So I found I have an FHA front spring on my '96--I triple checked and it is an FHA stamped code, not the HA code that is listed as being from the factory. It came on the car when I bought it and according to all the charts I can find is a 93.1 N/mm spring--a much earlier spring. That's fine with me as I'd been thinking about changing to a stiffer set of springs.

My rear is an RHZ code--which is where my confusion lies. Some places list it as a 39.9 N/mm spring, which would be a "matched" set. Some threads on here have listed it as a 26 lb spring and factory for a '96. Is anyone able to confirm which spring rate for the RHZ?

 

RHZ is 39.9 n/mm or 228 lbs - which is pretty soft.

 

Those are NOT normal for your car.

The rear is often softer in rate than the front (except 84 FE1)

FHB is one of the stiffest front springs, 84 Z51 had the stiffest rear, might be BMF but i think BMF is 84 standard suspension rear.

FHA front was used 89-90-91 in FE1, and FX3, both coupe and vert, with the exception of ZR-1. It is a 93.1 spring rate.

 

Yep, the RHZ should be 39.9. According to the Halverson chart, that RHZ spring came stock on convertible 96s and 96s with F45 suspension. If yours fits one of those descriptions, it's probably what came on your car. However, the FHA front spring seems to always have been paired with the RHZ rear spring in all the earlier cars it came in, so it's also very likely that someone swapped out both spring in your 96. If your 96 was one of the numerous options that had 60/26 f/r spring rates, this would have been a big improvement. Even the Z51 in 96 only came with 73.2/33 spring rates, so this would have been a performance upgrade even for that. The "Challenge" spring set that also came in 88-91 Z51 cars would be the next step up in rates, and they are 115.5/57.2. The only stiffer factory springs beyond that (that fit your car) are the 1984 rear springs at 72 (base) or 87.5 (Z51), but the early front springs don't fit 88+ C4s.

 

I had that 115.2 front spring in my 94 years ago, that is the FHB, i had that paired to an 84 FE1 (BMF code?) rear and Koni Yellow shocks. Car handled absolutely spot on, but didn't like bumpy roads all that much.

 

And as you note, that BMF spring was the soft rear spring in 84!

PS - Looks like we were typing at the same time above.

 

Quite possible, although i did edit a couple times due to reading the chart upside down and getting numbers mixed up all over the place!!

 

The car is a coupe and came with F45--I replaced the originals with Bilstein B8's when I got the car.

I'm thinking I'll go to a bigger sway bar up front then--I have a 24mm on the front now. Even though the charts list a 26mm for the front on '96 coupes I have the 24mm. I'd like to match the sway bar to the higher spring rate. Zip carries a 28.5mm front sway bar--I'm thinking that would be a good choice.

 

Were the originals stuffed and not able to be rebuilt? There could probably be a bit of a gain by resurecting the F45. Even Koni Yellows would probably be a step up.

I had a 24mm front sway on my 94, and it sat flat as can be.

My 93 has 24mm on both front and rear, it could do to have stiffer sway bars, but it is on coilovers, so you lose the anti-roll effect the leaf springs give.

 

It just depends on what your goals are for the car. With your spring combo, I'm guessing the car either understeers or is close to neutral right now. That Addco 28.5mm front bar (1-1/8") will add more rear grip by taking away from the front, and create more understeer. It is a pretty good value if you want stiffer front roll rates. Just be sure more understeer is what you're looking for. It seems like "theme" for setting up a performance-oriented later (88+) C4 is to get a 2:1 ratio between front and rear spring rates and then use a fairly stiff front bar with a fairly small rear bar. Your current spring split is about 2.3:1, which would lead me to guess that you may not want higher front wheel rates in roll (relative to rear rates) than you already have. But that's only a guess, and again it really all just depends on what you want to change/improve with the car.

 

Unfortunately, the F45 shocks cannot be rebuilt or replaced unless you get NOS set. From the literature I can find from back in '96 it seems that the F45 was more sophisticated than the FX3 system but provided less "feel" between settings. Although I do recall reading F45 was pretty effective at high speeds(read triple digits). That could be due to the spring rates they put on the F45 vs the FX3 systems or that the shocks were individually adjusted. Either way, it's not a system I would have ordered at the time or miss right now. I'm happy with the B8's--I don't intend to track the car. I just like to drive hard on curvy roads.

 

Yeah it is a bit more advanced than FX3.

I have FX3 with the coil overs, and in my case it is noticeable to me when i change the settings, i usually can tell what setting it is in without looking or feeling for the switch, i am reasonably confident i would be able to tell a difference in the F45 too, but never driven one.

 

How does FX3 work with coilovers? Did you custom fab the spring setup?

 

Matt, would this 2:1 f:r spring rate ratio also apply for a zr-1 with the heavier lt5? it was ? or, would a 2:1 still work given the approximately 65lbs wet weight extra over the l98? cheers for your thoughts

 

Good question. My question back atcha is if you have verified that this is the actual weight difference between an LT5 and LT1 car? The info is all over the map on this.

But my best answer is that it makes sense to have a slightly higher front spring rate for the ZR1 because of that weight difference. This keeps the suspension frequencies the same as with the lighter engine. Based on the Halverson chart, this appears to be how Chevy approached things, too. In exchange, it then makes sense to have a slightly stiffer rear bar relative to the front bar to keep the handling balance similar...especially with the wide 11" rear wheels. It looks like Chevy did that, too, using a 26mm tubular bar in front and a 26mm solid bar in back (stiffest rear bar ever used on C4s).

 

Very well. I didn't put them on there, Callaway installed them when my car was new.

 

Here's just some food for thought with what I've done with my car. I bought a base model 91 auto with 170k miles on it a few years ago for the main purpose of autocrossing it. It was autocrossed by the previous two owners who I know very well. I believe the the first of those two owners did most of the modifications to the car and the 2nd didn't touch anything. When I first got the car it was very, very loose. The rear would step out on me in every corner. The car was an FX3 car but the shocks were replaced with the Bilstein Z51 replacement shocks (B8's?). The springs an sways were never changed. I don't remember what the front bar was but the rear is the stock 26mm bar for the 91. The car was setup with an alignment with as much camber as was able to be achieved in front and 0 toe.

First thing I did was to put a larger stock front sway bar on it. I don't remember what size it was at this point though. That changed the car from being very loose, to having a bit of understeer. Next was to replace springs. I easily found a NYU rear spring on ebay. I didn't want to put it on until I could find a FHB spring to match it. It took me quite a while (almost 2 years), but I was able to locate one here off of the forum. I installed those springs and put poly bushings in the sway bars. The car is fairly well balanced now with a slight tendency to push at times. So in essence I have the 88-90 Z51 setup on my car but with a larger rear sway bar. I run with NCCC in the stock class which allows all the change I did. At the autocrosses in the stock classes, I can consistently beat base C5, C6, and some base C7s. I have a hard time beating Z06 or grand sports though. The car turns in sharply and stays flat. It really is pretty well set up right now and I like it.

Next thing for me is to get some poly or Delrin bushings. I don't think anybody is making the Delrin for our cars yet though.

Just trying to give you some information on what changes I did to my car and how it affected the handling.

 

That is all pretty good info there. Good experience there too, given what you do with the car.

I had an FHB and a BMF in my 94 with Koni Yellows adjusted to suit, but the swaybars were stock base.

It rode quite firm but sat very flat and i never really had any poor behaviour from it on the street. Had loads of grip and at that time in my life i still found it compliant enough to daily drive it.