Originally Posted by 12340987
without examining the math, I'm having a hard time believing these vehicles with power/weight ratios better than that of some supercars, can't exceed 140. I see posts of people hitting 130 in their stock civics, surely a bike is going to produce less drag than any car? Isn't 140 typically the figure that speed governors are set at?
Unfortunately at those speed it is quite the opposite, the car and the bike will have similar drag coefficients thus whoever with the most hp will come out on top.
Found this on the R1 forum that explains it way better than I can:
original post at http://www.r1-forum.com/forums/showthread.php?t=235116
Originally Posted by DoinTime
Total drag (Dt) and drag coefficient (Cd) are two completely different but related things. Cd is equal to the coefficient of parasitic drag (Cdp) plus the coefficient of induced drag (Cdi). Cdi is minimal on motorcycles though so it can be effectively disregarded.
Total drag (in lbs) can be found by multipyling the Cd, dynamic pressure* (q), and the surface area (S).
* dynamic pressure is made up from the density ratio (local air density accounting for local altitude, temperature, and barometric pressure measured against standard density) multiplied by the air velocity in knots squared. The resultant of that is then divided by 295.
The formula looks like this:
As you can see drag is determined by five factors. Cd, surface area, local pressure, local temperature, and air speed.
As the above poster mentioned, high end sports cars generally have Cd numbers in the mid 0.3s. Motorcycles however have Cd numbers close to 1.0 depending on the model bike, size of rider, and riding position. In looking at the formula you should be able to deduce one major point. Drag is increasing exponentially with speed.
Plugging in some generic numbers show this better.
Variables: Standard atmosphere (sea level, 15 degree C, and a barometer of 29.92), Car at .35 Cd. Bike at 1.0 Cd. Car surface area at 19 sq. ft. Bike surface area at 7 sq. ft. (surface areas and Cds are approximate numbers).
Bike Dt = 59.32 lbs
Car Dt = 56.35 lbs
Bike Dt = 237.3 lbs
Car Dt = 225.44 lbs
Bike Dt = 533.90 lbs
Car Dt = 507.20 lbs
In the case of cars vs. bikes the bikes generally have triple the Cd but 1/3 the surface area so Dt remains about the same regardless of speed. Cars kick the crap out of bikes at high speeds because at high speeds the predominant factor is aerodynamics. As speeds increase the rate of acceleration decreases and the superior HP/weight ratio that gave the bike superior acceleration at low drag speeds cease to be the deciding factor. At high speeds where rates of acceleration are minimal you need horsepower to overcome the drag factors. As we have seen, aerodynamic drag is approximately equal so whoever has the most horsepower wins.