The Next Level In Performance...
The Correct Spring Rates Are The Most Important Part To Getting Great Working Suspension
This page might be a lot to read- But it is the single most important page to read if you want to avoid wasting a lot of money with Suspension Shops. The most common mistake I see Suspension Shops or Tuners make is in using the wrong spring rates. Spring rates are determined by the weight of the bike and the weight of the rider- NOT THE SKILL LEVEL OF THE RIDER. Spring rates affect the ride height and angle of the bike, no matter what a persons skill level is, the bike is meant to ride at a certain height and angle. If the spring rates are wrong the bike could ride back high or front high, in this case the bike will never turn right. Another problem with the bike riding at the wrong height is that if the front spring rate is too soft, the forks will ride in the mid-stroke, this will make for very harsh ride. I have helped a lot of my friends by simply putting the correct springs in their bikes (that suspension shops have revalved and re-sprung with the wrong rate springs). After installing the correct rate springs and nothing else their bikes went from "not even being rideable" to "wow this is great" Before changing their springs they were convinced that the suspension shop did a horrible job valving it, when in fact it was just the wrong springs.
Alone with having the right springs, you must have the right preload on your fork springs. I can't believe how many shops don't know how to correctly measure fork spring preload. The fork Spring preload MUST be checked when the spring is installed. You cannot simply measure the size of the preload spacer you are putting in- IT DOES NOT WORK LIKE THAT. There is a great book that you can buy off of eBay or Amazon for under $30 that explains how to properly measure spring preload and also explains so much more "Race Tech's Motorcycle Suspension Bible" Fork Spring Preload Should be between 4mm - 6mm. If you have less then 4mm of Total preload, your forks will "wallow" when landing from jumps and do some other not so nice things. I can't stress enough how important Fork Spring Preload is.
Now on to spring rates... As a rule of thumb a 450F will use 1 spring rate higher (front and Back) then a 250F. So an example would be if a 175lb rider on a 250F uses .48kg Fork Springs and 5.4kg Shock Spring, on a 450F a 175lb rider would use .49 Fork Spring and a 5.5kg Shock Spring. This is due to the extra weight of the 450F over the 250F. You will notice that there is a range for the rear shock, this is because of the differences in chassis design. The sag must be able to be set correctly without the spring being loose. Some bikes like a lighter rear spring. Here is an example-  one of our riders went from a 2009 CRF450 to a 2014 CRF450, he ran a 5.4kg on his 2009 the sag set perfect and the bike handled great, on the 2014 the 5.4kg could not get the sag to set correctly.  SAG is very important.  Below is a list of spring rates for some rider weights and bike sizes. There are some exceptions and these spring rates do not apply to SuperCross.
Rider Weight Bike Size Front Springs Total Preload Shock Spring SAG In MM
150 - 160 lbs 125 (2-Stroke) .46 kg 4mm - 6mm 5.2 kg 100 - 103
165 - 180 lbs 125 (2-Stroke) .47 kg 4mm - 6mm 5.3 kg 100 - 103
150 - 160 lbs 250 (2-Stroke) .47 kg 4mm - 6mm 5.2 kg - 5.3 kg 100 - 103
165 - 180 lbs 250 (2-Stroke) .48 kg 4mm - 6mm 5.3 kg - 5.4 kg 100 - 103
150 - 160 lbs 250F .47 kg 4mm - 6mm 5.2 kg - 5.3 kg 105 - 108
150 - 160 lbs 450F .48 kg 4mm - 6mm 5.3 kg - 5.4 kg 105 - 108
165 - 180 lbs 250F .48 kg 4mm - 6mm 5.3 kg - 5.4 kg 105 - 108
165 - 180 lbs 450F .49 kg 4mm - 6mm 5.4 kg - 5.5 kg 105 - 108
185 - 205 lbs 450F .50 kg 4mm - 6mm 5.5 kg - 5.6 kg 105 -108