I'm not an expert either, but I'd be careful about being even that low. Your points about roll center are on the ball, but if the lower arm is horizontal at rest, that means when you go into "bump", the arm is going to be travelling straight away to a situation where it's pivoting "up". This is a damn hard issue to talk about without diagrams, isn't it?SLOroadster wrote:I'm by no means a wiz with messing with suspension geometry, but I do know the roadster can be improved. If you over lower a car you can drop the roll center below ground level and that is bad. To compensate for it you need stupid high spring rates. A quick way to tell if your front suspension is off, is to look at how the lower A arms sit with regards to the ground. Make sure the surface is level. If your lower A arm points upward toward the center of the car, the roll center is too high (or at least could be better) if the a arm slopes downward toward the center, your car is over lowered and the roll center is too low, likely below ground level. If the lower a arm sits horizontal to the ground, you are set. Its a quick and easy way to eyeball the setup. There is a formula to find exactly where it is, but you need to take a few measurements, and do some figuring.
Thing is, if you take the typical stock setup, the lower arm points up to the center of the car. Try this: draw a right angle triangle along the arm, up to the center of the wheel, and back across to the pivot point. Then draw a similar triangle within that - but where the spring is located. There's a ratio of wheel movement to spring movement, which is steadily decreasing (or rather, getting closer to 1:1) as the lower arm gets closer to horizontal. This ratio then starts to *increase* again as the arm passes horizontal and swings up - every mm the spring compresses the wheel can move more and more. This means that your effective spring rate at the wheel (which is where you're in contact with the ground) is now decreasing with every mm travelled, whereas before you got the arm horizontal, the spring rate was increasing. This is another reason why if you lower the car excessively, you need ridiculously high spring rates to compensate. Suspension setup is always a compromise.
Cheers,
Steve
