Folks, I need some help here. I'm in the process of building up my new TT bike, and have been doing some reading on aero drag, etc. I've seen John Cobb's name on enough legitimate stuff to trust that he knows what he's doing, but I just can't seem to replicate the numbers in his article here:
http://www.slowtwitch.com/Tech/The_Aerodynamics_of_hand_height_131.html
I'm not familiar with grams of force, but I'm assuming that refers to the weight of a gram on earth (ie, .001 Newtons), right? Now this part seems wrong to me:
I know I can't ride 19.17mph on 40w, but what am I missing? Likewise, I can't match any of his other numbers, nor are my answers off by a certain factor from his:
http://www.slowtwitch.com/Tech/The_Aerodynamics_of_hand_height_131.html
I'm not familiar with grams of force, but I'm assuming that refers to the weight of a gram on earth (ie, .001 Newtons), right? Now this part seems wrong to me:
...as I would expect the 4715 grams represents the total drag force, and not something that needs to be multiplied by the surface area, as the bolded portion would imply. Anyway, here's what's got me flumoxed:Our base position starting point was an average drag of 10.38 lbs. or 4715 grams of drag. This means that every square inch of exposed surface is “feeling” a pressure of 10.38 lbs.
Power = Force x velocity, so I multiply 4715 grams (force) times 8.57 m/s (19.17 mph) and divide by 1000g per kg to get the power in watts, right? I get 40.4 as an answer, as opposed to the 200w that John shows.Our base position starting point was an average drag of 10.38 lbs. or 4715 grams of drag. This means that every square inch of exposed surface is “feeling” a pressure of 10.38 lbs. While this particular rider is easily capable of producing very high watts, I have used a number to figure the time differences of 200 watts average. Our base line time for a 40k, or roughly 25mile event would be 1:17:45 @19.17 mph.
I know I can't ride 19.17mph on 40w, but what am I missing? Likewise, I can't match any of his other numbers, nor are my answers off by a certain factor from his:
Am I supposed to multiply by an assumed surface area, air density, or the gravitational constant or something? Seems pretty freakin' simple.This resulted in a drag of 9.176 lbs or 4165 grams. This gave a 40k time of 1:14:55 @19.9mp Next we closed the zipper. This dropped the drag to 8.804lbs or 3997 grams, or 1:13:59 @20.16mph (so, close up those zippers).