Coil Spring Tech


    is the formula for calculating the stiffness or rate of a coil spring.    

K = Spring rate in pounds per inch
W = Diameter of the spring wire in inches
G = 12,000,000 for steel springs (a constant)
N = Number of active coils (number of coils that are free to move + 1/2 coil)
D = Diameter of the coils measured to the center of the wire, in inches

Note the relationship between the number of active coils (N) and the spring rate (K).
If you cut off half the active coils the spring rate is doubled.

Applying the above to the stock springs on my 2000 Mustang convertible yields:

Front Springs
WNDKApproximately
0.65"8.754.25"398.63400 lb/in.

Rear Springs
WNDKApproximately
0.55"7.54.625"184.99185 lb/in.

Source: How to Make Your Car Handle by Fred Puhn, page 142

The table below lists some OEM and aftermarket spring rates and the approximate drops they will give if installed on 94+ Mustang.

  Comment or Rate (lb/in) Rate (lb/in) Drop (in.)
  Part No. front rearfrontrear
Ford Motorsports B M-5300-B 425-530 200-300 1.50 0.75
Ford Motorsports C M-5300-C 650 200-300 1.50 0.75
Ford Motorsports F M-5300-F 460-570 200-250 1.50 1.50
Ford Motorsports G (convertible) M-5300-G 460-570 170-310 1.50 1.50
Ford Motorsports R M-5300-R 700-850 200-260    
Eibach ProKit   425-530 200-300 1.50 1.50
Eibach Sportline   425-630 140-295 2.00 2.00
Eibach Racekit   700-850 200-260    
Jamex Sportsprings   425-450 180-225 1.50 1.50
2000 Mustang V6 OEM 400 190    
2000 Mustang GT OEM 450 210    
2001 Bullitt OEM 600 250 0.75 0.75
2003 Mach 1 OEM 600 250 0.75 0.75
2000 SVT Cobra R Eibach 800 750 (IRS)    
2001 SVT Cobra OEM 500 470 (IRS)    
2003 SVT Cobra OEM 600 600 (IRS)    
2003 SVT Cobra (convertible) OEM 500 470 (IRS)    
H&R Sport   490-575 250-285 1.60 1.50
H&R Super Sport   700-760 275-300 1.75 1.60
H&R Race   750-850 260-280 1.25 1.00
H&R Super Race   950-1050 260-300 1.25 1.25
Progress   580 250-300 1.75 1.75
Steeda Sport   650 200-250 1.25 1.25
Steeda Competition   750-850 250 1.25 1.25
Steeda Full Competition [front only]   850-1050 N/A 1.50 N/A

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