From another post, not the last member:
A bit of stretch is almost always a good thing, especially on soft sidewall tires, I agree completely. The exceptions are when tires are designed to be on a narrow rim due to some class rules like the cantilever Hoosiers I had on a 7" wide rim, the tread was over 9" wide but worked very well.
-----------------------
The roll center is a bit hard to express but easy to see when drawn out, it is simple a lever.
The line between the inner and outer points of the suspension needs to aim up towards the center of the car, when lowered it aims down. When you take that combined with the center of gravity there is a large leverage force placed upon the outside tires transferring nearly all the cars weight to them.
With most of the weight transferred, by leverage, to those two tires you then have basically just two tires doing nearly all the work.
Then add in bumps which cause the tires to loose contact to a degree, can be quite significant, you have very little left to keep the car on the road.
-------------------------
On another note, bump steer. That is an effect caused by lowering the car but not the steering linkage accordingly where it attaches to the steering knuckle. Now when you hit a bump, more so when turning, it will change the direction of the tire a bit, not what you want to have happen under a high g load, one side of the car can be steering differently than the other side(some of that is designed into the suspension, akerman effect but that is another subject as well)
-------------------------
So, compounding unstable steering, shifting the load to the outside tires, smaller than optimal tires, less than ideal suspension travel so hitting the bump stops at times, you create a very unstable car at speed in corners.
-------------------------
The is one simple word that best covers all of this and in fact everything in life.
Balance, the more in balance, the better everything is.
--------------------------
When I setup a care I take the utmost care to balance out everything possible to the best overall effect and then add in power in a balance way to compliment the car, not overpower it.
Making a car work on the street and track and be livable takes some effort but it can be quite rewarding, like passing a full on STI race car at the local road race track in a daily driver Forester XT that is on street tires, I really did that
Why? Because the car was balanced(and had $30k in mods that I did to it yet it looked nearly stock and got 28MPG)