Shim changes affect the entire force-velocity curve. Valving 000 gave a brief overview of the load-deflection and force-velocity 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 load-deflection curve. Below are the load numbers and a chart showing the load-deflection 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 load-deflection 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
10-30-05
2 - 40.30
23.30 base
2
68
05 RMZ 250
4
96
stk249s-v3
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
10-30-05
14 - 40.15
23.30 base
2
65
05 RMZ 250
4
93
stk249s-v4
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.
