|
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.
KTM
PDS Shock
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
that you have a basic understanding of a position sensitive system I will
explain how it applies to the KTM PDS shock. The KTM (White Power)
shock does not use a dampening tube with holes. Instead, they utilize
a very large tapered needle, much like the needle in a carburetor – only
much larger. This needle is mounted at the bottom of the shock body.
The center of the shock shaft is machined so that as the shock compresses
the needle will enter the shaft and slow the flow of oil up through this
passage. The passage is crossed-drilled above the first piston, much
like in the rebound adjustment system used on most shocks. The only
difference is that since there is no external adjuster screw, the oil flow
is controlled by the stroke depth of the shock.
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. |
