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SPRING
RATE and RIDE
HEIGHT
Here is a method of checking
spring rates yourself. You just need a bathroom scale, a scale (ruler),
a couple of pieces of bar stock, and the biggie, an H-Frame press. If
you have the rest of it, it probably wouldn't be too hard to find someone
to let you use their press for a few minutes. I don't know that the bottom
plate is entirely necessary. On these old scales, it seemed that the platen
wanted to deflect in the center more than I liked so I put the piece on
there. The bar on the top serves as a place to apply pressure and also
as a pointer for reading the scale. Assemble the items as shown and use
the adjuster to zero the scale. Bring the ram down until it just touches
the bar on the top and the scale has not began to register. Read the scale
for a dimension and then apply for to the spring with the ram. If the
spring is light enough, you can travel it a full inch and get a direct
reading. In other words, when you stop the ram at an even one inch of
travel, the scale will read directly what the spring rate is. If it says
187, then you have a 187 lbs. per inch spring rate. What you have to watch
out for is when the spring rate exceeds the scales capacity. That is what
is shown in the photo. That is a spring off of a Harley and in the photo
the reading is just about 147 lbs. What I had to do here is just travel
the ram 1/2" since the scale is only a 250 lb. capacity. If I had
of gone the full inch, I would have broke the scale. Since spring rates
are lineal, it follows that the spring rate for this spring is 294 lbs.
per inch. (2 x 147)
What you need right now, more than the spring rate, is to establish
the ride height and you have enough to information to do that right
now. You have coilover shocks that have 5 inches of travel (17"-12"=5").
When the car is complete, you need approximately half of the travel
of the coilovers in both jounce and rebound (compression and extension).
So, divide the total travel in two and you have 2-1/2" of travel each
way. Just take the fully extended length and deduct 2-1/2" for a ride
height length of 14-1/2". When the car is completed and the full ready-to-go-down-road
weight is on it. Then you will be able to evaluate what you need for
actual spring rates and the whole thing can be fine tuned for height
with the bottom spring seat adjusters. To be able to know what actual
springs you need at this point involves a lot of guessing. Here is just
a sample of what would need to be taken into account. Lets guess that
your car is going to weight 2000 lbs. and the weight distribution is
50/50. The rear weight would be 1000 lbs. That would be the wheels on
a scale weight. But the coilovers are not supporting all of that weight.
You would deduct the weight of the rear axle, wheels and tires and part
of the weight for locating devises like radius rods and track rods.
Lets guess that those amount to say 300 lbs. You deduct that from the
1000 lb. and have 700 lbs. that you need to support with the two coilovers
or 350 lbs. each. Since we want our springs to compress about 2-1/2"
to be in the middle of their travel, we divide the 350 lbs. by 2.5 and
get a spring rate of 140 lbs. per inch. There are other some other considerations
such as mounting angle and leverage that can come into play, but that
will give you a ballpark figure to start with. When it comes time to
put things together, just cut a couple of tube or pipe spacers that
are 2-1/2" long that will fit over the shocks rod and let them set the
desired ride height until you put on the springs.
by George Barnes
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