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.
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.
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
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.
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?
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.
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 :
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.
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.
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).
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.
