My thanks to Lew for the 3 point Ural weight figures of front and rear wheels and chair. Likewise to Greg for the suspension unit application years. I've advised the iKon factory in Australia that we can revise the Ural application years for the 7610-1664 adjustable rebound (dial-a-ride) unit to 1963-2017 in the fitment chart.
To answer your question Lew with regard to "Spring rate vs Load rate" in shock absorbers I have to say I never find it easy to explain simply and remain meaningful when it comes to matters of springing. However let’s have a go:
The spring rate or rates in the case of a progressive rate spring* is/are the amount of force/load that is required to deflect (compress) the spring a certain amount in the units chosen, e.g. lb/inch or N/mm.
Load rate is not particularly a term iKon use however if the context is a Ural outfit, then I would take it as what load will a certain spring work well with on a Ural. Then it all gets very subjective. The OEM springing, iKon springing, other brand springing are all likely to be different in some way and we’ll all say things like "That’s good for riders between 165lbs and 220lbs with pre-load adjustment to be able to add a passenger or gear in the sidecar or both." So for us to really select the correct spring rate** for a Ural customer, that customer has to tell us what load weight in terms of people and gear he or she is going to be placing on the outfit.
You won't get the best from any shock absorber unless you know the load weight. For example, a 100lbs rider, no passenger, no luggage is going to need a different rate spring than a 300lbs rider, 200lbs passenger and 200lbs of kit in the chair. Think of it like buying a made to measure suit rather than one "off the peg". Both fit, but one fits so much better than the other.
The fact is, manufacturers don't know what load (Lbs) is going to be put on their machines, so they have to opt for a spring rate which is 'average' or what they believe average to be.
The limitation to all that being if Ural say for example the load limit is 1300lbs for a particular model of outfit, then increasing the spring rate does not change that. The load limit is still 1300lbs as it is made up of more than just spring rate. It’s about all sorts of other things like frame strength, wheel bearings, bushes, tire load rates and so on.
* A progressive rate spring means the coils of the spring are close together at one end and 'progressively' become wider apart.
** iKon use the term 'spring rate' when referring to a hard or soft spring.