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Old 01-29-2013, 05:13 PM   #135
bgoodsoil
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Joined: Jul 2008
Location: Washington, DC, USA
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I found an interesting article on the new twinchamber/closed chamber versus the old open chamber KTM Forks.



Quote:
What Motorcycle Forks Work Best For You? link here
What motorcycle forks do YOU need? Should you buy an enduro bike with open cartridge forks or a MX bike with closed cartridge forks and convert it? How big or little is the difference between the two systems? And what is a cartridge anyway?

The suspension specialist in our family is my husband who worked several years in the R&D Department of WP at the KTM Factory in Austria. Even though I never personally considered putting different forks in my bike I know that a lot of guys wonder about upgrading their suspension.

The bottom line is that in extreme conditions and for pro riders the closed cartridge fork is the fork of choice. But then pro riders have a specialist technician to look after their forks. Hobby riders are better off with open cartridge as it is easier, cheaper and more reliable in the long run. Lets see why...

The cartridge is the part that contains spindles, needles, pistons and shims to make your fork act like a serious damping system and not just bounce around like a single spring would do. Just as it used to be on vintage bikes. When the fork compresses in, the oil is forced out the cartridge. When it rebounds, oil enters the cartridge again.

The closed cartridge system
In the closed cartridge fork the oil is displaced into a pressurised area at the end of the cartridge. The entire cartridge containing the damping system is therefore sealed off. The oil which lubricates the bushes, contains the fork spring and seals, is not mixing with the cartridge oil.
There are 2 oil systems:

•one in the cartridge
•one around the outside of the cartridge

That’s why it is called closed cartridge or twin chamber system.

Advantage of closed cartridge forks
This system offers more consistent damping due to the pressurised cartridge. Problems like cavitation (turbulence related air build up behind the piston) are far less. Cavitation can produce a split second loss of damping. Personally I have never noticed that in any suspension. If it occurs it might be affecting the performance of a pro rider though.
Closed cartridge motorcycle forks also work better in extreme conditions. Due to the two separate systems the oil inside the cartridge gets less contaminated by rub off from bushes, spring, inner tubes etc.

The pressurized system also helps to maintain damping at extreme damper speeds. Damper speeds of 8m/s are reached at pro level MX in comparison to road bikes where only 2m/s are reached. That is why all MX and all factory riders use closed cartridge forks.

Another consideration is cost. Closed cartridge is expensive to produce and money saved on MX bikes by not having lights, lighting coils, big silencers, side stands etc. helps to make up for the extra cost. Enduro also has a lower damper speed due to less big jumps.

Disadvantage of closed cartridge forks
The drawback of the closed cartridge is that it is more complex and requires more time and knowledge to maintain. Mechanics working on a closed cartridge fork have to be much more careful as it is easy to make small mistakes with bad consequences. Service intervals are shorter. Even though the oil inside the cartridge gets less contaminated. But the complexity of the system gives way for little hick ups in the cartridge. Regular checks are necessary.
The mechanic also needs more knowledge for personal settings in these motorcycle forks. It is more complicated to open the sealed cartridge to change settings and better skills are needed to find the optimum performance of the fork.

The open cartridge system
In this system the oil within the cartridge and outside is the same. The cartridge is at the bottom of the fork and remains under the oil at all times. The oil circulates in and out of the cartridge. There is one common oil chamber. That's why it is called open cartridge fork.
As we have seen the reason why enduro bikes come with this system is production costs. Enduro is also not as extreme as MX in terms of damper speed and therefore open cartridge units cope easily.

The open cartridge fork tends to offer slightly better comfort than the closed cartridge units due to little air mixing in the oil. This also means that damping may not always be as precise and consistent as with the cc units.

What is the right choice?
The average enduro rider does not need such a specialized motorcycle fork as the closed cartridge unit. An enduro bike comes well set up and is suitable for enduro right out the box. Obviously fitting springs to suit your weight is all that is needed in order to have an excellent set up enduro bike with standard open cartridge forks.
In order to use closed cartridge forks from a MX bike for Enduro it would need to be internally valved. The shock as well needs to be changed accordingly. This job is fairly complicated and if done wrong your closed cartridge fork would feel much worse than a standard enduro open cartridge fork straight out the crate.

Also don't forget that you need to service the closed cartridge forks more often. Makes no sense to use the better material and then have the performance suffer due to lack of maintenance.

Pro riders do not need too much comfort. What they expect from their motorcycle fork is consistent damping and big safety. If you go the closed cartridge route then make sure you have the time and money to experiment with different settings in order to get your cc units set up for enduro correctly otherwise they will be wasted. If you get the cc unit right, then you can be sure your motorcycle fork is as good as it can get and on par with the pros bikes.

http://www.dirt-bike-secrets.com/motorcycle-forks.html
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