Generally speaking, when it comes to determining which spring rate to go with, it comes down to a few different variables. The first thing you will need to determine the consistent weight that is being added to the bike for the majority of your riding (more than 50% of the time). Is that solo, two-up, solo with luggage, two-up with luggage, etc.?
Once you've determined the weight that is consistently added to the motorcycle (more than 50% of the time), you will need to compare that to our general recommendations of spring rate for the model of bike you have. On FL Touring models we recommend a heavy duty spring when the weight added to the bike consistently exceeds 500lbs, whether that is one-up loaded down with luggage, two-up, or two-up with luggage (whatever your riding configuration is the majority of the time).
For Dyna and V-Rod models, we recommend going to a heavy duty spring rate, at a little bit lower weight threshold due to the design of the chassis on these models. Shocks on the Dyna and V-Rod models are mounted at a steeper angle than the FL Touring models, which results in an increased amount of leverage being applied to the shocks from the swingarm. This basically acts to soften the shocks because of this increased leverage ratio. For these models we recommend going with a heavy duty spring, when the weight being added to the bike is consistently over 250lbs.
The next variable to consider, is riding style. Are you an aggressive rider, or are you searching for a "Cadillac" like feel when taking long trips? What feels comfortable to one rider is not necessarily the ideal ride quality for another rider. Two different riders of the same weight may have differing opinions on what feels too stiff or too soft. Some riders that are well below the recommended weight for a heavy duty spring, may feel the heavy duty shocks feel perfect for their riding style, while the next rider may feel the shocks are too firm. There is a lot of subjectivity in regards to what "feels right", and we offer the weight recommendations previously discussed as a baseline for our customers. These recommendations are based on customer feedback, and what we have found most often satisfies the majority of our riders out there.
Fortunately, if a customer orders the wrong spring rate, or determines they want a stiffer or softer spring, we do offer affordable options to have the shocks sent in to swap the springs. Shoot an email to firstname.lastname@example.org if this applies to you, and one of our customer service representatives will get you helped out!