Suspension spring rates

[JorgenL]

I'm looking into buying piggyback shocks for my café project and my main concern is the spring rates.
When I'm looking at Progressive Suspensions site the stock replacement springs are rated 75/120 and they are ok for shocks 12.5" to 14".
When I look at the budget piggybacks on Ebay they only say spring rate 200kg.
I know springs rates is normally measured in how many pounds it takes to compress them an inch or in metric how many grams to compress them one mm.
Now what does the 75/120 mean?

[cb650]

Dual spring?

[davesee]

just pulled this off some website


"for example: a 10lb to 25lb progressive spring will need 10lb to compress it the first inch, then 13lb the next inch, and so on, until the end of the travel; it will take 25lb to compress it the last inch. The benefit of this is that the spring can be soft enough at the start of the travel to offer a soft ride yet be stiff enough at the end of the travel to performance well during hard braking and turning."

here's a cool article on suspension on motorbikes in general

http://motorcycleinfo.calsci.com/Suspension.html