The Block-on-Cylinder Test is a mechanical evaluation method used to assess the wear resistance and frictional properties of materials, particularly lubricants and surface coatings.

In this test, a solid block, usually made of steel, is pressed against a rotating cylinder typically composed of an even harder material, creating a sliding or rolling motion between the two surfaces. This simulates frictional conditions found in many mechanical systems.

The wear and friction characteristics of a lubricant are then evaluated by measuring parameters such as friction coefficient, wear groove size, wear rate, and other relevant metrics. For a motorbike chain lubricant evaluation, we will use the following metrics: 

Engine’s Current Draw [A]: This metric is directly proportional to the friction force between the block and the cylinder in the test, as the engine draws more current to overcome the friction-related load. Therefore, the amperage indicates the amount of friction in the system. 

Seizure Groove Diameters [mm] (Length x Width x Depth): Typically, wear groove grows with increasing applied load, as more material is removed from the surface due to friction. 

Seizure Load [kg]: Is the maximum applied load at which rotation stops due to excessive heat generation, surface deformation, and ultimately, the welding of the two surfaces. 

Seizure Groove Area [mm²]: This is the area of the groove after the test stops. It is calculated as the product of its length, width, and, in our case, the ellipse area formula. It is used for seizure bearing stress calculation. 

Seizure Bearing Stress [kg/mm²]: This parameter represents the relationship between the applied seizure load (Ps) and the seizure groove area (As), which is the contact area between the static block and rotating cylinder at the seizure point. It is calculated as σs = Ps/As. 

Depending on the test setup, the applied load will change much slower, if at all, compared to the groove size and contact area. Essentially, the same load exerted on twice as large contact area halves the bearing stress. Therefore, this metric provides a single measure that considers at least two factors.

Furthermore, the bearing stress is a static measure, while the test involves motion. Thus, it is evident that the higher the seizure bearing stress, the better a lubricant separates two parts and mitigates friction.

Finally, seizure bearing stress can be related to the yield stress and plastic deformation of a chain, which can be calculated from its tensile strength and parts dimensions. This provides a better reference to our real-world needs when it comes to a lubricant choice.

Now, to refer the data to real-world conditions and interpret them somewhat useful, let’s break it down and compare with two most viable factors, when it comes to motorbike chains maintenance – replacement time and tensile strength.

The most common recommended stretch limit for sealed motorbike chains is 1%, and 2% for non-sealed chains over the length of a chain. If we applied these criteria to its link pitch, we could calculate how much wear, on average, is allowed in a single chain link, and compare it with the groove depths from the previous table. Let’s take the 1% stretch as stricter reference. 

The green shaded cells in table 1 contain grooves with a depth still below the smallest wear allowance shown in table 2 below - Meaning the chain is still good for service. 

Yellow shaded cells contain groove depths within the limit range in table 2, while the red shaded cells contain groove depths that are outside the allowable limits, meaning that such a chain is only due for scrap! 

Table 2

So, the depth of the groove serves as a somehow reliable indicator of how effective a particular motorcycle chain lubricant is. However, it's also crucial to determine at what point in the chain's operational strength limit it was created, as a lubricant should protect a chain from stretching within a wide range of loads. 

This is where a chain's tensile strength comes handy, although not in the standard force-measure, but rather in a measure of bearing stress because not only it informs us about the engine power and size a chain can support, but also reveals an intriguing fact: lighter chains, made of smaller parts, are often as strong, or even stronger than heavy chains. How is that? 

Despite the differences in tensile strength among chain designs measured in Newtons, Pounds, or Kilograms, they all share a yield stress range between 150 and 200 [kg/mm²]. This is because most steel alloys have their plastic deformation limits within this range. Subjecting a link's plate-pin connection of any given motorbike chain to such stress would result in deformation beyond its ability to return to the original shape, making the chain unserviceable. 

That's why chains are engineered with safety margins, ensuring there's a good buffer against possible failures, boosting confidence and safety. The industry standard typically requires chains to have a tensile strength at least four times greater than the maximum load they're expected to handle. 

In short, while heavy and massive chains can withstand larger tensile forces, they achieve this strength with larger parts, whereas lighter chains achieve it with stronger materials. To show this, we have gathered some manufacturer’s data on popular chain types and calculated their bearing stress rating: 

Table 3

The last column in table 3 contains maximum expected bearing stress exerted on a single pin-plate connection for a given chain design, assuming the safety factor of 4. These values can be now compared with the Seizure Bearing Stress values obtained in our test (table 1) for any tested lubricant, what makes it a much more useful measure for assessing if a lubricant meets our needs. 

In short, a good motorbike chain lubricant should at least be able to protect a chain within loads forced by your riding style. Hi-end lubricant, on the other hand, should protect the chain up to the maximum load it’s expected to handle.