# How to determine the compressed height of a spring?

I am looking at replacing the springs on my vehicle and am trying to get stiffer springs while avoiding a major change in ride height.

With my vehicle, one of the old common ways to do this was to install springs from a special edition (88SS) of the car (either junkyard pull or order OEM). These springs are now super rare and I am considering getting a set custom made.

I have the Wire Diameter, Coil Diameter, Free Length and Spring Constant of both my original springs and the 88SS springs.

What I'm trying to figure out is, given a certain amount of applied weight, what the height of the spring will be so I can compare the specs of the two springs and see how much of a difference in compressed height there will be.

I'll provide the specs below if you'd like to answer with the actual numbers (please show your work though), or the formula itself.

OEM Springs (F/R)
Wire Diameter: 13.8 mm / 13.8 mm
Coil Diameter: 170 mm / 110.0 mm
Free Length: 294.5 mm / 336.5 mm
Spring Constant: 146 lb/in / 188.2 lb/in

88SS Springs (F/R)
Wire Diameter: 14.5 mm / 13.5 mm
Coil Diameter: 170 mm / 110.0 mm
Free Length: 263 mm / 317.0 mm
Spring Constant: 207 lb/in / 224 lb/in

1984 Nissan 300ZX Turbo 50th Anniversary
Gross Vehicle Weight: 3,490 lbs
Gross Front Axle Weight: 1,630 lbs
Gross Rear Axle Weight: 1,860 lbs

(Side Note: the front weight is down about 60lbs from removal of the A/C and probably a good 5-10lbs from replacing the OEM air box with a K&N filter)

Spring Specifications courtesy of http://www.az-zbum.com/information.springs.shtml

• You should take the car over to a scale (truck scales are usually pretty easy to find and cheap to use) and get the true weight. Gross vehicle weight is total vehicle weight, when loaded to manufacturer maximum limits. Most larger cars are significantly under that when on the road (5 seaters designed to hold adults and baggage) and some smaller cars are often over it (like my MR2 that only permits 399lbs total for passengers AND baggage!). Aug 25, 2011 at 14:42
• @Brian, I was thinking about your scale suggestion and the issue I'm seeing there is it'd be measuring the unsprung weight also (LCAs, axles, wheels, rotors, tires, etc). I'm not sure if there is a truly accurate way to get your cars precise sprung weight without some work involved. Sep 1, 2011 at 2:34

You have the spring rate (you call it spring constant) so you can calculate it yourself. ;) The spring rate is the amount of force (weight/mass) the spring requires to compress the specified amount. For example, your OEM front springs compress 1 inch when you put 146 pounds on it.

So, with the numbers you provided the OEM height can be calculated. The car puts down 1630 pounds of the front axle. I'll asume this weight is evenly distributed and it's the unsprung mass, just for sake of simplicity. So, each spring carries 815 pounds. Because the front spring has a spring rate of 146 pounds per inch we can calculate that the spring will compress by (815/146) 5.58 inches. The unloaded spring is 11.59" inches high, so the loaded height of the spring is (11.59-5.58) 6.01".

So, if you want stiffer springs with the same ride height you need to have springs with the following specifications:

First, we have the desired compressed height: 6.01"

Second, we have the spring rate: 207 lb/in (88SS front springs)

Third, we have the weight per spring: 815 lb

So, these springs compress (815/207) 3.94". To have a ride height of 6.01" we must have a free length of (6.01+3.94) 9.95". This translates to a free length in milimeters of ((9.95*2.54)*10) 252.73mm.

Btw, very nice car! I love the 300ZX, but unfortunately it's very very rare in my country. :(

• Thanks, that was what I wanted, I figured I had the right data just which numbers to apply against each other. In my case, looks like using the 88SS's would add about .34 inches (taking into account my reduced weight) to the front but the rear will be only a .022 inch increase. I guess if I get the custom rolled springs I'll have them adjust the fronts a bit. Aug 24, 2011 at 13:32
• This calculation assumes that the spring rate is constant across the range of compression. I don't believe that is true, but I do not know how far off they would be. Aug 25, 2011 at 14:40