Home of K-fab

Technical Information

Chassis Tuning Wonder what things like camber, caster, toe in/out are? Read and learn.

Head Gear How to get a properly fitting helmet.

Jetting How to Jet your carb.

Shocks Theory on how they work, adjusting and setting them up.

Springs How they are rated, multi rate setups, formulae for figuring out rates.

Tire Information Charts on tire sizes and other useless info.

After running a Pilot as a Stadium Lite for the past two years, taking second overall in the Pace Series both times (winning once each year) and winning the CORR World Championship Round at Crandon, WI a couple summers ago, I've decided to share some of my Pilot's setup secrets.

First, the heart and soul of my Pilot is ATV's Long Travel Suspension kits and the 500cc conversion. BUT, you still have to tune these to work up to maximum levels. The biggest gain to be made in getting a Pilot to work well for racing is in the suspension modifications.

Let's bounce to our next subject! (har, har, har)

Springs are the energy input/output section of our shocks. These do nothing more than hold the car up off the ground and allow movement in the suspension. Boy that sounds simple, doesn't it? OK, so it's a little more involved than that. Let's take a look.

A spring is a mechanical device that absorbs energy through deflection and then expels that energy by trying to get back to it's nondeflected state. As the suspension is compressed, the spring is compressed. The spring takes the energy of the compressing suspension, stores it (although it may only store it for a few milliseconds) and then releases this stored energy in the action of expanding the suspension back. With the majority of the springs we use on Lites, this storing and releasing happens at a linear rate because they are wound in a constant spiral with each consecutive coil being spaced apart the same as the previous coil. If the distance between the coils gets smaller (or larger depending on which way you go) then the spring is considered to be a progressive wind - and that's a totally different animal that I'm not going to get into.

What does "Linear Rate" mean? It's pretty simple. Let's say you have a 15 inch long 100 pound rated spring (and let's assume that it will coil bind - where the coils touch due to compression of the spring - at a compressed length of 5 inches). This means that it takes 100 pounds of force to compress the spring one inch. So, it will take another 100 pounds (for a total of 200 pounds) to compress the spring 2 inches - so on and so forth. If you graph it out, it's a nice straight line. You can see that our spring can take 1000 pounds (or 10 inches of compression distance) to collapse it to the point of coil bind.

Why use dual springs? Dual rate setups allow for a softer initial overall spring rate that switches over to the stronger spring rate at a predetermined (cross over height) point. With dual rate springs you have a tender spring - usually a softer spring and a main spring. Because both springs are moving at the same time during compression it takes less force to move the shock one inch. An example would be using two 100 pound springs stacked on each other. If it takes 100 pounds to move our single spring one inch, then it only takes 50 pounds to move it 1/2 inch. Being that you have two 100 pound springs sitting on top of each other and they both are the same rate then they each only move 1/2 inch - both adding up to 1 inch. This movement only adds up to 50 pounds of force input to make the whole system move 1 inch.

How about I do a bit of plagurism? This information comes straight from Custom Axis. It covers the majority of the formulae and concepts:(the infomation is directed towards Quads, as that's where most of the shocks Axis makes goes, but you can apply it to any vehicle)

1.0 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 suspension's motion ratio determines how the spring(s) will operate, and the shock absorber 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 how a suspension system works, you need to know a little bit about motion ration, spring rates and shock absorber dampening.

1.1 Motion Ratios
The motion ration or ‘leverage ratio' is the path the shock absorber goes through its travel in relation ship to wheel travel. This is determined by the type of suspension hardware arrangement and geometry that the chassis manufacturer decides to use. The most commonly used suspension hardware is either a linkage type or a direct shock type, more commonly referred to as a ‘no-link'. The main difference between linkage and no-link type systems is packaging. Linkage systems in general utilize less space to operate, while no-links by nature of design usually require more space. Both types, however, have their assets and drawbacks. It is not the purpose of this manual to argue which is better. There are too many variables to consider. However, it is important to note that all linkage systems are not the same, all direct shock systems are not the same, and all shock absorbers are not the same.

1.2 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 200lbs. means it takes 200lbs. of force to compress that spring one inch. (One inch + 200lbs., 2 inches = 400lbs., 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 or poundage to compress both springs one inch. Fore example: When you compress two linear 200lb. springs stacked on top of each other for one inch, both springs are going to yield a linear rate of 100lbs. Both of the 200lb. springs will have compressed ½" 05 .050". By multiplying the spring movement .50" by the spring rate 200lbs., it will give you the working spring rate (200 x .5 = 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 short tender spring on top and a long main spring on 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 cross over height, the tender spring stops working and the initial rage 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 in corners.

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 final poundage to prevent extreme bottoming. Having the right spring combination and the correct cross over height is crucial to suspension performance.

1.3 Dampening
The function of the shock absorber dampening is to control the springs's reaction to input. This is done using a special piston called a dampening piston. It is attached to the end of the shock shaft inside the shock body. The dampening piston has special through passages or ‘ports' that allow fluid to pass from one side of the piston to the other. On either side of the piston there is a series of tuning washers or ‘valve shims' which seals off fluid flow in one direction and restricts or ‘dampens' fluid flow in the other direction. When the shocks compressed or retracted, the dampening piston moves through the shock fluid, forcing the fluid through these passages. Dampening is thus regulated by the assembly of the valve shims on either side of the dampening piston. Compression dampening regulates how fast the spring will compress, and rebound dampening regulates how fast the spring returns after being compressed.

The compression dampening should be taut, firm, but not harsh. Too much compression dampening and the ride will be stiff and choppy. Too much compression dampening could also cause the shock to become solid or ‘hydraulic'. This causes a number of undesirable effects, tow of which is blown seals and bent shafts. Too little compression dampening and the ride will be spongy and vague. Not having enough compression dampening will also cause you to blow through the travel too fast.

The rebound dampening should be on the slow side, but not too slow or the shock will ‘pack up'. Pack up means that after the shock has been compressed, the speed at which it returns is too slow to reach proper extension before the next compression stroke. With a gradual loss of shaft travel at each compression stroke, the shock could eventually run out of shock travel and bottom out. Not enough rebound dampening, the ride becomes springy with a buoyant feeling. In either case, not having the correct rebound dampening prevents the tires from not staying planted on the ground, causing them to skip, wander and bounce, which results in loss of traction and control.

2.0 External Shock Adjustments
A fully adjustable racing shock has four means of external adjustment: Preload, Tender Spring Crossover Height, Compression and Rebound Dampening.

2.1 Static Preload
Static preload is the amount of spring poundage your shock has in an unladen, fully extend condition. Basically, it's how much the spring or springs are compressed when installed on the shock. Example, you put a 300lb. spring on your rear shock and the spring has a free length of 10.00 inches before installation. After installation, you measure the spring again with the shock fully extended, and the compressed length is now 9 3/4 inches, or .25" of preload. Then multiply the spring preload by the spring rate and that will give you static preload (300lbs. x .25 = 75 lbs. of static preload).

The main purpose of preload is to raise or lower the vehicle's ride height by means of adding or subtracting spring preload poundage. NEVER add preload to prevent excessive chassis roll and bottoming. By raising or lowering the ride height, you are also moving the vehicle's center of gravity, or ‘CG', up and down as well as changing the vehicle's weight bias, either towards the front or the back. Optimum ride height is a balance between a center of gravity low enough to maintain good cornering stability and a chassis clearance high enough to prevent the frame from hitting the ground. It's ok to graze or scrape the bottom of your frame now and then, but you don't want it slamming the ground, knocking your hands and feet off.

2.2 Tender Spring Crossover Height
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 you can make, using the shocks' external adjustments. The crossover height moves the dual spring's point of progression in relationship to the shaft travel and motion ratio. Increasing the crossover height decreases 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 the 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.

Therefore, increasing the tender spring crossover height makes the suspension stiffer. Decreasing the tender crossover height makes the suspension softer. Your suspension should bottom out at least once somewhere on the track not so hard to where you feel the footpegs through your boots but enough to know that your using all the available travel.

SPRING FORMULAS

Spring rates are determined by how many pounds of force it takes to compress a spring one full inch.
To rate an unknown spring:
11,500,00 x wire diameter to the 4th power
---------------------------------------------------
8x(spring ID + wire diameter) cubed x active coils

example:
wire diam .362; Spring ID 2.575; active coils 9.2

11,500,00 x (.362)^4
-----------------------------
8 x (2.575+.362)^3 x 9.2

Your rate is 105.9

To figure active coils:
hold spring upright and start from the bottom.
When the flat end coil comes in contact with the first coil, that's zero. Up from there count the number of turns until it touches the other flat end coil. In most cases it won't end up on an even number. Divide the full turn of the spring into 10 units, take this and use it for the fractional determination of the spring coil where the flat coil touches again. You will end up with an active coil number such as 9.6 or 7.8 or 8.5 ....

To convert to metric Kg:
Divide pounds by 55.88
Divide inches by .03937

To figure out the combined rate using multiple springs:
The formula for two springs is 1/K+1/K2 =1/K3
For three springs it's 1/K+1/K2+1/K3=1/K4
(K=spring rate)

Example: You have an #80 Tender and a #370 spring combination.
1/80+1/370=1/K3 K3=#65.8

If you need to cut a spring to obtain a desired rate use this formula:
K1+AC=K2+AC (K=Spring rate, AC=Active Coils)
Example: a #60 spring with 8.5 active coils = #78.5 x 6.5 active coils