Now onto a bit more of an interesting topic.
There are two main components to understanding dynamic weight distribution. The first is what the car does to the wheels, and the second is what the road does to the car.
Lets start out with what the car does to the wheels.
When you accelerate in any given direction that acceleration produces an overall force on the car due to inertia. This force is best approximated as a single force going through the vehicles COG (center of gravity). You can do this because even if you break the car up into components and analyze their accelerations individually, their net effect will sum up to that same force in the same location.
Because the COG lies above the ground, any horizontal force you apply to it will end up creating a moment (torque) on the car. The higher the COG is, the larger the moment will be.
This moment is of extreme importance because the only way the car can counter it is by transferring the weight on its tires. If there is no weight left to transfer, the car will flip.
Here is an example of how it works.
Say you have a 3000lb car, that is 6 feet wide, and its COG is 3 feet above the ground. Lets imagine that it's pouring rain and some ballsy person is managing to pull 0.5g in a corner.
To find the force that is acting on the COG we do the following
F=(3000lb)*(0.5g)=1500lbs (yes, yes I am being horrible with my units but it is the easiest way)
So that means at this exact moment there is the equivalent to 1500lbs of force being applied to an imaginary spot 3 feet in the air trying to flip this car over!
To find the torque being applied by this force we do the following
Yes, That is 4500ftlbs of torque! Those are almost top fuel dragster numbers ;)
Now, to find the amount of weight transferred we simply divide by the distance between the wheels.
This means that 750lbs was lifted off of the inside tires and that weight was applied to the outside tires. So if the left and right sides were initially each holding half of the cars weight, they will now have the following weights.
1500lbs-750lbs=750lbs (on the inside tires)
1500lbs+750lbs=2250lbs (on the outside tires)
Keeping in mind that the more evenly distributed the weight between the tires is the more grip you get, it's easy to see that the harder you push the car into the corner, the less grip you are going to get!
Now, how could we improve this vehicle? Well, if we could make the COG height only 1.5 feet, then half of the weight would be transferred, and if half the weight is transferred, that will give much more grip in a corner.
But what if we went the other way? If we doubled the COG height... well quite simply at half a g force this car would be just on the verge of flipping... which means it would be a really shitty car.
This same concept also applies to accelerating and braking. And you can even see it happening in these scenarios. As you hit the brakes or throttle, notice how the front of your car goes up and down, that is the same moment/torque that is transferring the weight between your tires.
Pushing the concept further, it should be somewhat easy to see that as you slowly squeeze on the throttle while exiting a corner, or let off the brakes as you enter a corner, that there will be a substantial amount of weight shifting around, and that even if a car has close to ideal weight distribution in the pits it will never ever be close to it on the track.
Now for the second part of dynamic weight distribution... the road.
Simply put, no track is perfectly flat and smooth. Every little bump you hit will for a very short period of time load each tire that it hits. There will also be an equally short period of time where the tire will be unloaded. These bumps in the road make suspension vital.
The right combination of spring rate, damping and spring rate curve need to be optimized to maintain the highest amount of grip over these deadly imperfections. I will discuss spring rates and damping in another article.
There are more then just bumps however that effect weight distribution, hills, valleys and uneven roads also will add tire load, remove it and even transfer it.
It is easy to picture the load being removed on top of a hill from the "flying" sensation you get, it's also important to realize that at the top of the hill your traction will be less, so don't ever take a corner as fast as you normally would when going over a hill.
But you probably knew that already, what is of equal importance is when you are in a valley, that your tires will be loaded more. Many new drivers forget or don't even realize this, and generally take them at a less then optimum speed.
The final part which you probably haven't thought about is how hills also transfer weight. This is best thought of in yet another 2 parts.
First is comparable to the method I taught last time about using a hill to calculate COG. If you are traveling up a hill, more of the weight will be on the rear tires, and if traveling down a hill, more will be on the front tires. This means if you are driving a rwd car, be careful about pinning it when exiting a downhill corner. Similarly, in a banked, or a nasty counter banked corner, weight will be shifted down hill. If it's banked that means there is more weight on the inside tires, this helps compensates for the weight lost due to weight transfer in turning. On a counter banked corner, there is extra weight shifted to the outside wheels, which really is the last thing a car needs.
Second is transient weight transfer. Whenever you go on a hill, your vehicle will change it's orientation, i.e. it will point up down sideways, hell it might even roll a little bit. These changes in orientation don't happen without... you guessed it, weight transfer :P
Picture it this way, you are driving along and you encounter a hill, the front wheels will reach the hill first and there will be a moment where there will be more weight on the front wheels then the rear wheels, if the hill starts very suddenly this weight transfer is very strong and occurs over a very short period of time if you aren't expecting it, you can find yourself momentarily tractionless (aka skidding)... this momentary loss of traction can be followed by a much larger skid due to the fact that tires have less grip with skidding (as an after thought I probably should have put this effect into the first article).
And there you have it! Neglecting aerodynamics and downforce variation, you now hopefully understand weight transfer a little bit better :)