DeRisi Racing

Suspension Design

Having the right spring(s) and motion ratio is a very critical part of any suspension system. The spring(s) resist the forces of input from the ground to the chassis, the suspensions motion ratio determines how the spring(s) will operate, and the shock controls the spring’s reaction to those inputs. Obtaining a desired leverage curve and spring combination is the starting point of building a suspension system in relationship to the shock. To understand how a shock works in relationship with how a suspension system works, you need to know a little bit about motion ratios, spring rates, and shock damping.

Springs

Most of the springs you will see are straight rate or linear compression springs. Linear means that there is a constant progression of force in relationship to compression movement. For example: a linear spring with a rate of 300 lbs. means that it takes 300 lbs of force to compress that spring one inch.

(1 inch = 300, 2 inches = 600, 3 inches = 900, etc…)

With a dual rate spring combination you have two springs stacked on top of each other and they are compressed simultaneously. Because both are moving at the same time, it takes less force to compress both springs one inch. For example: when you compress two linear 200 lb springs stacked on top of each other for one inch, both springs are going to yield a linear rate of 100 lbs. Both of the 200 lb springs will have compressed ½” or .50”. By multiplying the spring movement .50 by the spring rate 200 lbs., it will give you the working spring rate (200 X .50 = 100).

The main purpose for using a dual rate spring combination is to enhance the progression of the chassis’ motion ratio. A dual rate spring stack consists of two springs, a shorter tender spring on top and a longer main spring on the bottom. This progressive rate system is used to produce a lighter initial spring rate for a desirable lower ride height as well as providing a smooth, supple ride over small surface irregularities. Then, at a determined point in the shaft travel, via the tender crossover height, the tender spring stops working and the initial rate then crosses over to the stiffer rate of the main spring. This progression to the stiffer rate is used to prevent harsh bottoming during high speed input, such as jumps or whoops, and also to prevent excessive chassis roll.

The important thing to remember is that springs are resistance poundage. It takes a given amount of preload poundage to establish a desired ride height, a given amount of spring poundage to prevent chassis roll and a given amount of spring poundage to prevent extreme bottoming.

Tender Spring Crossover

Tender spring crossover height is directly related to chassis roll and bottom out forces. Changing the tender spring crossover height is the most significant handling change that you can make, using the shock’s external adjustments. The crossover height moves the dual spring rate’s point of progression in relationship to the shaft travel and motion ratio. Increasing the crossover height decreases the tender spring travel, making the main spring crossover sooner in the wheel travel, providing stiffer spring poundage for more spring resistance during chassis roll and bottoming. Decreasing crossover height increases tender spring travel, making the main spring crossover later in the wheel travel, resulting in less spring poundage for softer spring resistance during chassis roll and bottoming.

Static Preload

Static preload is the amount of spring poundage your shock has in fully extended condition. Basically it’s how much the spring or springs are compressed when installed on the shock. Example, you put a 300 lb spring on your rear shock and the spring has a free length of 10 inches before installation. After installation, you measure the spring again with the shock fully extended, and the compressed length is now 9.75 inches (10” – 9.75 = .25” of spring preload). Then multiply the spring preload by the spring rate and that will give you static preload (300 lbs X .25 = 75 lbs of static preload)

The main purpose of static preload is to raise or lower the vehicles ride height by means of adding or subtracting spring preload poundage. Never add preload to prevent excessive chassis roll and bottoming.

Damping

The function of shock damping is to control the spring’s reaction to input. This is done with the compression piston. It is attached to the end of the shock shaft inside the shock body. The piston has passages that allow the shock fluid to flow from one side of the piston to the other. On either sides of the piston there are tuning shims or Valving shims stacked to control the fluid flow. When the shock is compressed it is forcing the fluid through the passages and has to push open the compression shims. When the shock extends back the fluid is forced through the rebound shims. You can tune the shock’s compression and rebound with the Valving shims using different shim thicknesses and diameters. Too much compression damping will give you a harsh ride, and too little compression damping will allow the shock to blow through the Valving and bottom too easy. Too much rebound Valving will make the shock recover too slow and cause a packing problem. (Shock will not recover quickly enough for the next bump) Too little rebound Valving will cause the shock to recover too fast and kick up or dance around.

There are a few different Valving stacks that you will see in different shocks. You have a single stage tapered stack, two stage tapered stack, and a three stage tapered stack. With the two and three stage stacks you have more control over the oil flow. On a single stack the shims are considered high speed Valving, on two stage Valving you have a low speed shim stack and a high speed shim stack, and on a three stage stack you have a low speed stack, a mid speed stack, and a high speed stack. The low speed stack will control low shaft speeds. Example hitting the takeoff face of a jump makes the shock compress slow and progressively that is controlled with the low speed Valving. The mid speed stack will control little larger forces to the suspension making it a little more progressive. The High speed stack will control the harder hits like flat landing a jump or roots and rocks, basically anything that makes the shock shaft move at high speeds. With the different stages of Valving the damping is progressive just like the multiple spring setups. The single stage Valving is similar to a single spring, the two stage Valving is similar to a dual rate spring, and the three stage Valving is similar to a triple rate spring setup.

Below are diagrams of each stage of valving.

Motion Ratios

The motion ratio or “Leverage Ratio” is the path the shock goes through its travel in relationship to wheel travel. This is determined by the type of suspension hardware arrangement and geometry that the chassis manufacture decides to use. The most commonly used suspension hardware is either a linkage type or a no-link type. Linkage systems generally use less space to operate where no-links have to have more space to operate. I.e. (Airbox clearance)

Obtain Leverage Ratio:

1. Set your bike on a stand and level the bottom of the chassis with a level.

2. Using the front as an example, you will need to take off your shock a tire.

3. Measure the shocks free length eye to eye. Write it down.

4. Block up the spindle to where the distance between the center of the upper shock mount and the center of the lower shock mount equals that of your shock’s free length.

5. Measure from the center of the spindle to the ground. Write it down.

6. Block up the spindle to where the distance between the center of the upper shock mount and the center of the lower shock mount equals that of your shock’s compressed length. (Compressed length = free length minus shaft travel.) DO NOT subtract for the bottom out bumper. Measure the full length of the shaft.

7. Measure from the center of the spindle to the ground. Write it down. The difference between the two shock mount measurements and the difference between the two spindle measurements is your “leverage ratio”. For example if your shock has 4.75 inches of shaft travel and you measured 9 inches of spindle movement, your leverage ratio would equal 1.89.

To Find Preload Poundage:

1. Put the shock in the vise and remove the lower spring retainer.

2. Measure the “relaxed combined” spring assembly. (The springs and dividers only. Do not measure the spring retainer or preload ring.)

3. Put the lower spring retainer back on and measure again. The difference is your spring preload.

4. Multiply your preload by your combined spring rate and that is your beginning poundage. Write it down. (Example: .25 in X 80 lb. Spring rate = 20 lbs. of preload)

Spring Rate Formula

To rate an unknown spring:

11,500,000 x (wire diameter) to the 4th power

8 x (ID and wire diameter) cubed X active coils

Example: wire diameter = .362; ID = 2.575; active coils = 9.2

Your rate is 105.9

Stacked Spring Rate Formula

To figure the combined rate using multiple springs:

The formula for two springs is: 1/K + 1/K2 = 1/K3

The formula for three springs is: 1/K + 1/K2 + 1/K3 = 1/K4

K=Spring Rate

Example: You have a 80 lb tender and a 370 lb Main spring combination

1 divided by 80 + 1 divided by 370 = 1 divided by K3

Kt = 65.8 lbs

We hope that this information will be helpful to everyone. If you have any questions on any of this material please feel free to contact me at anytime.

Thanks,

Santo DeRisi