Fellow Triumph Owner Pat Vilbrandt muses:
:: the free height of the coil springs on a TR4 varies by commission
:: number (but the spring rate is the same - I wish someone would explain
:: that to me?!).
If you want to know why Triumph used different spring lengths at different
times I can't comment. If you want to know how the rate can be independent
of free length, I can sort of help. The spring rate is a function of terms
including the diameter of the wire (BIG influence, as this term is cubed)
the diameter of the coils, the elastic modulus of the steel, and the number
of working coils. I have the formula somewhere, but not close to hand.
Stiffer: fewer coils, tighter wind, bigger wire
Softer: more coils, larger diameter spring, thinner wire.
Free length has no effect on the base rate. What the free length DOES
influence is how much the spring can be compressed before active coils bump
into each other, reducing their number, thereby increasing stiffness.
Of course, suspension travel length has never been a strong point of
Triumphs.
You may have seen springs with one half loosely spaced, the other end
having closer coils. These are "progressive" springs, which have one
rate when all the coils are free to react, but a higher rate when coils
close to each other bind up and turn into what is effectively a tube of
steel, no longer a spring.
mjb.
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