Here's a copy of an email I sent a while back on
springs.
If you have the money and the need, the PRI setup
can't be beat. Some of us have to watch our $$, so for
about $350, you can get 2 adjusatable seat Spax
shocks, plus 2 brand new springs. The trick is to
chooses springs that are a bit shorter (.5-1") than
you want when in the (fitted) compressed state. This
then allows you to make the adjustment to bring the
car up. Note that 1" of spring travel = roughly 2" of
wheel travel on our cars due to the geometry of the
suspension.
Anyway, here's what I sent, just based on my
experience, YMMV, hope it helps.
> >
> > The spring formula ( all 4 are the same )
> >
> > lc = lf - (w/r) -or-
> > lf = lc + (w/r) -or-
> > w = r( lf - lc ) -or-
> > r = w / ( lf - lc )
> >
> > lc = compressed length, inches
> > lf = free length, inches
> > w = load in pounds
> > r = spring rate, pounds per inch
> >
> > >From the Triumph Spitfire factory workshop
> manual:
> >
> > part number 209685
> > free length 12.59 in
> > fitted length 7.80 in
> > fitted load 718 lb
> > rate 150 lb/in
> >
> > 7.80 = 12.59 - ( 718 / 150 )
> > 7.80 = 12.59 - 4.78
> > 7.80 = 7.81
> >
> > Pretty close! Too bad you can't use these numbers!
> >
> > But you can determine the actual numbers for your
> car.
> >
> > First determine the compressed length. Use a good
> tape
> > to measure the fitted length, from mating face to
> > mating face on each spring. This will be tricky,
> but
> > be as accurate as you can, make sure you write
> down
> > the numbers.
> >
> > Next, if you are lowering the car, add some weight
> > until you get to the height you want. For weights
> use
> > bags of cement, sand, fertilizer, bricks, or
> friends
> > in the cockpit. Now measure and record the new
> fitted
> > length.
> >
> > Now for the nasty part, you have to remove the
> springs
> > in order to determine the free length and rate.
> >
> > First, mark the springs left and right, magic
> marker,
> > tape, ribbon, whatever.
> >
> > You can check the archives for spring removal,
> here is
> > how I have done it:
> >
> > 1) Set the ebrake, block the rear wheels.
> > 2) Loosen, but do not remove, the lower shock
> mount
> > bolts on each side.
> > 3) Remove the nuts that attach the studs on top of
> the
> > shock to the rubber bushings at the top spring
> perch.
> > You can leave the bushings if you want to.
> > 4) Jack up the front of the car at the center,
> under
> > the front cross member. Work slowly, checking for
> > clearance, binding of the brake hoses, etc.
> > 5) As the front raises, and the wheels drop, the
> > springs will lengthen, and the top studs of the
> shocks
> > will withdraw from the bushings at the top of the
> > spring perches.
> > 6) Eventually, the springs will be fully extended
> and
> > loose. Pull the top of the shock/spring assy
> towards
> > you, it will pivot on the loosened lower shock
> mounts.
> > 7) Wrestle the spring out through the center of
> the
> > top wishbone.
> >
> > Now measure and record each spring free length.
> >
> > Next, you must determine the spring rate. You can
> add
> > known weights and measure change in length.
> Problem is
> > that it may take a couple hundred pounds just to
> get
> > 1" of compression. A common scale may help here.
> Set
> > the spring on the scale, arrange a long 2x4,
> pivoted
> > at one end, the other end on top of the spring.
> Record
> > the weight reading on the scale. Now add the
> weights
> > onto the 2x4, you can hang them or whatever is
> safe.
> > Record the new scale reading, and measure the
> > compressed length. Subtract the unloaded scale
> reading
> > from the loaded reading to get the actual load
> applied
> > to the spring. Subtract the compressed length from
> the
> > free length to get the spring compression
> distance.
> > Divide the load by the distance to get the spring
> rate
> > in lb/in.
> >
> > Whew!
> >
> > So now you know your current springs' free length
> and
> > spring rates. Do not be surprised if these are
> > different from left to right.
> >
> > Use the normal (unweighted) compressed spring
> length
> > measurements, plug in your free lengths and spring
> > rates, solve for w, the load on each spring.
> >
> > w = r( lf - lc )
> >
> > Next is to select new springs that meets your
> needs.
> >
> > As an example, say you calculated your left spring
> > load as 820 lb. You wish to lower the car to a
> > compressed spring length of 7.00 in
> >
> > So, looking at the spring charts, a 14" spring
> would
> > need a rate of 820/(14-7) = 117 lb/in. Probably
> too
> > soft. A 12" spring would need 820/(12-7) = 164
> lb/in,
> > there is a 150 and a 175 listed. 150 would set
> lower,
> > 175 sit higher.
> > Let's try a 10" spring, needs 820/(10-7) = 273
> lb/in.
> > There is a 275 listed. That's a lot stiffer than
> > stock, but with less ground clearance, we will
> need a
> > stiffer spring. You should err on the side of a
> lower
> > ride, you can always raise the car with spacers.
> There
> > is no way to lower it if it's too high. (unless
> you
> > get threaded body coil-over shocks)
> >
> > By the way, to re-install the springs, basically
> you
> > reverse the steps. But go slowly, use a hydraulic
> jack
> > to lower the car. Have a helper guide the top
> shock
> > stud back into the hole, *watch your fingers*!
> Don't
> > ask how I know!
> >
> > If you do not feel safe or comfortable with this
> > procedure, get some help or advice. The amount of
> > energy stored in a spring is enough to cause
> serious
> > injury, *please* be careful and safe.
> >
> > This is certainly a lot of trouble. I'll bet that
> Ted
> > probably already knows what will fit.
> >
> > Anyway, I hope this helps.
> >
> > Carter
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