Contour Enthusiasts Group Archives Forums Modifications Suspension Cut springs!!!

Mcgainer, I read what you said about how cutting results in increased spring rate. Does that mean cutting a coil off my springs will make the car ride firmer than before? If so, wouldn't that improve the handling?

Sorry, but on a progressive spring cutting a coil would result in a firmer ride, but on a linear spring it wouldn't change the ride. The math formula F=-kX shows that it is a linear effect and that the force is a function of distance (X) that the spring can be compressed or stretched.
Less distance (i.e. cutting coils) means less available force. A progressive rate spring will most likely be some sort of a square function and it greatly depends on which end you cut the coils. Cutting the softer coils off would increase the overall rate and yield a firmer ride.
However, we don't have progressive rate springs, at least not stock.

I'm glad to hear that. I have always doubted that theory. Obviously with progressives it will change, but I knew in the back of my head that linears would not change, I just had no technical proof to prove it, just logic. Thanx for clearing that up.

I cut my springs today. I cut one full coil off of each spring. I got a three-quarter inch drop. I expect it will settle a little lower over the next few weeks. It rides and handles just the same as before, so far as I can tell.

Dudes (e.g. Warmonger and JohhnySVT)...when you cut any coil off a spring YOU RAISE ITS RATE. Period. Indisputable. Fact of physics. Progressive or fixed-rate, doesn't matter.

The equation that defines spring rate (not conjecture, not somebody's opinion, spring rate by definition) is:

K = w^4*G/8ND^3

K = Spring rate in pounds per inch
W = Diameter of the spring wire in inches
G = 12,000,000 for steel springs (a constant)
N = Number of active coils (number of coils that are free to move + 1/2 coil)
D = Diameter of the coils measured to the center of the wire, in inches

Notice the relationship between the number of active coils and the spring rate. Play with the math yourselves and see what happens to that equation when you reduce the number of active coils. For instance if you cut off half the active coils the spring rate is doubled.

I hope this scientific fact provides the necessary "logical explanation".

The questions regarding handling then become, as I've said before even in this thread, 1) does that increase in spring rate then make up for the reduction in travel caused by the cut? Many times, no, and the more you cut the more likely you are to have a mismatch. 2) You've lowered the CG, but have you also lowered the Roll Center so much that the CG now has an increased leverage against the Roll Center (yes, it happens) and your spring rate isn't enough to make an actual improvement, and Furthermore, 3) does the increase in rate make up for the wheel's new placement in the suspension's camber curve, which is typically horrible in a strut suspension? In other words, you've lowered the CG, but now you get 2 degrees if body roll in a corner and the camber curve dictates you don't gain 2 degrees of negative camber back as the suspension compresses, for a net loss in camber?

Things get complicated. Quite obviously a mild cut isn't the end of the world in these cars, but now you have the facts as to why cutting springs isn't the easy be-all-end-all suspension improver.

whoo thanx for the science

WEIRD SCIENCE

Well ok, thanks for the education. I stand corrected though the results of what I was trying to get across are the same....cutting the spring seems to be more significant in reducing overall spring force than the increases in spring rate from less coils would add to it. I played with the numbers and what stands out to me is the factor of 12^6 and a 4th order function of diameter in the numerator and a 3rd order function of diameter in the denominator.
In the end I think the spring would cease to fit correctly before the rate increases became significant enough to be beneficial.

Cal: Which end of the spring did you cut the coil off.

Thanks

Sarge, I cut the bottom coil off of each spring. By the way, I re-measured the drop today using a proper metal ruler, and the actual drop is three-quarters of an inch in the back, and half an inch in the front. Hopefully it will settle a little more in the front. Keep in mind that my car is a Zetec. V6 cars might get a different drop.