# Drag force equation

#### abudabit

##### New Member
Does anyone know the equation to aproximate drag force? I have an aproximation of my CdA (drag coeffcient * front area) for a tadpole I'm building. I want to find the drag at 40 mph.

The estimated CdA is 6.9-8.3 (assuming Cd is .9) depending on the rider.

Thanks. It didn't have the equation, but following a link from your link I found this calculator:

http://www.exploratorium.edu/cycling/aerodynamics1.html

And from the javascript I think I extracted the equation:

.5885 * (m/s)^2 * Cd * (Frontal Area in SqMeters) = Watts

mph / 2.2369 = m/s

sq ft * .0929 = sq meters

HP = 745 Watts

Came up with 120 watts, seems really low for 40 mph. I think there is something wrong with this equation or one of my conversions.

.5885 * (40/2.24)^2* (6.9 * .0929) = 120

Arggh, hate conversions.

Last edited:
Found the problem!

I didn't convert from Newtons. Here is the full equation:

.5885 * (m/s)^2 * Cd * (Frontal Area in SqMeters) = Newtons

Newtons * (m/s) = Watts

So I am looking at around 2142 Watts, or 2.9 hp countering the effects of drag.

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