blackozvet

I thought I would share some facts found researching Z51 spring rates and swaybar sizes that some people might find interesting.
I posted this info in c4 tech, but thought that some of the c4 guys in this section might find the info usefull.

i will put the year/front spring/rear spring/front bar/rear bar.
also quote in N/mm and Lbs/inch.

'84 - 102 nmm/582 lbs 87.5/499 25/23
widely regarded as the stiffest suspension put on the c4, front BMB/rear BMH code springs (but wait till we get to '88)

'85 - 63.5/362 57.2/326 30/24
after customer feedback GM engineers go softer on the springs, but bigger on the swaybars. NYU code for the new rear spring.

'86 - 66.5/380 57.2/326 30/22
slightly more front spring rate, 2mm less on the rear bar.

'87 - 66.5/380 57.2/326 30/22
no change.

These are the 4 years that the c4 Corvette dominates the SCCA championship and promptly gets banned from the series !
From what I have read a lot of racers used the 102/582 front spring with the 57.2/326 rear spring with 30/24 swaybars, although there was some mixing and matching (within the rules of course )

'88 was the beginning of the Corvette Challenge series, a race series for the Corvette only. The front geometry of the C4 was also changed.

'88 - 115.5/659 - 57.2/326 30/24
where did that front spring come from, it makes the 84 BMB spring look like it came off a luxury car ! The GM engineers had racing on their minds for sure with this combo.

'89 - 115.5/659 - 57.2/326 30/24
this is the year the R9G was offered for sale, a factory race car !
R9G 115.5/659 - 57.2/326 30/24
no doubt that spring/swaybar combo was being developed for racing.
'88 and '89 were the only 2 years of the Corvette Challenge Series.

'90 - 115.5/659 57.2/326 30/24
R9G 115.5/659 57.2/326 30/24
Corvettes were invited back this year to the World Challenge Series.

'91 - Z51 number dropped, replaced with Z07 code. No R9G available.
'91 - 115.5/659 57.2/326 30/26
the only year of the 26 mm rear bar, 2 mm more in the rear bar, engineers looking for more oversteer ?

'92 - 90.1/514 57.2/326 30/24
softer front spring, smaller rear bar.

'93 - 90.1/514 57.2/326 30/24
no change.

'94 - 90.1/514 57.2/326 30/24
no change.

'95 - 90.1/514 57.2/326 30/24
no change.

'96 - Z51 designation returns.
'96 - 73.2/418 33.0/188 26T/24
the 96 Z51 was the Cadillac version, spring rates verging on FE1 rates.

Interestingly, the rear Z51 spring rate did not change from 85 to 95, even though the front rates varied quite a bit.
Ride comfort in the C4 is mainly affected by rear spring rate, the engineers were obv happy with the 57.2/326 as a performance/street driven rear spring rate.
People who have used the 84 BMH rear spring 87.5/499 have stated it gives a harsh street ride, and tends to bind up the rear end of the c4 under cornering in racing.


Reference is from the Corvette Action Centre 84-96 suspension chart.

Does anyone else have any information (or corrections), especially spring codes ?
Hopefully this will be helpful to anyone looking at changing spring rates whilst using leaf springs in a performance/racing application.

SPRING CODES

FRONT 84-87
BMB 102
HMZ 63.5
FHU 63.5
HKX 39.9
HMP 39.9
HM2? 57.2
HMN 54.4

FRONT 88-96
FHA 93.1
FHB 115.5
FHC 93.1
AA 96.2
FSK 90.1
FSM 75.4
FSS 90.1
FSR 73.2
HA 60
JA 60

REAR 84-96
BMF 72
BMH 87.5
NYU 57.2
NYR 39.9
RCA 39.9
RHY 39.9
RHZ 39.9
RR 26




REAR SPRING CODES

Code #/in N/mm Year(s) GM part # GM Service Parts Catalog
---- ---- ------- ------- -------- ------------------------
BMH 499 [87.5] 1984 14045786 . . . . . . . . . . 7-H19


BMF 411 [72.0] 1984 14045785 . . . . . . . . . . 7-H19

NYU 326 [57.2] 1985-95 14080196 . . . . . . . . . . 7-H19 7-H22 7-I3

NYR 228 [39.9] 1985-93 22112253 . . . . . . . . . . 7-H19 7-H22 7-I3

RCA 228 [39.9] 1986-96 14106787 . . . . . . . . . . 7-H19 7-H22 7-I4

RHY 188 [33.0] 1992-96 17999322 . . . . . . . . . . 7-I4

RHZ 148 [26.0] 1994-96 22112253 . . . . . . . . . . 7-I4

___?d 148 [26.0] 1994-95 22112254 conv FE1 (SR) FX3 . 7-I3 no 3 letter code -
Code #/in N/mm Year(s) GM part # GM Service Parts Catalog
---- ---- ------- ------- -------- ------------------------
FRONT SPRING CODES

FHB 590 [115.5] 1988-91 10048553 . . . . . . . . . . 7-E9 7-E13 7-E19

BMB 521 [102.0] 1984 14045782 . . . . . . . . . . - not in microfiche

AA 491 [96.2] 1990-91 17988598 . . . . . . . . . . 7-E13 7-E19

FHA 476 [93.1] 1988-91 10048552 coupe FX3 FE7 + L98 7-E9 7-E13 7-E19

FHC 476 [93.1] 1988-91 10048574 conv FE1 (SR) . . 7-E9 7-E1347-E19

FSK 460 [90.1] 1992-95 17997091 . . . . . . . . . . 7-E19

FSM 385 [75.4] 1992-95 17997093 . . . . . . . . . . 7-E19

HMZ?a 379 [66.5] 1986-87 14106617 coupe FE7 ___ . . - not in microfiche

FSR 374 [73.2] 1992-95 17999031 conv FE1 (SR) . . 7-E19

FSS 374 [73.2] 1992-96 17999032 coupe FE1 (SR) . . 7-E19 7-E20

HMP 362 [63.5] 1984-87 14106617 . . . . . . . . . . 7-E5

HMP 379 [66.5] 1984-87 14106617 . . . . . . . . . . 7-E5

HA 306 [60.0] 1989-95 22145203 coupe FE1 (SR) FX3 7-E13 7-E20

JA 306 [60.0] 1994-95 22145227 conv FE1 (SR) FX3 7-E19

...?b 306 [60.0] 1995 22173029 conv FX3 (ER) . . 7-E19 no 3 letter code
...?c 306 [60.0] 1996 22173028 coupe FE1 (SR) . . 7-E19 no 3 letter code

HMN 278 [54.4] 1986-87 14094457 conv FE1 (SR) . . -

HKX 276 [54.0] 1985 14094458 . . . . . . . . . . page 7-C24 microfiche

HMP 261 [51.8] 1986-87 14094458 . . . . . . . . . .

Solofast

Your data has one error in it. The 90 Z51 has a 24mm rear bar. This error is in several of Hib Halversons charts, but he has corrected that error in others on the web. The only year that had a 26mm rear bar in the Z51 was 1991.

With a combination of too much rear bar and/or spring coupled with the relatively high roll center does not "bind up', it jacks up and the sticker the rubber you have on the car the worse it is. There are plenty of pictures on the web of C4's with positive camber on the outside rear wheel when they are cornering at high G's, and that's just the way it is.

I drove an early production 84 when they just came out and wrapped it down a fast on ramp and felt it jack up a bit, step over and then set back down... When I threw the keys back to the GM engineer that loaned me the car I told him it was great, but that it jacked in the back... He started to vociferously deny it, until a second guy from the proving ground tapped him on the shoulder and said... "He's right, it jacks a bit"... Then he said that that rear spring was gone next year, so they knew the issue existed from the start...

The big rear spring on the 84 was matched with a smaller rear bar (23mm). The 91 bigger rear bar was a disaster. That car jacked in the back like an old VW with that setup.

Bottom line is that the fundamental limits to the rear suspension in stock form is that there are combinations of rear spring stiffness and bar that cause the car to jack up.

Lowering the rear roll center would help, as does lowering the car a bit, but you can't lower a C4 very much before you are on the bump stops, so there are limits there too.

blackozvet

does that apply to both z51 and R9G ? (I will edit that part)

it looks like in the rear you can have a stiff spring or big bar, but not both !

Solofast

Don't know about R9G, only that the Z51 bigger bar was only one year and that was 91.

Yup, it's a matter of total roll stiffness. It's kinda like the old skit about the Chinese restaurant that Buddy Hacket used to do... You can have one from column A and one from column B... No you round eyed iriot, you can't have 2 form column B!!!

blackozvet

its like the laws of nature, too much of a good thing always ends up being bad !

froggy47

Describe "jacks up" as you are using it.

Number90

On my build sheet I have listed the front springs FJY with rear springs RCA.

I am not fully understanding the window sticker and the build sheet but it appears my Vette is a Central Office order from Warren MI but delivered to Hubler Chev in Indianapolis. VIN 900051

It's not a Z51 cuz it's a vert but has many of the parts, G92, V01, B4P, KC4/ZJS. I am not sure about everything in the Z51 but it appears to be some of it or those parts that can be ordered on their own possibly. Also my lock to lock steering is 2.25.

The first public sale record I am finding is in Dallas TX with 3750 miles on it. I am thinking if it was not a track car at the '86 Indy 500 then it may have been a dealer trade and dealer car like sales manager driven. No solid proof yet tho either way.

Any recommendations, clarifications and info on my '86 C4 Vert would be greatly appreciated.

Thanks,
John

blackozvet

Hi John,

the FJY is 54.4 n/mm or 311 lbs/inch
the RCA is 39.9 n/mm or 228 lbs/inch

they are your usual FE1 type springs which are average in spring rate by comparison.

Im pretty sure that the 2.2 steering ratio is non z51 also.

Number90

Very good, thank you.

MatthewMiller

Can I check some math here? I'm trying to verify the rate of a FHB front spring I have. For the FHB, I see that one listed everywhere as 115.5, but shouldn't that convert to 659lb/in instead of the 590lb/in that's listed?

Also, when that FHB spring is paired with an NYU rear (326lb/in), does that comprise a set of "Challenge" springs, which is the same as 88-91 Z51 rates? Do I have that right?

blackozvet

yes, the FHB at 115.5 equals 659 lbs, where did the 590 lbs come from ?

the challenge cars had to run as they came out of the factory (except for compulsory safety mods) and that included the z51 suspension springs , shocks and sway bars.

MatthewMiller

It's in the last listing of spring codes, from the GM catalog:
Now that I'm looking at that list further, I think a lot of their calculations are wrong. I think many their front rate conversions are wrong. Strangely, their rear spring rate conversions are correct. The ones that are wrong are wrong by a factor of about 0.895, so either someone's slide rule got buggered up when this chart was first assembled, or they tried to factor in the motion ratio for some of the conversions, or...something.

But unless I'm missing something, the conversion factor for N/mm to lb/in is 5.71 (rounded to nearest hundredth). I might be missing something, because a lot of website unit converters don't give answers that are proportionate to 5.71/

Thanks for that!

blackozvet

I was warned that some of the stuff that came out had errors in it,

I use this online calculator,
http://www.convertunits.com/from/pound/inch/to/N/mm

Im not sure why they used N/mm as a measurement, no one else in the modern (hot rod) world does !

CorvetteCam91ZO7

I know that this is an older thread but really helpful !

69autoXr

Do all 88-96 front springs have the same arch? Specifically I'm wondering how the FHA 93.1 base front spring compares to the FSK 90.1 92-95 Z07 front spring. Do they give the same ride height?

Kubs

When I converted my FE1 spring to the Z51 on my '90 there was a difference in arch of the spring and right height was similar. When I took all the weight out of the car to make it a track car there was still too much arch and I had to mount the spring end UNDER the control arm to get ride height correct.

RedHot85Vette

Steering Ratios Base vs Z51
Base: 2.36 Turns / Z51: 1.96 Turns
As Per the MVMA-C-85 Document, Pg. 73

VikingTrad3r

Kubs, apologies for bringing this old thread to the forefront but you mention here that you run your transverse spring UNDER the lower control arm. Would you be able to describe (pics would be awesome but i think you have sold your car!) of how the heck you achieved this? I have searched....havn't found anything.

Some of us who are lightening and racerfying our cars would love to know!

Thanks!

MatthewMiller

I'm sure Kubs will answer, but I was going to also suggest that you ping Tom400CFI about this question, since I believe he has at least contemplated the same thing. You basically hang the spring ends from the LCAs with bolts, same as the rear spring setup. At least I think that's the idea, but this is the part where Kubs should jump in!

Kubs

Matt is correct. I ended up using a wood style bit to drill a hole in the fiberglass spring, and a hole in the control arm pocket where the spring normally sits. I used the Banski delrin bolt kit intended for the rear to hang the spring under the arm. I had to cut the bolt shorter so it wouldnt hit the shock, but it was otherwise pretty straight forward.





Also, I dont have a picture of this, but when the spring was in my full weight street car I cut the rubber off where it meets the frame to lower it. Now I needed the spring to hang lower to lay a little more flat when loaded. I put a small (I think 1/2" thick if I recall) block of aluminum in the frame pocket to lower the spring back down a little.

This picture was after doing the bolt mod with NO ENGINE in the car! Very low.