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All About Springs
To start with, a spring is just a steel bar
with a very good memory, coiled up to form the spring shape. Just like
a steel bar, the larger the bar is in diameter the stronger it will be.
Also, like a steel bar, the longer the bar is the weaker the spring will be.
A good example of this can be found if you compare the diameter of the
spring wire and the length of that wire on the typical Japanese rear shock
to the diameter of the spring wire and the length of that wire on the KTM
PDS shock springs. The KTM PDS spring wire is shorter and much larger
in diameter. For a 180 pound rider, a typical KTM PDS spring would be
approximately 8.6 kgs/square cm to approximately 5.0 kgs/square cm for that
of a Japanese bike. For those of you who prefer American measurements
to the metric measurements, 8.6 kgs/square cm = 480 lbs/square inch, and 5.0
kgs/square cm = 280 lbs/square inch, almost 60% stiffer. To convert
kgs/square cm to lbs/square inch, multiply the kilograms by 55.88 (5.0 times
55.88 = 280). Springs are primarily sold using the kilograms per
square centimeter notation.
What do the measurements of 5.0 kgs/square cm
or 280 lbs/square inch actually mean? Springs are rated by how much
pressure, in pounds or kilograms, is required to compress the spring l inch
or 25 mm (l centimeter). A pressure of 280 pounds is
required to compress a 280 lbs/square inch spring one inch. Each
additional inch of compression requires an additional 280 pounds pressure.
The average rear shock compresses approximately 3 inches to get
approximately 12 inches of rear wheel travel. It would take
approximately 840 pounds to compress that spring three inches. It
would take 1440 pounds to compress a 480 lbs/square inch PDS spring three
inches.
Fork springs are rated the same way as shock
springs, except they are much softer. The average fork springs for a
180 pound rider would be .42 kgs/square cm or 23.5 lbs/square inch times 2
because you have two fork springs. So, in reality, it would be the
equivalent of having a single .84 kgs/square cm or 47 lbs/square inch
spring in the front. Much lighter than the rear spring. The front
forks typically have 12 inches of travel. To bottom front forks
equipped with a pair of 23.5 lbs/square inch springs would require 282
lbs/square inch of force per fork leg for a total of 564 lbs/square inch.
The difference between a 5.0 kgs/square cm
(280 lbs/square inch) and a 5.2 kgs/square cm (290 lbs/square inch)
shock spring
is 10 lbs/square inch. The difference between a .42 kgs/square cm
(23.5 lbs/square inch) and a .44 kgs/square cm (24.5 lbs/square inch)
fork spring
is l lb/square inch times 2 = 2 lbs/square inch. A general rule is
that a change of .2 kgs/square cm for a
shock spring
and .02 kgs/square cm for
fork springs
is needed for every 20 pounds variation from normal rider weight. |
