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I've heard that the old process of using static (unloaded) sag to determine rear spring rate is changing. The new process uses actual spring preload in millimeters to help make the decision and goes like this;
1) Set race sag as usual
2) Measure spring preload
3) If preload is between 2 to 8mm spring is probably good (2mm being more on the stiff side, etc)
4) Less than 2mm spring is too stiff, more than 8 and it's too soft.
Heck, 2 mm is practically nothing. Most springs will almost rattle on the perches at that setting but that's the new trend, even if it means your unloaded sag is huge (45-70mm!). I tried it on my bike and it made a huge difference in my spring selection, from 4.5 to 6.0. Wow! And it worked much better to boot. Obviously, this process favors stiffer rates everything else being equal.
Anyway, just curious what others think about this.
1) Set race sag as usual
2) Measure spring preload
3) If preload is between 2 to 8mm spring is probably good (2mm being more on the stiff side, etc)
4) Less than 2mm spring is too stiff, more than 8 and it's too soft.
Heck, 2 mm is practically nothing. Most springs will almost rattle on the perches at that setting but that's the new trend, even if it means your unloaded sag is huge (45-70mm!). I tried it on my bike and it made a huge difference in my spring selection, from 4.5 to 6.0. Wow! And it worked much better to boot. Obviously, this process favors stiffer rates everything else being equal.
Anyway, just curious what others think about this.
The Shop
You should be able to get your race sag within 1-2 turns max in either direction.
To confirm my spring rate is good for my weight I then check static sag is appx 30-40mm ish. This is where I have always tried to be with 100-105mm of race sag and corresponding free sag. I am 215 pounds without gear and I am running a 5.8 rear on both my 2016 KX450 and 2006 KX250, free sag is 32mm on the 450 and 36mm on the 250 which makes sense considering my 450 weighs more than my 250 two stroke...so I could probably use a 6.0 on the 450 but I am currently dropping weight so it was a compromise.
If your free sag is too much you need to go up in spring rate and if it's 20mm to fully extended it is too light.
If it is too stiff you will have to much free sag and it will adversely affect your handling do to your chassis being to low in the rear effectively raking your forks out too far... it will be stable in a straight line but good luck turning!
If you are over compressing a light spring rate to get correct race sag you are storing too much energy within the spring that causes your suspension to not operate properly.
You will probably get about as many opinions on this as what two stroke oil to run along with mine as this has always worked for me. So unless slipdog or Ross from Enzo comes on here and says I'm doing it completely wrong I'm going to keep using this method for me!
Personally, i would much rather the factories provide the correct fork and shock spring for your weight from the factory. When buying a bike, you tell them your weight and the bike comes set up for you.
Spring rate too light: you must crank the preload down considerably to achieve the proper race sag. At this amount of preload, the spring pushes back harder than it should. Thus, the bike has little or no free sag when you are not sitting on it.
Spring rate too heavy: you achieve race sag without much preload on the spring. Thus, it is not pushing against the subframe very hard while you are not on the bike and sags too much.
BTW, I got this info from Enzo.
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