Sportpix, i know your big into the suspension stuff. but im going to disagree with you. My car- 2007 Mustang GT Premium w 17" wheels. Still have the stock 235/55/17 pirellis. My suspension consists of the Eibach Sportline Kit, and BMR LCA's. Stock shocks and struts...
I have had this suspension on the car for 8000 miles. With a live axle rear end, no matter how fast or slow you go into a corner and hit a bump, the rear end is going to want to shift. Stock height or not. Although my car is much lower than stock (2") I have only bottomed out TWICE. And that was do to my own neglect. I was not watching my speed when driving into a big dip in the road. And I hit an un-avoidable pot hole. I do not feel that my car is dangerous in hard cornering situations. I feel that my car handles on rails. I am very happy with the outcome of my suspension. The only time ive felt at risk, was due to entering corners much too fast. Again, also my own fault. Although the car is close to the bump stops, the spring rate is raised to compensate. And i have never once had a problem with the front tires rubbing the fenders or splash shields. I wanted my car to be slammed, and it is. Its clean, level and it handles well. Im going to put some D-specs or AGX's in this summer to stiffen it up a bit and then do some sway bars and that will be all for me.
Hi 88_stanga,
All live axle cars will exibit some grip issues when encountering rough pavement. Some are better than others and the speed at which the bounce, slip, grip happens depends on the wheel and tire weight, other unsprung mass, size, type, quality of the tires used, spring rates, damper rates, ride height etc.
All live-axle chassis rear ends using a Panhard bar for lateral location must by definition experience some lateral axle movement defined by the arc described by the length of the Panhard bar. The amount of lateral movement is dependant on the total suspension travel from full compression to full droop, and the more travel you have the more lateral movement there wll be. Additionally the more total suspension travel the more unbalanced this movement will be using the stock mounting points. This is unavoidable in a Panhard rear axle car but this lateral movement is not the cause of the usual grip, slip, grip handling of a live axle car when experiencing rough pavement. This is caused by the solid rear axle's lack of independence side to side and is the only practical advantage an IRS rear suspension has over a live axle aside from the differences in unspring mass which of course affects other issues.
Any Mustang on stock dampers with any spring you choose will make the chassis bounce (at certain speeds depending on the spring rate and sprung and unspriung mass) up causing a momentary loss of traction because the damping rate is so high in compression damping that it prevents the springs from absorbing the impact. This makes the chassis feel and behave skittish both front and rear when traversing rough or bumpy pavement. This is just a fact of life on stock S197GT dampers and is the same problem that folks who have installed FRPP's suspension will tell you. ALL of the people who have dumped the stock or FRPP Multi-Matic struts and dampers and installed D-Spec, Koni Sports or Bilstein HD struts and dampers have experienced the same massive improvement in ride quality and handling. This same experience holds true across the board with any S197 Mustang GT stock, Sport or Comp spring set regardless of wheel or tires.
Your car is bottoming out all the time if you are driving on the streets regardless of if you know or acknowledge it. You can do some simple clay clearance testing and confirm this or you can simply disassemble your struts and rear dampers and have a look at the surfaces the bumpstops are rubbing on. I can assure you that they show signs of bottoming out. Additionally if you have to slam on your brakes and avoid dips in the road your suspension ride height is simply too low not matter what you believe.
Consider this, if your stock car had 6" of total suspension travel and normally sat with a static ride height that used only 2" of the 6" suspension travel the car will have 4" of bump travel available. You then take away an additional 2" of additional travel with lowering springs and the car now has only 2" of bump travel possible before bottoming and going solid on the framerails. In order to insure that you can avoid bottoming out under most conditions your spring rate would have to be doubled to assure that you can prevent bottoming out. This only applies to a linear rate spring as it is virtually impossible to design and build a progressive rate spring that is well matched to the particular car and ride height. The problem is that as suspension travel gets smaller the spring rate heel of the curve has to shift very precisely or the rate or ride height will be way off. This is not to mention the impossible task that the dampers need to perform with a progressive spring design. This is why the working spring rates on virtually all race cars is linear, you can't effectively dampen the springs due to the rates bouncing all over the place.
If your car really is as good dynamically as you believe then you could not likely enter corners too fast. As far as I can tell you have never been on a road course and don't really know how slow your car is in it's current condition. Go out and attend an HPDE and let a qualified instructor drive your car as it is and he will tell you your car as it sits has problems and that is is bottoming out when driven at all hard and that you need dampers.
Your car's small tires cannot possibly rub because they are only on 8" rims and the tire section widths are over 2" narrower than the tires we are talking about.
HTH!