph slower yesterday than in July.

The only difference I can think of is temperature. The race this summer was in the low-mid 80s and humid. It was mid 60s yesterday. Does temperature make that big of a difference?? Enough so that I can put out 10%+ more power but still be 2% slower? My power is measured at the hub so any loss of power through the drive train shouldn't be a factor.

If you can give me Barometric Pressure, Temp and Humidity (or Dew Point) I can calculate the air density for each day.

You could also find several online calculators to do the same thing.
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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Could be. Air thins as it gets warmer, enough so that pilots who don't pay attention to temperature can end up dead as, at the margins of performance, a plane that will take off at one temperature will not fly at 20 degree warmer, all other things being equal.

I'm sure BestBikeSplit would tell you for sure but, this free power calculator I've always found pretty accurate shows a .3 mph difference between 80 and 60 degrees at 250 watts. http://www.kreuzotter.de/english/espeed.htm
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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Whilst temperature (and also humidity) does make a difference, air pressure dominates.

There's a calculator in Golden Cheetah if you have that data.

Red is Faster - making you and your bike faster
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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Entirely possible.
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Re: Air Temperature and Drag [atasic] [ In reply to ]
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atasic wrote:
Entirely possible.

Not entirely possible.
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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Same power meter?
Same clothing?
Same helmet?

“I am the light of the world. He who follows Me shall not walk in darkness, but have the light of life.” -Jesus

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Re: Air Temperature and Drag [nickwhite] [ In reply to ]
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Same Helmet
Upgraded Clothing (LG Aero Tri Suit vs. 2 Piece sleeveless)
Same Power Meter

So it looks like all of these conditions basically can be rolled up into air density. Using the calculators I estimated the air density to be .07222 lb/ft3 in July vs .07596 lb/ft3 yesterday, and confirmed the wind conditions were almost identical. I think something else must have been going on like potentially a minor brake rub which I'll check when I get home from work. All of the calculators are showing that even with the increased air density, my additional power should have netted me a gain of about 0.8 mph instead of a loss of speed.
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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Riding position... lost of fitness/flexibility?
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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IME, with powertaps in past, is that if it is much warmer than the temp you transported the bike , when at an event, if you don't rezero after PT has warmed up to ambient temps, the meter will read low. if you are foolish enough to chase watts, that becomes a recipe for disaster(don't ask how I know) my point is, it is entirely possible that last summer's numbers are low and/or the current ones high. maybe they are better now on temp drift(they kept claiming they were back when I had them to no avail)

alternately, have you checked the calibration of the meter to see if it is off?
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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Out and back? Lots of turns? Post winter crap on roads and less time riding outside. You may cornering slower.
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Re: Air Temperature and Drag [Rambler] [ In reply to ]
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That is very interesting re: temperature fluctuations with the PM. As I understand it the hub based powertap automatically calibrates itself so I assume it is calibrated accurately. The course is a loop with 3 or 4 turns, so while it is entirely possible that I was rusty in my cornering I don't know that it would make that big of a difference.

I double checked my rear brakes last night and the clearance was extremely tight. There was a slight rub for 2-3" on each rotation that I hadn't noticed when I checked the brakes the morning of the race. I'm wondering if under power and with my weight on the bike the rub would have been exaggerated enough to rob 20-30 watts. That seems most likely to me at this point, but who knows.
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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Powertaps will auto zero when you coast, *if* they are set up to do that. It's an option.
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Re: Air Temperature and Drag [rruff] [ In reply to ]
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I think on new stuff the default is to be on, I posted this info on the PowerTap thread after a email conversation with them :

WD Pro wrote:
Some quick updates from me based on some communications I have been having from a very helpful guy at Powertap (Cody) :-)

My questions related to the GS hub (and therefore I assume my info is also relevant to the G3 and any other variants of hub with the latest dual ANT+ and Bluetooth cap).

The hub comes set with ‘auto zero’ function being set to ‘on’ as default. So for those that can’t see this option on their head unit (i.e. me with a Polar V650) there is no need to concern yourselves with this feature, it will just work invisibly for you in the background. The recommendation for the hubs was never to turn this off (possibly why I can’t even see the option if I connect through the latest app). Only the pedals ‘could’ need this function temporarily disabling.

The torque offset values have changed recently. My ‘Offset Raw Torque Value’ after what the app said was a ‘successful calibration’ is 2046.97 Nm. This was confirmed as normal and the response was even accompanied with an identical screen shot from the app of a Powertap technician showing a value of 2047.88 Nm.

Hope this helps someone reading this in future :-)

WD :-)

WD :-)
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Re: Air Temperature and Drag [gd28] [ In reply to ]
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gd28 wrote:
very similar wind conditions, .

For years I noticed that I was slower in colder conditions, but by colder I mean in the 40s and lower compared to the 60-70 range. Above I was slower due to heat stress. I always felt it was due to increased density of the air at colder temps.

I would look at the wind factor. You state they were similar. Similar is not exactly the same and I find that just a few MPH more on the wind speed can really make a big difference as can the wind shifting around. Just my two cents.
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Re: Air Temperature and Drag [RChung] [ In reply to ]
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RChung wrote:
atasic wrote:
Entirely possible.


Not entirely possible.

It's funny to me that this got overlooked

The above poster is a physiologist employed by Pearl Izumi. However, statements are not made on behalf of nor reflective of PI in any manner... unless they're good, then they count.
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Re: Air Temperature and Drag [xtrpickels] [ In reply to ]
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Umm, wind? Anyone?

What was the weather like?

Don't be concerned, if your power went up and your placing for the bike leg went up you're laughing.

Other factors could include tyres and tyre pressure, rubbing brakes, an off zero power meter - lots of possibilities.

It's most likely that you've improved, just your speed on the day hasn't increased.
Last edited by: 42x16ss: Mar 22, 17 0:47
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Re: Air Temperature and Drag [42x16ss] [ In reply to ]
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It's not just the strength of wind, it is also the direction.
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Re: Air Temperature and Drag [SkippyKitten] [ In reply to ]
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SkippyKitten wrote:
Whilst temperature (and also humidity) does make a difference, air pressure dominates.

There's a calculator in Golden Cheetah if you have that data.

I thought that temperature dominates over air pressure, at least if you stay on the same level above sea.
This
http://www.kreuzotter.de/english/espeed.htm
often-used calculator does have temperature as an input, but not air pressure (only as a function of height above sea level).
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Re: Air Temperature and Drag [TriguyBlue] [ In reply to ]
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TriguyBlue wrote:
It's not just the strength of wind, it is also the direction.

I've considered making a mobile weather station using a raspberry pi board for precisely this reason. Currently I dislike how to capture the data for TT's since not all courses are close to an airport or weather station, and the course is not always in the same conditions as those airports/weather stations.
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Re: Air Temperature and Drag [longtrousers] [ In reply to ]
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The variations by elevation are small. The meteorological variations are bigger.

Just some numbers:
1020 vs 1000 hPa yields about 0.02 variation in rho.
20 vs 25°C also yields about 0.02 variation.

Key thing is to use measured pressure, not sea level corrected.

Wiggins' hour record was reduced because it was a high pressure day.

Red is Faster - making you and your bike faster
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Re: Air Temperature and Drag [42x16ss] [ In reply to ]
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From the closest weather station at the time of the bike leg:

3/19/17: Temperature 66.2 F, Dew Pt. 62.6 F, Pressure 30.22 in, Wind Speed 8.1 MPH, Wind Direction S
7/24/16: Temperature 84.2 F, Dew Pt. 71.6 F, Pressure 29.98 in, Wind Speed 6.9 MPH, Wind Direction S
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Re: Air Temperature and Drag [SkippyKitten] [ In reply to ]
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SkippyKitten wrote:
The variations by elevation are small. The meteorological variations are bigger.

Just some numbers:
1020 vs 1000 hPa yields about 0.02 variation in rho.
20 vs 25°C also yields about 0.02 variation
100m altitude v 250m altitude results in about 0.02 variation in rho

SkippyKitten wrote:
Key thing is to use measured pressure, not sea level corrected.
Knowing the sea level pressure value (standard for weather reporting) is fine. You just need to know the altitude. Barometric pressure doesn't vary a lot over large distances (just look at the isobar values on a weather map), so getting a value (and trend if changing) from a local weather station is not so bad.

Air temp isn't overly hard to record, humidity doesn't really matter much.

But wind is very difficult to get right since common anemometers (e.g. a Kestrel) can't record wind speeds low enough (e.g. most need at least 0.6m/s to register).

e.g. a headwind at rider level of only 0.22m/s (not enough to even feel it) adds about 1 second per km for a 1200W/m^2 rider.

Or to put it into roughly equivalent terms to an increasein rho of 0.02kg/m^3, a headwind at rider level of only 0.1m/s will have the same impact.

0.1m/s, doubled to 0.2m/s allowing for boundary layer velocity gradient at 10m above rider level (where official wind measurements are made), is rated 0 / calm on the Beaufort scale.

We are unable to sense such a low level of air movement. Smoke rises vertically.

SkippyKitten wrote:
Wiggins' hour record was reduced because it was a high pressure day.
On a more favourable day, perhaps another 400-500 metres.

http://www.rstsport.com
http://www.aerocoach.com.au
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Re: Air Temperature and Drag [AlexS] [ In reply to ]
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Thanks for that Alex

You're right of course to say that you can use sea level pressure if adjusted for the elevation. I was (badly) trying to note that the pressure from a weather station is normally adjusted to sea level.

Red is Faster - making you and your bike faster
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Re: Air Temperature and Drag [xtrpickels] [ In reply to ]
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xtrpickels wrote:
RChung wrote:
atasic wrote:
Entirely possible.


Not entirely possible.


It's funny to me that this got overlooked

This thread resembles media coverage of facts.
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