I think they get firmer, but by how much I'm not sure.Im talking about my stock 96 Z's

I want to experiment and cut about 1 1/2 coils off all four.

 

They get stiffer. How much stiffer depends on how many active coils were present before and after you cut (this can be trickier than it appears at first glance).

OTOMH, cutting 1.5 coils sounds a bit much, as in "riding on the bump stops". Perhaps you could count the number of active coils and their loaded spacing and make a few other measurements (or provide the overall motion ratios and loaded coil spacing). Then a reasonable estimate of the resulting drop can be made and/or your amount cut adjusted to approximately result in a given drop.

Norm

 

I'd suggest you go a max of 1/2 a coil at a time ...especially on the fronts. The drop you get is much greater than the amount of coil you take off.

 

Even cutting the fronts in increments of half a coil might be too much, as cutting half a coil there can easily result in more than an inch difference in ride height. For example, on my G-body, cutting that much would drop it much closer to 2" (so I think in terms more like 1/8-coils there). While the F-body is probably not quite that sensitive, it won't be as nice as half an inch per half coil either.

If I happen to see an F-body up on a lift anytime soon, I'll try to provide a more useful correlation between coils cut and ride height change.

Norm

 

Springs get stiffer when you cut them. And they do so in proportion to what you started with and what you cut off. If you have 10 active coils and whack 1 off, you've just made the spring 10% stiffer than it was.

While I never recommend cutting springs, I REALLY wouldn't recommend cutting 1.5 coils off. Possum is right, what you cut off the spring, especially in front will not = the same amount of drop in ride height.

Also consider the springs have to hold the car up. If you cut them, you could lose more height than you increase rate to compensate. We're about 290-300 pounds per inch on the front springs. If you lower the car 2 inches, but only increase the spring rate say 40 lbs/in. you will end up hitting the bumpstops a lot more often and a lot harder......

 

ummmm i dont understand how cutting a spring will make it stiffer..... we will talk in a hypothetical situation here...
take 10 "active" coils on a 1000 lb/in spring each "active" coil is essentially producing 100 lbs/in if you remove one of those coils then the total would only be 900 lbs/in.... this is just my opinion and i have no facts to base it other than just looking at it... if there is a spring shop somewhere that manufactures springs and could test it on say some old truck/car or just old springs that are laying around and give positive results with a guage that would be helpful

 

No, listening to people who already know the correct answer would be helpful to you.

Either that or picking up a book or two that cover the subject if you're so interested.

 

Go easy on the guy, he's just trying to understand the logic of it.
Think of it this way, if you set a weight on top of your 10 coil spring that makes each coil deflect 1/10 inch the spring will compress 1 inch. When you cut off a coil the same weight will compress the spring only 9/10 of an inch, therefore it is 11.111etc. percent stiffer.

 

I'm afraid that trying to explain the logic to a self-admitted newbie is only going to cause further confusion. You either end up working with flexibilities (the reciprocal of stiffness) or computing the stiffness of a single coil and doing springs-in-series math to find a total rate. Neither is, IMO, a particularly easy way to get the point across in one shot. So Jon's answer was appropriate, if a wee bit blunt.

Arguably the better way is to simply present the established formula for use and see what questions follow. In that vein, the formula for the stiffness of a coil spring is:

[Spring Rate] = {[Shear Modulus]*[Wire Diameter]^4} / {8*[Number of Active Coils]*[Mean coil Diameter]^3]}

Shear Modulus is a material property, generally in the 11E6 psi range for steel, though it could be slightly higher or lower depending on the specific alloy. Wire diameter and coil diameter must be measured carefully, since the result is a 4th power function of the former and a cube function of the latter.

BTW, I did happen to see a late-model F-body up on a frame lift since my above post (wheels drooping, so I couldn't even guess at the loaded coil spacing). Although I didn't get to take measurements, the motion ratio for the front springs looks to be slightly more than 0.5 (I'm guessing 0.55, maybe 0.6?), which suggests that if the loaded front coil spacing is 1.5" (measured between the centers of the wire of adjacent coils) you could expect the car to drop by about:

1.5" * 0.5 coil / 0.55 = 1.36".

In the rear, the motion ratio is 1.0, so for a loaded coil spacing of 1.5", cutting half a coil would drop you by about 0.75".

It's necessary to use your own specific loaded coil spacing, as the 1.5" I used above is only for the purposes of illustrating the quickie math.

Maybe somebody can post a more accurate motion ratio for the F-body as well (Jon?).

The effect of the increased stiffness was intentionally left out for simplicity, and that will tend to work in your favor if you're cutting - if you cut per the simpler math you'll end up with a slightly higher ride height if anything, which is better than ending up too low and having to start over with another pair of springs.

Norm
Engineer/occasional Internet mechanic

 

Roughly .581. It does change slightly with ride height but using .58 will get you close enough.

Pretty good guess though Norm.

 

After chewing on "[Spring Rate] = {[Shear Modulus]*[Wire Diameter]^4} / {8*[Number of Active Coils]*[Mean coil Diameter]^3]}" for a few minutes, I'm wondering, isn't that formula lacking one variable? Namely, the "pitch" (angle of the wire vs the direction of compression) of the coil? Or am I missing something?

 

A lot of theory here, which is great, but the bottom line is buy some springs from reputable people that have already figured out the numbers.

I trimmed the spings on two of my cars and while they worked ok, I wouldn't do it again.

 

Marks' "Standard Handbook for Mechanical Engineers" provides a correction factor for "heavy closely spaced coils", which relates the wire diameter to the mean coil radius. However, this is related to the stresses in the spring at the given load, not the deflection under that load. This would be important if you were designing a spring from scratch, but is far less so for minor modifications performed on an acceptable existing design. It is also noted in Marks that this factor "k" can be taken as unity "for lighter springs", though I couldn't find any criteria to establish what that might mean other than what the referenced 1929 ASME Transactions paper might provide.

This correction factor also finds its way into the formula for resilience U, which may have something to do with how much more likely it is that a given modified spring will settle in service over time as compared to if it were left in its unmodified state.

Norm

 

I run a cut stock rear spring. I wanted 130lb springs, and a little lower.. just so happens stock (114lb) springs are perfect when cut. Fronts, not sure about.. use the above formula to find out!