My understanding (so possibly wrong :lol: ) but shock length is a matter of travel. If you measure the length between the shock mount at full height, then at full compression (and add a bit) then that's the length and travel of the shock that you need. For a given amount of movement (travel) in the suspension, the shock length remains the same and higher / lower or softer / stiffer is then all about the spring rate and length (that's the tricky bit and where I ran into indecision before :roll: ).
So.... If 16" bodies are working for people then that's probably worth sticking with 8)
The springs are then a bit harder. Imagine if you fitted the shocks with no coils then dropped it back on the ground (ride height at rest) and wound the adjuster ring to half way before measuring the distance between the adjuster ring and the tophat at the, erm, top. Let's call it 8 inches (I've no idea if that's the right number, don't copy it!).
You could fit 8" 80lb springs or 8" 500lb springs and your bus would sit the same height at rest on both (because neither are compressed). But the 80lb ones would ride soft and the 500lb ones, well, wouldn't.
If you fitted longer springs at the same rates or wound the adjuster up, then you would raise your bus a bit as you are now preloading the springs. For the same increase in spring length / movement of the adjuster, the increase in ride height will be less on the 80lb springs because the spring compresses more easily than 500lb ones.
But even with the longer springs, the ride will stay the same as on the 8" springs - preload doesn't alter spring rate.
So here's the trick. The spring rate determines ride quality. If you find the right (comfy) rate, it's the same regardless of spring length or adjuster position. If your bus sits too low on your favoured rate, then you either need longer springs or to adjust the spring seat. Too high and it's shorter ones (or adjust the seat the other way). If you run out of spring seat adjuster movement (all the way to the top / bottom) then you've definitely hit the point of needing different length springs.
So... if 125lb is comfy and it doesn't rub on full bump, then it's the right rate. If it's mushy and hitting the arches on bump then fitting longer 125s won't do anything and you need stiffer (higher rate) springs. For the length, if there's still adjustment left on the lower seat and you're happy with the ride height then it's good.
All of that's the simple version (big note - my understanding of it, anyway :lol: ). There is one more bit about preload, which is that if you got the spring length / adjuster position right, you could dial out that initial sag you get when you climb aboard which would be cool but that's where things get tricky and I manage to get myself confused...
