In previous examples, we have been talking about the stiffness of shim stacks as one might calculate on the Shim Program. The program is not used to compare stacks, it's just used to predict how proposed shim changes affect the stacks overall load-deflection curve.
As previously stated:
There are many factors that affect a dampers performance. Fluid dynamics, pressure drops, friction, heat, piston design etc. For the most part, we are not trying to alter these factors when revalving suspensions. What we do alter is the number and placement of shims in the stack. That is what suspension revalving is. . . . . . adjusting the stiffness of the shim stack to alter the oil flow through the piston. This in turn has a direct affect on the damping forces of the suspension unit
After the shim stack is installed in the damper, we need to look at how the shim changes affect the actual damping forces, or the damping action as felt by the rider.
Currently, tuners correlate a specific shim change to rider feedback. For example, the rider may say a shock is harsh. The tuner removes a low speed shim, and if the riders then says the shock is plusher, the tuner would correlate that particular shim change to having a direct affect on harshness/plushness. In actuality, the tuner does not know exactly how the shim change altered the force-velocity curve. He is only guessing. Herein lies the problem. If he guesses wrong, it only confuses matters later.
For example, removing a low speed shim affects both low speed and high speed damping forces. Which end of the curve was responsible for the reduction in harshness?
The suspension dyno allows the tuner to know exactly how the shim changes affect the force-velocity curve, eliminating some of the guess work. (notice we said some)
Dyno forces can be used to make direct comparisons between stacks. Ideally, for an accurate representation of actual forces, we would want to dyno the damper over its entire force-velocity range.
The dyno's force-velocity curve coincides with the Shim Programs load-deflection curve.
Chart 7
Below is an example of the Shim Programs load-deflection curve and how it can vary from the dyno's force-velocity curve. The shim stack is the stock compression valving from an RM 125 shock.
stock RM 125 comp
stack
16-44.2
34.15
34.15
40.2
38.2
36.2
34.2
32.2
30.2
28.2
26.2
24.2
22.2
20.2
def
Shim Program
load (lbs/in)
.003
16.7
.006
33.5
.009
50.6
.012
68.1
.015
86.0
.018
104.6
.021
124.0
.024
144.3
.027
165.6
.030
188.9
.033
214.3
.036
241.0
Chart 8
ips
dyno force
(lbs/in)
5
180
10
238
20
354
30
470
40
586
50
702
60
817
70
933
80
1049
90
1165
100
1281
Chart 9
The load-deflection curve on Chart 8 shows a slight increase in loads as deflections increase. The dyno's force-velocity curve on Chart 9 shows a very slight decrease in forces as the velocities increase.
To reiterate, the program is best used to predict how shim changes affect the stacks load-deflection curve. The dyno is best used to show exactly how the changes affected the dampers force-velocity curve.
Dyno forces and rider feedback
The dyno's force-velocity curve represents the actual forces of the damper through the velocity range. Using the RM 125 dyno numbers from above, Chart 10 shows how the force-velocity curve might translate to the suspensions performance on the track (very approximate for this example).
Using the same approach as before, we'll pick the 5 ips velocity for low speed and the 100 ips velocity for high speed. If we wanted to increase the bottoming resistance, we might increase 1281 lbs (at 100ips) to 1350 lbs. For this example, we have managed to leave the force at 5 ips at 180 lbs.
As shown in Chart 10 , the entire force-velocity curve is going to change depending on the stiffness of the added shims. Stiffening high speed damping also stiffened mid speed. There is nothing we can do about that. We can't add a shim and have it work only through the last half of the velocity range. It's always going to do what it's going to do through out the entire range.
ips
dyno force
(lbs/in)
5
180
10
238
20
354
30
470
40
586
50
702
60
817
70
933
80
1049
90
1165
100
1281
Chart 10
At this point, you might say, "Why can't I add a shim to the high speed stack and have it kick in toward the second half of the velocity range? After all, that's what two-stage stacks are supposed to do. " Depending on the shim stack design, you may or may not be able to do this. Chart 11 (hypothetical) demonstrates what it might look like if you were able to specifically target the high speed in this manner.
Chart 11
This particular curve would minimize any changes to low speed and maximize changes to high speed. If you are already familiar with valving techniques, you may be lead to believe this is possible with a two stage stack. We'll look into that in a later section.
