GameboyRMH said:
One real use of spring preload is using it to counteract your rebound damping, effectively reducing it. It would be a very odd circumstance where this makes sense, but it's something you can use preload for.
Not sure I follow how you'd use that. Can you expand? Remember that the spring rate doesn't actually change with a preloaded spring. The only thing that happens is that when the shock is fully extended, it'll take a bit of extra load to start moving it if you're really, really lightly loaded. Say you've got 50 lbs of preload on your spring, this means the shock won't start to compress from full extension until you get to 50.1 lbs of load. But from that point on, it will be exactly the same.
I've realized (too late to put in the video) that when most people are talking about increasing preload and getting better handling, what they're really doing is moving the perch up (more preload) and then shortening the shock body so the ride height is the same. What's really going on here is that you're increasing the amount of compression travel, and in most street vehicle applications that means less time on the bumpstops and thus better handing. The preload is a side effect, not the cause.
I was going to mention the bowing problem with very long springs, but you can't package a spring in a Miata that's long enough to have that problem and it was a video for a Miata audience :) I think we use 12" springs in one of our ND applications.
Running multiple rates with multiple springs in spacers is a fun experiment. I did it on the Targa Miata years ago. You have to lock out the softer springs at static ride height, though, otherwise you cannot properly damp the springs. You either have to set up the shock for the light rate or the heavy rate or neither. The offroad guys who are doing it have bypass shocks that change damping depending on where they are in their travel.
Two-piece shocks have one advantage other than cost, which is packaging flexibility, although making use of it involves making even more tradeoffs in suspension travel, and if money were no object you could always design a better single-piece shock with similar packaging. Still, this has been very handy for me when trying to package springs and sway bar mounts around extra-wide wheels on a tight budget for a car that doesn't need much suspension travel.
If you're trying to put something where it doesn't belong, sure. A set of well-designed one-piece shocks will already be the correct dimensions for the application they're designed for, like you noted. It's not a money-no-object thing, either - some companies will happily build you a shock to your specific dimensions or provide the parts to build your own. AFCO, for example, lets you select your body length, shaft length, top and bottom fixing types at a pretty affordable price. You just have to figure out what those have to be. That's what I have on the MG, a set of very custom AFCOs that are exactly what I asked for.
There are places where you can get away without much suspension travel because you're not using much. But then someone else comes along with more suspension travel and is able to go faster over the one bump you had to lift for. Superspeedways are very much an unusual case, it's my understanding that they'll actually run the cars in full coil bind. Effectively no suspension at all.
) but for adventurers at home, it is possible. 
