All bikes come from the factory with shiny chrome forks stanchions, and then somewhere along the line, we get the idea that a new coating will make them much better. Is this real, or do we just like shiny new things and different colours?
Firstly, why do all the manufacturers use Chrome from the factory?
The answer is very simple; cost. Applying chrome to steel is a relatively low cost process compared to more complex procedures used for Titanium Oxide (TiO2), Titanium Nitride (TiN) and Diamond Like Coatings (DLC). Applying chrome is done in a five step process.
1: Clean the steel tube.
2: Dip the tube in an acid to etch the surface i.e. chemically remove any surface impurities such as iron-oxide (rust).
3: Electro plate the steel tube by giving it a negative charge, and dip it into a bath of positively charged chromium based liquid. The positively charged chrome particles bond with the negatively charged tube.
4: Rinse clean the tube after taking it from the bath.
5: Polish the surface to leave a smooth finish.
Complicated as this procedure sounds, it’s actually easier and cheaper than TiO2, TiN or DLC. So what is the benefit of these coatings over steel?
When you think about it practically, there are probably two main areas that we would like to try and improve:
1. Friction.
2. Robustness to damage.
At a stretch we could also include reducing weight, but even chrome coating is only 0.03mm (0.0012 inches) thick, so there isn’t a whole lot to be gained from changing to a different material. Let’s take a look at friction then.
Happily, there is a unit of measure we can use for this, it’s called the “coefficient of friction” and it represents the relationship between two forces. In this instance it’s the force needed 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 on concrete, and it takes a force of 5N to move it, the coefficient of friction is 0.51. There are even standardised tests for measuring friction, one of them is called the ASTM G99 and you can watch a video of it here if you are as boring an engineer as I am.
Basically, the material of a standard size is placed on a test bed and rotated against the bed and the forces are measured. The force needed to start an object moving from a static position is higher than the force needed 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 is moving, we can get a measurement of the dynamic friction.
Now, that we know how the test is done, 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.
Ehhh OK. It looks like DLC has some potential, but why would we use TiO2 or TiN when the friction values from chrome are lower?
Let’s look at the benefits of TiO2, TiN and DLC over Chrome.
TiO2 offers better corrosion resistance.
TiN forms a very hard surface and offers better wear resistance.
DLC offers extremely low friction and better wear resistance.
You might have noticed I haven’t mentioned anodizing or Kashima coating, and this is because this mainly applies to aluminium rather than steel.
When we think about aluminium, while it is a very light material compared to steel, it is also a very soft material. Ideally we would really like to improve it’s hardness so that the wear resistance is improved, and it would also be nice to have lower friction when it comes to suspension.
A way that the hardness of aluminium can be improved is through anodising. Anodising is quite similar to chrome plating by:
1. Cleaning: The aluminium is thoroughly cleaned to remove dirt, grease, or surface contaminants. This ensures uniform anodizing.
2. Etching: The aluminium is etched with an acidic or alkaline solution (like caustic soda) to remove surface imperfections.
3. Desmutting: A solution, often nitric acid, removes residues or oxides left after etching.
4. Anodising: The aluminium is placed into an electrolytic bath, commonly made of sulfuric acid, chromic acid, or phosphoric acid, depending on the application. The aluminium acts as the anode (positive electrode), and a cathode (negative electrode), typically made of lead or stainless steel, is placed in the bath. A DC electric current is passed through the solution. Oxygen ions from the electrolyte combine with the aluminium atoms on the surface to form aluminium oxide (Al₂O₃).
It is the oxide layer that improves the hardness of the aluminium, but by how much?
Thankfully, there is another standardised test we can use which is called the Rockwell Hardness Test. In this test, a diamond point tipped tool is pressed into the material, and the test machine can measure how far into the material the pointed tip has gone.
The results are:
Untreated Aluminimum: 15 to 30 HRB
Anodised Aluminium: 60 to 65 HRC
This means that the anodised aluminium is 4-5 harder than untreated aluminium, great for wear resistance!
But what about our old friend friction? Well the answer isn’t wonderful when dry, and reasonable when lubricated:
CoF’s Untreated Aluminium: Static: 0.3 – 0.45, Dynamic: 0.2-0.4, Lubricated: 0.1-0.2
Sealed Anodised Aluminium: Static: 0.4-0.7, Dynamic: 0.3-0.5, Lubricated: 0.1-0.25
The surface oxide layer created during anodising is actually quite rough.
And this is where Kashima coating comes in. Kashima coating is a proprietary anodising process by a company called Miyaki Co. Ltd. in Japan. I don’t know why it’s called Kashima, initially I thought that might be the name of the person who developed the process, but I can’t find any information to confirm that.
Anyway, the Kashima process was developed to give aluminium the improved hardness from anodising, while at the same time reducing the friction, and this is done by having an additional lubricating molecule as part of the anodising process. How much does it work? Well take a look at the picture below from the Kashima website.
Back to the point of the article, are special coatings for your suspension worth it?
As usual, it comes back to your available budget, and/or the level you ride or race at. If you are a professional race team looking for every 1% of performance improvement, then DLC coated inner tubes and Kashima coated outer tubes are the way to go. If you are an average weekend warrior then maybe your money would be better spent keeping your suspension serviced, or buying the corrects springs for your weight and ability.
I hope you enjoyed this article 😀
Kind Regards, Jens