Mismatched spring / shock combo

[Steve85]

My car originally came with the performance handling package Z51, which included stiffer springs, bigger bars and revalved shocks. I have 5-6 year old "Z51 Spec" Bilsteins, so they are valved for the stiffer springs.

I noticed the rear squatting a lot more than I would expect on so I got curious and checked the spring code. I have the regular spring in the back so it was swapped out at some point. :x

The front/rear rates for Z51 are 63.5 / 57.2. The rear spring on the car is actually 39.9.

Bars are 30 / 24 (Z51 spec) and the bar that came with the softer spring suspension was 20. So I have a mismatch of bar/spring/shock, with the spring being much softer than spec'd for in combination of the other parts.

How would you expect this to effect handling?

I can't explain it well, I it just feels "sloppy" in the rear. :bootyshake:

[D_Eclipse9916]

Typically a softer spring in the rear improves rear traction and tends to make the car understeer a bit.

That said, older shocks + mismatched spring rates (especially stock "soft" springs) can cause a car to do very funky things that no one can predict. How much spring a car typically needs is a "best guess" scenario, engineers typically work out the ballpark, and then you swap springs to get the balance right.

When you say sloppy, I assume when you nail into the throttle there is a lot of squatting. That's fine if the car is hooking up and the car is not plowing.

[Steve85]

Sorry, super late reply but I found something in the shed.

I think you nailed it, "unpredictable" is really what I meant by sloppy. It almost feels like the rear is half a second behind the front if that makes sense. Like the front "sets" and the rear is delayed, and then the feel changes when it does. I dunno, I just know it felt weird :?

So fast forward to this weekend and I'm cleaning out the shed and look up in the rafters and there's a spring. I checked the code and it's the Z51 spring. One of the times I changed the rear gearing I used a rotor-to-rotor complete rear and obviously didn't swap the spring. Must have been in the early drag racing days.