Shim changes affect the entire forcevelocity curve. Valving 000 gave a brief overview of the loaddeflection and forcevelocity curves, and how shim distortions alter the curve. (It's advisable to read Valving 001 if you have any questions about the terminology used on these next few pages)
Shim thickness
The thickness of the shims in the stack have an affect the curve.
 Theoretically, thinner shims get proportionately stiffer as deflections increase.
Shims of various thicknesses and diameters are often arranged in a stack. An understanding of shim characteristics is needed so the tuner can predict how each shim contributes to the overall stiffness of the stack.
A commonly used shim conversion formula is:
s = (X)(m1)/(m2)
Where:
S= The number of shims you need to use.
X = Number of shims of the size you want to replace.
m1 = Factor of the shim you want to replace.
m2 = Factor of the shim you want to replace it with.
Thickness Factors (m):
.10 mm = 1.0
.15 mm = 3.4
.20 mm = 8.0
.25 mm = 15.6
.30 mm = 27.0
Example:
Convert 7  .15’s to .20's.
s = (7 * 3.4) / (8.0) = 3  .20 mm shims
This conversion is fairly accurate, but only at the lower end of the loaddeflection curve. Below are the load numbers and a chart showing the loaddeflection curve for a series of shims converted using the above formula.
Starting with one  40mm diameter x .30mm thick shim:
 qty 1  40 x .30
 = 1.7  40 x .25
 = 3.4  40 x .20
 = 7.9  40 x .15
 = 27  40 x .10
Using the Shim Program, we calculated the load numbers for the shims from above. For accuracy of the calculations, we calculated the fractional number of shims. Obviously, you can't have 3.4 shims, but we will use that number for the calculations.
 At .003 inch deflection, all shim stacks have about the same load (about 2.0 lbs).
 As the deflections increase, each stack stiffens at a different rate.
 Shim distortion does not change the fact that thin shims get proportionately stiffer as deflections increase, but distortion may influence the rise in stiffness.

27  40.1 
7.9  40.15 
3.4  40.2 
1.7  40.25 
1  40.3 
def 
Shim Program
loads (#/in) 
Shim Program
loads (#/in) 
Shim Program
loads (#/in) 
Shim Program
loads (#/in) 
Shim Program
loads (#/in) 
.003 
2.1 
2.0 
2.0 
2.0 
2.0 
.006 
4.4 
4.2 
4.2 
4.1 
4.1 
.009 
7.4 
6.7 
6.6 
6.3 
6.3 
.012 
11.2 
9.6 
9.2 
8.7 
8.7 
.015 
16.2 
13.2 
12.3 
11.4 
11.2 
.018 
22.7 
17.6 
15.9 
14.5 
14.1 
.021 
31.0 
23.0 
20.2 
18.1 
17.3 
.024 
41.3 
29.4 
25.2 
22.1 
20.9 
.027 
54.0 
37.2 
31.0 
26.7 
25.0 
.030 
69.3 
46.3 
37.8 
32.0 
29.6 
.033 
87.6 
57.1 
45.6 
38.1 
34.7 
.036 
109.2 
69.5 
54.6 
44.9 
40.5 
Table 1
Chart 1 shows the difference in stiffness of the shims.
Chart 1
The chart above shows the shim loads greatly increasing at the end of the deflection range. When shim stacks are measured or calculated, they tend to produce a steeper loaddeflection curve than when they are installed in a damper. When the shims are installed in a suspension unit, the curve tends to flatten out and become more linear.
Below are dyno results from a series of tests that will demonstrate this. According to the shim conversion formula above, 2  40.30 shims should be equal to 16  40.15 shims. We ran several tests, and concluded that 2  40.30 shims more closely matched 14  40.15 shims. Results are below.
Using a KYB 46mm shock:
stack 
ips 
dyno force
comp in lbs 
103005 
2  40.30
23.30 base

2 
68 
05 RMZ 250 
4 
96 
stk249sv3 
5 
108 

10 
162 

20 
256 

30 
341 

40 
427 

50 
515 

60 
604 

70 
694 

80 
768 

90 
870 

100 
957 













stack 
ips 
dyno force
comp in lbs 
103005 
14  40.15
23.30 base

2 
65 
05 RMZ 250 
4 
93 
stk249sv4 
5 
103 

10 
157 

20 
256 

30 
346 

40 
434 

50 
523 

60 
614 

70 
714 

80 
784 

90 
891 

100 
981 

Table 2 
Chart 2
Conclusion: The 14  40.15 shims were slightly stiffer than 2  40.30 shims, indicating that thinner shims get proportionately stiffer as deflections increase.
 The Shim Program calculated the load increase to be 69.5lbs  40.5lbs = 29 lbs.
 The dyno tested the force increase to be 981lbs  957lbs = 24 lbs.
The difference in these numbers appear to be very close, but this may be a coincidence. The Shim Program load differences do not generally mirror the dyno differences. We are currently working to define the exact relationship between the program and dyno.
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