ebay - elcheap coilover springs

[JohnL]

cutting springs doesnt make the spring rate any stiffer than it already is, you'll need to remake/heat them to make them stiffer. cutting springs just reduces the range of the compressions that the spring can make, so it gives you the illusion that it is stiffer. same amount of weight will still lower the car the same as before (as the spring will still have the same spring rate that it was originally wound to), only difference is that they'll bottom out sooner with more people, as they will reach their maximum compression sooner (at this point they'll just look like a cylinder with screw-type grooves).

Sorry, but this is absolutely incorrect. Cutting coils off will always in every case increase the spring rate. A coil spring is simply a torsion bar that has been wound up into a coil, so, when the spring compresses the coil wire elastically deforms in exactly the same manner as does the wire in a torsion bar, i.e. the wire is twisted.

If you don't believe this then try this thought experiment (or you could do it for real if you were really sceptical / keen!):

Take two identical coil springs and cut a coil or two from one. Now uncoil them so that we make them into straight torsion bars. The one we cut will of course be shorter than the one we didn't. Now clamp each bar at one end so it can't move at that end, and now attempt to twist each bar with a torque wrench through say 90°. The shorter bar will require more torque to twist to 90°.

Still don't believe? Lets take it to an extreme to more clearly demonstrate the principle. Cut the short bar much shorter still, say to about 100mm long and twist again. You'll find the very short torsion bar will now be hugely stiffer than it originally was. This principle applies whether we shorten the coil wire length by a lot or by a little.

Things such as coil number, coil diameter and free length only affect spring stiffness in so far as they affect the length of wire in the coil (ignoring any dead coils that may be present in a 'progressive' spring). It may be however that coil pitch angle may have some very small affect; imagine a spring with a very steep coil pitch angle, the steeper the pitch becomes the more force passes through the wire in a sort of 'end-on' manner, though unless pitch angle is severe the effect is probalby quite small or the spring would attempt to rotate the spring seats as the spring compressed.

Having said this, I think you may be correct in that in some cases the shortened spring might possibly bottom out more easily than the unshortened spring, despite being stiffer. This is because the shorter spring lowers the ride height, but the stiffness increase may not be enough to compensate for the shorter distance from the top of the damper body to the bumpstop.

A cut spring will compress / lower less with X additional weight than the same spring uncut, because it is stiffer. The cut spring will probably ride lower with this extra weight but this is because the initial ride height will be lower.

If you cut coils off a spring this will reduce how far the coil can be compressed before it becomes 'coilbound', but the real limiting factor here will be the damper body and bumpstop lengths, i.e. the ride height will be lower and suspension travel less, but the 'bottomed out' height will be the same.

Heat treatment will make no difference to the spring stiffness, but it may have a significant affect on how far the spring can compress (wire twist) before it becomes permanently deformed or how long the spring might last. Nearly all steels (with the possible exception of some very exotic alloys that won't be found in springs etc) have the same elastic stiffness regardless of composition (alloy) or heat treatment (note that stiffness is not the same as strength). So, any two coils (or torsion bars) that are made from differing alloys or with differing heat treatment but having the same physical dimensions will have the same stiffness.

This doesn't mean they will be the same though. The difference will be in how far the spring can be compressed before it becomes either 'sagged' (permantly deformed) or actually breaks, i.e. where its elastic limit lies. Another likely difference may be in resistance to fatigue failure, whether this be from sagging over time or eventually breaking. Heat treatment / alloy also affect such things as shear strength, malleability etc, but this isn't an issue for springs.