We are lucky to live in a time that has so many options available for upgrading a suspension. There are so many companies that make high performance components like coilover system, springs and shocks; and they are made for pretty much any application. It is possible now (though, for a price) to simply go online and order a set of 3-way adjustable dampers that are built for your car and are ready to install when you get them. Just a few years ago, such a shock (damper) was only reserved for professional racing and had to be custom valved and built for your application. But, with all these choices, you need to be educated to make a decision to find what’s best for you. In this post, I would like to focus on springs.
There are really two basic types of springs available on the market – linear springs and progressive springs (which are also called variable rate springs). Each type has different purposes and behaves a certain way. Knowing how these springs behave can help you make a better decision when buying aftermarket springs for your car. The names of these springs describe the spring rate, or how they resist the force being applied to them.
Linear rate springs:
Linear springs are the easiest to understand. When you think of a spring, this is typically what you are thinking of. As the load on the spring increases, the spring compresses an amount directly proportional to that load. So, if the spring is rated at 100lbs per inch, in will compress 2 inches when 200lbs is applied. The following graph describes this relationship which I’m sure you already understand:
In general, the most performance oriented and springs designed for racing are going to have a linear rate. The consistent nature of linear rate springs makes the car stable, optimizes traction, and makes the car more predictable for the driver.
Progressive (variable rate springs):
These springs are sometimes called variable Rate springs. The purpose of non-linear springs is to provide more compliance in the suspension over rough surfaces. These springs are designed with low initial spring rates but rapidly increase as the spring is compressed. The theory is that this allows the car to travel smoothly over bumps and road imperfections but still be tight enough to provide good handling and prevent the chassis from bottoming out. Because of this, these springs are popular in aftermarket lowering springs and coilover system intended for use on public roads.
Variable rate springs are easy to spot because the springs are not symmetrical. Typically, one end of the spring is wound tighter than the other end with the more tightly wound end being the softer part of the spring. As the spring is compressed, the soft coils on the tightly wound side will collapse, coming into contact with each other. When this happens, the rest of the spring starts to compress at the second higher rate. Sometimes two separate springs are used at the same time with a special perch in-between them to achieve the desired effect. Depending on the design, many different curves are possible. Some of the possible curves could look like:
This type of curve would be most likely on a spring with two very different rates – the stiff side of the spring is not compressing until the soft side has fully compressed.
This would be a truly progressive spring. The space between the coils would most likely be decreasing (or increasing depending on your point of view) at a consistent rate.
Another variation could produce this curve. This starts out linear and then smoothly transitions into a higher rate.
While progressive rate springs may not be common in road racing, in off-road racing like rallying, they come into their own. In off road forms of racing where suspensions have longer travel, they can be tuned to come into effect when the car lands after a jump and still maintain softness over the general stage.
Progressive springs are not common in road race cars because these cars typically operate on smooth surfaces. On a car designed for smooth tarmac, these types of springs actually decrease performance as they induce some slop into the suspension. Furthermore, high level road race cars do not have enough suspension travel to really take advantage of progressive springs. These cars typically rely on bump stops (which actually is a second spring with an extremely high rate) to keep themselves from bottoming out and to protect their suspension components.
Progressive springs are popular with lowering springs for street use because most cars are street bound. High rate linear springs produce a harsh ride which is undesirable for everyday driving. Everything in cars is a compromise, and you can’t have the highest performance springs without sacrifices some place else – in this case ride quality.
How about both linear and progressive springs at the same time?
I have also seen some spring kits that come with linear springs at the front and progressive springs at the rear. When this is done, it is most likely to limit the possibility of snappy oversteer and to delay it until the car has fully transitioned into the corner.
The downside to progressive springs.
There is a big downside with progressive springs – and that is that they are hard to damper effectively. A linear spring will always have a consistent rate which is easy to match to a damper, but a progressive or dual-rate spring will most likely go out of the effective damping range as it crosses into higher or lower rates. Trying to damper variable rates is actually a problem on all cars because anti-sway bars act as variable rate springs. The problem with sway bars is that their rates are added to the coil springs only when the car is cornering. This is compounded further when variable rate springs are introduced.
Even though this down side exists, don’t be to too afraid of getting a progressive spring systems if your car is street bound. Just be sure to do the proper research on a spring kit to know what to expect after it is installed. Personally, since I have a dedicated fun car, I have installed linear springs because my car is not driven for any purpose other than going fast, and I make do with a harsh ride on the street.
What you should know:
What you should know is that linear springs are going to provide the maximum level of performance for a car on pavement at the cost of a harsh ride. Progressive springs are an attempt to have a smooth ride and good handling at the same time but are not optimal at both. It’s up to you to decide which is going to be best for your application.
6 Comments to "Linear vs progressive rate springs"
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Great article. Summarizes what can be a very technical topic into an easy to understand, easily digestible read. Although I was set on linear springs, I think I will now compromise with progressive rate springs as in reality only about 4-5 days a year my car will see a road course and not a shopping center.
I don’t think many people pick and choose the spring rates they are going to run on their car. Because of this, they are mostly at the mercy of aftermarket spring kits that come from companies like H&R or Eibach. Most of these are designed for the street, and even with linear springs, they usually are not that rough. If you do a lot of sporty driving, even on the street, I would go with linear rates. Just because its linear doesn’t mean its track only, and many sports cars come from the factory with linear springs.
Thanks for the information! I’ve thought about getting progressive springs to replace the springs in my car. I didn’t know that they are difficult to damper effectively. It seems like I would be better off installing linear springs since they’re a lot easier to damper.
2002 B MW 525i (NON SPORT)
When front wheels contact road irregularities, from small bumps to shallow pot holes, it results in a sharp jolt. Are there any replacement springs that will remedy this condition? I will replace all 4 shocks also.
It is really great post. It is really great study about spring. Because liner spring have more damping capacity and the suspension system is mostly affected by the damping so i will prefer linear spring in future.
The second curve is quite obviously wrong. It shows 3 rates, initially soft, then hard, then soft again.