The load numbers from the Shim Program relate to the stiffness of the shim stack, not to the actual damper forces. The rider feels the accumulated forces produced by the piston, shims, oil, etc. all working together to provide the damping action. The shims are just a small, but important part of the total equation.
The damper forces felt by the rider can be measured and analyzed with the help of a suspension dyno. The dyno allows us to measure the fork or shock at various velocities, giving us a force-velocity curve of the damping forces. These numbers relate directly to what the rider feels on the track.
Velocities
We need a standard of measurement to correlate the damping forces felt by the rider with the forces that can be reproduced on the dyno. This standard is velocity. Think of velocity as the speed of the piston, not the speed of the wheel.
Fork piston and wheel velocities are the same.
Due to the rear linkage (leverage ratio), the shock piston and wheel velocities are different.
We will use data acquisition equipment to measure the piston velocities over various size bumps at the track. Patterns will develop and we'll see that specific track obstacles produce velocities within specific ranges. We'll fit these various velocities into the low, mid or high speed damping ranges.
Velocities are measured in meters per second (m/s) or inches per second (ips).
To convert ips to m/s, divide by 39.37.
Low speed, mid speed and high speed
Low speed, mid speed and high speed will be used in the context of the stroking speeds of the piston. When the rider hits a bump, the piston moves a certain speed. Smaller bumps produce slower speeds, and faster bumps produce faster speeds.
There are no definitive ranges for low speed or high speed. Everyone will have a different opinion of what fits into what category. For example, how do you decide where low speed ends and high speed begins.
To make life easier, think of low speed, mid speed and high speed as a curve, with low speed on one end, and high speed on the other. We don't need to define where one speed ends and the other begins as long as we always think of it as the low & high speed curve.
If we added a shim to increase the LS loads, the entire load-deflection curve is going to change depending on the stiffness of the added shim (as demonstrated in Table 3 from the previous page). There is nothing we can do about that. We can't add a shim and have it work only through the first half of the deflection range. It's going to do what it's going to do to the entire curve.
Here is a table and a graph line for a typical load-defection curve.
def
Shim
Program
loads (lbs/in)
.003
9
.006
19
.009
29
.012
39
.015
51
.018
63
.021
75
.024
88
.027
101
.030
115
.033
130
.036
144
Chart 4
For a simple example, we'll pick .003 inch deflection for low speed and .036 inch deflection for high speed.
If we wanted to increase low speed damping, we might increase the 9 lbs (at .003 def) to 12 lbs, and if we wanted to decrease high speed damping at the same time, we might decrease 144 lbs (at .036 def) to 120 lbs. In essence, we would reposition the curve by raising the low speed end and dropping the high speed end.
Chart 5
The above example is hinged in the middle. We could take away more from high speed and have something like this.
Chart 6
The transition point where the lines cross is where the damping curve remains unaltered (or the force numbers remain the same).
