Wow!! Thanks Carter !
I've printed your response and
will use it to help with choosing
the new springs.
Larry
Carter Shore wrote:
> Larry,
> I could not find the posting I sent on springs, but
> here's the gist of it:
>
> 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
> Yahoo! Auctions - buy the things you want at great prices
> http://auctions.yahoo.com/
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