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.