[cbr250rakid]

While I was using the spring rate calculator I noticed the stock rate is 0.490 kg/mm. I'm 180 lbs and the spring rates for my weight were 0.80 kg/mm for street use and 0.85 kg/mm for track use. I went with the 0.80 kg/mm because if I want to lighten the bike ie. muffler and battery, then the suspension will get a little tighter. I can also adjust a bit more by playing with different oil weights. It should make a world of difference.

r u sure about a .8 fork spring?
that's almost double the size spring from stock!
when i'd respring for my dirtbike, all i needed was a .02 - .06 jump over/under to from stock size.

 

[cbr250rakid]

I got thinking about the stock spring rate and wondered what rate 0.49 kg/mm is for .... I'm guessing that the stock springs were put in just to keep the bike from falling over while on the kickstand.

you can come close to finding this out by measuring the sag.
---------------------------------------------------------------------------
Rider Sag = Measured sag of bike and rider w/ rider and rider's gear.
Free Sag = Measured sag of just the bike, no rider, no rider gear.

Front -
Rider Sag - 30-35mm (25-30% of Full Travel)
Free Sag - 15-20mm (60-70% of Rider Sag)
Rear -
Rider Sag - 20-30mm (race), 30-35mm (street) (25-30% of Full Travel)
Free Sag - 5-10mm (extremely light bikes use less) (15-25% of Rider Sag)
----------------------------------------------------------------------------
me = 162 lbs.
w/ stock front springs:
my rider sag = 45mm
my free sag = 27mm

so, since i'm 10mm over the rider sag, then for each mm over, that will equal a certain amount of weight over the spring's ideal rate. (fyi, springs are often rated in kg/mm). if it's true that the stock spring rate is .49kg/mm, then for each mm over, that's .49kg extra weight, or about 1 lb over.
to drop from 45mm to the 30-35mm rider sag range, that's a 10-15mm difference.
so i'd have to be roughly 5 to 7.5kg lighter (11 to 16.5lbs). then from my math, the stock fork springs will work best for someone who weighs between 145-151 lbs.