| Step 1: Motion Ratio | ||
| A-arm Suspension ?(See diagram 1) | ||
| MR | Motion Ratio |  |
| d1 | Distance from spring centerline to control arm inner pivot center (in) or (mm) | |
| d2 | Distance from outer ball joint to control are inner pivot center (in) or (mm) | |
| Beam Axle Suspension ?(See diagram 2) | ||
| MR | Motion Ratio |  |
| d3 | Distance between spring centerlines (in) or (mm) | |
| d4 | Distance between tire centerlines | |
| Angle Correction Factor | ||
| ACF | Angle Correction Factor |  |
| A | Spring angle from vertical (see diagram 1) | |
| C | Spring Rate (lbs/in) or (N/mm) | |
| Step 2: Wheel Rate (non beam) | ||
| Wheel Rate: | ||
| WR | Wheel Rate (lbs/in) or (N/mm) |  |
| C | Spring Rate (lbs/in) or (N/mm) | |
| MR | Motion Ratio | |
| ACF | Angle Correction Factor | |
| Step 3: Suspension Frequency | ||
| Suspension Frequency | ||
| SF | Suspension Frequency (cpm) |  |
| WR | Wheel Rate (lbs/in) or (N/mm) | |
| Sprung Weight | Vehicle corner weight less unsprung weight | |
| Tip 1: Calculation of Wheel Rate for a given frequency | ||
| WR | Wheel Rate (lbs/in) or (N/mm) (see step 2) |  |
| SF | Suspension Frequency (cpm) (see step 3) | |
| Sprung Weight | Vehicle corner weight less unsprung weight | |
| Tip 2: Calculation of Spring Rate needed for a given Wheel Rate | ||
| C | Spring Rate (lbs/in) or (N/mm) |  |
| WR | Wheel Rate (lbs/in) or (N/mm)(see step 2) | |
| MR | Motion Ratio | |
| ACF | Angle Correction Factor | |
| Diagram 1 (A-arm suspension) |
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| The motion ratio is a lever arm effect of the control arm acting on the spring. If the spring is mounted at an angle, the reduced motion of the spring must also be taken in account. |
| DIAGRAM 2 (beam axle suspension) |
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| The motion ratio of a live axle setup is shown here. Over two-wheel bumps, the motion ratio is 1:1. Over single wheel bumps and during body roll, the motion ratio as shown in step 1. The motion ratio is only used for calculating roll resistance, not for suspension frequencies. |
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Determining Rate on Eibach ERS (Coil-Over Springs) | ||
| In order to determine the actual spring rate in the working range of the spring, all Eibach ERS coils are tested between 20% and 70% of the spring´s total travel. This spring rate can be measured easily using the following procedure:
(For our example we will use a 1200.250.0500 (12", 2.5"ID, 500lb/in) ERS spring) | ||
| 1 | For the spring to be rated, please refer to the specifications listed in our new Application Guide or catalog and record the Sc (deflection/travel) measurement. This number represents the total available travel from free height to coil bind. In this example, our spring Sc measurement is 6.25". | |
| 2 | Next, determine the testing preload compression. Calculate starting compression travel by taking 20% of 6.25" (which equals 1.25") and the ending compression travel by taking 70% of 6.25" (which equals 4.375"). The actual travel between these two points (3.125") is where we determine the spring rate. | |
| 3 | Preload the spring 1.25" and record the force measurement. Continue to compress the spring an additional 3.125" (total compression of 4.375") and record the force measurement. Calculate and record the difference in the force between the two points (1.25"><4.375"). In our example the difference would be approximately 1565lbs. | |
| 4 | Calculate the actual spring rate by dividing the difference in force (1565lbs) by the actual travel between the two points (3.125"). The rate should fall between 490 and 510 lbs./in (1565 / 3.125 = 500lb). | |
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