What is that black coating on my dirt bike fork tubes and shock shafts, and what does it do?
By Jon Light
Use of coatings on dirt bike fork tubes and shock shafts is commonplace, but what are they, and what do they really do? To better understand their purpose and function let’s take a look at stock dirt bike suspension engineering philosophy, limitations, and how those limitations are overcome.
When designing a production dirt bike suspension system, engineers must produce a single product that provides a satisfactory blend of ride comfort and good handling to suit the needs of a wide range of riders. One set up must please riders with differing weights, skill levels, and in a variety of riding terrain types. While this “one size fits all” approach generally satisfies, it often does not meet the specific ride comfort, terrain type, and handling preferences of the individual. The problem is that engineers must bias the suspension set up to handle the extreme end of the rider range; they must prevent the suspension from bottoming out when heavy riders hit bumps at high speed, or hit very hard bumps in general. This makes the suspension unnecessarily stiff for lighter riders as they encounter the same riding conditions.
To solve this problem, riders can turn to suspension tuners. Essentially, the suspension tuner’s job is to achieve a suspension set up that strikes the best balance of plush ride and excellent handling for a specific rider’s weight, height, skill level, and the type of terrain they ride on. Whether going over slow speed rolling bumps, or high-speed sharp hits, the ride needs to be plush and the handling controlled. Tuning involves a combination of equipment, engineering, experience, and perhaps a bit of black magic.
Rob Hendrickson of RG3, a renowned suspension tuner for championship winning Motocross, Supercross, Arenacross, Pro Off Road and Elite Freestyle riders, says they start by listening to the rider. “They will tell you about the problems they are experiencing with their ride and handling, and let you know how they would like the motorcycle to perform. To achieve the rider’s desired result we make equipment changes and adjustments based on the rider’s weight, height, skill level and terrain, and then combine that with basic engineering principals and what we have learned from hundreds and hundreds of hours of suspension testing experience” Rob informs.
Tuners then turn their attention to the equipment; springs, forks, and shocks. “It’s a matter of properly controlling the fluid flow through the forks and shocks, using the correct springs, and controlling suspension breakaway friction” says Rob.
First comes the choice of a correct spring rate, which is determined by factoring the riders weight, and type of riding to be done – for example whether they do trail riding, Motocross, Arenacross, or Supercross.
Next come the forks and shocks. Typically, stock motorcycle forks and shocks come with pistons that have very small valve venturis to prevent the suspension from bottoming out at high speed when heavy riders hit bumps, the bike takes really hard hits, or in G-out situations. The valves are then matched with a shim stack to create a damping curve that works with the spring rates of those components. At low suspension speeds this set-up can work reasonably well, and provide a plush ride. However, at higher speeds, when the suspension must react more quickly, it will not flow enough oil, and will experience hydraulic lock. The result is a harsh ride and poor handling. To remedy this, suspension tuners install new pistons with larger valve ports that flow more oil. Since the valves now flow a higher volume of oil, the shim stack is heavily relied upon to control the oil flow at high-speed. The larger ports must be matched with an appropriate shim stack to establish the high-speed damping curve. The modification allows the damping to be more thoroughly controlled throughout the fork or shock’s range of operation, providing a plush and controlled ride at low and high-speed suspension movement. One of the main criteria that distinguish excellent suspension tuners is their ability to best optimize the relationship between the shim stack and the valving.
Coatings are a component of suspension tuning that often aren’t well understood. Manufacturers frequently use special coatings on the fork leg inners, and sometimes the shock shafts, to try to minimize the amount of stiction (static friction) in the suspension. In this case, stiction is defined as the friction between the fork tube, when at rest, and the fork seal. The same goes for the shock shaft, when at rest, and the shock seal. The higher the stiction, the more force is required to set the suspension component into motion. By reducing stiction, the transition between the suspension being at rest and in motion is more smooth and fluid; the snap of breakaway is greatly reduced, and bumps are more smoothly absorbed. Coatings will reduce sliding friction of suspension components too; especially under high side loads that cause binding. This helps tuners; Rob at RG3 says they are able to “tighten up the fork and shock valving when binding friction is reduced.”
Coatings also provide protective surfaces on fork seals and shock shafts that guard against formation of scratches, nicks, and sand lines. This provides a smooth surface for the fork and shock seals to slide over, which extends their service life, and reduces oil leakage.
A coating frequently applied to dirt bike fork tubes and shock shafts is Titanium Nitride. Titanium nitride (TiN) is gold in color, and is an extremely hard ceramic material deposited on metal surfaces as a very thin film. The most common method of Titanium Nitride thin film creation on steel is physical vapor deposition (PVD). With this method, pure titanium is converted into a gas, reacted with nitrogen in a high-energy, vacuum environment, and deposited on metal as a thin film. Titanium Nitride reduces friction, and when used on steel has a coefficient of friction of 0.3. As a reference point, steel on steel has a coefficient of friction of 0.7.
Titanium Nitride provides the benefit of a hard surface barrier against nicks, scratches, and sand lines. However, the limitation is that it is non-flexible and brittle. If the fork tube or shock shaft is ever bent or deformed, the Titanium Nitride is prone to crack and then flake off. This increases friction. As well it creates a rough surface that prematurely wears fork and shock seals, resulting in oil leakage.
A better performance coating technology for steel, stainless steel, titanium, aluminum and other non-porous metals is Diamond Like Carbon, or DLC. The "Diamond-Like" term was coined simply to describe its very high indentation hardness, which is between that of carbon and the soft facets of natural diamond. DLC has no other properties, which make it appear like natural diamond, CVD diamond or PCD other than this high hardness and excellent scratch resistance compared with "ordinary" hard materials. This coating type is a dark carbon color, and is made up of carbon atoms that are deposited in molecular formations that have no brittle fracture planes. So they are flexible and conform to the underlying shape being coated, while being as hard as diamond. If the underlying metal is bent or deformed the DLC will flex with it, rather than cracking and flaking off. Like Titanium Nitride, most DLC is deposited by physical vapor deposition (PVD) process. In this case argon gas molecules are introduced into a vacuum chamber and converted into ions by removing electrons. These energized molecules are then directed at a target made of carbon. The molecules collide with enough speed to physically dislodge solid carbon particles. This target material travels in solid form, with various particle sizes and energies, and is deposited on the metal surface as a thin film 2 to 4 microns thick. DLC wears as a graphite and has a coefficient of friction of 0.2 when used on steel. DLC has a wear factor about 300 times less than steel and about 10 times less than titanium nitride, which means that the DLC film will last much longer than a titanium-nitride-coated or an uncoated part.
With the PVD process target material is dislodged by momentum transfer of energetic ions hitting a solid target. Target material travels in solid form, with various particle sizes and energies, to coat to a substrate.
The tall block (blue) creates a shadow on its left side, where the evaporated material (green) does not fall.
DLC coating is a dark carbon color.
However, not all DLC is created equal. There are inherit shortcomings with PVD applied DLC. One is that the target carbon material particles are of different size, which when viewed on the coated part at the microscopic level, shows a lumpy surface with grains of various sizes. Large grains sometimes extend the full depth of the coating, leading to a potential defect situation where an entire grain can be pulled from the coating, leaving a substantial hole. Another issue is the part being coated must be rotated to get even coverage in the carbon particle “spray” (imagine the coating process as paint being sprayed from a can). It is difficult to get consistent even coverage with this technique. Also, DLC coatings are very difficult to bond directly to metal surfaces. As a solution, Titanium Nitride is frequently used as a “base coat” on the metal. The DLC is then bonded to it. Overall, this will reduce friction, as the DLC provides the upper surface of the coating. However, the Titanium Nitride is not flexible and will crack and flake off if the metal is deformed (taking the DLC with it), thereby negating the flexibility benefits of the DLC.
PVD applied DLC at 7000x. Crystalline grain structures ranging from 0.1-1.6 microns are present on the surface. The crack is caused by growth plate defects formed during coating, as grains are seen on the exposed walls of the crack. This can be a typical PVD shortcoming. When a 1.6 micron grain rips from a 2 to a 4 micron thick film during use, it will look as though the film was gouged at that point and down the path of applied force.
There is a deposition technique that resolves the issues of PVD applied DLC, which is termed Plasma Assisted Chemical Vapor Deposition or PACVD. Extremeion, of Denver North Carolina, a supplier of DLC coatings to champion motocross, supercross, and arenacross teams, has perfected the PACVD process with their Carbon Raptor DLC coating. George Barr, President of Extremeion, states; “(With PACVD) the chamber fills uniformly with a hydrocarbon precursor gas. The gas is ionized into plasma by an electrical charge, somewhat like neon ionizing in a neon lamp. The plasma condenses a carbon molecule at a time onto the part by temperature differential between the plasma and the cooler substrate, somewhat like beads of sweat on a cold drink container on a hot day, and is attracted to the surface of the part by electrical attraction. All exposed surfaces of the part receive uniform coverage, and our proprietary techniques allow us to successfully bond Carbon Raptor directly to metal surfaces.” Carbon Raptor is applied in a very smooth and controlled fashion, and therefore lacks the grain structure prevalent with PVD applied DLC. Since there are no grains to pull out of the film during use, wear is much more even.
The chamber fills uniformly with a precursor gas. Gas is ionized into plasma by an electrical charge, somewhat like neon ionizing in a neon lamp. Plasma condenses a molecule at a time onto the part by temperature differential between the plasma and the cooler substrate, somewhat like beads of sweat on a cold drink container on a hot day, and is attracted to the surface of the part by electrical attraction.
Carbon Raptor at 7000x. Carbon Raptor is very smooth over the area, as it is applied in a very controlled fashion, and therefore lacks the grain structure prevalent with PVD applied DLC.
There definitely are suspension limitations with even the best stock dirt bikes. By employing the expertise of a good suspension tuner, and quality equipment, a good suspension can be transformed into an incredible suspension. Perfectly suited to a particular rider’s weight, skill level, and the type of terrain they ride on.