Getting ready to install Eibach Pro-Kit springs and wanting advice on if other components will be needed. Primarily will I need a front end alignment kit and a new rear panhard bar?

Also, I looked all over for spring rates on an IROC. Here's what I've measured and calculated. The front spring rate is about 450 lbs/in, I'd say plus or minus 10%. The rear springs are around 217 lbs/in, but that is using a constant spring rate calculator. This will be slightly higher since it is a variable diameter coil.

Eibach may not like this, but here are the calculated rates for the Pro-Kit. The fronts are about 600 lbs/in. and the rears (again with a constant rate calculation) are about 280.

FYI, I am a mechanical engineer with spring design experience, albeit on smaller force requirements, so these rates are trustworthy.

 

Eibach says their rates for their springs are as such.

Pro Kit: 714 lb/in linear front 1 inch lower front
109/177 lb/in progressive rear 1 inch lower rear

Sportline: 700 lb/in linear front 1.6 inch lower front
80/137 lb/in progressive rear 1.3 inch lower rear

Drag launch: 337/514 lb/in progressive both front sides
80 lb/in linear driver side rear
80/217 in/lb progressive passenger side rear

 

Welcome to TGO, gsmcgehee.

I'd have an alignment myself. Typically, adjustable panhards are recommended for lowered cars. Before purchasing, however, you might consider a before and after measurement to see if the rear is indeed off center. Might save a few bucks.

JamesC

 

those numbers are about what I remember for the eibach's, I seem to remember the stock performance springs to be around mid 5xxlb front, and low 1xx range rear. I remember actually measuring them and calculating the rates and I got something like that also.

 

The impression I get is that GM has constantly tinkered with the spring and bar rates for the 3rd Gen cars, and that the Firebirds did not necessarily share the same setups as the Camaros.

From memory of a relatively in-depth article when the 3rd Gens first came out, the Z28 front springs were 550-ish and the rears 180 or so. I believe that the IROCs were closer to 700 front at one point, don't know about the IROC rears.


How closely have you been able to obtain dimensions for wire diameter, mean coil diameter, and number of active coils? I'm not picking on you, just that 3rd and 4th power functions will magnify the effect of any measurement 'errors' or tolerances, and 'N' can be hard to determine precisely. Ignoring a paint thickness of only 3 mils is worth over 3% in terms of rate.


Do your calculated rear spring rates represent the low rate for the "progressive" by including the maximum number of active coils or are they taken at some other height where part or all of a coil has gone inactive? If all of the the closely-spaced coils are closed up at static ride height, they effectively become 'helper springs' and the overall spring behavior becomes essentially linear over much of the working range at the rate of the widely spaced coils (comparable to the main spring in a coilover application).

Loosely related, but are those "progressive rear springs" true progressive springs or something that's better described as "bi-linear" such as with the close coils all at the same spacing?


Norm

 

Thanks to all for the additional information and help. I will take several measurements before and after, and will definitely re-align the front end after.

Norm - I have very closely measured the springs from Eibach since they are off the car. I did not consider coils that go in-active after installation, which of course raises the rate. I also do not have a formula that deals with variable rates on the rear as noted in the original post. I do understand the science of springs, therefore my notes on plus/minus10% on my calculations. Maybe more on the rears.

My main purpose in changing springs is to help handling for autocross - but did not want to go with the full lowering you get with Sportline. The main handling issue I have (both driver and car induced) is losing the back end under braking. I should be able to report back after this season to tell anyone if the springs helped (already have Koni sport shocks).

 

Maybe it's not the spring rate, per se, but more about how the roll steer varies as a function of rear ride height and forward pitch under braking. As the rear rises, most stick axle suspensions tend toward having less vehicle roll understeer and can cross over into giving you "loose steer" with the rear wheels actually pointed to run a couple of degrees wide. This is separate from and additive to whatever slip angle the rears may be instantaneously running at.


Norm