All standard steel springs are black and are marked with their spring rate in lbs/in (in 50 lb increments) and the total travel of the spring itself. Standard black springs should only be paired with shocks having less travel than the travel marked on the spring itself.
How do I choose the correct size?
There are three important things you need to know about your set-up:
1. The Stroke of your shock - This is the actual travel of the shock itself. To measure this, measure the visible part of the shock shaft in Inches 1" = 25.4mm (This is the part that disappears as the shock compresses)
2. The Maximum Spring Free length that your shock will take. To do this wind the preload off completely and measure from the inside of the spring collar to the inside of the preload nut.
3. Spring rate in Lbs. - Spring rate refers to the stiffness of the spring and will vary due to rider weight, riding style/discipline, Leverage ratio of the bike, and suspension design. As a general rule you are trying to achieve the correct amount of sag. Sag is the amount your suspension travels when you're sitting on your seat, feet on your pedals and hands on the bars. This should be around 1/3 of the total travel on a DH bike and 1/4 of the total travel on an XC All mountain bike (This can vary due to suspension design and personal preference)
Once you have established the answer to 1, 2, 3 you need to choose your spring using the following rules:
1. Spring Stroke should be Equal to or Greater than Stroke of the shock
2. Spring Length should be less than the Max Free length of the shock (This is so the spring physically fits)
3. Spring Rate should be correct to suit your body weight, bike and riding style
Example. If your stroke is 2", your rate is 450Lbs and your max free length is 130mm. You need - 2.25" x 450Lbs x 125mm i.e - If you don't see a spring that has the exact stroke of your shock you can use a spring with a longer stroke, as long as the free length will fit.