I drive a 2000 Z28 that has been lowered with the Pro kit, boxed billet LCA's and adj. PHR. I never really noticed my car doing this before, however... after driving my girlfriends Z06 for a while, I notice my car doing something weird under the follwing condition.

Lets say I have just nailed the gas (already going 50mph) and the weight has shifted to the rear, I then hit the brakes hard to stop. The rearend seems to get really light (nose diving) but it also gets squirrely, almost to the point that it feels like I am not in control of the car.

What gives? I was considering upgrading swaybars but this really doesn't seem to be a swaybar issue. Although I will probably upgrade them anyway. Am I just spoiled driving the Z06? Do I need bigger brakes on my car?

 

1. Yep, you are probably spoiled.

2. Check your shocks, maybe they are worn out. Then they get bouncy and might have leaks.

 

When you are braking aggressively, you transfer alot of weight to the front of the vehicle. This has several effects on the vehicles suspension.

As you transfer weight forward, you begin to remove weight from the rear tires (and the contact patches). If the rear gets light enough (say you lift the tires completely off the ground) the car will want to spin. It will basically "trip over the front tires" and spin.

I once saw a study about braking dynamics (of "stock" cars) where they found that under hard braking, there was only about 60 lbs on each of the rear tires. The remaining weight had transferred to the front of the vehicle. This was in one series of tests with a specific vehicle, etc. There are lots of variables to the weight on the rear tires under braking (just wanted to clairfy that one size dosen't fit all). Interesting....

The other thing to consider is the the front alignment. As the front suspension compresses, the alignment will move slightly. We have not yet invented the perfect suspension that does everything we wish it would do. Meaning, the way the suspension is designed (to maintain negative camber during cornering) will cause it to change the way the tires meet the road when the suspension gets compressed (by transferring all that weight forward). This change in alignment can impact vehicle stability. In otherwords....it may get "squirrely".

You can help the situation by increasing spring rates (which the Z06 already has), correcting alignment specs (you may have to detirmine what adjustments are needed), and by making changes to shock valving (or replacing worn shocks). Bushings can have an impact on alignment as well. Bushing deflection will cause undesireable changes in alignment. Upgraded bushings will help. However the design of the front suspension (specifically, the lower a-arm) only has one bushing that can be replaced with a poly bushing, the other side needs to be rubber or a heim joint to allow for proper suspension travel. The Z06 has already addressed the bushings as well.

Since you already have springs, I'd consider shocks as the next place to start (as well as an alignment). However, if you are experiencing "nose dive" you may need slightly more spring rate (generally speaking). The Eibach Pro kit works reasonably well on these cars (even very well at times) so I'd stick with it for now. I'd consider Bilstien HD's (possibly revalved from Sam Strano at www.stranoparts.com) or Koni's (there are many others as well) as an upgrade.

Of course, this all assumes that the wheel bearings are not excessively worn, the balljoints are tight and the tie rods are still in good shape. If the front suspension parts are not doing their job, none of the rest of the suspension will work properly.


Sorry this is so long, I hope it helps.

Kevin

 

P.S. Upgraded brakes will actually aggrivate the problem. You may see increased weight transfer (up to the limits of the tires grip) and more wandering.....

 

Thanks trackbird. I think since I am running the stock shocks since I have lowered it (3 years), this is the first place to look as an improvement. I think the stock suspension (ball joints, wheel bearings, tie rod ends) are good. I'll look at replacing the shocks. Thanks for the suggestions of which ones to get and who to contact. Your post wasn't too long, it gave me alot of insight as to what I need to address. thanks.

 

Glad I could help....don't forget an alignment after the new shocks are on there. You'll be amazed at the difference a mild perfomance alignment will make.

Kevin

 

All LS1 cars have a brake hop under very hard braking. Feels like the rear is gonna come around...


Braking smoother (earlier) seems to help somewhat for me. Others have reported mixed results with Torque arm replacement.