How to service dirt bike forks

The different fork designs.

Many different fork designs exist across the various models of dirt bikes, new and old alike. The challenge to achieve desired suspension performance in many demanding conditions drives the evolution forward. As suspension improves, new things are learned which in turn yields new designs, yet not every new design is an improvement. This being said, suspension concepts tend to remain basically the same. Given the many different types of forks available, it is important to understand what you have before you attempt service work.

Fork designs, within the scope of performance dirt bikes, are typically the following depending on vintage and bike model.

  • Conventional forks: Conventional forks have the outer fork tubes on the bottom. The inner fork tubes are clamped in the triple clamps. These types of forks tend to be on older machines or other non motocross dirt bikes. The term conventional here is generic in the sense of the basic fork structure. Fork internals can vary across bike models with conventional forks.
  • Inverted forks: Modern motocross bikes all typically have inverted forks. As the description implies, inverted means flipped relative to conventional forks. There are many different types of inverted forks. Inverted forks became prevalent during the early 90's.
  • Open chamber inverted forks: Open chamber means the damping oil shares the spring space. The same oil that is adjusted in height for spring rate tuning is also the oil that dampens the fork action. By nature then, it is possible for air bubbles to accumulate in the oil which can make suspension damping inconsistent.
  • Twin chamber sealed cartridge forks: This design of fork separates the damping oil from the spring chamber by incorporating an internal sealed damping cartridge. This is great because damping oil can now remain void of any air, making for much more consistent damping. Complexity of sealed cartridge forks is a bit more than open chamber forks.
  • Single function forks (SFF): Single function forks means that each fork leg has a single function, so to speak. One fork leg serves as the spring and the other fork leg serves as a damping unit.
  • Pneumatic spring forks (PSF): Pneumatic spring forks are forks that have no mechaincal springs. Compressed air is used as the spring which reduces weight and makes possible track side spring rate adjustment with nothing more than a shock pump. One possible down side is the rate may need adjustment frequently as the air pressure can change drastically depending on temperature. Arguably, the lack of a mechanical spring means that linearity adjustment is taken away given that air pressure change is progressive (which I discuss in another article).
  • Other designs: Other design types will likely come as we are already seeing variations with PSF forks with more air chambers such as negative chambers. There are also forks that have single function fork legs related only to damping. For example, some KTM's have spring forks where one side serves as compression damping and the other side rebound damping.

Below are some excellent videos about how to service various types of forks.

Basic service of sealed cartridge mechanically sprung inverted forks.

In this video, Scott Gustafson with Dirt Rider Magazine shows how to replace leaky fork seals. The scope of the video extends to replacing fork bushings, how to change fork oil and fork springs replacement.


Replacing seals on inverted sealed cartridge mechanically sprung forks. 19:11 minutes.

Some additional tips by DIRT BIKE RESOURCES:

You can pop the fork tubes apart without the use of heat if you must. Your best option is to follow the advice in the video, however. The heat expands the outer tube making it easier to separate the tubes with minimal risk of damaging the slider bushings. I've done several of these without heat and had no issues.

If you loosen (but not remove) the fork caps before removing the forks from the bike, you'll struggle much less. To do this, loosen only the top triple clamp bolts, then loosen the top cap and inner plug. Loosen them enough that the o-ring seal is not exposed. This is so you will not spill oil out until you are ready to drain the tubes.

Bushing inspection here focuses more on the non coated side of the bushings. However, the coated side is the side that slides and can wear out as well. Every bushing I have replaced was due to wear on the coating side. At some point, the coating can break free from the bushing. Once it begines flaking off, wear is accelerated.

How to service open chamber inverted forks.

This video by Dirt Rider Magazine shows how to replace fork seals, how to replace fork bushings, how to change fork oil and how to replace the fork springs. Open chamber inverted forks, demonstrated here by Scott Gustafson, do not have sealed cartridges and are typically easier to service than sealed cartridge types. Open chamber inverted forks are found on many 90's models motocross bikes.


Servicing open chamber mechanically sprung inverted forks. 19:40 minutes.

Some additional tips by DIRT BIKE RESOURCES:

Mr. Gustafson explains bushing inspection, but realize the coated part of a bushing often wears out. This coating is an antifriction coating likely containing Teflon or similar material. He indicates the non coated mounting side as the wear surface. This is not to say the mounting side can't wear, as it certainly can after a long period of time. However, the coated side can also wear out and often does before there is a fitment problem on the press fit side. It depends on various factors.

Before sliding the spring in, you can tie a long piece of mechanics wire onto the rod end. This way, you can easily pull the rod up through the spring.

You can pop the fork tubes apart without the use of heat if you must. Your best option is to follow the advice in the video, however. The heat expands the outer tube making it easier to separate the tubes with minimal risk of damaging the slider bushings.

Be careful not to over tighten the fork top caps. This can make removal a problem next time.

How to service conventional (non inverted) forks.

This video shows how to service conventional forks from a CBR600. It is an excellent example of conventional forks found on many older full size dirt bikes. Every bike model has potential differences, so you should always refer to the appropriate service manual. The service procedures demonstrated here by James Wright-Roberts show you how to replace fork seals, how to replace fork bushings, how to change fork oil and how to replace the fork springs.


How to service conventional forks. 34:56 minutes.

How to service Single Function Forks (SFF forks), damper side.

This video by Rocky Mountain ATV shows how to replace fork seals, fork bushings and fork oil on the damping side where there is no spring. The SFF forks serviced here are the Showa SFF used on some Suzuki models.


How to service the SFF damper side. 12:11 minutes.

How to service Single Function Forks (SFF forks), spring side.

This video by Rocky Mountain ATV shows how to replace fork seals, fork bushings, spring and fork oil on the spring side. There is no damping function in this spring side fork tube. The SFF forks serviced here are the Showa SFF used on some Suzuki models.


How to service the SFF spring side. 7:57 minutes.

PARTS AND SUPPLIES

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