85 Z51 Front Spring Replacement Selection / Rate

[DaveP85C4]

I haven't been on here in awhile. Forgot how cool it is compared to other Corvette forums....

I need to replace the front spring in my 85. (Very original unmolested car, and has the award to verify it). It is breaking. My main concern is that the correct original ride height be retained. It is a Z51 car. Because of the reduced rate from the fracture, it is also over-steering. It would be nice if the rate re-established the almost perfect balance this car had for the first 30 years I owned it.

Hib's Chart shows a rate of 63.5 Nn/m for an 85 Z51 front. Corvette Brakes lists two springs: 300-320 or 321-340 and a "sport" of 450-550. I spent some time looking for an online conversion, but I couldn't do it.

Anyone able to do the conversion for me from the 63.5 into ftlbs? Any other sources for springs in the early C4's? I don't trust used, they will be as old as the one that is breaking.

Thanks in advance. I'll try to drop in more frequently.

 

[forrest]

I haven't been on here in awhile. Forgot how cool it is compared to other Corvette forums....

I need to replace the front spring in my 85. (Very original unmolested car, and has the award to verify it). It is breaking. My main concern is that the correct original ride height be retained. It is a Z51 car. Because of the reduced rate from the fracture, it is also over-steering. It would be nice if the rate re-established the almost perfect balance this car had for the first 30 years I owned it.

Hib's Chart shows a rate of 63.5 Nn/m for an 85 Z51 front. Corvette Brakes lists two springs: 300-320 or 321-340 and a "sport" of 450-550. I spent some time looking for an online conversion, but I couldn't do it.

Anyone able to do the conversion for me from the 63.5 into ftlbs? Any other sources for springs in the early C4's? I don't trust used, they will be as old as the one that is breaking.

Thanks in advance. I'll try to drop in more frequently.

using a basic engineering APP. on my phone I came up with 63.5 Nm/m converts to 749.36 once ft. or 46.835 pound ft. OR 562.022 pound inch
I hope this helped

Here is a link to a double checked sit Convert newton meters to foot pounds - Conversion of Measurement Units

 

[DaveP85C4]

using a basic engineering APP. on my phone I came up with 63.5 Nm/m converts to 749.36 once ft. or 46.835 pound ft. OR 562.022 pound inch
I hope this helped

Here is a link to a double checked sit Convert newton meters to foot pounds - Conversion of Measurement Units

Thank You.

I'm pretty good with Ohm's Law, and measurement conversions, but have zero experience with force conversions. I couldn't make the results "fit" the numbers listed for the Corvette Brakes springs. I'm now guessing the "sport" spring with it's "450-550" rating is in pound inch, and is closest to the Z51 rate of 562 that you calculated for it from Hib's chart.

Thanks again. It helped a LOT.

 

[catbert]

If you are a numbers matching purist stop reading now........

If I had an early C4 with a broken Z51 spring, I'd call it a blessing and replace both units with good base or Z52 (1987 units) springs. The Z51 package's only saving grace was skidpad performance and track performance on an absolutely smooth surface. The Z51's are uncomfortable overkill for the street, and even most autoX applications. They tend to skip and bounce over irregularities rather than adhere. When I raced early C4s, I took off both the Z51 springs and shocks, and replaced them with Z52 spec units. It was the stealthiest cheat on the circuit. Other drivers had no idea why I was so fast on most courses. The ride height with Z52 was never out of spec for a Z51 car, and it was checked fairly often.

The prevailing "wisdom" back then was composite springs didn't have the "wear" issues of conventional springs, and undamaged, used springs were perfectly acceptable. I never saw a failure of used replacements, even on the track. Around here C4 guys still routinely replace defective springs with good, undamaged, low mileage used units. The Vette shops do the same thing.

 

[DaveP85C4]

Thank you for the reply and thoughts.

I had an 88 Convertible with Z52. I agree the 88's handling performance was superior to the 85's, but I attribute the difference to the better tires and no-scrub geometry, and the structural enhancements of the 88. I've owned the 85 since new. After the first few years, my objective with the car became "preservation in its original state". Not sure how or why, it just did. It has since been through the NCRS "System" and received a Crossed Flags Award for originality in 2008. My car is one of two NCRS Crossed Flags 1985 Corvettes on the planet. I'm not a "purist" per se, everyone is entitled to do what they wish with their car(s). I've beat the hell out of, and modded the heck out of many of my cars. Just not this one. Anyone who has never attempted to keep an original car "original" for over 30 years has no idea how difficult it really is to accomplish. Mostly just keeping stuff from falling on it in storage. It's damn difficult, perhaps that's why so few cars survive 30+ years in their original state. The harsh ride of the 85's Z51 package is well known. In an original car, the correct ride should probably be preserved too.

I'm reluctant to try used springs because mine began to fracture 10 years ago with 35,000 miles on it after a rather docile life including a 10 year period where it was never started, just occasionally pushed in or out of the garage. But I'll take your input that used springs have been known to give satisfactory service when reinstalled under advisement. I wouldn't mind retaining the "correct" spring code, just because of what the car is and what the objective for it has been. On the other hand, It's been to the Valhalla of Corvette judging, was accepted, and will never be subject to that scrutiny again. So "correctness" at this point is secondary to making the car reliable to drive again.

When I drive it, I tend to extract the performance that's available. I was cautioned that if the spring were to break while the suspension was loaded up I could suffer a loss of control and the results could be detrimental to the originality I've worked so hard to retain. I want to change the spring for reliability, but improvements in handling are not a requirement.

Thanks again. It is appreciated.

 

[Hib Halverson]

If you are going to downgrade the car's handling in exchange for a nicer ride, you can't do it just by changing springs.

You also must change the stabilizer bars and probably the shock absorbers.

As for the conversion...

The FE7 spring rate for MY85 is 102 N/mm. That converts to 582 lbs/in.