General Tech - How to figure out coilover spring rates?

Greg said:

So I have a coilover spring rate question... basically, I would like to get a pair of the lower coils to verify that everything is going to fit/clear on my front end before I weld up the coilover mounts.

I have been trying to figure out a baseline that others have used for a similar vehicle, but that seems like the numbers can vary so much between similar vehicles.

Can anyone provide some additional information on figuring out spring rates for coilovers?

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Your combined spring rate with two coils before you hit the dual rate stop is much softer than either of the coils alone. For example, if you have a 100-pound coil over a 200-pound coil in your dual rate system, both coils will see the full 200 pounds when loaded. The 100-pound coil will compress 2 inches, and the 200-pound coil will compress 1 inch. Therefore, 200 pounds will compress your shock 3 inches in this system. Using the formula 200/3 = 66.67, a 100 over 200 will give you a total of 66.67 pounds per inch spring rate, until you hit the dual rate stop. Once the dual rate stop is engaged, the top spring is taken out of the equation, and your rate jumps to whatever the bottom spring is, in this case, 200 pounds. This increases stiffness significantly before hitting the bump stops.

To figure out what springs to buy, you first need to know the weight sitting on the springs. If the shocks are vertical, you can simply measure the sprung weight. An even better method is to use a coil of a known rate, install it, and measure how much it compresses. This will precisely tell you the weight on it.

Once you have an estimate of the weight on each shock, the next step is to determine how much you want the shock compressed at ride height. Some prefer setting it in the middle of its travel at ride height, while others might want different amounts of compression or extension.

To avoid using tender springs, ensure the springs are slightly compressed at full extension of the shock so they don't go slack and fall out of the cups. If you know the weight and how much you want the shock to compress at ride height, divide the weight by the number of inches you want the shock to compress to get your desired spring rate.

For instance, let’s say you have 14" travel coilovers. You want them to sit in the middle of their travel at ride height, and you've determined that your front coilovers are each holding 500 pounds of unsprung weight. You don’t want to use tender springs, and you want the 500 pounds to compress your springs 7 inches. Using the formula 500/7 = 71.43, you get the required spring rate. To avoid tender springs, you might opt for a combined spring rate a bit under the calculated amount, adding some extra tension to reach ride height.

The formula for finding a combined spring rate is:

CR = (R1 * R2) / (R1 + R2)

CR = Combined spring rate
R1 = Spring rate of the top spring
R2 = Spring rate of the bottom spring (and total spring rate after the stop)

For around 70 pounds spring rate, you might use 100 over 180 for a 70-pound rate, or 80 over 200, or 140 over 140. Any two springs that add up to 280 will yield the same combined rate. Check with Summit Racing or other suppliers for affordable options. Often, you’ll find one rate for 25 bucks and another different rate for 80. You should be able to identify a workable combination at a reasonable cost.

Be prepared to make adjustments; you won't get it perfect on the first try. You’ll likely end up replacing at least one spring on each corner. Make your best calculations and purchase less expensive springs initially, expecting to have extras in your garage.

Also known as deflection rate, spring rate measures a spring's strength, determined by wire diameter, coil number, and steel quality. It is the weight required to compress the coil spring by one inch. OE spring rate varies by vehicle model, engine size, drivetrain (4W vs. front-wheel), transmission type (automatic vs. manual), and other vehicle specifics affecting weight and ride. A single model can have different packages and trim levels that impact spring rate.

When it comes to spring designs, it's vital to match OE specs by application to restore OE ride height specs. Even a 15% reduction in ride height can cause noticeable sag, impacting wheel alignment and leading to accelerated tire wear.

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Contact us to discuss your requirements for steel auto shock absorber. Our experienced sales team can help you identify the best options for your needs.

Here are some additional things to know:

Quality Matters

A spring manufacturer uses its own formula to achieve the spring rate by adjusting steel strength, wire diameter, or coil number. Simply adding more coils to compensate for lower strength steel may not match the OE spring rate. Additional coils can create touchpoints at or before full suspension compression, causing coil bind and potentially immediate suspension component damage.

Rate Differences

Spring rates can differ from the left to the right side of the vehicle. Vehicle design heavily influences the manufacturer’s determination of proper spring rates. For example, engines may be offset to the passenger side to make room for the accelerator and brake pedals. This affects the spring rates for each side of the vehicle. When dealing with individual coil springs or strut assemblies, mismatching the OE spring rate can compromise vehicle performance.

If you want to learn more, please visit our website for information on automotive spring manufacturers.