How to Choose The Proper Spring Rate for Your Car

Some people want their car to handle like a formula racer, while others want a quiet and non-intrusive ride.  Choosing the correct coilover spring rate is crucial in providing the intended handling and ride characteristics.  Fortunately, this just got easy.   The bottom of this page provides a link to an excel spreadsheet which will calculate the proper spring rates (front and rear) for you.  However, you must first determine the front and rear weights of your car, unsprung weight, wheelbase, etc.   Methods used to obtain these inputs are discussed below.

Axle Weights

The first of these is front and rear axle weights.  The easiest way to obtain this information is at a truck scale.  I use a local scale that weighs grain trucks.   First, I drive my car onto the scale so that only the front wheels are on the scale.  This is the front axle weight of the car.  I then drive across the scale until only the back tires are on the scale.  This is the rear axle weight of my car.   Simple, no?  Depending on the drop off at each end of the scale, you may have a slight error in the weight readings.  Preferably, the approach and departure are level with the scale mechanism.

Sprung & Unsprung Weight

"Sprung" weight is a term used to describe the parts of an automobile that are supported by the front and rear springs. They suspend the vehicle's frame, body, engine, and the power train above the wheels. The "unsprung" weight includes wheels and tires, brake assemblies, the rear axle components, and other structural members not supported by the springs.  Suspension parts such as control arms, anti-roll bars, shocks, and struts are a percentage sprung weight and a percentage unsprung weight. The actual percentage depends on the application.  More unsprung weight makes it difficult to keep the tire in contact with the ground and to maintain ride comfort.   A heavy wheel/tire combo can skip and bounce over uneven pavment actually making handling worse instead of better.  Mathematically, the relation between the two for a complete axle is:

Sprung Weight = Axle Weight - Unsprung Weight

There are a number of methods use to approximate sprung & unsprung weight.   The easiest is to assume that unsprung weight accounts for 15% of the total vehicle weight.  My 240-Z weights about 2650 lb.  Fifteen percent of this is 400 lb., and divided by four is 100 lb. of unsprung weight for each tire.  Easy, but probably the least accurate.

Probably the most common method is to remove and weigh the suspension/brakes/tires.  One rear suspension/tire assembly of my Z weighs around 110 lb.  However, the unsprung weight is actually less than this.  The reasoning is that some of the weight is carried back to the springs by connections to the body.  For example, struts and control arms transfer weight back to the body where it is semi-sprung.  If we assume 20 lbs. is carried back to the body, the unsprung weight is 90 lb.

Another method is to remove the spring to body connection point, letting the suspension dangle by its control arms onto a scale.  For a Z car, place the car on jack stands, so that the wheels are about six inches off the ground.  Next, place a bathroom scale on a floor jack directly under the tire and remove the slack.   Remove the three nuts holding the strut to the body.  Gradually lower the jack until the strut bolts have pulled free from the body, and the suspension and tire is supported by the scale.  The scale reading will normally be a little light due to the preload in the control arm bushings.  For this reason, feel free to add 20 to 30 lbs. to this reading.  Using this method, I found the rear strut & tire of my Z weighed 70 lb.   I compensated for the bushing preload by adding 30 lb. for a total of 100lb.

If you know the rate (lb./in) of your springs and their free length, you can determine "sprung" weight as follows.  With car's full weight on the tires, measure the distance between the ends of the compressed springs.  This is the length of your spring under sprung weight.  Plug these into the following equation and you'll have sprung weight:

Sprung Weight = (Free Length - Compressed Length) * Spring Rate

Wheelbase

Now we need to know the vehicle wheelbase.  You can measure this directly with a tape measure, or consult a service manual.  My 240-Z has a wheelbase of 90.6 inches.

Front Spring Natural Frequency (Hz)

What kind of ride do you want?  Ride quality and handling characteristics depend on the natural frequency.  I know that sounds like a scary term, but it's really not.  Simply stated, the natural frequency is how fast the car bounces up and down when hitting a bump in the road (assuming no shocks/damping).  If you want a Cadillac ride, a natural frequency of 1 cycle/sec is your match.  If you want a Miata ride, then you need around 1.5 cycles/sec.  An all out road racer will use a 2 cycle/sec frequency.  FYI, a stock 240-Z has a frequency of approximately 1.25 Hz. 

Of course, there are other factors to be considered other than ride quality.  If you are lowering your car, you must increase the frequency to prevent bottoming.  Increasing the frequency means stiffer springs.  Stiffer springs are necessary to absorb the same bump energy, but with less suspension travel.  A stock Z has about three inches of bump travel.  If you lower it an inch; well, I think you see my point.

Average Driving Speed

Average driving speed is the last of the variables needed.  This along with the wheelbase measure, is used in calculating the rear spring frequency.  Bear with me and I'll try to explain.  The frequency selected in the previous paragraph is for the front springs.   The rear springs will get a faster frequency.  The reason is to prevent pitching.  Pitching is the "bucking" feel when driving at highway speeds.   Anyone who has ever pulled a loaded trailer down a concrete highway will know what I'm talking about.  Here's what happens.  The trailer tongue load increase the weight at the back of the tow vehicle.  This effectively slows the natural frequency.   As the truck and trailer cruise down the highway at 65 mph, it encounters expansion joints in the road.  First the front tires hit, then the back tires.  If the front tires bounce and rebound before the rear tires, you get a bucking action.  It can actually get pretty violent if your truck has soft rear springs.  You can prevent this by selecting a rear spring frequency that rebounds at the same time the front springs do at your average driving speed.  In other words, they hit at the same time.

Enter Data Into the Program

Okay, that's it.  Now all we have to do is input this data into the spreadsheet, and voila - your spring rates have been selected!

Take this link to the excel spreadsheet:   Spring Rate Selector.  By the way, the default values in the spreadsheet are what I used in calculating the coilover rates for my SSZ.   I assumed 100 lb. of unsprung weight at each wheel, with a design frequency of 1.6 Hz.

Questions or Comments?  E-mail me at:  [email protected] .

Copyright �1996, Brian's Garage. Property of Brian Holdeman, P.E.
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This page was last edited on 03/09/02.

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