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Triumph Spitfire & GT6 Spring Rates and resultant Wheel Rates and Roll Stiffnesses
from: Paul Geithner's Triumph Spitfire Technical Information
compiled by Paul Geithner Text Box: qText Box: d1Text Box: d2Text Box: SsinqText Box: WText Box: SText Box: lower A-armText Box: trunnion attachmentText Box: frame attachmentText Box: damper attachmentText Box: LrestText Box: DperchText Box: d3Text Box: ARB attachment
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
(ft-lb/degree) [3]
Spitfire mk1, a.k.a. Spitfire 4 (except FC3214-FC39527) & mk2 209685 green 12.6 7.8 150 0.56 84 147
Spitfire mk1, a.k.a. Spitfire 4
(FC3214-FC39527)
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 Standard-Triumph
c. "Triumph GT6 and Vitesse 2-litre Workshop Manual", p/n 512947, by Standard-Triumph
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 ft-lb/degree = (wheel rate*track^2)/1375
Text Box: DText Box: cText Box: bText Box: a
FRONT ANTI-ROLL 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
(ft-lb/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
Text Box: G = shear modulus of spring steel = 30000000 psi N = number of leaves W = width of leaves T = thickness of leaves L = length of spring (eye-to-eye)
Sources:
a. direct measurement and inspection
b. "Triumph Herald 1200, Vitesse and Spitfire Workshop Manual", p/n 511243, by Standard-Triumph
c. "Triumph GT6 and Vitesse 2-litre Workshop Manual", p/n 512947, by Standard-Triumph
d. "Triumph Spitfire and GT6 - A Guide to Originality" by John Thomason
e. Rimmer Bros. parts catalogs
y. Addco parts catalog
[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 ft-lb/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 (eye-to-eye) (in)
[a,f]
track width (in)
[d]
wheel rate factor [8] total semi-elliptical rate (2x each side's spring rate) (lbf/in) [9] spring rate each side (lbf/in) wheel rate (lbf/in) roll stiffness
(ft-lb/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 Standard-Triumph
c. "Triumph GT6 and Vitesse 2-litre Workshop Manual", p/n 512947, by Standard-Triumph
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; semi-elliptical 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]; semi-elliptical swing spring rate for late GT6 is interpolated from Spitfire swing spring data.
[10]: fixed spring roll stiffness in ft-lb/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 (ft-lb/degree) rear
(ft-lb/degree)
total vehicle
(ft-lb/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%