Find someone with a good set of portable race car scales and find out how much weight you're carrying on each wheel. Several of the round track racing equipment suppliers such as Racer's Wholesale have a huge selection of coil springs of different heights, diameters, and weight capacities. Find something that looks like it will fit your application and try it. Fortunately, racing springs aren't terribly expensive if you happen to guess wrong the first time.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!