Computrack Boston

How Often should I service my Suspension?

One question every suspension tuner has to answer is “How often should I service my suspension?” The answer depends on several factors. Do you race? Do you log a lot of miles? How critical is consistency of performance? These and other factors must be considered when implementing a suspension service schedule. Let’s start with what happens to the components of the suspension during use.

The main components of the suspension on a motorcycle are the rear shock(s) and the front forks. Both have a spring(s) and a damper. The springs support the bike and allow the wheels to move independently of the chassis. The damper controls the extraneous wheel and chassis movement by converting the unwanted kinetic energy into heat energy and dissipating it to the atmosphere.

Modern springs are made either of Chrome-Silicon steel wire or Titanium wire and last for millions of cycles. They require almost no maintenance except for some possible setting or sacking, as it’s called. When a spring sacks it loses a few millimeters in length, but does not lose its rate. So about the only maintenance is to check the preload after the first few duty cycles. For race bikes one race weekend, for street bikes by the 600 mile break in is fine. In most cases, the springs are pre-set when manufactured and will not lose any length. Any loss in length is compensated for by adjusting the preload. Once this initial setting, or sacking is corrected, there is no additional maintenance needed for the springs.

The other component of the suspension system is the Damper. (Not Dampener) The damper (also called a shock or fork cartridge) is the part that needs the most attention. The job of the damper is convert kinetic energy, or motion, into heat energy and dissipate it the air. While there are many different systems implemented to accomplish this, they all share the same basic physics principles. By using hydraulic oil and a piston moving through the oil, the resistance of movement of the piston through the oil changes the kinetic energy into heat. It is similar to trying to run through waist deep water at the beach. The resistance is varied by utilizing a piston with ports that are covered by a stack of thin flexible washers called shims. These shims come in various materials, thicknesses and diameters. By using different diameters and thicknesses, the resistance to flow can be increased or decreased. The resistance force is called the Damping force. Damping forces are velocity sensitive. That means that the faster the piston tries to move through the oil, the higher the resistance force, or more Damping force. It is what happens at the edge of the piston where the oil is squeezed in between the piston port lip and the shim stack is what’s most abusive the oil.

As the oil passes between the piston port lip and the shim stack, the spring coefficient of the shim stack is trying to close off the port and squeezes the oil with a shear force. This shear force then breaks up the molecular chain of the oil and, in layman’s terms, “Spaghettifies” the oil. When this happens the oil flows more freely. It does not lose viscosity (resistance to flow), but instead the long strings of spaghetti like molecules pass between the piston port lip and the shim stack easier than the interlocking molecular clumps that the oil was designed and manufactured to have. This phenomenon happens with all oils, but is more pronounced in heavier weight and non-synthetic oils.

Here is a real world example of how I learned this. In 1998 we were experimenting with different piston designs in Penske Racing shocks. Their standard 1/1 or one degree/one degree linear piston was and still is a great all around piston. It gives good drive grip, good bump compliance and has very even pressure balancing. They also had a piston called a Digressive Piston. This piston had a much steeper Low Speed Nose. That is the piston developed much more low speed damping force and therefore gave additional drive grip. The digressive nature of the damping curve made it very plush, as the damping force was reduced at higher shaft speeds. This, however, resulted in the shock bottoming out too easy. To solve this Penske combined the best attributes of the 1/1 linear piston and the Digressive to create the VDP Piston.

VDP stands for Velocity Dependant Piston. As piston speed changed, so did the nature of the damping curve. So now we had better drive grip, good bottoming resistance and a very plush feel. The problem was that after about 4-5 laps the shock would lose its damping force. The back of the bike would pump up and down through the stroke like pogo stick. After trying everything I could think of to solve the problem I called Penske for help. I was referred to Jeff Ryan (now the JR in JRi Shocks) and he suggested three things to help solve the fading problem. First was to switch from the standard Penske Shocks oil of Silkolene PRO RSF 5 weight to the 2.5 weight. Second was to make sure there was absolutely no air bubbles in the oil at all. Last was the most interesting. The VDP piston had separate high and low speed shim stacks. The low speed stack was three, 8 thousandths shims. He had me change to 24, four thousandths shims. The spring coefficient of the shim stack remained the same, that is to say the “spring rate” of the stack was the same and tried to close off the ports with the same force.

Right from the first lap of the next outing the performance of the shock was dramatically improved. First, drive grip increased as the chassis was more geometrically stable. Second the feel of the rear tire contact patch was increased. The rider reported a better connection between the tire and the throttle due to increased feed back and feel. Last was the fade issue. Whereas with 3 .008” shims, the shock would fade after 4-5 laps, now with 24, .004” shims there was no noticeable fade after 12 laps, and after a 30 minute Endurance race the shock felt the same as it did on lap one.

So the question is, why? Why did changing to a greater number of thinner shims result in reduced fade and increased performance? To keep it simple, it was the shear forces within the shims. As the thicker shims bent, the force they exert on the oil is “harsher” than the reduced shear forces within the thinner shims. This resulted in less of the aforementioned “spaghettifiying” of the oil. That means that the oil was left in more of the molecular clumps state and maintained its resistance to flow.

Another reason to service your suspension regularly is cleanliness. As the piston moves up and down inside the shock body (or the fork cartridge) it scrapes way a little of the anodizing off the body, loses a little of the Teflon off the piston band, and leaves those deposits in the oil. There are also bushings in the fork and shocks that have a bronze base and Teflon coating that also wear and deposit in the oil. Some of these chunks can become wedged in between the piston and shim stack and keep the shim stack from closing off the piston port. Effectively this is like turning the external adjusters wide open. In forks, where the springs are inside the damper, the coils can scrape on the inside of the fork tube and leave even more deposits in the oil.

Lastly, regular service of suspension components allows the tech to find problems before they can destroy expensive components. Over the years we have found thing like, the nut that holds the piston and shim stack on the damper rod coming loose. Is some cases we have found it after it came off and the piston and valving gouged the cartridge or shock body beyond repair. We have seen shims bend beyond the breaking point and have shattered into little pieces. In a very rare case we found a broken fork spring.

So there are many reasons to have your expensive suspension components serviced at regular intervals. Maintaining consistent damping performance and avoiding expensive repairs are two of the most important. Many of the After Market suspension companies we represent call for 20 hour service intervals of their forks and shocks. This is purely a race schedule. For the street we recommended every 10,000 miles. A regular maintenance schedule will allow your tech keep track of wear intervals of moving parts and prevent unnecessary damage. Modern suspension systems are better than ever, but no system is exempt from the perils of neglect and the cost of maintenance is much less that cost of replacement.