KTM PDS Shock

*** 03/04 KTM Whitepower Setup ***

I need to discuss a change on the 2003 and 2004 KTM (White Power) shock sag set up.  White Power made a change in these shocks.  They installed a 12mm long top-out spring inside the shock.  Because of this change, the proper race sag should be between 112 and 118 mm.  If you try to use the 100mm normal race sag, you will start compressing the top-out spring.  Since the spring can’t be compressed to zero, you can’t use the same race sag measurement as on the older bikes.  The factory directions still incorrectly recommend the 100mm race sag setting.

In the last column I discussed how a speed/pressure sensitive dampening system works, and I said I would explain how a KTM PDS shock works in this column.  However, we must first define a position sensitive system.

A position sensitive system controls dampening by how far the damper (in forks OR shock) travels, not by its speed/pressure.  The most common type of position sensitive dampening system is still used in the forks of most road bikes and low budget trail bikes.  In these bikes, the inner fork tube (usually the upper tube which is chrome plated) encases a smaller tube called a dampening rod that is bolted into the lower fork leg.  The dampening rod’s upper end is enlarged and has a nylon ring around it so that it can slide up and down inside the upper tube, much like a piston in a cylinder.  This dampening rod has a series of holes drilled crosswise along its length. These holes will close off as the dampening rod travels up into the upper tube.  The more the fork is compressed the more holes are closed off, restricting the amount of oil forced out of the dampening rod, and thereby stiffening the dampening.  In theory, the fork is compressed more on big bumps than on little bumps so that the fork stiffens as it gets deeper into the travel.  As the fork rebounds, the holes are uncovered - allowing less dampening. One of the main problems with this system is that the rebound circuit provides very little rebound dampening. 

Now to complicate the matter a little more, White Power added a second piston above the cross-drilled shaft.  When the shock travels only a small amount, the oil flows up through the center of the shaft, exiting the cross drilled holes - therefore missing the first piston completely.  Oil is now between the two pistons.  As the pressure increases the shims on the second piston will start to flex, allowing the oil to pass through the piston in the process I discussed in my last column.  As the shock continues to compress deeper into the travel the needle enters the hole in the shaft thereby restricting the amount of oil that is bypassing the first piston. When the pressure increases enough it will cause the shims on the first piston to flex, thereby allowing oil to pass through the first piston and then on through the second piston (since the shims on that piston are already open.)  The first piston is used in the deeper part of the shock travel and is valved stiffer than the second piston.  The second piston is used in the early part of the shock travel and is valved lighter.  This shock still has the rebound bypass adjustment system above the second piston, which works just like a standard shock.  

The PDS shock works with a combination of the speed/pressure sensitive and the position sensitive systems.  The reason for using both systems in the shock is that the late model KTM’S (1998 to present) do not have a complicated link system hooked to the shock.  KTM needed to use the position sensitive system to hydraulically make the shock stiffer so that it would not bottom easily and could be valved such that it would be ridable. The link system is also a type of position sensitive system.  As the links travel through its arc, compressing the shock, it makes the shock stiffer through mechanical advantage.  

The front forks created by my own Terrain Tamer modification and some of the front forks made by Marzocchi utilize both the speed/pressure sensitive and position sensitive systems for the compression dampening function while the rebound dampening function is strictly controlled by the speed/pressure sensitive system.  This combination allows very plush dampening in the first part of the fork travel but stiffer dampening in the deeper part of the travel, which very effectively controls bottoming resistance.