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  fork rebuilds Lnav
   4CS fork kit explaination  
  • This is copied from desktop / 4CS_kit_explaination.xlsx

    WP 4CS FORK  
The 4CS fork has the same basic components found in traditional twin chamber forks, with the exception of a recirculating check valve.  
Some of the components are shaped different and may be in different locations, but their basic functions are the same. For example, the 4CS has a smaller cartridge rod as well as smaller mv and bv piston diameters. These differences do not make for a bad fork, nor are they directly related to why the 4CS fork does not perform as well as expected. As with any suspension unit, understanding and controlling the different components is the key to proper setup.

Here is our approach. We dyno and pressure test the dampers. This allows us to break down the overall damping force into 4 basic parts.
1 - mv force
2 - bv force
3 - gas force
4 - drag force

After testing hundreds of different valving configurations (not just shim stacks, but any and all internal parts that affect damping), we begin to see patterns. In the case of the 4CS fork, one pattern we noticed MISSING was low speed damping control. We have tested KYB, Showa and WP bladder forks. All of these forks have decent low speed damping and provide a way to control it. The 4CS does not.

Here is a quick comparison of mv and bv forces for a typical plush Yamaha fork vs. stock '15 SX 250 4CS fork. We tested at 4 velocities: 3 - 10 - 70 - 100 ips (inch per sec) and recorded the peak force in lbs (pounds).  
For arguments sake, we'll call 3 and 10ips low speed. It's easy to see the 4CS has attempted to swap low speed control from the base valve to the midvalve. This sounds reasonable since the compression adjuster now controls the mid valve circuit.

Comparing base valve to base valve, the 4CS has little low speed compression force (compared to the KYB). Low speed compression force usually comes from the base valve, and we can make the assumption that this is desirable. We are basing this on patterns where forks with low speed control coming from the base valve generally have a good feel. Forks that have too much low speed control from the midvalve usually become harsher.

We're not going to jump on a soapbox and say the fork will never work without low speed from the base valve. We have simply performed our pressure analysis and found this noticeable difference. Since the goal is to 'understand and control the damping curve' (via its different components), we have something to shoot for. To make a long story short, we shot for more low speed control and it works great. We have developed 4CS parts that adds back the low speed (from bv) and makes the fork function in line with what we are all familiar with.

4CS is an acronym for 4 chamber system. This is nothing new. Most forks and shocks we work with have 4 chambers. To demonstrate, it's easiest to start with a shock.  

We might as well look at some pressures. If you fill the shock with 145psi, the pressure in each of the 4 chambers would be 145psi.  

When compressed, the 4 chamber system creates different pressures in each chamber. The fluids goal is to equalize the pressures as quickly as possible. The fluid flows through the pistons and past the shims, creating the damping force. Here is an example. You don't have to understand the concept, just notice the different pressures in the chambers.  

Twin chamber forks function the same. We give the chambers slightly different names to reflect they are from a fork.  

Fork pressures are lower, but the principle is the same. Compressing the fork creates different pressures in the chambers, and the fluid flows through the piston and shims, equalizing the pressure as quickly as possible and creating the damping force.  

We have all heard of cavitation. This occurs when the mv stack is too stiff, and the pressure in the rebound chamber drops too low, causing the oil to foam. Simply speaking, cavitation will occur when the pressure in the reb chamber drops below the pressure in the bv chamber. In this example, notice that P.r is less than P.bv.  

Now things get funky. The 4CS fork has the same 4 chambers, they are just arranged a little different. Using the same pressures from above, we can see the location of the chambers.  

The 4CS functions exactly the same as a twin chamber fork, except they have an added feature. The fork is designed to circulate. If the fork were to cavitate, there is a check valve located at the top of the tube that allows fluid to circulate from the P.bv into P.r. In this example, notice P.r is less than P.bv.  

Unfortunately, the fork does not function very well. Tuners have tried different combinations of shim stacks, bv pistons, mv pistons etc., all in an attempt to get the fork to work better. And everyone will have different opinions on what needs to be done.

Here is our fix. Based on rider feedback coupled with our dyno and pressure analysis, we focus on increasing the low speed compression damping as well as improved control over the mid/high speed damping circuits.

We have developed a 4CS kit that addresses all these issues. We attack the lack of low speed by adding more base valve compression force (via 'flow control' bv pistons and related parts) and making it clickable with an adjustable base bolt. We also add modified midvalve parts that give better control over the small diameter mv piston.

The parts are matched/balanced to create the desired pressures and related forces throughout the entire damping curve. Everyone that has tried revalving knows you often swap symptoms. One setting works good here, but not there. Another setting works good there, but not here. That is because all valving changes effect the entire damping curve. Once you are able to identify and control the damping curve, making changes to specific areas without adversely effecting other areas is possible.

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 fork rebuilds rnav
4CS fork kit installation
4CS kit install instructions
4CS kit tools
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4CS kit explaination