The PDS debate



By Matt Andruk  Owner/Lead Tech Active Ride suspension

In 1998 KTM debuted the no linkage design with the PDS system.  PDS stands for Position Damping System.  The system was designed by OHLINS by the patent was bought by KTM/WP to be used under the WP name. The system attempts to work by creating an increase in damping as the suspension compresses to mimic the slow mechanical progression of a linkage system.  Contrary to popular belief the KTM does have a linkage ratio of sorts, albeit a relatively flat one. This is possible because of the shocks angle. As the suspension compresses the shock's bottom mount "moves" out towards the rear tire in relative terms. What this means is that as the shock progresses it lays down more altering the swing arm's leverage against the shock. It doesn't physically moves it just alters the center of gravity of the swing arm to provide less leverage against the shock making it stiffer.

Many rides complain about the rear shock on a KTM. The real problem with the system is the idea of controlling position sensitivity with damping rather than consistent mechanical spring resistance. This is a problem because this the equivalent of having a small man on your shock with a screwdriver increasing your damping while you ride anytime the bike gets deep into the stroke. For example Imagine going at a high speed into a steep hill. As you g-out and maintain a forward motion your rear shock is supporting most of the weight and sinking into the stroke. If at this point there is any sharp bumps, roots, or rocks they are met with a large increase in damping and simply can not be absorbed. This is also a problem when you land off a jump with square edge holes on the landing. This bumps will most likely cause your bike to kick and remove all confidence in your motorcycle.

The other problem is that since it has a relatively flat linkage ratio it requires stiffer low speed valving.  This kills the bikes ability to absorb small bumps and maintain traction. It also seems to throw off the balance of the bike making the forks work harder because of the extra leverage this stiff setup provides.Some suspension shops who don't address the system as whole end up reducing the number of face shims on the primary piston in order to gain small bump absorption. What they end up doing is causing the bike to ride too low in it's stroke kicking in the secondary damping too often and making the bike a bucking bronco. I remember the first time I discovered this in testing. I reduced damping initially and increase damping on the secondary piston. This was a horrible mistake. This is what makes the PDS shock tricky to valve correctly and it isn't something I would trust to any local shop.

The last problem that is encountered in the PDS system is HEAT. The large shock shaft that displaces large amounts of oil, the twin piston design (in damping and piston band friction) the floating piston nitrogen system and the four stroke exhaust now prevalent in the sport have all increased the heat of the shock to astronomical levels. This is problem because in any shock heat is the enemy. Heat causes the shock oil to breakdown causing inconsistent performance. Many shops like to install a bladder in these shocks and I think at the heat levels and the porous nature of the rubber this is a mistake. There isn't much if any increase in performance by installing a bladder. Most of the difference felt is by incorrect bleeding by the do it yourself suspension tuner who makes these kits popular.

There have been many attempts to solve this problem. Reducing the progression of damping, Increase the progression with a long needle or removing the pds system all together.  The factory WP shock uses a small secondary piston that is similar to a bottoming cone inside your fork. This completely eliminates secondary damping in all but the roughest of conditions. Now in order for the system to work you must run a progressive spring. I generally follow this rule of thought and it's the method I use in my business. I think the long needle idea works well too I just don't think it's the best idea. I think most of the benefit from this system comes from predictability. It allows for more consistant valving and has it's merits.

I have some other ideas that I would like to pursue however with the possibility of KTM going back to a linkage I don't think this idea is a good
Idea with the money/time invested. The costs would be rather high as well for the full system I might as well offer it as a KIT shock. However the parts could be broken down and purchased one at time for a more affordable option.

I am interested in helping out KTM owners to their suspension woes. If you are interested and want me to develop parts for your bike. E-mail me with your thoughts. Who knows if you are in the Tampa Bay area I might use you as a test rider!

Matt@active-ride.com


Understanding your Clickers
By Ross Maeda  KYB Consultant/Owner Enzo




TWO KEY COMPONENTS

To understand how to start adjusting your suspension, you have to understand the basic components. Both the forks and shock have two key elements: the spring and dampening.

The spring is load or position-sensitive, which basically means it’s job is to hold the rider’s weight. The dampening — which is what the clickers control — is a speed-sensitive element.

The spring is really just a dumb piece of metal that’s bending. It’s like a trampoline, if you stand on a trampoline, it holds you up; but if you jump on it, it goes down a lot deeper and throws up equally as high. If you just had a spring on a motorcycle — and no dampening — it would be like a car going down the freeway without shocks; it would bounce up and down for miles.

Adding dampening to the suspension is like putting that trampoline in water. It will still support your weight, but you don’t get the springy, bouncing effect.

START WITH THE RIGHT SPRINGS

You can’t compensate for a spring that is too stiff or too soft with the adjusters. So before you do anything, check the weight range indicated for your spring and, if needed, make a change there first. Your local suspension shop can help you with this.

WHAT YOUR CLICKERS DO

As I said, your clickers are there to make adjustments to dampening; to control speed-sensitivity. Going in (clockwise) on your clicker makes that adjustment tighter/slower. If you go out (counter clockwise), it makes that adjustment looser/faster.

For example, if you are bottoming out in low-speed situations, go in on the low-speed compression clicker to compensate (increase compression dampening).

The suspension on a modern motocross bike has dampening adjusters (clickers) for:

• Fork compression
• Fork rebound
• Shock low-speed compression
• Shock high-speed compression
• Shock rebound
• Check your owner’s manual for the exact location of each adjuster.

Shock AdjustmentsIt’s important to remember that your shock has adjusters for both high- and low-speed compression. The low-speed adjustment is for situations with low amounts of stroke, and the high-speed is adjuster impacts situations with large amounts of stroke.

Let’s say you’re accelerating through a section with very deep bumps, and your shock is going from full extension to 3/4 quickly; to change how soft or stiff your shock feels in that situation you would adjust your high-speed compression clicker.

If you’re in a situation where your shock is using short amounts of stroke, like braking bumps, you want to adjust your low-speed compression adjusters.

FORK ADJUSTMENTS

Your forks only have a single compression adjuster, and a rebound adjuster. Adjusting the fork compression will affect how quickly or slowly your front end compresses, while adjusting the rebound clicker changes how fast the shock springs back to full extension.

Changes to fork dampening can have a big impact on how well your bike handles in corners. If your front suspension feels too soft, go in on your compression. If your fork is coming back too fast, go in on your rebound adjuster.

MAKING ADJUSTMENTS

Some people are afraid to adjust their clickers, but I tell them if you can count, use a screwdriver, and tell the difference between clockwise and counter-clockwise; you can adjust your clickerrs. Just remember, clockwise is tighter or slower, and counter-clockwise is looser or faster. It’s that simple.

To get started, go to your local track and run a few laps. Then come back to the pits and make adjustments one at a time. If you think it’s one thing, try making an adjustment two or three clicks at a time, then go back out and ride. If that didn’t help, go the other way; or if you don’t feel it, keep going more.

If you get lost you can always go back to the stock settings by checking your owner’s manual, and most suspension shops will be happy to give advice over the phone.

Suspension Fluid FAQ's

By Matt Andruk Owner/Lead Tech Active Ride Suspension

Countless times someone will call or come up to me and ask "What kind of fork oil do you use?" I will tell them we use genuine KYB oils..Then he will say.."Well I use so and so's magic fluid and it works awesome!" I am writing this to explain some things about oil.

What is the purpose of fork oil?
To understand what you need from a suspension fluid you must first need to understand what is required of it. The basic job of your suspension fluid is to provide a fluid platform for damping resistance. In other words it's there to slow the suspension movement by flowing through a series of small orifices as your suspension cycles through it's stroke. Another job for the fork fluid is to lubricate the bushings and seals to reduce friction.

Can I get an increase in performance just by changing oil?
Of course. If your suspension fluid has over 10hrs on it you will see a noticeably smoother action from your suspension. If you change weights or brands that are able to maintain it's damping character longer you will notice an increase in performance. There are three performance aspects of fork oil. First, it's viscosity rating. This is the number that shows it's hydraulic resistance to flow. Thinner is softer thicker is stiffer. It's ability to maintain that viscosity rating and finally it's ability to lubricate. Any other claims to performance are far fetched and likely untrue.


What is viscosity?
Viscosity is the thickness of the oil. Fork oil weights are derived from the industrial standards used for hydraulic applications, called the Saybolt Seconds Universal (SSU). This measurement uses a similar viscometer arrangement as used to determine a cSt value, but grades the oil using a much more sensitive viscosity calibration scale.The thickness of hydraulic and fork oils are listed as the Saybolt Seconds Universal at 100 degrees C/viscosity index. Let's say that the numbers listed on the bottle read as 85/150. It means that the oil's SSU value at 100 degrees C is 85. Then, the flow of the oil is measured at 40 degrees C. The second number-150-is the value given to the difference in flow between the two temperatures. This is called the viscosity index (VI). This is important because the higher the VI rating, the more stable the weight of the oil remains when it gets hot.
How does this apply to motocross forks? The friction created by sliding metal parts and oil flowing back and forth through valves creates heat. The more consistent the weight of the oil remains the less likely the fork's damping is to change as a moto progresses. If the oil gets hotter and thinner, the forks will get softer and faster.

Instead of getting a revalve can I use thicker or thinner oil?
We do not change oil weights for a couple of reasons. First heavier oils tend to heat fade faster and more noticeable as a race progresses. Also the stiffness gained is mostly through bleed ports and is similar in feel to adjusting your clickers.  What that means is it is basically the equivalent of a large clicker adjustment change.  It tends to move the damping scale up. The only real benefit to thicker oil is it's ability to cling to the components inside your fork. However this only lasts until the oil starts to thin.


Can I use fork oil in my shock?
No.  Fork is designed to work in low heat unpressurized environment. It is designed to cling and lubricate and prevent foaming. Fork oil is not designed to withstand the heat inside a shock and will eventually breakdown. Shock oil is also generally thinner (even if the weights are the same) to allow for more consistent performance.

What about 2.5 weight fork oil?
The thinner oils generally are too thin. they tend to fling off the metal tubes and spring inside and wipe away too easily between the fork seal and the inner chrome tube. (There is always a very thin layer of oil on the visible portion of the lower chrome (or DLC) tube). The reduction in damping is generally not worth the extra friction and bushing wear this oil causes.


What kind of oil do you use?
We use genuine KYB oils in all the work that goes in and out of our shop. The reason being that every year KYB collects samples of all available suspension fluids and runs them through analysis and testing.  Nothing has ever even come close to the performance and durability of their oils.


My suspension feels soft and wallowy do I need my oil changed?
Yes along with having a rebuild of the components. If there is a drastic change in performance over time most likely your suspensions bushings,seals,shims, and piston bands have worn out. If you just change the oil it would be similar to the effects of just changing the oil in your bike if your clutch was slipping.