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Showa Shock Sag Calculations (DVD: SCRF450R03D)

Step 1 Measure the extended length with the rear tire elevated
EL = The Extended length
EL = 21"
Step 2 calculate the average static sag with the rider off the bike

DRC = Distance after the shock returns after compression
DRL = Distance after the shock returns after extension
AFD = The average difference between measurements is the Average Friction Distance
SS = Static Sag

AFD = (DRC - DRL) / 2
AFD = (19-1/8" - 19.5") / 2
AFD = 3/16"
Now add the distance after the shock returns from compression to the average difference.

AD = DRC + AFD
= 19-1/8" + 3/16"
= 19-5/16"

SS = |AD - EL|
SS = |19-5/16" - 21"|
Static Sag = 1-11/16"
We want the static sag to be 1-1/8" therefore we need to increase the pre-load until the difference is 1-1/8".

Step 3 Set the Race sag
Step 1 Measure the extended length with the rear tire elevated
EL = The Extended length
EL = 21"
Step 2 calculate the average static sag (with the rider on the bike)
DRC = Distance after the shock returns after compression
DRL = Distance after the shock returns after Lifting
AFD = The average difference between measurements is the Average Friction Distance
SS = Static Sag

AFD = (DRC - DRL) / 2
AFD = (16-5/8" - 16-7/8") / 2
AFD = 1/8"
Now add the distance after the shock returns from compression to the average difference.
AD = DRC + AFD
AD = 16-5/8" + 1/8
AD = 16 ¾"
RS = |AD - EL|
RS = |16 ¾" - 21"|
Race Sag = 4-1/4"
We want the static sag to be 4" therefore we need to increase the pre-load until the difference is 4". Setting the Race Sag correctly is more important than setting the Static Sag. If the spring rate is correct for your rate then setting the Static Sag will get you in the ball park of the Race Sag specification.