First off, a disclaimer. Don't take any of the following as absolute truth,
I'm too tired to double check all my references. Chris K. has seen my office,
he can appreciate the effort required to find a specific bit of information
piled somewhere in the debris. And I just poured myself a shot of some cheap
bourbon left over from the Beach Party, maybe I should have gone for the
Woodford Reserve...
Anyway, I'm somewhat surprised to see Spitfire springs mentioned tonight, as an
hour or so ago I was at the shop staring at the red prod racer, thinking how
little I actually knew about the car, and considering getting it on the scales
and checking wheel rates. Speaking of getting it on the scales, I was pleased
to read the latest "Sportscar" magazine and see that there is a proposal to
put the D Prepared Spitfire on a diet for next year. Cool.
What Bob B. says about tie wraps on the perch is true. Too bad I don't have a
photo online of Killer's front springs. Having a spring come off the perch at
full droop may or may not be an issue. If it has nowhere to go other than
where it should when it is under load once again, no problem. If there is room
for it to flop about and possibly end up mis-seated then you need to deal with
it. Depends on your particular shock and perch combination.
Oh oh, I can see this missive may be getting out of hand. I'll try to be
concise. Yeah, right, Fat Chance.
There are a few critical dimensions related to coil springs. The ones that
carry the most interest for those of us with the coil-over setup of the small
chassis Triumph are rate, free length and fitted length. Gee, come to think
of it those are the most interesting for those of us with the big TRs as well!
But only two of those numbers are independent. These three numbers are in
themselves easy to understand. Spring rate is simple, defined as the distance
a spring compresses when subjected to some force. This is usually expressed
in terms of pounds per inch for those of us using non-metric standards. The
425 pound front springs on Killer require 425 pounds of force to compress 1
inch. Brainless, right?
Free length is also a simple concept. Take the spring, put one end on a flat
surface, and measure how far the other end is from that surface. If you've
ever taken apart a Herald/Spit/Gt6 front shock and spring and lived to tell
about it, you might have noticed that the stock Spit spring is about 14" long
when not crunched up on the shock. This is from memory, I'm likely off a few
inches, but the concepts remain the same.
Fitted length is the distance from one end of the spring to the other as it
sits in place at rest on the vehicle. That stock Spit spring with the 14"
free length may only be 10 inches long if measured in place on a car.
And there's the rub -
Two springs with the same fitted length could have different free lengths
and different spring rates.
Two springs with the same free length could have different fitted lengths
and different spring rates.
Two springs with the same spring rate could have different fitted lengths
and different free lengths.
I've not gotten into wheel rates versus spring rates. I'll turn off the
pontificating for now, and work on getting some real numbers for the next
installment. Those of you who actually know this stuff feel free to chime in.
mjb.
|