
Triumph Spitfire & GT6 Spring Rates and resultant Wheel Rates and Roll Stiffnesses  
from:  Paul Geithner's Triumph Spitfire Technical Information  
compiled by Paul Geithner 


FRONT COIL SPRINGS  
native vehicle  spring part number [b,c,d,e] 
identifying marks [a,b,c,d]  free length (inches) [a,b,c,d] 
installed length (inches) [a,b,c,d]  spring rate (lbf/in) [a,b,c,d] 
wheel rate factor [2]  wheel rate (lbf/in) 
roll stiffness (ftlb/degree) [3] 

Spitfire mk1, a.k.a. Spitfire 4 (except FC3214FC39527) & mk2  209685  green  12.6  7.8  150  0.56  84  147  
Spitfire
mk1, a.k.a. Spitfire 4 (FC3214FC39527) 
210566  light blue  12.2  7.4  150  0.56  84  147  
Spitfire mk3 & mkIV  214144  purple  12.2  7.5  150  0.56  84  147  
Spitfire 1500 [1]  TKC1884  purple & white  13.4  8.0  180  0.56  101  177  
GT6 (all)  212425  n/a  12.5  8.1  200  0.56  113  197  
aftermarket Spitfire 1500 [x,1]  TT4302  n/a  9.2  6.9  330  0.56  186  324  
Sources: a. direct measurement and inspection b. "Triumph Herald 1200, Vitesse and Spitfire Workshop Manual", p/n 511243, by StandardTriumph c. "Triumph GT6 and Vitesse 2litre Workshop Manual", p/n 512947, by StandardTriumph d. "Triumph Spitfire and GT6  A Guide to Originality" by John Thomason e. Rimmer Bros. parts catalog x. Moss Motors/TriumphTune catalog 

[1]: lengths and rates for Spitfire 1500 and aftermarket springs are direct measurements  
[2]: wheel rate factor = sin(theta)*(d2/(d1+d2))^2; see figure  
[3]: roll stiffness in ftlb/degree = (wheel rate*track^2)/1375  


FRONT ANTIROLL BARS  
native vehicle  spring part number [b,c,d,e] 
diameter (inches) [a,d] 
spring rate (one end, lbf/in) [4] 
wheel rate factor [5] 
wheel rate (lbf/in) [6] 
roll stiffness (ftlb/degree) [7] 

fixed spring Spitfires and GT6s  207093  0.6875  61  0.35  21  39  
swing spring Spitfires and GT6s  217033  0.875  160  0.35  56  102  
aftermarket 1" Addco [y]  707  1  273  0.35  96  175  


[4]: ARB rate in lbf/in = 500000*D^4/(a*0.4244*b^2 + 0.2264*c^2); see figure  
[5]: wheel rate factor = (d3/(d1+d2))^2; see figure  
[6]: NOTE: ARBs contribute only to roll stiffness, nothing to droop and bump stiffness  
[7]: roll stiffness in ftlb/degree = (wheel rate*track^2)/1375  
REAR TRANSVERSE LEAF SPRINGS  
native vehicle  spring part number [b,c,d,e] 
spring type [a,b,c,d,e] 
composition [a,b,c,d,e,f]  fitted spring
width (eyetoeye) (in) [a,f] 
track width (in) [d] 
wheel rate factor [8]  total semielliptical rate (2x each side's spring rate) (lbf/in) [9]  spring rate each side (lbf/in)  wheel rate (lbf/in)  roll stiffness (ftlb/degree) [10,11] 

Spitfire 4, mk1, 2, 3  305894  fixed  7 leaves: 0.2188" thick 1.75" wide various lengths 
41.25  48  0.74  166 [a,b,d,f,z] 
83  61  103  
Spitfire
mkIV, early (up thru FH50000) 
159640  swing  5
leaves: 0.25" thick (bottom two) 0.375" thick (top three) 1.75" wide various lengths 
41.25  48  0.74  323 [a,z] 
162  119  41  
Spitfire
mkIV, late (FH50001 and beyond) and all 1500 
50  0.68  110 [d,z] 
41  
GT6 mkI  307324  fixed  8 leaves: 0.25" thick 1.75" wide various lengths 
41.25  48  0.74  305 [f,z] 
153  113  189  
GT6
mkII (a.k.a. GT6+) early GT6 mkIII (through KF20000) 
308499  fixed  6 leaves: 0.234" thick 1.75" wide various lengths 
41.25  49  0.71  185 [a,f,z] 
93  66  114  
late
GT6 mkIII (KF20001 and beyond) 
159654  swing  5 leaves: 0.3125" thick (bottom two) 0.375" thick (top three) 1.75" wide various lengths 
41.25  50  0.68  383 [z] 
192  130  87  
aftermarket Camber Compensator (Joe Curry model)  n/a  pure swing  1 leaf: 0.25" thick 2" wide 
37  48  0.59  115 [a] 
58  34  0  
Sources: a. direct measurement and inspection b. "Triumph Herald 1200, Vitesse and Spitfire Workshop Manual", p/n 511243, by StandardTriumph c. "Triumph GT6 and Vitesse 2litre Workshop Manual", p/n 512947, by StandardTriumph d. "Triumph Spitfire and GT6  A Guide to Originality" by John Thomason e. Rimmer Bros. parts catalogs f. TSSC "The Courier" issue #158, article "Suspensions Sussed" by John Thomason, August 1993 z. calculated 

[8]: wheel rate factor = (fitted spring width/track width)^2  
[9]: published fixed spring rates compare closely with calculated rates where k = G*N*W*T^3/3(L/2)^3, see figure; semielliptical swing spring rate of 323lbf/in for later Spitfire is actual measurement, which when converted to wheel rate correlates precisely with published 110 lbf/in wheel rate by Thomason in "Triumph Spitfire and GT6  A Guide to Originality" [source d]; semielliptical swing spring rate for late GT6 is interpolated from Spitfire swing spring data.  
[10]: fixed spring roll stiffness in ftlb/degree = (wheel rate*track^2)/1375; swing spring roll stiffnesses calculated by participation of fixed leaf only; calculation compares with claim by Thomason on p. 77 of "Triumph Spitfire and GT6  A Guide to Originality" [source d] that the roll stiffness of the Spitfire swing spring is 25% of that of the Spitfire mk3 fixed spring, and also compares well with the hypothesis that net loss of roll stiffness with the swing spring is offset with net gain in roll stiffness with a stiffer ARB and that total vehicle roll stiffness is comparable between predecessor fixed spring and successor swing spring swing axle vehicles (e.g., Spitfire mk3 vs. mkIV and GT6 mk1 vs. late mk3).  
[11]: NOTE: camber compensator contributes only to droop and bump stiffness, nothing to roll stiffness; data is for camber compensator model sold by Joe Curry.  
FACTORY VEHICLE VERTICAL (BUMP/DROOP) STIFFNESS  
native vehicle  total
front wheel rate (lbf/in) 
total
rear wheel rate (lbf/in) 
total
vehicle vertical stiffness (lbf/in) 
%
front vertical stiffness 
%
rear vertical stiffness 

Spitfire 4, mk1, 2, 3  169  123  291  58%  42%  
Spitfire
mkIV, early (up thru FH50000) 
169  239  408  41%  59%  
Spitfire
mkIV, late (FH50001 and beyond) 
169  220  389  43%  57%  
Spitfire 1500  203  220  423  48%  52%  
GT6 mkI  225  225  450  50%  50%  
GT6
mkII (a.k.a. GT6+) through early GT6 mkIII (through KF20000) 
225  131  356  63%  37%  
late
GT6 mkIII (KF20001 and beyond) 
225  261  486  46%  54%  
FACTORY VEHICLE ROLL STIFFNESS  
native vehicle  front (ftlb/degree)  rear (ftlb/degree) 
total
vehicle (ftlb/degree) 
%
front roll stiffness 
%
rear roll stiffness 

coil  ARB  total front  
Spitfire 4, mk1, 2, 3  147  39  186  103  289  64%  36%  
Spitfire mkIV  147  102  250  41  291  86%  14%  
Spitfire 1500  177  102  279  41  320  87%  13%  
GT6 mkI  197  39  236  189  424  56%  44%  
GT6
mkII (a.k.a. GT6+) through early GT6 mkIII (through KF20000) 
197  39  236  114  350  67%  33%  
late
GT6 mkIII (KF20001 and beyond) 
197  102  299  87  386  78%  22%  