This will be a four part article that covers what we know about GPL's suspension system. The first part will cover the springs, the second will cover the bump rubbers, the third will cover ride height, and the fourth will cover what this knowledge might imply for setups.
Much of this information was orginally discovered by Gene Fryman. I have independently confirmed Gene's findings and added to them.
GPL normally uses a suspension travel of 19 centimeters (7.5 inches) from full droop to full bump. However, it may be possible for the suspension to be compressed more than 19 centimeters and the code extrapolates the suspension force beyond the 19 centimeter compression point.
A big misconception has been that spring rate and wheel rate (as specified in the setup menu) are different. This is normally true in real life, but in GPL, they are exactly the same as the code does not apply any mechanical advantage from the suspension arms. So when discussing the rates in GPL, wheel rate and spring rate can be used interchangeably. Figure 1 is a picture of a real world shopping cart spring that is a very good depiction of how GPL's wheel and spring work together. This clearly shows a real world suspension where wheel rate is the same as spring rate.
GPL simulates non progressive springs that are vertically mounted . The spring uses a constant rate measured in pounds per inch of spring compression. As the spring is compressed, the force it exerts increases at a linear rate. Althought the code slightly tilts the springs for rather obsure reasons, for our purposes we can assume that the springs are always vertical. Again, Figure 1 is good depiction of this.
Figure 2a is a graph that shows the force exerted by a 100 pounds per inch spring as the suspension is compressed. The vertical axis is the spring force in pounds and the bottom axis is the suspension deflection in centimeters. The graph shows the linear increase in spring force that continues in a straight line up to and beyond the 19 centimeter suspension deflection point.
Figure 3a is graph that shows the force exerted by a weaker 80 pounds per inch spring. You will note the more gradual slope and lower force than with the 100 pounds per inch spring.
Edited by Lee200, Jul 11 2014 - 08:55 AM.