Thread: Why X5K High Stress Steel for coil springs

are they the same spring rate?

Originally Posted by uninformed

are they the same spring rate?

They are relatively comparable yes. They are both designed for 40mm lift and both heavy duty to suit dual battery, winch and bullbar.
In practice, when I measured them after swapping, they are sitting within 3mm of each other.

when researching what springs I should buy for my Disco, I read lots of old threads and reviews complaining of sagged or broken King Springs. I'm not sure when they switched but there are no threads or reviews I can find within the last 4 years or so where the writer complains of those problems.
I did end up getting the King springs.

Not judging these springs, but adding some notes about coil springs and the material.

In the blurb about the spring material being high stress steel, sounds to me like marketing spin. High stress steel is not a term used for mechanical properties of materials - the correct term is tensile strength. Stress is not a material property, it is result of load and area (stress = load / area). Strength is the stress that the material can withstand.

Because the spring has fewer active coils the wire diameter has to be reduced to obtain the desired spring rate (rate increase when number of active coils is reduced, and reduces when wire diameter is reduced).

It is the ratio of allowable stress / working stress that determines when or if the spring will fail.

Reducing either or both the number of active coils and the wire diameter will increase the stress induced in the wire when the spring load/deflection increases. The design strength of the wire material has to be chosen so that it is greater than the maximum working stress when the spring is compressed by a load - in other words the spring manufacturer is forced to choose a stronger material for the wire if the stress is higher.

Another way to look at these issues is:

When a coil spring is compressed by a load the wire twists. This twisting is called shear strain and the shear stress increases because stress and strain are a function of the elastic modulus. What is happening when you hit a bump is that the shock/energy is converted into strain energy.

The strain energy capacity of a round wire coil spring is a function of the volume of material in the wire used for the spring and the stress/strain at the allowable compression/deflection. If the volume of wire is greatly reduced (as in these springs) the material stress will be greatly incresed.

Obviously the spring manufacturer sees some advantage in producing springs like this, and I'm not disputing those. What I want to point out is that unless you compare the ratio of allowable stress / working stress, it is not possible to say that any spring made from higher strength material will have greater or lesser life if other changes increase the working stress.

Originally Posted by Bush65

Not judging these springs, but adding some notes about coil springs and the material.

In the blurb about the spring material being high stress steel, sounds to me like marketing spin. High stress steel is not a term used for mechanical properties of materials - the correct term is tensile strength. Stress is not a material property, it is result of load and area (stress = load / area). Strength is the stress that the material can withstand.

Because the spring has fewer active coils the wire diameter has to be reduced to obtain the desired spring rate (rate increase when number of active coils is reduced, and reduces when wire diameter is reduced).

It is the ratio of allowable stress / working stress that determines when or if the spring will fail.

Reducing either or both the number of active coils and the wire diameter will increase the stress induced in the wire when the spring load/deflection increases. The design strength of the wire material has to be chosen so that it is greater than the maximum working stress when the spring is compressed by a load - in other words the spring manufacturer is forced to choose a stronger material for the wire if the stress is higher.

Another way to look at these issues is:

When a coil spring is compressed by a load the wire twists. This twisting is called shear strain and the shear stress increases because stress and strain are a function of the elastic modulus. What is happening when you hit a bump is that the shock/energy is converted into strain energy.

The strain energy capacity of a round wire coil spring is a function of the volume of material in the wire used for the spring and the stress/strain at the allowable compression/deflection. If the volume of wire is greatly reduced (as in these springs) the material stress will be greatly incresed.

Obviously the spring manufacturer sees some advantage in producing springs like this, and I'm not disputing those. What I want to point out is that unless you compare the ratio of allowable stress / working stress, it is not possible to say that any spring made from higher strength material will have greater or lesser life if other changes increase the working stress.

Thanks for the great tech as always John.

Other spring makers refer to the steel more correctly as X5K high tensile OR high strength micro-alloyed spring steel.
e.g: OME Spring Features & Benefits | ARB USA Blog

I cannot find who makes it or the specs...

The principal advantage (spruked by King) seems to be saving weight. I cannot see the point on a landie as the coils are hardly a significant proportion of the vehicle's weight. I would prefer a coil that has the longest design life! (after seeing a poor old bloke stranded at well 17 for several weeks with 2 broken coils).

ARB blurb on X5K

X5K Micro-Alloyed Spring Steel

New generation high strength spring steel. This steel is enhanced by additions of silicon, chromium, and vanadium nickel, along with lower carbon than conventional spring steel grades.

Elsewhere I can find this discussion of the properties of various spring wire:

Chrome Vanadium wire

This is the most popular alloy spring steel for improved stress, fatigue, long endurance life conditions as compared to high carbon steel materials. This material is also suitable for impact and shock loading conditions. Is available in annealed and tempered sizes from 0,8mm to 12mm. It can be used for temperatures up to 220 oC. Will not generally change dimensions under heat. Can be plated.

Chrome-silicon wire

This an excellent spring material for highly-stressed springs requiring long life and/or shock loading resistance. It is available in diameters 0,8mm to 12mmm and can be used from temperatures up to 250oC. Will not generally change dimensions under heat. Can be plated.

Originally Posted by Bush65

In the blurb about the spring material being high stress steel, sounds to me like marketing spin. High stress steel is not a term used for mechanical properties of materials - the correct term is tensile strength.

I think the important part to look for in order to identify this type of spring is 'X5K', this is what many spring manufacturers use to reference this type of material.

Originally Posted by twr7cx

They are relatively comparable yes. They are both designed for 40mm lift and both heavy duty to suit dual battery, winch and bullbar.
In practice, when I measured them after swapping, they are sitting within 3mm of each other.

I dont think that automatically qualifies them as having the same spring rate............

Originally Posted by twr7cx

I think the important part to look for in order to identify this type of spring is 'X5K', this is what many spring manufacturers use to reference this type of material.

But we don't know that they will be more durable. If they made them the same, then they would, but as John has pointed out the lightweight (fewer coil) design means higher stress.

The failed coils I mentioned above were made by a manufacturer who (now) uses X5K steel. The springs probably predate the switch, however...