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Progressive Vs Linear springs

July 13, 2017
By

Linear springs:

Linear springs require the same amount of force per inch of compression throughout its full travel.

Almost all OEM’s use linear springs for three main reasons:

1.The automaker has the luxury of designing the strut, tower, and suspension geometry around their spring and shock. Therefore if they want the car to be higher or lower, they can modify the rest of the suspension rather than relying on the spring to provide the drop.

2. Predictable suspension travel for the driver.

3. They provide the least amount of NVH or Noise, vibration, and harshness and longevity. Unlike progressive springs they do not have dead coils, and therefore coils do not rub against each other (longevity), nor do you get any NVH from the coils rubbing.

Not only do you normally find linear springs on most OEM applications, but springs are also almost always linear in true on-road racing applications.

 

Progressive springs:

Progressive springs require different amounts of force per inch at different areas of travel of the springs compression.

The reasoning behind progressive springs is:

1. To provide (or advertise) a high spring rate, but to also allow for a lower spring rate at normal cruising loads.

2. To take up spring slack, more specifically to keep spring force on the strut assembly at full unload (like going over a hill fast, or one side unloading during heavy turning).

3. To provide a high spring rate at the end of the travel to prevent from bottoming out (especially when lowered).

 

Progressive Vs linear:

There are only two legitimate reasons to have progressive springs:

For offroading; particularly to provide a high spring rate at the end of the suspension travel to prevent from bottoming out. Even so, in most professional offroading applications, teams will use true dual rate coil over systems. Where the second set of coils will not start compressing until the first set is fully compressed.

 

This allows for a sharp transition between travel rates; allowing for at-least some predictability:

 

For lowering your car with disregard for handling; if you are going for looks over function depending on the make and model of the car, often times the only way you can get the ride height you want (using springs) is to have dead or soft top coils. Otherwise, the spring would not be long enough at full unload to apply force to the top and bottom of the strut and would rattle around.

Although with McPherson suspension systems, full unload is more determined by the shock’s bumpstop at full extension than the length of the spring. As long as the spring has enough travel to cover the full extension of the shock, then taking up slack becomes irrelevant.

 

In every other case, liner springs are superior:

On road handling; We always hear that the lower the center of gravity the better the handling, but like most things in life, there are sacrifices, to have a low center of gravity means you must also have a high spring rate (also good for handling) to prevent the car from bottoming out on the ground. However, the higher the spring rate the harsher/stiffer the ride will be on a daily basis if the car is driven on the street.

Unfortunately we cant have our cake and eat it too, one of the main selling slogans of progressive springs is that you get a softer spring rate at normal compression loads and higher during heavy cornering, this sounds good right? Wrong. One of the most important aspects of good handling and having a well handling car is to have predictability. With a linear spring the more you turn you know the next bit of travel will be the same as the last, with progressive you’re constantly guessing where you are in the range of your suspension travel, and to what extent the rate is changing on you. Even worse, the spring rate changes both way, this includes when you unload the spring, it will spring back on you unpredictably; even further, if you are cornering over a crest, one spring will be loading non-linearly while the other side is unloading non-linearly. Those “dead” coils they told you about that are taking up the slack are now all of a sudden active during unload.

Recommendation: If street driven; get as high of a spring rate in linear springs as you can handle day to day, don’t be focused on your center of gravity.

 

Lowering your car with regard for handling:

Further to the above (on road handling), dont get convinced that you need progressive springs to take up slack.

Slack is determined by shock travel, and every shock has a range of spring lengths they can accommodate.

The full travel of a Bilstein HD shock for an E30, is 88mm or 3.46 inches.

The full unloaded length of an E30 spring for the shortest option (E30 Evo M3) is 305.7mm or 12.035 inches.

That 12.035 inches includes the spring needing to take up an additional 2-2.5 inches of travel based on the elevation of the strut tube relative to the lower spring perch.

So in total, the spring at minimum needs to be 2.5 inches + 3.46 inches = 5.96 inches (likely even less because of some compression from top perch). With the lowest spring (6 coil Evo M3) being 12.035 inches, slack is of no worry.

If you have a make of car where the measurements get you concerned, start looking into sport shocks with shorter travel.

Measurements below arent for an E30, just an illustration of travel differences between HD (or stock) shocks and sport shocks.

 

 



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