Published On: January 21, 2025

IIn MXGP it has become commonplace: a fancy colored coating on the fork legs of the factory machines. Amateur riders who want to throw money at it can use the same technology. All bikes come standard from the factory with shiny chrome forks, but is a different coating really that much better? Is it a fad or do these expensive coatings really work? 

Why do manufacturers use chrome on their fork legs as standard?

The answer is very simple: cost. Applying chromium to steel is a relatively cheap process compared to the more complex procedures used for titanium oxide (TiO2), titanium nitride (TiN) and diamond-like coatings (DLC). Applying chromium is a five-step process.

The titanium oxide coating on a Honda CRF

The steel pipe or tube is first cleaned. Then the pipe is immersed in an acid to etch the surface. This chemically removes impurities such as iron oxide (rust). Then the steel pipe is given a negative electrical charge and immersed in a bath of positively charged chromium-containing liquid. The positively charged chromium particles bond with the negatively charged pipe.

The tube is then rinsed clean and polished to a smooth surface finish. As complicated as this procedure may sound, it is actually simpler and less expensive than TiO2, TiN or DLC.

Warom TiO2, TiN of DLC?

From a purely practical perspective, there are probably two main areas we want to try to improve: friction and damage resistance. We could also try to reduce weight, but chrome plating is only 0,03mm thick, so there’s not much to be gained by switching to a different material. So let’s look at friction.

Titanium nitride coating combined with outer legs treated with Kashima coating.

Luckily, there is a unit of measurement we can use for this, it’s called the “coefficient of friction” and it represents the relationship between two forces. In this case, it’s the force required to move an object on a surface divided by the weight of that object in Newtons. For example, if you have a 1kg (9,81N) block of rubber lying on concrete and it takes a force of 5N to move it, the coefficient of friction is 0,51. There are even standardized tests for measuring friction, one of which is called ASTM G99.

Basically, the standard size material is placed on a test bed and rotated against the bed and the forces are measured. The force required to get an object moving from a static position is greater than the force required to keep an object moving, so the test must overcome the static friction (sometimes called “stiction” in the motocross world) and then as the material moves we can measure the dynamic friction.

Now that we know how the test is performed, here are the values:

  • Chrome: Static: 0,16-0,20. Dynamic: 0,10-0,20.
  • TiO2: Static: 0,35-0,40. Dynamic: 0,30-0,35.
  • TiN: Static: 0,40-0,55. Dynamic: 0,30-0,50.
  • DLC: Static: 0,05-0,20. Dynamic: 0,05-0,15.

It seems that DLC has potential, but why should we use TiO2 or TiN if the friction values ​​of chromium are lower?

The typical black color of DLC coating

Let’s look at the advantages of TiO2, TiN and DLC over chrome. TiO2 offers better corrosion resistance. TiN forms a very hard surface and offers better wear resistance while DLC has extremely low friction and better wear resistance.

We haven’t talked about anodizing or Kashima coating yet, and that’s because that mainly applies to aluminum and not steel. When we think about aluminum, while it’s a very light material compared to steel, it’s also a very soft material. Ideally, we’d like to improve its hardness so that it improves its wear resistance, and it would also be nice to have lower friction when it comes to suspension.

One way to improve the hardness of aluminum is by anodizing. Anodizing is similar to chrome plating, this is the process:

Cleaning, etching, removal of oxidation and the anodizing process itself. The aluminum is placed in an electrolytic bath, usually made of sulfuric acid, chromic acid or phosphoric acid depending on the application. The aluminum acts as the anode (positive electrode) and a cathode (negative electrode), usually made of lead or stainless steel, is placed in the bath. A direct current is passed through the solution. Oxygen ions from the electrolyte combine with the aluminum atoms on the surface to form aluminum oxide (Al₂O₃).

It is the oxide layer that improves the hardness of the aluminum, but by how much?

Fortunately, there is another standardized test that we can use, the Rockwell hardness test. In this test, a tool with a diamond point is pressed into the material and the testing machine can measure how far the point has gone into the material.

The results are:

  • Untreated aluminum: 15 to 30 HRB
  • Anodized aluminum: 60 to 65 HRC

This means that anodized aluminum is 4-5 times harder than untreated aluminum, great for wear resistance! But what about our old friend friction? The answer is not great when the surfaces are dry, and reasonable when lubricated:

  • CoF's untreated aluminum: Static: 0,3 – 0,45, Dynamic: 0,2-0,4, Lubricated: 0.1-0.2
  • Sealed Anodized Aluminum: Static: 0,4-0,7, Dynamic: 0,3-0,5, Lubricated: 0.1-0.25

The oxide layer that is created during anodizing is actually quite rough. And this is where the Kashima coating comes in. Kashima coating is a proprietary anodizing process developed by the company Miyaki Co. Ltd. in Japan. Why it is called Kashima is unknown.

Anyway, the Kashima process was developed to give aluminum the enhanced hardness of anodizing while reducing friction, and this is done by adding an extra lubricating molecule as part of the anodizing process. How does it work? Just look at the image below from the Kashima website. It goes without saying that the Kashima coating is only used for outer legs as it is a process for coating aluminum.

Are special coatings for your suspension worth it?

As usual, it all depends on your available budget and/or the level you ride at. If you’re a professional race team looking for every 1% performance improvement, DLC coated tubes and Kashima coated tubes are your best bet. If you’re an average amateur rider, your money might be better spent maintaining your suspension or buying the right springs for your weight and power.

Thanks to Jens Köpke, CEO of Motoklik, for providing us with the available information.

Source: Motoklik.com
Tekst: Jens Köpke and Danny Hermans