2) SUZUKI DR-Z 400E/K FORK/SHOCK SPRINGS
 
The springs stiffness' ratio R' = shock spring stiffness / one fork spring stiffness) is 13.0, with both values in N/mm or kgf/mm. The original ratio is about 12.0 .
 
This is my personal table reporting springs stiffness' related to rider's weight (neglecting the fork air stiffness, near to 0.1 N/mm):
 
fork--------shock-------rider's net weight (kg)
4.1-----------54----------------65
4.25---------56----------------72.5
4.4----------58----------------80
4.55---------60----------------88
4.7----------62----------------95
4.85---------64---------------102.5
5.0----------66---------------110
 
 
Example: if your weight is about 105 kg, then the springs right for you should be 4.8 N/mm (fork) and 64 N/mm (shock) if you want a medium-soft tuning, 5.0 and 66 if you want a stiffer one.
 
 
 
These values come down from this facts:
 
a) The stock shock spring stiffness (SS) is 54 N/mm, as reported by the Suspension Network database. This value should be measured with the spring pre-loaded by a weight of about 50 kg.
 
b) The fork springs stiffness' (FS) that is nearly perfect (in my personal tests) with the stock shock spring is 4.1 N/mm.
 
c) R' = SS / FS = 54 / 4.1 ~= 13, thus every rear/front springs that respect this stiffness' ratio should work as well as the 54 / 4.1 springs.
 
d) Multiplying shock spring stiffness by the factor 3.7593 we will obtain the TW = total weight = motorcycle + rider weight.
 
e) Subtracting from TW the bike weight + gear weight (I assume 130+8 kg) we will obtain the RW = rider's weight (kg):
RW = TW - 138 = (SS * 3.7593) - 138 .
 
The reasoning is correct if every point is satisfied.
 

1) INTRODUCTION TO THE "SPRINGS STIFFNESS' RATIO"
 
Springs stiffness' ratio is a strange concept for a lot of people, even for some suspension engineers, I suspect. It come down from this idea: the dynamical behavior of a motorbike has an optimum when:
 
(Front suspension stiffness) * (distance of front wheel from BB) = (Rear suspension stiffness) * (distance of rear wheel from BB)
 
BB = barycentre of the byke + rider system
 
This relationship is verified when the stiffness' are in the ratio R:
 
(Rear suspension stiffness) / (Front suspension stiffness) = R ,
 
where R = (distance of front wheel from BB) / (distance of rear wheel from BB)
 
If the rider's barycentre is exactly over the bike barycentre, then increasing the rider's weight the value R is constant. This is my hypothesis.
 
On the other hand, suspensions stiffness' must be increased when rider's weight increase, in order to avoid bottoming problems and so on.
 
R is not a springs stiffness' ratio, but a suspensions stiffness' ratio. For our purposes it's better to have:
 
R' = (Rear spring stiffness) / (fork spring stiffness)
 
"Fork spring stiffness" means "one fork spring stiffness"; in the "Front suspension stiffness" are involved all the two springs, with a factor to account that the fork isn't vertical.
 
Normally it's very difficult to calculate the R' value in a real bike, thus it's better to perform drive tests with various springs. It's not difficult to find out the front spring that better fit the rear spring. A set of  54 - 4.1 is almost perfect for 70 kg riders, thus the R' ratio become:
 
R' = 54 / 4.1 = 13.17 ~= 13
 
This value should be valid with every rider's weight. Maximum error admitted on the springs stiffness' is 5%, maintaining a given hydraulic set-up.  There is, however, the opportunity of balancing an R' out-of-range value by changing the compression dampening of both shock/fork, i.e.: if R' is higher than normal, it's better to increase the fork comp. damp. and/or reduce the shock comp. damp. . The opposite operation will be performed if R' is lower than normal. All this can account for the big differences in R' values between american and italian settings. Most american websites springs calculators works on R' = 11 or around this.
Because the transmission ratio between the rear wheel and the shock is variable, all this stuff is referred to the standard value of SAG. If the SAG is altered, the R' can change slightly. Bigger changes in R' will happen if rear suspension links are modified or substituted.