Setting spring preload

[Guest macks]

G'day guys

 

I've all but finished my coilovers using 300zx front shock inserts, and I'm at the point of welding the perch for the threaded sleeve. If my understanding of the previous thread ( http://www.hybridz.org/phpBB2/viewtopic.php?t=7803&start=15 ) is correct, my springs should be preloaded so that the loaded weight of the car compresses them to the mid-point of shock travel ... correct?

 

At the moment I've got the struts assembled and the threaded sleeve loose, but sitting flush with the bottom of the spring, which is flush at the top spring mount which is bolted to the top of the shock. So, I should weld the perch for the sleeve in place the preload amount above where it is now, determined by:

 

(using the unloaded top spring perch as the reference point, downwards +ve)

 

corner weight/spring rate - preload = shock travel / 2

 

Does this sound right?

 

example: 650 lbs / 175 lbs/inch - preload = 3"

-> 3.7" - preload = 3"

hence the perch point should be 0.7" above where the bottom of the threaded sleeve sits with an unloaded spring flush top and bottom. I think.

[Guest macks]

c'mon, someone's gotta know - I've gotta finish it this weekend or I'm in trouble

[TimZ]

You're making this too hard - the preload will work itself out. You just need to look at how long your spring will be when it is compressed at its normal static loading.

 

For your example, the spring will be compressed by about 2.9" (650/225). You didn't mention how long the spring was (this is very important), but let's suppose that it is 10". So, at your normal static loading, your spring will be 10" - 2.9" = 7.1" long.

 

Now, all you need to do is place the lower perch such that the middle of the threaded sleeve (or at least some part of it) is 7.1" from the upper perch when the suspension is at the desired ride height (presumably the middle of your shock travel).

 

One thing to note - in this example, if you assume that the strut had 7" of total travel, you might notice that there was no preload on this spring. The "ideal" bump travel of 3.5" is longer than the spring was compressed at static loading. This isn't a huge deal - you can assume that the bump stop eats up some of that travel, and take this into account when you section the strut. Or, you can just give up about 0.4" of total travel. As you start going to stiffer and stiffer springs though, this can become an issue.

 

Another very important thing to look at is how long the spring will be when it is fully compressed (coil bind). You must use a spring that is long enough that the lower perch is farther below the upper perch than the fully compressed length of the spring. In other words, if your spring is still 3" long when it binds, then your lower perch must be at least 3" from the upper perch when the suspension is fully compressed.

 

Sorry it took me a while to notice this thread - it's been a busy weekend.