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Coil Spring Rates

Thursday, July 25, 2019 - Ken Pruett

Coil Spring Rates

One of the most popular upgrades for enthusiasts is changing the coils. The vehicle manufacturer had a limited view of suspension set up when the car was designed and many of the factors they took into consideration might get a different answer from enthusiastic drivers. In fact most manufacturers sell their vehicles with multiple suspension packages, which usually include different spring rates, anti-sway bars and suspension damping. Some people are just looking to change the appearance of the car by changing the ride height and others are looking to change the way it rides.

One of the biggest issues with choosing your upgrade is that the manufacturer never discloses the factory spring rates, so there is no relative baseline. Whether it is lowering coils or sport coils, it seems like everybody has questions about the coils, and they frequently ask about the spring rate.

While the rate can have some affect on ride height at times, it is primarily about ride quality. Ride height must be designed into the coil unless you have adjustable spring perches (which stock vehicles never do). Rates are to set ride quality and load control. In a perfect world you would have a different set of linear coils for each condition you use the car, but this world is far from perfect and changing coils is not simple.

"What is a coil spring rate, and what does it mean for me?"

It’s OK, you can ask and we won’t forum flame you. This is also something that many in the forums discuss endlessly and mostly in ways that are not very helpful. So let’s go over the basic terms and how they work in the real world.

Linear - (sometimes called straight rate) This means that at the intended installed ride height (which is controlled by the weight of that corner of the car and the suspension rate) the rate of the spring remains constant through the expected range of motion. Let’s unpack those words.

Each spring is designed to work for a specific weight at ride height. This is not measured outside the car (which would be more convenient for the customer) but from the top to the bottom of the coil. Keep in mind that many (most) vehicle suspensions also operate with leverage rates so 1 inch of coil height is frequently not one inch of ride height.  The spring is placed in the test fixture and then pre-loaded (compressed) to the weight/resistance of the load that would occur at normal ride height in that specific vehicle suspension.

The resistance (weight) of the spring is measured at this load. Then if you compress the spring another inch the resistance will go up the amount of the linear rate. Each inch of travel through the designed range of motion should result in the same change in resistance (weight) and the amount of that change is the rate. This is confusing because the resistance (weight) keeps rising but it is rising at the same rate. Typically a linear rate spring “appears” to have the same distance between the majority of the coils.

Arbitrary imaginary spring
750lb = 9 inches
900lb = 10 inches              Spring rate = 150lb per inch
1050lb= 11 inches

Remember that if you cut the spring (shame on you) the rate will change. This is because the amount of flex in each inch of the spring wire totals up for the whole spring. If you decrease the number of inches of coil wire (number of coils) there will be less flex total since there are less inches to flex. So you just threw all the math out the window.

Progressive - The term “Progressive Rate Coil” is different than progressive spring, because all coil springs are progressive by nature. Standard coils progress at a linear rate. A progressive rate coil does something different.

Since we covered linear rate it is simpler to discuss what is different here. A progressive rate coil does not have a single rate number like a linear rate coil. If you see a progressive rate coil advertised with a single rate it is typically the rate at the designed ride height/corner weight, but this rate will rise when you hit bumps (compress) and lower when you go over a rise (lengthen). Typically a progressive rate coil will have a winding that changes progressively in distance through the length of the coil that is visually obvious.

You may find a progressive rate coil that matches our 150lb per inch example above when it is compressed to 10 inches (900lb). But the resistance will not be the same 150lb rise at 10 inches. It may instead have gone up 165lb. Also when you raise the coil (decrease load) an inch, it will decrease less than the linear 150lb (maybe it lowers the resistance 135lb at 9 inches).

765lb = 9 inches
900lb = 10 inches              Spring rate = 135lb,150lb,165lb per inch
1065lb= 11 inches

It is immediately obvious that progressive rates sound great on the surface, but they are extraordinarily complicated to design. Most actual race vehicle builders despise progressive rates since they feel they can never get the suspension to “set” and stability/traction is less predictable (also wheel chatter issues have been seen).

Dual Rate - Dual rate refers to coils that have been designed to have two specific different resistance rates. These are more common in “overload” applications for real world reasons.

A dual rate coil has two different stated rates at two different loads. Dual rate coils are typically visually obvious with two different spring winding distances. Do not confuse this with designs that have coils with no winding gap at all (which are called “dead coils”).

In this case the coils with the lower rate will compress (collapse) and lose their gap first and then the stiffer part of the winding will begin compressing. This is frequently accomplished in the aftermarket by stacking two separate coils with a guide (or on a strut). You can also see “bump” coils that act as a type of bump stop on performance suspension units. This can be an effective design for vehicles that are frequently loaded, but is not optimal for vehicles with more predictable loads.

One common complaint with these is that the transition between the two rates can be uncomfortable or sudden/harsh. This may not bother you in your Saturday garden truck but can be annoying on a family road trip. So is hitting the bump stops.

The majority of vehicles come from the factory with linear coils installed, though they are frequently installed with non-linear suspension designs. This means that the suspension may behave like it has a progressive springs, but the spring is actually linear and the suspension is progressive. Some heavier vehicles (light duty trucks) may come with dual rate springs but these are frequently leaf springs (principal is the same but shape is different). Progressive spring designs are predominantly aftermarket options but are factory installed on some high end vehicles.

So that is the basic summary of what the different rate terms mean, which is only a very small part of complete coil design. It is pretty easy to explain these small aspects but remember that there are other complicating factors (like dead coils and suspension rates) which make this question far less omnipotent. The spring rate doesn’t set the ride height, which is what most enthusiasts are looking for. The rate controls the stiffness of the ride. The spring rate number is just a single factor in the function of your suspension but knowing what this number means will lift a bit of the veil.

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