Thanks Nick, I'll take note of that. I need not to be afraid of spring, seems I was traumatized by that 5Kg stock rear spring
Got an answer from the spring makers, they'll do it in about a month. They will wind it up to spec so they need to know length, diameter, rate, and travel. It will be the occasion for me to measure everything and try 5wt oil, before changing the springs.
While waiting, I had fun studying Racetech's bible spring chapter, and drew a little graph showing spring rates. Please feel free to correct if my calculations are off.
The horizontal axis shown spring compression, in mm. The vertical axis show weight applied on spring, in KG. I drawed the lines for a pair of springs each time (double the weight, for the same length).
I added preload force ( F = Rate x Preload length, ex: 0.42Kg/mm with 10mm preload: 4.2Kg, times 2 for your pair of spring, 8.4Kg of preload), which does not affect the rate (the slope of the line), but moves the line up or down.
The first line I drawed, in black, is the stock spring. 0.345Kg/mm with 28mm of preload (Dear KDXgarage, I saw posts from you talking about 28 or 38mm, and from other post with measurements I guessed 28mm).
Now, I got on my bike and measured free sag (26mm) and race sag (57mm). I crossed this with the Stock Spring rate, and went horizontally to deduct the applied weight of free sag and race sag. This weight being independent of the spring rate, I should now be able to predict free and race sag with other spring rates and preload values.
I highlighted in yellow the recommended race sag range from Racetech. I added recommandations from other tuners, but they feel a bit odd to me.
I should add now that this graph only shows the spring rate, omiting the "air spring" effect that is tuned by the oil level. This curve, according to RT's book, is quite like an exponential, with almost no effect at the start of the stroke, and huge effect at the full compression. However, this air spring rate is the same whatever spring rate we're using so we can forget it and compare springs.
Now the fun begins: I drawed the common used spring rates, with 4mm preload. 4mm preload with various spring rate gives about 3.2Kg (.40 springs) to 3.7Kg (.46 spring). In comparison, the stock weak ass .345 spring, once preloaded to 28mm, produces almost 20kg of preload force!
I drew small circles where the lines cross the stock line.
The graph shows clearly that there is lots of ways to get the same Race Sag, heavy spring with small preload or light springs with stock preload. However, albeit having the same sag measurement the soft spring will be stiffer in the first half of the stroke and weaker after that.
We can see here that a .40 spring (blue line), will be weaker than stock until mid-stroke (145mm), and stronger to the end of the stroke.
Even the .42 spring is lighter until being 105mm in the stroke.
I added a line for .41 springs with 10mm preload to show the effect (dotted black line). While having the same full-stroke force a the .42/4mm, it will be harder throughout most of the stroke, albeit quite close.
Lastly I compared the force needed compress the springs 290mm, based on the stock spring. (Mistake on the .46, it's 270kg, +23%). The difference seems a bit small to me (but what do I know?) compared to the huge effect of oil level at the end of the travel. So maybe the bottoming problem should be dialed in with oil level. We'll get to that once the spring is sorted.
Now, I could just get whichever spring rate has been advised for 30 years but, isn't it enjoyable to get some stuff down on paper and verifying it afterwards? I'm going for .42 springs.
I should add now that while this graph compare spring rates(in the event that it is not completely wrong), it does not compare riding performance! I do not have the experience to tell if weaker or stronger springs in some parts of the strokes will achieve better or worse behavior, but my goal is to measure and test, trying to understand the relatioship between rates, preload, sag and bike handling. This will require lots of riding.