So, this is an old debate that needs to be put to rest.
The first thing that comes to mind when people hear about cut springs is 'ricer bounce'. This is very much a reality of cutting springs, you will end up with a bouncy ride if all you do is cut your springs, but cutting springs isn't a bad thing.
In any sort of performance driving body roll and extreme weight transfer is not helpful. Many people buy new springs without understanding how they work. Stiffer is better in a general sense, and will help reduce aggressive weight transfer, but you don't need to rush out and buy new springs.
A spring is a long bar, and if you remember being the asshole in class who snapped pencils and rulers than this concept should be fairly obvious. If you weren't, no worries, go grab something long and thin, bar like and try it yourself.
When trying to bend a bar, it's much easier to bend a long bar. Holding it at the ends, you have quite a bit of leverage to increase along the length of the bar, and it bends easily in the middle. Trying to bend a short bar, you struggle, the leverage is weak as it's multiplication along the short length is minimal.
A spring is a bar, just curled. It's strength or resistance to change/deformation is determined by a few factors:
Coil thickness: The size of bar used, the thicker the bar, the harder it is to bend.
Wind diameter: The diameter of the curl of the bar, how big of a circle it makes. This affects it's length:
Number of winds: This is the number of curls it makes, again it affects it's length.
So what it boils down to, is the length of the bar, and it's thickness. That's your spring rate.
So what happens when I cut it? Well you obviously can't add thickness to the bar to improve it's strength, but what you can do is reduce it's length. By cutting spring away, the bar becomes shorter, adding resistance to deformation.
Do I go willy nilly on it? No, it's a very simple process. If you have a micrometer you can make all the necessary calculations to change your spring rate to a very close approximation of your desired rate.
How do I do that?
1. Measure the thickness of the coil using a micrometer. Write that **** down.
2. Measure the diameter of the wind. This is to the outer edge. Write it down homie.
3. Count the number of *active coils. The number of coil winds helps determine the length.
4. Throw it into a calculator, you can find one here: http://www.engineersedge.com/spring_comp_calc_k.htm
5. This is an extra step, but some people deal in metric or imperial values, here's a converter for use to walk between the two worlds.http://www.hraefn.net/projects/spring_rates.php
*So why the astrix? This is slightly more complex, but in a spring, there is active and inactive coils. If a coil is allowed to move freely, it's active, if it's pressed against something, and can't move, it's inactive. So check the shape of your spring, and spring perch to see how much of the spring is touching, and reduce that from the coil wind count.
Now if your not in the mood to cut, or out of cutting blades, you can create your own dead coils. This is a very old trick, and like so many old and very useful tricks from the past, have got lost in the minds the minds of the elders. Pass on your tricks you jerks! You can create your own dead coils by placing rubber stoppers, u-bolts and other devices around your springs to stop coils from moving. This makes these coils inactive, and again, shortens the length of the active coils, increasing spring rate. Just be careful though, these items need to be well engineered to take the abuse they receive.
What about ricer bounce, why does that happen? This happens because a lack of education about shocks, the spring now is no longer matched to the rate of the shock, and often will blow the shock. Ricer bounce doesn't occur just from cut springs, it comes from all inappropriate increase of spring stiffness without a change in the rates of the shock. If you were to throw some really stiff aftermarket springs into your car, they could even be original equipment upgrade, but they will more than likely destroy the shock if you leave the original shocks in. Why is this? Here's a crash course:
A spring bears the weight of the vehicle, but it has unwanted reactions when compressed or decompressed. It wants to get back to it's original position as fast as possible as soon as possible, so yes, it wants to 'spring back'. What's the shock do then? The shock does not hold the weight of the vehicle, it's purpose is to monitor and control the speed at which the spring compresses and decompresses. These are two similar, but separate processes. The resistance to compression, is called 'Bound'. Bound slows the spring from compressing to fast, when hitting a small bump on an otherwise smooth road, having the wheel suddenly lift from a sudden jolt/hit, means the tire would no longer be touching the road for a moment. This is not acceptable, or helpful, the shock helps resist this by slowing the speed at which the spring compresses, "Chill out bro!". The resistance to decompression, the spring expanding, is called "Rebound", it too is important. It slows the speed at which the spring expands after it's been compressed. Although the spring is more than willing to compress, it becomes more resistant to more compression the more it's compressed, it wants to expand again! If there way no rebound, the spring would push your car back up very quickly, this will cause wobble, unpredictable handling, and in sometimes, cause your car to go airborn! The rebound controls this speed, so it is an expected reaction, it opens again at whatever rate the shock is designed to allow it to.
So what does this have to do with ricer bounce? Well, your shock from factory is designed for a weaker spring. The bound is very stiff, and the rebound is very soft. The spring has very little resistance, so it wants to compress easily and has very limited force to expand again. The bound is high in the shock, resisting compression, but the rebound is very low.
When you add stiffer spring, the shock is not matched, The car will resist compression to much, and when it finally compresses, it has too little rebound control, and the spring will expand very rapidly, causing this "bounce". In many cases, the shock will 'blow', losing it's resistance in one or both directions. This is either from the lowered stance of the car, allowing the shock to 'bottom out' causing damage to the shock, and/or the spring is to strong for the shock and it will rip the valves and seals of the shock, again blowing it.
So what does this all mean? It means you can save a lot of money on suspension if your informed.
How do I use this information?
-By understanding how springs work you can properly adjust your own spring rates at home.
-You can now use your skills to junkyard hunt. Remember the shock on an unmodified car is designed to match the spring. Many shocks are physically compatible between different cars. By understanding the strength of their stock springs, you can pick up used shocks that will match your now, newly cut springs.
I hope this clears up some myths around cut springs, and makes people a bit more comfortable with them. Yes there are some more issues regarding fitments and other things, but for the most part this works. I'm not super authority, and there is a bit deeper science to this, but use this as your introduction and feel free to dig deeper into the subject.
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^ it depends on valving and suspension travel.
No point going into details, just take everything with moderation.
What would be a good way to determine what rates a shock can take? Other than ‘better than stock’, ‘stages’, etc?
I have Tociko Blues for example. Is there a spec that can tell me this?
[…] the introduction of cut springs we begin our dive further into the complex concepts of […]
Very few suspension companies list the rates of the shock. Mostly because it’s useless information for the average person, and if they get it right, it’s very easy for a competitor to just match it. Though there are suspension dyno’s around you can get them tested.
The best thing to do is to test things out. I really should write up a guide to desired suspension use versus design. However, this is a complex issue with consideration to driver style, vehicle usage, and most important, tire choice.
Heh, Didn’t think there was. Probably saftey BS that keeps them from telling us.