Originally Posted by
kjoer
It indeed does sound weird, my guess is it being backwards generates more overall roll?
Nope. Let me see if I can explain it. There are two reasons for the effect.
When you add vertical load to a tire, it gains traction. But if you put them both on a graph, you will see that the traction goes up less than the load is going up. For example, take a tire with a 1 pound vertical load on it, let's say it can take 1 pound of lateral load before it slides. Now double the vertical load to 2 pounds. Does it now take 2 pounds of lateral load to make it slide? No, it will slide at a lesser load, let's say 1.8 pounds.
Secondly, if you move static weight forward in the car, you are putting more vertical load on the front tires but you are also moving the center of gravity forward. Obviously I am talking about the fore/aft position of the cg, not the height. The cg is the point on the car where centrifugal force pushes on it to try and push the car towards the outside of the corner. Let's see an example of how this might affect things.
Imagine a 1 pound car. It has the exact same tires all the way around and its weight distribution is exactly even, 1/4 pound of weight on each tire. Its fore/aft cg position will be right in the middle of the car between the tires. Now let's say skid pad tests show this car can take a 1 pound lateral load before skidding, meaning it can corner at 1g and it's perfectly balanced handling wise so when it skids, it's a four wheel drift out of the circle, not a spin out or an understeer out of the circle. If you doubled the car's weight to 2 pounds, the tires will gain more traction but not as much as the weight will increase the lateral load so the car will now skid with a 1.8 pound lateral load which is LESS than 1g with a two pound car.
Now let's go back to the one pound car but let's move some component forward so that the front tires now have more weight on them. This would initially seem to increase front traction so now this car will spin out when it skids instead of a nice, balanced, 4 wheel drift right? No, that's wrong. Remember, traction goes up more slowly than the vertical load does. So what happens is that the now more lightly loaded rear tires have lost less traction than the fronts have gained. But this alone is only part of the story since we are still seeing a loss of rear traction and a gain in front, even if it isn't an even trade.
That's where fore/aft cg location comes in. Imagine the original, balanced car is stationary on the skid pad and you are in the middle of the skid pad with a pool cue to use to try and push the car from the side out of the circle. The end of the pool cue is placed right on the cg (middle of the car) and the tires and load are the same so when you push on the cue, the car just slides all four tires. Now move the weight forward. The cg moves forward so you have to move the spot on the car that you push with the cue forward. This means you are now pushing harder on the front tires than on the rear. This would be balanced out by the increase in front traction and the decrease in rear traction resulting in another perfect, four wheel slide except that as I said before, the rears have lost less traction than the fronts gained. So the front tires are going to slide first. The car will now understeer.
I hope that makes sense. I am not the best at clearly explaining complex concepts of vehicle dynamics but I have studied it extensively and this is the basic consensus of ALL well informed engineers on the subject. This is why nose heavy front engined cars fight understeer issues while tail heavy rear engined cars like 911s fight with oversteer.
Also, this subject can get even more confusing due to another effect. As I described above, moving the weight to the rear should make the car oversteer but there is a situation where this could be backward. Moving the weight back will make the car looser in a steady state corner but with a rear wheel drive car, the increase in rear weight could reduce wheel spin under power, improving rear traction. This is why offroad dirt cars have such a high rear weight distribution. So in this situation, moving the weight to the rear could result in more steering off power but less on power. Especially when powering off of a slow corner, but might have even MORE on power steering exiting a fast corner.