First off, skidding is simply where the tires can no longer supply enough frictional force to counteract the forces acting on your tire. They no longer have static friction with the ground, but instead kinematic friction, which is a fancy way of saying that they just start dragging like any ol object. A rotating wheel usually has static friction, because the contact point that is on the ground is not actually moving.. skidding changes this.
Once your tires start skidding, they act like a regular sliding object would, ie they continue in whatever direction they were going, with friction acting in the opposite direction of that. Eventually this friction force slows the tire down enough that the sideforce drops below the limit of kinematic friction, and your car regains its footing.
So there''s two types of skid that you want to model : when the front (turning) wheels skid, and when the rear wheels skid. They both work exactly the same, but you''ll probably want to seperate them, because the car will handle differently depending on which wheels are skidding.
In both cases, you determine if the car is skidding out by the amount of force being exerted in a non-vertical direction on the tire.
so if the force we''re exerting (on the rear tires from accelerating, the front tires from turning, etc) is greater than this maxforce, our tires will go into a skid until they go under the threshhold -- now, an important point is that the value where your tires regain their grip is at much less force than you could originally put on them. Try it yourself. Take a book and put it on an inclined board. Raise the board up until the book starts sliding. You''ll have to lower the board much lower to stop the book from moving farther down. This is the difference between our static and kinematic (moving) friction
So, lets apply this to our car.
You''re cruising along at 100kph (yeah I don''t use metric normally either, but I learned physics in the metric system). You start a sharp turn to the right. You turn the front wheels, lets say, 15degrees, which is enough to bump your sideways force over the limit we looked at earlier.
Now, the ground will apply a friction force on your tires, opposing the motion of the car straight forward.
Once again, you can check if a tire is past its adhesion limit by checking the sideways force limit :
maxforce on tire = weight in kg being put on this tire * 9.8m/s^2 * static frictional coeffeicient
When you''re actually in a skid, you use the same formula, except to use the kinematic frictional coeffeicient instead of static. (just a lower number)
So, as this force is applied, your car slows down forward. You probably also want to model partial grip, where your tires, even though they are skidding, still accelerate you in the direction you''re pointing, just at a very reduced rate.
Another common thing is for the rear end to skid out. Once again we just use the same checks, see if there is a non-vertical force greater than our limit, if so, the rear tires will no longer track, but will continue in the same direction they were before, their speed being reduced every timeslice by the frictional force until it drops low enough to reduce the sideways force.
An example of this would be when you are moving, and just after starting your turn, either punch the accelerator or pull the emergency break, putting the rear wheels into a skid. The rear wheels and thus the rear end of the car continue on their previous path, while the front wheels track the curve, resulting in a very familiar slide-out, where the back of the car is moving sideways compared to the direction the car is pointing.
To get this to work right, you will have to model 3 things :
The front tires
The rear tires
The car body
If the front or rear tires go into a skid, the differences in the location of your two ends defines the angle of the car. This gets a bit more complicated also, because then you also need to model the force between the components -- for instance, the rear tires are in a skid, going sideways in comparison to the front tires. The front tires will provide some additional frictional force to stop the skid, since they are attached to the rear tires through the body, and the relative locations of the tires and body cannot change (without your car getting wrecked, of course.
)There''s tons of other things to try modelling in your car also, such as weight distribution, roll, springs, etc. But I think realistic skids and power is more than enough to chew on at start.
-- Remnant
- remnant@mailandnews.com
