Please check my math

[IdahoZ]

I purchased a 1972 240Z from asomeone who had started a restoration with some performance upgrades and never finished the project. The car has new aftermarket performance springs on it that never saw the street. I have decided to go with new coil overs and was wondering if I could figure out what type of springs are on the car. I am just looking at the front springs at this time, and after a little measuring and testing, I think they are 140 lb/in springs (in the front) and would keep or return a 240Z to stock height. The car was in pieces when I bought it, so I could not really tell how the car would sit with all of the parts on it? I would like to have someone check my math so when I sell these springs, I can describe them accurately. The PO bought most of the parts for this car from VB and MS auto. The springs are blue if that helps.

 

240z (1972)

 

Stock Spring Information (out of the service manual)

 

Front

Normal Load: 562 lbs

Number of Coils: 10

Active Coils: 8.5

Wire Diameter: 0.417 inches

Coil Diameter: 4.35 inches

Free Length: 14.7 inches

Comp Length: 8.0 inches

Compression: 6.7 inches

Spring Rate: 83 lb/in

 

Performance Spring Information

 

Front

Normal Load: 562 lbs

Number of Coils: 8.75

Active Coils: 6.8

Wire Diameter: 0.450 inches

Coil Diameter: 4.325 inches

Free Length: 12.0 inches

Comp Length: 8.0 inches*

Compression: 4.0 inches

Spring Rate: 145 lb/in**

 

*compressed length was calcualted @ http://www.engineersedge.com/calculators/comp_spring_pop.htm

 

**spring rate based on the spring rate calculator at http://www.proshocks.com/calcs/coilsprate.htm and double checked with a spring compression test on bathroom scale (took 140 lbs to compress spring 1”)

 

Summary: The stock spring on the LH side is 2.7 inches longer than the aftermarket spring (14.7" vs 12"). The stock spring will compress 2.7" more than the aftermarket spring (6.7" vs 4"). So the car should sit real close to stock height with the aftermarket 145 lb/in springs, right?

[blueovalz]

Looking quickly through your calculation, they look correct. Now for the important thing. Is the weight on the spring correct? The sprung weight at the corners is vastly different than the corner weights. When you remove the weight of the wheels, brakes struts, etc, you'll find a lot less weight is on that spring. You can get a pretty close verification of this by measuring the compression of the strut. My 200 lb/inch spings had about 2" of compression on the front of my 240, which was about right after I removed the unsprung weight.

[IdahoZ]

I am only assuming that the "Normal Load" called out under the front spring specifications in the service manual is the "Sprung Weight"? All things being the same with the exception of the springs, the unsprung weight would not be enough to make a differance in ride height as long as I use the same weight as the service manual when calculating my ride height with the after market springs? I'm trying to figure out what the spring rate is and approximatley how the car will sit with said springs, as this is the information any potential buyer would probably ask me. I want to sell these aftermarket springs and do not want to mislead anyone, i.e. tell them that they will lower the car 1" when they probably will raise it back up to stock height if their existing springs are old and sprung. I have heard of people buying performance springs that promise a ride height drop, when in fact they do not drop the car at all, and in some cases they raise the car back to stock height. I guess the easiest thing to do is just provide as much information about the spring as I can and inform the buyer they need to make sure they will work for their application.

 

Thanks.

[blueovalz]

Yeah, I'd say you've got enough info there to where any potential buyer should be able to make an "informed" decision.