Road Bike, Cycling Forums banner
1 - 20 of 25 Posts

·
Registered
Joined
·
183 Posts
Discussion Starter · #1 ·
How much effect does air temperature have on a time trial? It seems to me that you should be faster in warmer, less dense air. In two time trials this year I have failed to beat my previous time when I "should have" improved. The only "excuse" I have is that the temperature was quite a bit colder for the slower times.
 

·
waterproof*
Joined
·
41,745 Posts
It can be significant. I forget the exact formula, an old wizened TT guy told me one time, and that year we were both riding the same weekly series and I could tell he was right by looking at our times as the summer turned into fall.
 

·
Registered
Joined
·
499 Posts
If you are just considering the "density" of air based on different temperatures then that is way, way too simplistic of an analysis. I live and ride in the heat in Florida and can tell you, without a doubt, that I am faster on cooler days. The heat, and particularly the heat combined with humidity, has a huge effect on efforts because your body has to send blood to your skin to control overheating. That decreases the amount of blood that is used to get oxygen to your muscles. Over time your blood volume decreases as you sweat more and more to shed heat.

If you want to get really technical, you also need to factor in how the humidity effects the amount of oxygen in air. Cool, dry air has more oxygen per liter of volume than hot, humid air--you get less oxygen to your muscles in hot and humid weather.
 

·
Registered
Joined
·
183 Posts
Discussion Starter · #4 ·
Air

Heat, humidity, and how you feel are probably bigger factors than physical factors in most cases, that's for sure. I was just wondering, those things aside, how much difference the air density makes.
I know from flying how "nice" it can feel on a clear crisp winter morning. In aviation they refer to "density altitude" and it makes a huge difference in landing and takeoff distances. You figure the takeoff distance for every flight based on the plane's weight, fuel, payload, and the density altitude. You don't want to try to take off and get to the end of the runway and realize that the plane has not left the ground yet but you are going 150 knots!
 

·
Registered
Joined
·
21,832 Posts
Some numbers

Poppadaddio said:
How much effect does air temperature have on a time trial? It seems to me that you should be faster in warmer, less dense air. In two time trials this year I have failed to beat my previous time when I "should have" improved. The only "excuse" I have is that the temperature was quite a bit colder for the slower times.
Air density is significant but there are competing factors. As our Florida friend tells us, too hot wipes you out as well. IME from doing a lot of time trials over the past couple of decades is that 80 F (27 C) and high humidity is about the optimum. Not so hot that you can't dissipate the body heat, and the humidity further lowers air density.

Just for reference, 50 F (10 C) air is nearly 6% denser than 80 F. That is a LOT more drag (6 % more! :))
 

·
Registered
Joined
·
12,278 Posts
Kerry Irons said:
Air density is significant but there are competing factors. As our Florida friend tells us, too hot wipes you out as well. IME from doing a lot of time trials over the past couple of decades is that 80 F (27 C) and high humidity is about the optimum. Not so hot that you can't dissipate the body heat, and the humidity further lowers air density.

Just for reference, 50 F (10 C) air is nearly 6% denser than 80 F. That is a LOT more drag (6 % more! :))
I remember you noting this once when I was having difficulty in colder temps, which might have also been cold induced asthma.

Back to the subject at hand, I tend not to worry too much about the different weather conditions. In fact, sometimes I go out on various high wind, low wind, and everything in between since I like to know how I perform. Sure, I could get a power meter, but I like time trialing and paper is cheap. :p

Still, if you're worried about this during a race, it will effect everyone, though probably at different amounts depending on aerodynamic form and power on that given day (which in and of itself can vary). My area basically races the same TT route twice a year and on some days, you'll see that the weather was more favorable or less favorable, but the rider placement tended to be roughly the same.
 

·
Registered
Joined
·
183 Posts
Discussion Starter · #7 ·
Kerry Irons said:
Just for reference, 50 F (10 C) air is nearly 6% denser than 80 F. That is a LOT more drag (6 % more! :))
That's what I was wondering. Where did you get that information? I rode a
TT when it was 45 F and came in slower than the year before. 6% is real, not just psychological.
 

·
Registered
Joined
·
120 Posts
I think the temp can make a real difference and I try to gage my start times accordingly. However you need to take wind into account too. If it's a morning TT, there could be as much as a 15 degree spread from the first rider off, to the last as the sun comes up. However, if the course is exposed, then the wind may be more of a factor than the temp. The wind tends to pick up as the temp rises. In an evening TT, the temp is falling as it gets later, but the wind tends to decrease also. So I take all this into account as to when I register, in order to get a start time that has an advantage. That doesn't mean I get it right though, I'm learning all the time.
 

·
Registered
Joined
·
17 Posts
I did six TT races in the span of seven weeks here in CO. Same course. Same gear every race. We had two races in which the temps were 40 degrees or colder. Almost every rider in my cat was 30-60 seconds off times posted during other weeks when temps were 50 or above. In addition, one of those cold days was the 2nd to last race in the series, when, in theory, most racers are getting in to form. In fact, most put up faster times on the warmer days even though the winds were significantly stronger than on the colder days. It's not a scientific analysis, buy my first-hand observation is that cold air is slower. Cold air on windy days slows you down even more :(
 

·
Registered
Joined
·
21,832 Posts
Data source

Poppadaddio said:
That's what I was wondering. Where did you get that information?
For me, I got the information from my ancient Perry's Chemical Engineer's Handbook, 4th Ed, page 5-61 :) But you can find the equation for drag in any fluid dynamics or aerodynamics texbook. The power to overcome aerodynamic drag on a bicycle is cubic with respect to velocity, and linear with respect to frontal area and air density.

Calculating air density just involves PV=nRT, where T is in absolute temperature (whether Kelvin or Rankine).
 

·
Registered
Joined
·
499 Posts
Alex_Simmons/RST said:
Physiological issues aside, heat affects air density, which, for a given power output will impact speed:



Changes in altitude and barometric pressure also affect air density. Humidity has only a very minor impact on air density.

Wind has a pretty big impact on speed as well:

From your chart, I would suggest the wind density/drag factor plays a clear back seat to the body's need to shed heat in most circumstances. Going from 41 degrees farenheight to 86 degrees farenheight affects speed by less than 1 mph. If you go from 41 to 86, and are considering an hour long time trial, I'd bet the body's need to work harder to cool would affect your performance (speed) more than the 1 mph.

Now if the temperature increase is from just say 41 to 51 for that hour, then your body's need to work harder to shed heat would not play as big of a role. Of course the speed difference from air density from 41 to 51 is almost de minimus anyway.

The other factor would be the time spent time trialing. If it is a really short one, such as a 8km prologue, the need to shed heat is not near as great and air density may become the more important factor.

Are you able to quantify the effect of temperature/air density on the amount of oxygen in the air we are breathing to help fuel our muscles with oxygen. There is absolutely an effect, but I don't know the degree when you are talking about temperature. Of course changes in altitude can have a huge affect and we all know that. But what about temperature? (I know that motorsports racers love cold, dry air and that is where cars and dragsters are fastest).
 

·
Cycling Coach
Joined
·
1,734 Posts
Gatorback said:
From your chart, I would suggest the wind density/drag factor plays a clear back seat to the body's need to shed heat in most circumstances. Going from 41 degrees farenheight to 86 degrees farenheight affects speed by less than 1 mph. If you go from 41 to 86, and are considering an hour long time trial, I'd bet the body's need to work harder to cool would affect your performance (speed) more than the 1 mph.

Now if the temperature increase is from just say 41 to 51 for that hour, then your body's need to work harder to shed heat would not play as big of a role. Of course the speed difference from air density from 41 to 51 is almost de minimus anyway.

The other factor would be the time spent time trialing. If it is a really short one, such as a 8km prologue, the need to shed heat is not near as great and air density may become the more important factor.

Are you able to quantify the effect of temperature/air density on the amount of oxygen in the air we are breathing to help fuel our muscles with oxygen. There is absolutely an effect, but I don't know the degree when you are talking about temperature. Of course changes in altitude can have a huge affect and we all know that. But what about temperature? (I know that motorsports racers love cold, dry air and that is where cars and dragsters are fastest).
Some riders perform better the hotter it is (up to a point), so I don't think you can make a blanket statement on the physiological impact. And riders can acclimate to different temperatures as well.

I was simply pointing out the physical impact of different temperatures on speed for a given power.

The impact on partial pressure of O2 is so small as to have no physiological effect. It's only large changes in altitude that have such an impact.

Air density due to changes in barometric pressure and temperature matter - I coached a rider to a world masters hour record (48.317km) and we were very interested in conditions on the day as we were adjusting target lap splits accordingly.

The fastest days are very warm, low pressure, windless days - think summer storm cell before the wind gets up.
 

·
Cycling Coach
Joined
·
1,734 Posts
Gatorback said:
From your chart, I would suggest the wind density/drag factor plays a clear back seat to the body's need to shed heat in most circumstances. Going from 41 degrees farenheight to 86 degrees farenheight affects speed by less than 1 mph. If you go from 41 to 86, and are considering an hour long time trial, I'd bet the body's need to work harder to cool would affect your performance (speed) more than the 1 mph.
Personally I would perform a lot better power wise at 86F than at 41F. And I would certainly be an awful lot more aero than the extra clothing needed for riding at 41F.
 

·
What Would Google Do.
Joined
·
1,578 Posts
it never amazes me the lengths some people will go to make excuses for lack of performance, denser air? pfft..
 

·
Registered
Joined
·
1,846 Posts
Kerry Irons said:
.
Just for reference, 50 F (10 C) air is nearly 6% denser than 80 F. That is a LOT more drag (6 % more! :))

Minor factual error
.

Air density would be a function of temperature if the earth were a closed container, but it isn't. Air density is a function of barometric pressure, which does trend somewhat with temperature, but is more affected by altitude and humidity, water vapor being less dense than nitrogen or oxygen.

Altitude also affects local pressure and air density, which why many speed records are set at altitude. So expect to ride faster on hot humid days in Denver than you might on a cold clear day in Boston, all other things being equal, which they're not.

Note also temperature, humidity and reduced oxygen all also have physiological effects , so what you gain on one hand, you might lose on the other.
 

·
Registered
Joined
·
21,832 Posts
Nope

FBinNY said:

Minor factual error
.

Air density would be a function of temperature if the earth were a closed container, but it isn't. Air density is a function of barometric pressure, which does trend somewhat with temperature, but is more affected by altitude and humidity, water vapor being less dense than nitrogen or oxygen.
Nope. A cubic foot of air, anywhere on the surface of the earth, is 6% less dense at 80F than at 50 F. Full stop. You have severely misinterpreted PV = nRT.
 

·
Cycling Coach
Joined
·
1,734 Posts
FBinNY said:

Minor factual error
.

Air density would be a function of temperature if the earth were a closed container, but it isn't.
Air density is most definitely a function of air temperature. It has quite a large impact in fact.

Here are two examples, same pressure, humidity and altitude, air pressure at 10C (50F) and 30C (86F).

Temperature (Tc) 10.0 Celcius
Relative Humidity (RH) 70.0 %
Barometric Pressure (P) sea level 1,015.0 hPa
Altitude above sea level 25.0 metres
Air Density 1.242 kg/m3

Increase Temp to 30C:

Temperature (Tc) 30.0 Celcius
Relative Humidity (RH) 70.0 %
Barometric Pressure (P) sea level 1,015.0 hPa
Altitude above sea level 25.0 metres
Air Density 1.151 kg/m3

1.151 / 1.242 = 93%

I would say that's quite a big difference.
 
1 - 20 of 25 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top