seach " 84 -96 suspension chart " ... this shows all the spring rates and sway bar sizes ... i have some questions ...

 

What questions?

 

the 84 ef1 has a rear spring rate 72.0 and a rear wheel rate of 28.6 .. the 85 z51 has a rear spring rate 57.2 and a rear wheel rate of 30.6 ... wouldnt the 84 with 72.0 spring have a higher wheel rate then the 85 with a lower 57.2 spring , 30.6 rear wheel rate ?

z51 - 85 , 86 , 87, has 57.2 spring with a 30.6 rear wheel rate ...

z51- 88 has a 57.2 spring but has a rear wheel rate of 33.5 ..

why is this .... the only thing i see thats different is offset .. 6mm ..

i dont know how to put the chart on here , sorry ...

 

Wheel rate in heave/squat/lift* is not the same as the wheel rate in roll**. We should assume that this chart is giving us the wheel rate in roll, which includes the roll resistance of the swaybar. Note how much bigger the swaybar is on the 85 Z51, 24mm vs the 84 FE1's 20mm. A swaybar is a torsion spring, and its rate is a function of the wire diameter ^4 (to the 4th power). So the 85 Z51 swaybar is 207% as stiff as the 84 FE1's bar, and will contribute a lot to the wheel rate in roll.

ETA: Ugh! That chart is jacked up somehow, I think. The swaybar difference could account for the different wheel rates between the 84 and 85 models you cited. However, the 86 and 87 Z51s have the same rear spring rate but only 22mm swaybars vs the 85's 24mm swaybar, but they are shown on the chart as having the same wheel rate. So I'm not sure how they arrived at those numbers!

* "Heave" is when all four corners of the suspension move the same amount in the same direction, like when you reach the bottom of a dip. "Squat" is both rear corners moving up the same amount (compression), and "Lift" is when both rear corners move down the same amount (rebound). In all of these cases, the rear spring is providing all of the rear wheel rate - the swaybar is not contributing to the wheel rate at all.

** "Roll" means the left corners are moving in the opposite direction of the right corners. In this case, the swaybar does contribute to wheel rate.

 

In developing a basic spring setup, you first step is determining your Motion Ratio. A different formula is needed for the type of suspension your vehicle utilizes: A-arm or Beam axle. ... Wheel Rate is the actual rate of a spring acting at the tire contact patch.

 

I'm not seeing connection between your post and the original question or my reply. The OP was asking why the rear spring rate on the 84 is higher than the spring rate on the 85 car, even though the published wheel rate on the 85 is higher. The two cars have identical suspension geometry, ergo they have identical motion ratios. EDIT: the 88 rear supension might have a different motion ratio for the either the spring or swaybar rate, so that could account for its higher wheel rate than the earlier years that had the same spring rate.

Static wheel rate is the sum total of forces from all the parts that provide static resistance to suspension compression, as measured at the wheel hub. Primarily, this is provided by the spring for heave, squat, and lift at the rear. But in roll, the swaybar adds its force to the spring's force, thus the wheel rate in roll is higher. Small contributions to the wheel rate are also made by the gas pressure in each damper, and by bind in the stock rubber bushings (of if using urethane in the dogbones, then a lot of rate is added in roll!).

 

There has been problems with that chart in the past, its documented. OP, what are you building or trying to accomplish?

 

im building a 84 and want to use 85 , 86 springs . but i came across that chart and threw me off , 84 with higher spring rate and lower wheel rate .. then 85 with lower spring rate and higher wheel rate ... i wanted to use a 57.2 spring in the rear and a 63.5 or 66.5 spring in the front ..

84 ef1 has a 63.5 front spring with a front wheel rate of 21.5.
85 z51 has a 63.5 front spring with a front wheel rate of 26.0 ..

thanks for asking !

 

Maybe I'm missing something here but this build is being done I'm guessing using "used components" that are likely 25+ years old?

When I'm interested in specifications I generally use the MVMA specifications published in the production year by the manufacturer. All of this information is available currently BUT is it actually relevant here? I believe you also need to understand that these specification whether from an Internet compilation or from the MVMA information are "design specifications".

How compliant and what tolerances were considered OK at production?

 

Lets make this simple. GM went to a LOT of trouble to get the suspension "right" on the C4, hence, all the changes. If I was building a 84 and wanted different springs,etc....I would go with a 85-87 setup. Those cars rode better and had no loss in handling compared to the 84 cars.

If your into handing you WANT the Z51 setup. WVZR-1, brings up a good point and God only know if the springs and bars are still what they where built as but the fiberglass springs seemly do not loose their "springness" unlike steel springs over time and cycles. GM wore out their test equipment trying to wear out the fiberglass springs.

I think the most important thing is with a stock rear suspension on a C4 (mounting bracket locations,etc...) there is a limit of how stiff you can go in the back before the car becomes snappy loose. Has to deal with the high roll center in the back. Unless you change your brackets with stuff from Doug Rippie Motorsports I would not go much stiffer than stock Z51 in the rear.

I've tired this in my 93 Z07 car and it was NOT happy.

Please keep in mind the sway bars adjust at the limit handling and are easily changed I would not get too focused on them. Shoot, you can make dramatic differences in handling with just how the sway bars get mounted to the chassis. Example: My front 30mm S bar has shims between the bracket halves. Takes out a lot of understeer and makes the car easy to adjust. A tight auto-x lot and I run two shims on each side. Go to a road course and that would be too loose so NO shims. As my car is setup its very neutral at road course speeds but still stable.

 

xracer33, the "easy button" solution would be to contact VBP and get a set of "challenge" springs. Those are equivalent to the factory Z51 Challenge car setups, and will be a good basis for street-driven C4 that still handles well. Based on your stated goal, that's what you're looking for.

 

Lets make this simple. GM went to a LOT of trouble to get the suspension "right" on the C4, hence, all the changes. If I was building a 84 and wanted different springs,etc....I would go with a 85-87 setup. Those cars rode better and had no loss in handling compared to the 84 cars.

thats my thoughts too ...

yes i plan on building camber and strut rod brackets , and rear toe rod ...
and using 85,86 z51 springs and bars , but the question is the rear 85 z51 rear spring is 57.2 and the 84 base is 72.0 and has less wheel rate then the 85 with less spring rate . the way i see it is if a 85 z51 spring is used in a 84 it well have less wheel rate then the base 84 ...

base 84 wheel rate 28.6
z51 85 wheel rate 30.6 have you looked at the chart ???

thanks

 

The chart is just wrong. I'm not 100% sure which wheel rate is really correct. But it doesn't matter because the geometry didn't change at all from 84-88 at least. And in fact, I think the rear geometry remained the same for all C4s (all the springs interchange). So the motion rate never changed. There's no need to lose any sleep over the wheel rates on that chart. Just get the get the spring rate you want. Basically, you have to ask yourself if you want an 85+ Z51 setup or the base setup, and then get springs accordingly.

 

thats how i see it , wrong !! 86 springs is what id like to use , that should be softer then the 84 ef1 set up .... all i see is were 84's rode really stiff and some tried an 84 spring in a 86 and it rode bad . i cant compare ride with any c4 vettes cuz i havent ridden in any , havent even driven this 84 . just want it good out of the box , ive redone all the interior , fixed the dash panel ,fiberglassed parts of the dash that were broken , tring to build seats . this has sat for ten years so it needed some work .. and gotta stiffen up the chassis with targa brace or something like it .

 

84-87 Rear suspension had a higher roll center compared to the 88-96 cars. They modified the strut rod brackets on the differential to lower it down some. This coincided with the change to zero scrub radius suspension up front.

The motion ratio's changed a TON up front when they did this change but basically nothing in the rear.

 

havent got to the front yet , just started the rear , so you think 86 z51 setup is good for an 84 ..

 

Yes

 

k thanks for the help

 

Good for what use of the car? I kind of see that spring combo as a good all-round compromise: decently compliant for the street while providing decent pitch/roll resistance for autocrossing and tracks. If all-round use is your goal, it's a good starting point. A base 96 spring set will ride a lot softer, but the later C4s also had more anti-dive and anti-squat built in to compensate. A VBP Extreme spring set like I run is a good step up for competition use and allows ride height adjustment up front, but most people find it pretty harsh for street use (I am more tolerant than most in this regard).

 

all around is a good start, but i wont know what i like till i drive it ...again thanks a base 96 would alot softer , a 96 has less anti dive and squat ...