Progressive Rate Springs
The diagram shows three types of springs commonly used in the offroad industry. This post will discuss Progressive Springs. I am going to provide specific values for spring rate, load at ride height etc. for discussion purposes only. These are not actual numbers because most manufacturers have proprietary spring rates and loads that they believe are ideal for a quality ride or are tuned for their specific systems (so does MC). All numbers are examples only.
Progressive Rate Springs- Many top end long travel suspension manufacturers use Progressive Rate Springs. There are two ways to produce a progressive rate spring.
1) Continuously Variable Rate (Pitch)....or the winding of the spring coils such that the distance between each subsequent active coil (coil pitch) of the spring increases as you progress from the top coil to the bottom coil. In other words, the coil pitch continouosly changes throughout the entire length of the spring. Production of this type of progressive rate spring is rare as it requires specialized CNC spring winding machines.
2) Combining 3 or more Spring Rates (Pitches)....most Progressive rate springs are produced using a combination of 3 or more linear spring rates (coil pitches) wound into a single spring coil. Some manufactures use marketing terms like Triple Rate or Quad Rate springs, however these are really just progressive rate springs with a fancy name. Let me explain what I mean...when one uses three spring rates (coil pitches) in a single spring, each spring rate must also have a transition rate (coil pitch) between each of the three spring rates, therefore a spring with three spring rates must effectively have a total of at least 5 spring rates . Once you have that many different spring rates in a single spring coil it is effectively an estimate of a progressive rate spring. In other words, Progressive Rate Springs, Triple Rate Springs, Quad Rate Springs are all the same thing with just different names. I will refer to all of these as Progressive Rate Springs in this post.
Progressive Rate springs are generally more expensive to make, harder to design, harder to inspect and do not have the ride height consistency of linear rate springs. They are excellent springs for long travel suspension systems. Look at the Progressive Spring in the image above. Notice that the distance between the active coils (Pitch) steadily increases as you progress from the top coil to the bottom coil. This is how you can identify a true progressive spring from a marketing progressive spring.
A true Progressive Rate Spring is progressive by RATE. Meaning the spring rate increases the further the spring is compressed.
Unfortunately a Linear Rate Spring is progressive by FORCE. Meaning the load force of the spring increases the further the spring is compressed. I have seen some manufacturers claim there springs are progressive and technically they are progressive by force but not by rate.
Notice how the last two active coils are the furthest apart (I will return to this later in the post).
So I am going to use the same spring assumptions I used in the last long travel suspension example:
Assume the desired spring rate for the perfect ride is again 150lbs/in
Assume the compressed length for the Progressive Spring at ride height is again 10
Assume that the Free Length for the Progressive Spring is again 18
So in summary we need our spring to compress a total of 8 (18 - 10). When it compresses this 8 and reaches the desired length of 10 it needs to handle a load of 600lbs and have a rate of 150lbs/in at that specific point.
The following is a chart of what is happening with the progressive spring as we compress it to the desire ride height. I hope this chart works on your screen.
Spring Compression___Spring Rate at Compression__Force at Compression___Length of Spring
___2_____________________20 lbs/in________________40 lbs______________16
___4_____________________40 lbs/in________________120 lbs_____________14
___6_____________________90 lbs/in________________300 lbs_____________12
___8____________________150 lbs/in________________600 lbs_____________10
So this is an example of a progressive spring with a continually changing rate that will work in the example long travel suspension used above.
As you can see the Spring Rate is changing as the spring is compressed and aproaches the desired ride height. This is what makes it more difficult to provide a consistent ride height because the Rate and the Force of the spring are both changing at the point that the user wants his/her ride height. Remember I said it makes it more difficult not impossible.
In addition, as the Progressive Spring in the image above, compresses every coil of the spring will move the same distance resulting in the top coils slowly collapsing on top of one another before the lower coils collapse on one another. As a coil collapses on a previous coil it becomes fully supported or solid and no longer acts as an active part of the spring at all. This collapsing of coils onto one another is the exact mechanical feature that causes a Progressive Rate Spring to act as a Progressive Rate Spring.
In other words, as spring coils are removed from being active the fewer remaining active coils result in the spring becoming stiffer. It is the same concept as a long rod being easier to bend (softer) than a shorter rod of the same diameter (stiffer).
This is another way to identify if you truly have a progressive rate spring or not. If the coils of your springs do not progressively stack directly on top of one another (touching) during a good portion of the compression cycle of the spring you likely do not have true progressive rate springs. Or your spring has not reached its progressive rate stage within the compression cycle that you viewed.
Progressive Rate Springs have a very nice feature in that they substantially increase in rate as you compress beyond ride height. This helps to reduce the force of impact of the axles on the bump stops under extreme off road conditions. Nice feature over linear springs.
Now for the number one reason MC did not choose to design our springs as Progressive Rate Springs.
We use a lot of up travel in our suspension systems therefore our springs get very close to the fully compressed (all coils touching) condition. Progressive Rate Springs do not like to be fully compressed as they are susceptible to sagging. If you can picture an almost fully compressed Progressive Rate Spring, all of the top coils will be collapsed on one another and inactive while only the very last active coil will be active. In this condition the largest amount of spring stress is concentrated in the final active coil. Substantially more stress than what is in the first coil that collapsed and became solid long before the fully compressed condition. This large concentration of stress in a single coil can cause the spring to fatigue and sag over time.
Therefore MC chose to use Dual Rate Springs. That write up is next.