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FORK SPRINGS SHOCK SPRINGS AIR SPRINGS AND MORE

Updated: May 7


SPRINGS ARE THE FIRST AND MOST IMPORTANT SUSPENSION COMPONENT




Suspension101 Shock Spring

Suspension101 Fork spring set

A motorcycle will not perform with improper spring rates and preload.

There are two parts to spring rate, the wire coil and the air in your fork that is compressed the closer you get to bottoming. This is called the air spring.

So lets learn about fork springs, shock springs, air springs and more!


AIR SPRING

The air spring is controlled by adjusting oil level, in turn adjusting how large or small the air chamber is. The oil level changes the air spring rate which has the most effect at the bottom 1/3 of the stroke.




STRAIGHT RATE SPRINGS

When put on the spring rating machine, these springs have a very consistent or "linear" increase in rate as they are compressed.


For Example:

Compress the spring 10mm to 20mm and you get .50kg on the scale.

Compress the spring from 20mm to 30mm and you will still get .50kg on the scale.

This spring increases .50kg for every 10mm it is compressed, so with 20mm of compression you now have a total of 1.00kg of spring force.




WHEN DO WE USE STRAIGHT RATE SPRINGS?


Straight rate shock springs are used on rear linkage suspension, this is a good fit because the linkage firms up the stroke as the rear end compresses (unlike a PDS bike).

Straight rate fork springs are used on all bikes, in particular high quality or performance builds. The stroke is controlled with a combination of valving and springs.





VARIABLE RATE SPRINGS

These springs increase in rate as they compress.

For Example:

Compress the spring from 10mm to 20mm and the spring rate is .50kg.

Compress the spring 20mm to 30mm and you find the spring rate increases to .52kg.

So the first 10mm of compression was .50kg and the second .52kg, so with 20mm of compression you now have a total of 1.02kg of spring force.

As variable springs go deeper, the rate will continue to increase.




WHEN DO WE USE VARIABLE RATE SPRINGS?

Variable rate shock Springs are most times used on PDS, non linkage bikes, due to their lack of rising rate. The PDS design does not ramp up firmness as it goes deeper into the stroke like a linkage bike does. So, to slow the rear down as you go deep into the stroke a variable rate spring is a fitting addition.

Variable rate fork Springs might be used for a budget, "DIY" build that you do not intend to improve the damping characteristics on to control of the forks.

Even some re-valves with insufficient valving may benefit from variable fork springs.

Variable rate fork springs are a bandaid but can be an improvement over stock performance.


Suspension101 does not use variable rate fork springs, we control fork performance properly with a balance of correct straight rate springs and customer adjustable valving, through use of the Gobblers, for a more linear fork action.



ADVENTURE BIKES benefit from variable rate shock springs even if it is a linkage bike. While the linkage does firm up as it goes deeper into the stroke, a 700lb bike with rider and gear makes the increase in ratio insufficient.

The Adventure motorcycles have the same shock sizes as the mini MX bikes! To bring these shocks to a dependable high level additional tools, like variable rate shock springs, have to be introduced.


So, what are the advantages of a variable rate shock spring on nearly all ADV bikes?


Reducing heat

A variable spring slows the shock down as it compresses reducing heat, making fluid last longer and your shock body last longer.


Safety

If you hit something unexpectedly the spring will assist the valving in keeping the rear from going too deep. Why is it bad when the rear goes too deep?

When the rear goes too deep into the stroke the spring energy will overpower the rebound valving and the rear will kick back rapidly. The Variable rate spring is a step in the right direction in alleviating this dangerous situation.

Surak the dog at Suspension101

Preload adjuster range

When you adjust for weight changes (saddle bags, tents, your fat passenger, or your dog Surak!) your preload adjuster will have more lift range due to the increase in rate as the spring is compressed. (See our article on sag to learn why you should change your preload in the first place).




Reduced cavitation

Some ADV bikes have shock seal failures because of cavitation. The variable spring will slow the shock down resulting in reduced cavitation. There are other helpful modifications to make these shocks more reliable and long lasting, and the variable spring is one part of that solution.





SPRING PRELOAD

It is often good to make an extreme example to show a point.

Above we mention a straight rate spring that increases .50kg with each 10mm of preload.

If you have a total of 10mm of preload on the springs installed in your bike then the spring will give you just .50kg of resistance when you hit a bump.

In other words the spring will not allow the fork to move until you apply .50kg of force to the forks.


Now let’s preload the spring 250mm upon installation. Now the spring will give you a whopping 12.5kg of resistance! Keeping the fork from moving until you hit something really hard at high speed.

So if you hit a small rock at a moderate pace the fork may not move at all with 250mm of preload. Ouch!

A stiffer spring and light preload would be a much more comfortable ride everywhere, and be capable of high performance at race pace.





Racetech Variable rate shock spring

DUAL RATE SPRINGS

Some springs are called dual rate springs. A couple of the coils are very close to each other as you can see in the image on the left. This means that very early in the compression stroke the spring changes like a switch from light rate to full rate.

Eibach "variable rate" springs are much closer to dual rate springs.

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