The stock Marzocchi fork springs are part number 5141418/P. I could not find any OE alternate rates or any aftermarket supplier of suitable springs. Although it is possible to have custom springs wound, I did not really know what rate I wanted and at a cost of $175+ per try, I sought a different solution.
The Marzocchi forks have a spring in each leg, whereas the Tech forks use a single spring. Since I have never seen a spring rate published for either of these forks, I was forced to measure the dimensions of the springs and calculated their rates with a spreadsheet.
The design formula for a wire spring is: k = G*d^4 / 8*D^3*N
k = spring constant
d = diameter of the wire
G = a constant equal to the shear modulus of the material (depends on metric or Imperial units)
D = average coil diameter
N = number of active coils
Being raised to the 4th power, the diameter of the wire has the strongest influence on the spring rate. (A manufacturer of springs once told me that they measure the diameter of each batch of incoming material to 0.0005" when calculating the number of turns required.)
When reverse-engineering an existing spring, D = OD - wire diameter. But when winding a spring on a mandrel, D = ID + wire diameter. This value is also quite influential as it is raised to the 3rd power.
Being a linear term in the equation, the number of active coils is less critical. When calculating a spring's rate, the number of active coils is generally taken to be about 2 fewer than the total number of coils (including partial turns).
Note that, although it may be counter-intuitive, the more turns a spring has, the softer its rate. This will be important in the next section.
The stock Marzocchi fork springs have a 0.149" (3.785mm) wire diameter, OD of 1.2135" (30.8mm), comprising 26.75 turns, with an overall length of 303mm. This calculates to 23.26 pounds-force/inch or 0.415 kgf/mm or 4.07 N/mm. We need to double this value to account for both springs (which gives 0.83 kgf/mm).
I did the same calculation for the Tech forks (which only have a single spring) and got 43.7 pounds-force per inch or 0.78 kgf/mm. This was my target rate.
Unfortunately, the OD of the Marzocchi spring is 31mm which is very uncommon when looking for a substitute spring from another bike. In fact, the only readily-available spring I could identify with that OD is the 2018-2019 YZ65. However, a wide range (2002 – 2011) of KTM 65s use a slightly smaller 30mm OD spring. And, just over the limit are YZ85 / KX100 springs with a nominal 32mm OD.
The ID of the Marzocchi tube is 31.75mm which gives a bit of latitude. Due to a stroke of great luck, I had a pair of springs from a YZ85 among my spares which showed an OD of 31.4mm and a rate of 0.32 kgf/mm. I used one for the experiment described below.