Coil Springs

All dimensions are in inches

Spring rates are in pounds per inch

Frequencys are in cycles per second or hz

Decimal points are dots... not commas,,,

The formula calculation pages all have typical values entered as an example, change any or all values to your needs and hit Calculate to see the results

Motion ratios are how much the spring moves compared to how much the wheel moves. This is normally a number less than one. For example enter .5 instead of 2 to 1.

If you get wierd stuff back, it is because the conditions are not right. For example, too high a spring rate for the amount of travel to bump, or the situation is impossible.

If you see negative numbers, that means the spring is coil binding and the results went to infinity.

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Eibach Springs, Inc.

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Corona, CA 92879

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Motion Ratio

What is d1 ?

What is d2 ?

The Motion Ratio is

Coil Spring Intro

Besides holding the vehicle off the ground and at its ride height, the spring is the device used to store the engery from that big bump you just hit. The engery that goes into the spring is later returned with practically no loss. The dampers do the work of converting that engery to heat, If there is too much or too little spring the dampers are not being used to their fullest. Therefore it is important to match the rate of the spring to the weight of the vehicle and to the load it will see and the intended driving style.

How the springs are mounted make a very big difference as to the rate needed. With the variety of vehicle types, suspension systems, type of use and terrain, there is no end to the possibilities.

The only place to start is with the Motion Ratio. This Ratio expresses the relationship between the way the spring is mounted compared to the wheel

It is best to cycle the suspension and measure the spring travel to each inch of wheel travel. This is alot of work. A "quick and dirty" way is to measure the a-arm points (or suspension link) and use those measurments directly.

Measure from where the spring mounts to the pivot point of that arm. Call this Distance 1 (d1)

Next, measure from the ball joint to the pivot to get the total length. Call this Distance 2 (d2)

Next divide d1 by d2. This number will normally be less than one. This is the "Motion Ratio".

We are almost there

You might have noticed in the diagram the spring is leaning. We need to include the amount of lean in our calculations. The more a spring leans, the weaker it appears to the wheel.

The angle we care about is the angle from straight up and down. Straight up and down is zero degrees. For example, if the spring is leaning 15 degrees from straight up and down, the angle is 15 degrees.

We will call this angle the "Spring Angle".

Natrual Frequency Intro

With all the possible combinations of springs, motion ratios and vehicle weights, natural frequency is the only way to compare cars.

A periodic motion is one that repeats itself in all its particulars after a certain interval of time. An example is the up and down motion of a weight hanging or resting on a spring. An oscillation is the period of time required for the spring to extend, compress past its starting point, then return to its original starting point. Another example, when driving around town and seeing a car with worn out shocks, the car will bob up and down when driving down the road, that is the natural frequency of that end of the car.

Once a desired natural frequency is known, it is possible to work out the springs needed to have that frequency. By checking the natural frequency of one vehicle, another vehicle can be set up the same, even if the weight and motion ratio is different.

Next, we need to think about sprung weight

The part of a vehicles weight that is supported by the springs. The chassis, body, engine, transmission are all sprung weight of the typical vehicle.

The easiest way to measure the sprung weight is to remove the springs and set the chassis on scales (which are located between the front wheels and between the rear wheels) and let the suspension rest on the ground. The scales need to be between the wheels so they will measure the proper front and rear weight distribution.

The sprung weight is the weight that is held up by the springs on that corner of the car. For example, if the front sprung weight is 1000 pounds, there are 500 pounds on each front wheel, 500 would be the number to enter in the form.

Food For Thought

There is a balancing act between springs and shocks. A spring has the most force when it is deflected the most. A shock works when there is movement. When you hit a bump and compress the suspension toward bump, there is a period of time when the suspension stops and then it starts to return back to its ride position. At that instance the shock is doing nothing. If you are running around with too much spring at that instance, the shock can not do anything about it. You need to be pretty close as to the initial spring rate

By calculating frequency and keeping track of its amount, changes in its amount and the direction of changes, you can start to “get a feel” for what you are trying to accomplish. Think of natural frequency as a way of predicting the behavior of a suspension. Lower number/frequency suggests slower response time and taking more time to react. Higher number/frequency likewise suggests faster response time. There is a balancing act regarding response time. If the response time is too fast it could become harsh and have less traction. It is important to know where you are

You can also guess the sprung weight and just try different values. If you know corner weights you can subtract the unsprung parts.

The part of a vehicles weight that is not supported by the springs. The tires, hubs, solid rear axle would be 100% unsprung weight. The upper and lower A-arms, axles that use constant velocity or u-joints would be 50% unsprung weight , because one end is hooked to the chassis and the other end moves with the wheel.

Shocks and coil springs would be a combination of sprung and unsprung weight, the amount depends on the motion ratio they are mounted at.

Spring Needed For

Single Frequency

Frequency ?

Sprung Weight ?

Motion Ratio ?

Spring Needs To be

Spring Angle ?

Springs Needed For

Dual Frequencys

Bump Frequency ?

Droop Frequency ?

Sprung Weight ?

Motion Ratio ?

Top Needs To Be

Botton Needs To Be

Spring Angle ?

Springs Needed For

Triple Frequencys

Bump Frequency ?

Droop Frequency ?

Sprung Weight ?

Motion Ratio ?

Top Needs To Be

Botton Needs To Be

Spring Angle ?

Middle Needs To Be

Ride Frequency ?

Natural Frequency of

Single Spring

Spring Rate ?

Sprung Weight ?

Motion Ratio ?

Wheel Frequency Is ?

Spring Angle ?

Natural Frequencys of

Dual Springs

Top Spring ?

Bottom Spring ?

Sprung Weight ?

Motion Ratio ?

Bump Frequency Is ?

Droop Frequency Is ?

Spring Angle ?

Natural Frequencys of

Triple Springs

Single Spring Wheel Rates

Spring Rate ?

Motion Ratio ?

The Wheel Rate is ?

Spring Angle ?

Dual Spring Wheel Rates

Top Spring ?

Bottom Spring ?

Motion Ratio ?

Bump Rate Is

Droop Rate Is

Spring Angle ?

Triple Spring Wheel Rates

Top Spring ?

Middle Spring ?

Bottom Spring ?

Bump Rate is

Ride Rate is

Droop Rate is

Motion Ratio ?

Spring Angle ?

Just For Fun

Misc Spring Calculations

Find Coil Rate

Spring O.D. ?

Diameter Of Wire ?

How Many Coils ?

Spring Rate is

Rate of Leaning Spring

Spring Angle ?

Spring Rate ?

Actual Rate Is

Estimate Single Spring

Needed

Sprung Weight ?

Motion Ratio ?

Wheel Travel ?

Distance To Bump ?

Spring Angle ?

Spring Preload ?

Spring Needs To Be

Estimate Dual Springs

Bump Wheel Rate ?

Sprung Weight ?

Motion Ratio ?

Wheel Travel ?

Distance To Bump ?

Top Needs To Be

Bottom Needs To Be

Spring Angle ?

Needed

Spring Preload ?

Estimate Triple Springs

Needed

Coil Spring Systems

Single Spring Systems

Dual Spring Systems

Triple Spring Systems

Help and Information