# How to find the coefficient of drag of a car model?

1. Apr 6, 2013

### theBEAST

Can we assume that the coefficient of drag of a car model is constant using one of the values from:
http://en.wikipedia.org/wiki/Drag_coefficient

The reason I ask this is because the coefficient of drag of a car varies with velocity. So wouldn't it change depending on how fast the car is moving?

Furthermore, I would like to use this to model the physics of a car using a differential equation which includes the drag force, thrust force, friction floor form floor and m*a. Is this a valid simulation?

2. Apr 6, 2013

### cjl

Actually, part of the point of a drag coefficient is that it is relatively static and doesn't change much with velocity.

3. Apr 6, 2013

### theBEAST

http://faculty.wwu.edu/vawter/PhysicsNet/Topics/Dynamics/Forces/DragForce.html [Broken]

It says that it only holds true "when the flow is turbulent the Reynolds number is large, and the drag coefficient CD is approximately constant."

Last edited by a moderator: May 6, 2017
4. Apr 6, 2013

### cjl

True enough. For most cases involving a reasonable size of object in air though, the flow will be turbulent, and the reynolds number is large. Are you talking about using small replicas of cars, or full size? Also, what speed range are you referring to?

5. Apr 6, 2013

### theBEAST

Yeah small replicas of cars, for example RC toy cars. Speed range will be around 1m/s to 5m/s.

6. Apr 6, 2013

### cjl

Hmm. In that case, it'll probably be fairly difficult to consistently measure drag, since at that speed, drag force will be quite small. In addition, at that low speed, you probably will be in a laminar flow region, and possibly even a flow regime where viscous forces are important (very low reynolds number), so you will tend to have a velocity-dependent drag coefficient. Unfortunately, you would really need to measure it experimentally to have an accurate idea for its effect - it isn't something that would be easily calculated.

7. Apr 6, 2013

### theBEAST

What are some ways to measure coefficient of drag?