Originally Posted by glass slipper
You're welcome, glad it worked out for you.:thumbs:
To address your spring rate question, the stock spring rates for the '90-'91 ZR-1 is 549 LBs/inch front and 228 LBs/inch rear while the '92-'93 ZR-1 is 430 and 188 front and rear. Of course those are the spring rates, not the wheel rates which are affected by the mounting position of the spring on the suspension relative to the center of the wheel and the angle at which the force of the spring is exerted to the suspension. Wheel rates are always lower than spring rates on the C4.
As you can see, comparing spring rates of the coilovers to the leaf is an apples to oranges proposition...you really need the wheel rates to make a direct comparison, but let's try anyway.
For the front, the coilover mounts farther outboard on the lower control arm than the leaf giving it a higher wheel rate for the same spring rate. But the coilover is mounted at an angle meaning part of its' force has a horizontal component while the leaf exerts its' force in a pure vertical direction, this gives the coilover a lower wheel rate for the same spring rate. Without running the numbers, I'm going to call it a wash and say the spring rates can be compared directly although I suspect the coilover results in a slightly higher wheel rate. That being said, your present setup of 400 LB/inch is very close to the '92-'95 ZR-1s.
For the rear, the coilover mounts farther inboard by a small amount and is at a slight angle which gives the coilover a lower wheel rate for the same spring rate. But the long lower lateral link (minimizes the affect of the slightly inboard mount) and the low angle to vertical of the coilover results in just slightly lower wheel rates. Again, I haven't run the numbers, but we know for sure which direction it goes. That being said, your present setup of 250 LB/inch is very close to the '90-'91 ZR-1s.
Something else to consider is the horizontal component of force the coilover exerts on the suspension. On the rear, it's minimal and probably doesn't cause enough impact to be detected by the driver. On the front, the angle is enough to result in a horizontal force sufficient to compress the lower control arm bushing in the horizontal direction a measurable amount. When cornering, this causes an increase in compliance understeer that is sometimes confused as "slop" by the driver as the lower control arm on the outside of a turn compresses the bushing an amount equal to a factory setup plus the amount that the coilover caused. Because the tie rod connects to the front of the spindle (referred to as "front steer"), this action results in excessive "toe-out" on turn-in. That's actually a good thing since the factory setup exhibits compliance understeer also, but you get too much of a good thing and the car doesn't handle as well. Of course on a race car, they have solid steel bushings/heim joints meaning this is of no concern to them. Once again though, what works on the race track doesn't really work so well on the street unless you're willing to make some big sacrifices...which you don't seem so willing to do at this point. Obviously, poly bushings will significantly reduce the impact of coilovers to compliance understeer returning the handling back to "normal"...again at the sacrifice to ride quality.
You're so close to the factory spring rates, my recommendation would be to go back to the factory springs and make improvements via shocks, stabilizer bars, alignment, and tires. The factory front spring has the added benefit of additional roll resistance via the dual mount design. This is often incorrectly referred to as "crosstalk" in a negative connotation, but it was a novel way of increasing roll resistance without increasing spring rates giving a softer ride. It worked so well, GM added it to the rear suspension of the C5 which is why your C5 rides so much better than your C4 while handling better. A sticky autocross tire like the Kumho V700s have a tread depth of 6/32" and will last for 2-3 years at the rate we drive these cars, you'll be amazed at their grip. Learn to do alignments yourself so you can go back and forth from an agressive autocross alignment to a street alignment...it's really easy once you learn and the C4s are the absolute easiest because of the front steer arrangement and the adjustments being made on the upper control arm. All you do is add/pull shims once you optimize either the race or street alignment you want. Just pick which one you want to optimize and you can get very close to other by just doing the shims. I optimize the race setting and live with a little wear on the street. I can literally go from race to street in less than 20 minutes at the track.
If you really want to lower it, stick to the '90-'91 ZR-1 springs or get some '88-'91 Z51 springs...the lower you go, the stiffer the spring has to be to keep you from hitting the bump stops on big bumps. Once you get to the bump stops, spring rate goes to infinity...not a good thing for handling or ride. One other thought to consider, when you lower the car, you change the "z" and "d" heights of the front and rear suspensions which takes the suspension out of the optimum position for the camber curves as the suspension goes through its' travel. If all you're after is the "look", then it's no big deal. If you have super stiff springs that limit wheel travel anyway, again it's no big deal. And one last thing...as you lower the car, you increase anti-dive in the front suspension and decrease anti-squat in the rear suspension. Your C5 gets increased anti-dive and anti-squat as you lower it...it's just geometry.:cheers:
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