Does the cutting of a spring make it softer or harder?

[cj]

It sounds like a simple question, but i've heard argumented opposite opinions. Please, share your experience.

[Knudsen]

It would be the same as before, only a bit shorter. You dont change the compound of the metal by cutting it..

[Eircamae86]

Id never do again, arch rubbing, bottoming out, crap handling etc

[Dorito86]

Actually, it depends on whether the spring you intend on cutting is progressive or not. If not, the shortened spring will be as stiff as before, only will bottom out more easily. If it was progressive, it depends on what end and how much you will cut off.

It's too backyard engineering for my tastes though...

[Mux213]

Actually, it does change. The thing is, as force is applied each winding compresses a certain amount.

So say that you have 5 windings and uncompressed your spring has a lenght of 50cm. Under load, say 250kg the spring compresses to a length of 40cm.

With a lineair spring this means the spring has a stiffness of 250kg/100mm = 2.5kg/mm.

It also means that under this load each winding compresses from 10cm to 8cm (thus 2cm compression under 250kg per winding).

If you cut one winding and end up with 4 windings you get 40cm length uncompressed (4x10cm) and 32cm length compressed (4x8cm).

This means that you now have a stiffness of 250kg/80mm =3.125 kg/mm

The spring has become stiffer and you get an 8 cm lowering of your car.

Well thats the theory as far as I've understood is, no idea if this theory is correct, I'm no expert

[Apolan]

Mux you are correct. Cutting the spring shorter increases the overall springrate, making it stiffer. The load on the whole spring is the same as for every coil of that spring. So the more coils you have spring will be softer (given that the thicknes is the same), because each coil will compress a certoin amount. If you cut say half of them away, the ramining half will compress just the same as it had before, but now having less coils you have lost half of the "compress distance". therfore it is twice as stiff.

[SekiguchiUeno]

yes this theory is correct i think, but this little stiffening is not enough for this big drop in height.

[Apolan]

true. thats why many have the bottoming effect.. if you have a spring with 5 coils, and it is 50 cm's long and has a springrate of 2 cm/kg.

Cutting one coil off, will shorten the spring for 10 cm's but only raising the spring rate for 1/5 th. so the "new" spring wil have 40 cm's and a springrate of 2.4cm/kg.

Thats how i understood the theory. Correct me if im wrong.

[dAYTONA]

You are correct Mux.

[cj]

Thanks everyone for their replies. I did cut the rear springs and the car felt much stiffer. I still may be wrong since i also changed the rear shocks to harder ones (Tokico HTS).

From what i've read, to measure the stiffness of a lienaer spring one would need the following parameters:

1. wire diameter
2. coil diameter
3. number of active springs
4. "modulus of rigidity" - a factors that concerns material physical property - but in the case of cutting it's value is constant

I do not know the exact equation, but while trying different on-line calculators I noticed that if you change only the "number of active coils" (meaning you have cut some) then the math result is a stiffer spring.

For illustration i've uploaded an Excell model bellow: