derby said:
The spring rate does become a little firmer as it is lowered. But it is not by preloading, it's because the there are fewer coils to compress. The different in firmness while lowered is not very much.
This is correct. Highdell, you're correct about how springs work, but you've got a misconception about how u-turn works.
When you lower a u-turn fork, what you're doing is reducing the number of coils in between the upper spring perch and the lower spring perch. It has the same effect as
cutting the spring, not
compressing the spring.
When you cut a spring in half, the remaining half-spring does not have the same rate as the whole spring had. If 20 pounds was enough to compress the original spring by one inch, then it will compress each half-spring by a half-inch - the spring rate for the cut spring is now twice what the original spring rate was.
If you want to use the torsion bar analogy, imagine clamping the middle of the torsion bar instead of the far end. The torsional stiffness of the
material is unchanged, but since you're now operating on a shorter length of bar, the resulting torsional stiffness is increased (doubled).
The lower spring perch in a u-turn fork is threaded, so that rotating the spring causes the spring perch to move up and down the spring. It's kind of hard to put into words, but I just swapped springs in my Pike and operating the mechanism in my hands made it really obvious how it works. It's pretty clever. It does
not compress the spring but it does shorten the effective portion of the spring, just like cutting would.
