After all the mess I went through with the spings on the front end, I thought it might help someone else if I went though some of the formulas and math needed to figure out what springs you should be using when you have made major modifications to your suspension. It might minimize the amount time you spend out in the shop looking at your ride and saying WTF! How did that happen?!

Basic Coil Spring Suspension Calculations:

I found these formulas in a great article covering coil spring over shocks at RideTech. http://www.ridetech.com/info/spring-rate-calculator/  They also have a handy table of vehicle weights and distributions to help give you an idea of where to start. The weight table is at: http://www.ridetech.com/info/coiltech-2/ Even though the article is mainly about coil over setups, the formulas work just as well for a normal coil spring/shock installation.

–  Information that you need to have to begin –

• The total weight of the vehicle – TW (typically 2800-4800 lbs)

Example: TW = 3800 lbs

• What percentage of vehicle weight in on the front wheels – %Front (typically 50-60%

Example: %Front = 55%

• How much is the per wheel unsprung weight (i.e. wheel, tire, disc, calipher etc) – UW (typically 70-120 lbs)

Example: UW = 120 lbs

• Lower Control Arm Pivot to Shock Mount Distance – PS (typically 5-9 inches)

Example: PS = 8.25 in

• Lower Control Arm Pivot to Ball Joint Distance – PB (typically 10-20 inches)

Example: PB = 15.5 in

• Spring or Strut Angle – SA (typically 80-90 degrees)

Example: SA = 90°

• Spring Rate – SR (typically 200-700 lbs/inch)

Example: SR = 303 lbs/in

• Unloaded Spring Height – UH (typically 12 -18 inches)

Example: UH = 17.05 in

– Formulas and Calculations –

• Weight on Front Axle:  FW = TW x % Front

Example:
TW = 3800 lbs
% Front = 55%
FW = 3800 lbs x 55% = 2090 lbs

• Weight on Front Corner: CW = FW / 2

Example:
FW = 2090 lbs
CW = 2090 lbs/2 = 1045 lbs

• Sprung Weight: SW = CW – UW

Example:
CW = 1045 lbs
UW = 120 lbs
SW = 1045 lbs – 120 lbs = 925 lbs

• Motion Ratio: MR = PS/PB

Example:
PS = 8.25″
PB = 15.5″
MR = 8.25″/15.5″ = 53%

• Load on Spring: WS = (SW/MR) x Sine(SA) (Note: Sine is the trignometric function)

Example:
SW = 925 lbs
MR = 53%
SA = 90° –  The Sine of 90°= 1.00
WS = (925/0.53) x 1.00 = 1738 lbs

• Preload on Spring: PL = WS/SR

Example:
WS = 1738 lbs
SR = 303 lbs/in
PL = 1738 lbs/303 lbs/in = 5.74 in

• Loaded Height of Spring: LH = UH – PL

Example:
UH = 17.05 in
PL = 5.74 in
LH = 17.05 in – 5.74 in = 11.31 in

Calculate Spring Rate From Spring Measurements:

I had found a very good formula, I think it was from the Eaton Spring website, that helps you calculate a coil spring’s, spring rate if you have the following information about the spring:

• The inside diameter of the coil spring: CD (typically 3.5 – 4.5 in)

Example: CD = 4.09 in

• The diameter of the wire used to make the spring: WD (typically .5 – 1.0 in)

Example: WD = 0.656 in

• The number of active or free turns (where the wire is not touching anything): AT (typically 7 – 9)

Example: AT = 8 turns

 With the above information you then calculate:

• The spring’s Mean Diameter: MD = CD + WD

Example:
MD = 4.09″ + 0.656″ = 4.746″

• Then calculate the Spring Rate: SR = (11250000 x WD^4)/(8 x AT x MD^3)

Example:
SR = (11250000 x 0.716 x 0.716 x 0.716 x 0.716 )/(8 x 8 x 4.796 x 4.796 x 4.796) = 419 lbs/in

The above information was taken from the spring set that was in my 1956 Dodge Wagon when it came from the factory. Without this formula, there was no way that I could calculate spring heights or what would happen if I were to cut 1 1/2 coils from the spring. 

 Given this spring rate of 419 lbs/in, and the unloaded spring height of 14.75 inches I can calculate using the 1st set of formulas that the original springs should have gave me a loaded spring height of 10.6 inches which was no where close to full droop spring pocket dimension of 12 inches that would bottom out the stop on the upper control arm. As it sits now, the wagon is lighter than the 3800 lb curb weight that I was assuming, however it still was not light enough to bottom out the upper suspension stop so I should have come to the conclusion that something was binding the front suspension. Unfortunately, all this came after the fact since I (or should I say THE DUMMY) didnt really look at any of this until after I had chopped the springs … DUH.

When I cut 1 1/2 coils from each of the front springs, I changed the spring rate of the spring from 419 lbs/in to 515 lbs/in and I also reduced the unloaded spring height from 14.75 in to 12.75 in.

This did have a major affect on the front end! Not only was the unloaded spring height of 12.75″ very close to the full droop distance of the front suspension, but the loaded spring height had dropped down to 9.38 inches. Since the stop on the lower control arm bottoms out at 10″ spring height the old girl was firmly bottomed out on the stop and even with the light curb weight, the front end was almost laying on the ground.

OH, and one more thing that I found out at the Eaton site. There are three basic styles of coil suspension springs – ones with pig tail ends, like the rear springs on a lot of GM full and midsize cars, tangential ends like the coils used on the ever popular Mustang II front ends and finally the square or box end coils as used on the front springs of GM cars. The catch here is that they only recommend that you cut coils from a tangential spring end since the upper and lower spring pockets in the suspension are designed for that style.

When you cut square end springs (like the ones in my Dodge and all GM products) they end up being tangential, but the lower spring pocket is designed for square ends so the spring ends up taking the full load on the very end of the cut part of the spring, instead of larger surface area of the square end spring. This in itself really cuts down the effective spring rate of the cut coil, which could lead to early failure or at least some screwy handling on rough roads.

So, unless you have a Pinto/Mustang II front end under your hot rod, spend some time working with the formulas and the Moog spring catelog. Dont be like my shop dummy and whip out the cut off wheel and start chopping before you know it will work!

Another Excel spreadsheet that you might find helpful is this one with specs on all of Moog’s production coil springs: Moog Coil Springs 

Once you have a Moog part number for the spring you wish to use, you can go to RockAuto and use their part number search to find the spring, then select the buyers guide to get a listing of all of the cars, models and years that used the spring in production. Typically RockAuto has about the best prices for OEM parts that I have found. You may also want to check Summit Racing or JEGS as they sometimes have specials going.

So with that, I think I am about ready to put the dash board back in and start wiring things up. And I need to order brake lines and fittings along with A/C hose and fittings, and some fuel line and fittings would be nice as well! Merry Xmas to Me!

Hopefully, I can keep the Shop Dummy busy elsewhere so that I wont have any major wiring or plumbing issues, hard to say though…. hard to say!