Question is, when lowering the rear with longer bolts does this also reduce spring rate? or soften the ride any?thanks

 

Has no effect on the spring rate. Just sits lower on the spring

 

...and it causes the rear tires to toe out when the rear squats, because when you lower it, the rear the suspension geometry gains bump steer... a very unsettling experience.

 

toe or camber?

wouldn't an alignment fix that anyway?

 

It can be fixed for when the car is stationary....but suspension movement have a different effect on the rear since the geometry is altered from stock. Nothing to worry about or to not lower it over though.

 

As the wheel moves up and down; the suspension rods move in an arc... making the tire/wheel move inboard/outboard slightly. Unless the steering rod is set to move along this same arc when the wheel is moved inboard/outboard, the wheel will steer (toe-in/out).

The suspension is set from the factory so that the suspension rods arc pretty much matches the arc of the steering rod. This insures that any inboard/outboard movement that occurs is corrected by the steering rod, negating any toe-in/out.

But, when you lower the car, the rods are moved away from their optimum position... and when the wheel/tire moves up and down, the in/out movement of the wheel is excentuated. The arc of the steering rod is no longer matched to compensate for this extra movement, resulting in toe-in/out as the wheel moves up/down.

In the particuar case of the lowered C4, when the suspension goes into compression, the tires will turn "toe out"... which is very unstable (feels like your rear end wants to pass your front).

You can manage this in two ways: 1) is to dial in quite a bit of extra toe-in (-1/8 or -3/16) so that when the car squats, it'll never get to a toe out condition. But this is a little hard on tires. 2) the better way is to get a Goldstrand or Exotic Muscle kit and shim the toe-rod so that it's arc comptensates for the altered arc of the other suspension parts. (in my case I had to shim .665 inches to get the bump steer right)

 

If you lower it by adjusting the packing pieces where the spring bolts to the diff i.e. put them all below the spring it doesn't affect the geometry at all.

 

I'd be interested in this procedure. Can you provide further info as to the shim location and thickness calculation. Or just a simple link to a site will do.

 

It wouldn't matter where you lowered the spring, the wheel would still be lowered relative to the suspension links.

Let me explain with an example. Put your arm out in front of you and then bend at the elbow, allowing your fist to rise up about 2". (this represents your wheel traveling up into compression). Do you notice that the fist, while traveling up, also travels inboard slightly?

Now repeat, this time starting with the elbow bent at 45 degrees, then raise the fist 2". You'll notice that the fist now travels inboard a lot more than it did on your first try. (this represents your wheel traveling into compression on a lowered car)

To compensate for the greater in/outboard movement of the wheel (fist) on the lowered car, you'd need to reconfigure the arc of your steering rod (tie rod or toe rod).

JrRiflecoach: You could call the folks at Exotic Muscle and ask them about thier C4 toe rod kit. They'd be able to tell you what all is needed. Basically, you just bolt up a bumpsteer plate and measure the deflection of toe as you raise and lower your suspension into compression and extension from your static ride height; then shim your toe rod up/down until you get an arc that keeps your wheel pointing straight ahead.

 

Craig do you have a picture of the shims installed?

 

Thanks, had not heard of the "plate" you refered to.

 

Your burgandy C4 is a beautiful car... I hadn't seen a burgandy one in many years. Mine's boring-black...

 

Picture of CJ's shim:

 

One popular term for the condition that Craig Brian Johnson has
mentioned is Bump Steer. (Edit: oops - I see now that he refers
to it as such.)

Some discussion appears here

Longacre offers several variations of Bump Steer Gauges. The link
shows the gauge and has a link to instructions for use

.

 

How much was the Vette lowered to need .665 inches of shims

 

"considerably"

 

http://www.longacreracing.com/articl...sp?ARTID=13#Q1

The above site has a nice article and has links for bump steer plates to buy.