# Automotive Vehicle dynamics -- Accelerating FBD

Tags:
1. Jul 11, 2016

### Eduardo1234

Im starting to learn about vehicle dynamics by watching video lectures here , and also reading the books by Gillepsie and Jazar. I´ve got a fundamental question about the FBD of the vehicle.

According to Gillepsie :http://imgur.com/a/lGXxw
http://imgur.com/a/lGXxw

The vehicle is accelerating uphill.
But aren't the tangent forces to the wheel inconsistent to the way the wheels should turn when going in that direction.Or this way of FBD is simplifying the rolling analysis in some way just to analyse the movement of the center of mass in de x direction.

Last edited by a moderator: Jul 11, 2016
2. Jul 12, 2016

### CWatters

What do you mean by "tangent forces to the wheel"? The diagram names all the forces, can you tell us the name of the force you think is wrong? Are you referring to the friction ("tractive") forces Fxf and Fxr ? They appear to be correct (up the slope).

3. Jul 12, 2016

### jack action

If you isolate the ground and do a FBD, the friction force will be in the opposite direction, i.e. the car pushes on the ground. But when you isolate the car, on the FBD, the friction force seen by the car is the ground pushing on the car.

For the rolling resistance, by definition, it must always produce a reacting torque (not shown in next figure) slowing down the rotation, otherwise it wouldn't make sense.