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TT question(7 posts)

TT questionvtecbike
Apr 28, 2002 9:01 AM
Here's one for the more experienced cyclist to ponder...

Yesterday, I did an 8 mile time trial test on the same course I did my last 8 mile test on a month ago.(same schedule on the bike during the week before the test)

two exceptions: 10-20 mph gusts of wind and 90% humidity during yesterday's test (temperature was the same for both tests.

Imagine my frustration at the end of the TT when I looked down and finished the test in the exact time to the second as I finished it a month ago!!!
So much for progress, huh?

During yesterday's TT test, I started with the tailwind and finished in a headwind.
Ques........Would the wind and humidity make much of a difference in performance (shouldn't the headwind going back be negated by the tailwind going out???)
Is all my training over the last month (intervals and races) of no benefit???
Thanks for any insight into the frustrating (and windy) matter..
TT timesKerry
Apr 28, 2002 4:39 PM
First, you really shouldn't expect huge improvements in just four weeks. Second, higher humidity makes the air less dense, but also can lead to overheating of the rider, so it depends on the temperature whether it helps or hurts. Third, windy conditions always make for a slower ride. Try this simple math problem: 25 mph on the way out and 15 mph on the way back does not equal a 20 mph average. On your 8 mile course, you're spending 16 minutes at 15 mph and 9.6 minutes at 25 mph, a total of 25.6 minutes and an 18.5 mph average. Finally, your TT times will vary +/- 20 seconds over this distance just based on how you feel on a given day. If you want to track your average, ride a TT every week and make notes about wind and weather. The best weather for a TT in my experience is ca. 80F/27C and high humidity with zero wind. Since we don't get these conditions often around here, there may be only one TT per season (out of 15-20, riding one per week) with those conditions, and if I'm not having a good day that day, I have to wait until the next year to see how my personal best is doing over time!
TT timesbrider
Apr 29, 2002 9:27 AM
Actually, higher humidity makes the air MORE dense, and thus will make for more aerodynamic drag (that's why Merckx did his hour record at Mexico City -- lower air density). And the wind factor -- unless the wind is at a dead-tail wind, it's going to effect you through the entire distance. And the drag is a function of the SQUARE of the speed, so the tail wind benefits you as a square root of the speed. You essentially get 4 times the detriment in a headwind as you get benefit in a tail wind.
Increasing humidity = lower densityKerry
Apr 29, 2002 4:18 PM
Merckx did Mexico City because the air is less dense at higher altitudes. Water weights 18gm per mole. The average molecular weight of air is 29 gm (21% oxygen at 32 gm per mole, 79% nitrogen at 28 gm per mole). As the humidity increases, water displaces oxygen and nitrogen molecules from a given volume of air, and the average molecular weight of air drops as water is added. The density is by definition proportional to molecular weight, so decreasing average molecular weight means decreasing density. If you have a psychrometric chart and a thermo book, you can do the calculations. The density reduction is not huge, but it has an effect, just like temperature and altitude.

While wind drag is a function of the square of the speed, the power to overcome the wind is the drag times speed, so it ends up being a cubic function. Thus, a true 5 mph head wind at 20 mph increases power requirement by 72%, while a true 5 mph tail wind decreases by 44% the power required to go 20 mph.
Damn! Try to work that one out on your next hill interval! (nm)allervite
Apr 30, 2002 8:19 AM
a good site for infoDougSloan
Apr 29, 2002 8:48 AM
Thanks for all the good info!vtecbike
May 1, 2002 5:02 AM
I'll take my laptop on my next interval session