# Car Physics questions

1. Apr 21, 2007

### C0d3r

Hello, I'm looking for some information so I can write a simple car simulation program.

This program will be a simple model with 3 things:
* aceleration
* gear shifting (with dif. ratios for each gear)
* ground resistance

So that the user can do a simple simulation like a normal car in a straight line, where at some point gear shifting is necessary.

Some specific questions I have at the moment:

* What actually happens when the car gears shifts up?

From what I've played in games like Need For Speed, when the gear is shifted up, the car's RPM reduces. But I'm not sure how this influences the whole system.

* How do we determinate the gradual increase in aceleration if we keep the acellerator down?

I'm guessing this is related to the current gear.

2. Apr 21, 2007

### Andrew Mason

Gear shifting has to do with the torque characteristics of the internal combustion engine. An internal combustion engine has low power at low speed. So in order to accelerate the car at low speeds, the engine has to go fast - hence the low gear. As the car speed picks up, the gears are increased.

What you would need is a table showing the power produced by an engine as a function of speed. You would also need to know the ratio of engine speed to car speed for each gear. That would enable you to determine the power being deliverd to the car at any speed in any gear. That power is the rate of change of kinetic energy of the car:

$$P = \frac{dE}{dt} = \frac{d}{dt}\frac{1}{2}mv^2 = mvdv/dt = mva = Fv$$

AM

3. Apr 22, 2007

### C0d3r

So the RPM indicated in the car meter is the engine speed? Or is it the speed of weels rotate?

Thanks.

4. Apr 22, 2007

### C0d3r

So when the driver presses the pedal, he is given more Torque/Force to the engine?

And what's the rate of change of that force?

5. Apr 22, 2007

### Andrew Mason

The tachometer displays the engine speed in rpm. The relationship between engine speed and car speed (or wheel speed) depends on the gear being used.

AM

6. Apr 22, 2007

### Danger

The rear axle and tire diameter also contribute to the gear ratio, although in most cases those don't change during driving. (Sometimes they do, but not in the situation that you want to simulate.)