FREQUENTLY ASKED QUESTIONS ABOUT SPRINGS
What is Spring Rate??
Spring rate is a measure of how stiff a spring is. It describes how hard you have to push to make it move a given distance. Spring rate units are Pounds / inch in the English system or Kg / mm in the metric system. So, if you rear shock spring is 280 pounds / inch (or 5.0 Kg / mm), you must compress it with a force of 280 pounds for it to become 1 inch shorter. You would have to push it another 280 pounds to make it compress another inch. Thus if you compress it a total of 560 pounds, it will compress 2 inches.
Do Spring Rates Ever Change??
No, Never. Over time the spring may take a slight compression set (become shorter), but the rate will never change. In fact when the spring makers wind a .44 Kg / inch spring for a 01 Suzuki or Kawasaki it is 470 mm long. They can heat it and completely compress it for a set amount of time and it will "take a set" (shorten) by 10mm and become a Yamaha .44 Kg / mm by 460 mm spring.
What Do I Do If My Bike Sags More Than It Used Too??
Add Preload. A highly stressed spring like our race bike springs will shorten slightly over time. On your rear spring you simply turn the spring preload adjusting nuts to add preload. On your front springs you would have to measure them and preload them with washers to make up for any slight compression set they have experienced. (At Too Tech we always measure the front spring length before installing them back in the Forks.)
Do Suspension Springs Wear Out or Why Do Springs Break??
In our Motorcycle world springs rarely break. A well designed steel spring will basically last forever. How long a spring lasts is based on how much stress is produced in the wire when it is compressed to it's minimum length AND how many cycles (times) it will experience this load. Engineers have charts to describe how many millions of cycles a given spring material can take at a different levels of stress before they fail. (See chart below) As the stress in the wire increases, the number of cycles until failure decreases. The larger the spring wire diameter is, the less stress the wire will experience for a given spring rate. Motorcycle suspension springs use relatively large diameter wire for the loads they see in service thus they seldom experience enough stress in the wire to cause breakage. Some aftermarket companies try to use cheap wire which can't handle high stress levels without severely shortening their life expectancy. But most of these companies have seen the error in their ways and now I am not aware of any companies with chronic spring breakage problems.
The chart below is used to predict how many cycles the spring can experience before it fails. First we must determine which chart line to use. This is determined by analyzing how the spring will be used. The second picture down on the left side shows a spring cycling from no load to it's maximum load (or maximum negative stress). Our springs have almost no load at full extension so the second picture best describes the stress pattern our springs will see (from 0 to max and back to 0). This stress pattern is considered normal, so we use the standard graph marked "R=0".
The chart on the right is for some mild steel. If it were a graph of spring wire it would look similar but the allowable stress loads would be higher. The X axis (horizontal axis) on the bottom denotes the number of cycles the spring will experience before it breaks while the y axis (vertical axis) describes how much stress the wire receives during each cycle. If you follow any one of the lines you will notice that as the number of cycles increases, the allowable stress the spring wire can handle goes down. Follow the "0" line. Note that if we want to design for only 1000 cycles, our material could handle about 105,000 PSI stress in the material before it failed. If we want to design for 100,000 cycles, we can only stress the wire to 80,000 PSI. Notice that at about 500,000 cycles the line levels out at a stress level of about 75,000 PSI. This means that if we subject this material to a maximum stress of 75,000 PSI, it can be cycled indefinitely and never fail.
If a steel spring is designed properly (utilizes the proper wire size) for a given application, the spring will never experience stress levels higher than the horizontal part of the line. Thus a properly designed spring will last forever. This is a unique property of steel only. For instance aluminum, titanium, and other exotic materials continue to "age" regardless of their stress levels (there is no horizontal part of the line). Because of this constant aging, engineers must use large cross sections to keep the stress levels in the material very low. As long as the stress levels are very low, these materials can last a very long time. (Think about this as your aluminum bars get to be a few years old!!!)
Do I Change The Spring Rate When I Turn My Rear Spring Adjuster Nuts To Change My Preload??
No, only the preload is changed. The spring rate remains the same. Increasing the spring preload will raise the ride height, so the bike will be taller and ride higher in the stroke. But it will not get stiffer any faster as you use up your travel. For the shock to get stiffer as you use travel, you must change to a higher spring rate.
Why Does A Heavier Rider Need A Higher Spring Rate??
The bikes suspension is a combination of spring rates and damping rates. It can be optimized for only one weight of rider and bike. As the rider weight changes the spring will have to be changed to maintain the optimized combination. The relationship between the total weight of the bike and the rate of the spring determines the Natural Frequency of the system. To maintain the same Natural Frequency ratio, if the rider gains weight, the spring rate would need to be increased.
Natural frequency can be calculated or derived by measuring the static deflection of the bike when it is taken off the stand. Measure the amount the front triple clamps move downward from full extension when the bike is removed from the stand and the rider stands on the foot pegs. This sag, plus about 3/8" to account for fork spring preload, determines the SD or static deflection in the formula below. Now put a heavier rider on the bike and measure the new static deflection and put it into the formula. You will note that as the Static Deflection gets bigger, the Natural Frequency gets lower.
To maintain optimal suspension handling, you want the NF to remain the same so larger riders need stiffer spring rates.
NF = Natural Frequency in minutes (divide NF by 60 to get NF in cycles per second.
SD = Static Deflection of the machine (if the spring starts with no preload)