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Three Things About Suspension Oil

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Posted By: Mason Copeland

Nov 4, 2022

2-Minute Expert is an NSMB series that distills technology and other concepts into short, easily digestible articles.


We can't see what's in our forks and shocks, which makes their inner workings a beautiful mystery of shims, valves, and pistons that help fuel the fun on two wheels. What's also lurking inside, though, are petroleum-based oils that are harmful to mechanics when not handled carefully, and harmful to the environment if not disposed of properly. We all know that taking care of our suspension is essential for a smooth-running bike that performs well, but in doing so, we should try to make choices about the products we use that are better for ourselves and the planet.

Let's cover a few things about suspension oil that - if you're like me - you probably didn't know before now.

Suspension oil weights are not standardized

One brand's 5wt is not the same as another brand's 5wt, and anyway 'wt' or 'weight' isn't even an accurate way of measuring the performance of a suspension oil. If you take nothing away from the tech stuff below, it should be that if you're trying to pick what suspension oil to use, don't go by ratings that use 'wt' - it's not going to be that useful at least as far as comparisons across brands. The reasons behind this are beyond the scope of this article, but the basic gist is that the SAE weight system is inconsistent due to the way the SSU/VI systems are measured.

Let's review a few terms before we go further.

Viscosity is the resistance of fluid to flow, measured in square centimeters per second, aka centistokes (cSt). The Society of Automotive Engineering (SAE) uses cSt measurements and converts it to a weight value like 5wt, 10wt, which may sound familiar because that's what you're used to seeing on a container of motor oil. However on motor oil which is listed like 5w30, '5w' stands for '5 winter' so this is where some of the confusion arises.

In addition, fork oil weights aren't measured using the SAE scale. Instead, they rely on the Saybolt Seconds Universal (SSU), a standard used for hydraulic applications. The SSU uses a similar measurement standard to the one used to determine a cSt value, but oil grades are based on a more sensitive scale of viscosity calibration.

How does the Saybolt Scale (SSU) Work?

The thickness of hydraulic and fork oils are listed as the Saybolt Seconds Universal at 100 degrees C/viscosity index. For example, if the numbers listed on the bottle read as 75/150, that means the oil's SSU at 100º C is 75. The flow of the oil is then measured at 40º C. The second number (150) denotes the difference in flow between the two temps, aka the viscosity index (VI).

The Viscosity Index (VI) measures the stability of fork oil between 40 and 100 degrees Celsius. The higher the VI rating, the more stable the weight of the oil remains when it gets hot - in other words a high VI is important for your fork to perform consistently as heat builds up. Again, though, fork/suspension oil weight is also not measured using the SSU scale. Yes, this is confusing, and that's the reason why WPL would love to see centistokes used across brands, because cSt is a standardized measurement.

That's a lot of indexes and scales, but the important takeaway here is that because of the variables of SSU/VI vs. SAE scales, using a 'wt' scale doesn't give an accurate measurement of how a fork or hydraulic oil will behave. Effectively, one brand's 7wt oil could be lighter than another brand's 5wt, and any two different 5wt oils are not alike. This means that if you - or your shop mechanics - use a different brand of oil to service your fork, or a higher or lower viscosity, it could lead to different results.

Why should you care?

Because with standardized suspension oil measurements, it'll be easier to choose what works well for your riding style and conditions, and make adjustments according to changing needs. Without it, you'll be guessing.

WPL suspension oil

Suspension oil has three main roles

The three roles of suspension oil are to provide damping, dissipate heat, and provide thermal and chemical stability to prevent degradation over time to the oil itself or the components it comes in contact with.

Regarding the prevention of degradation, a key term to be familiar with is lubricity, which indicates a substance's ability to reduce friction. A suspension oil's lubricity is therefore important from the perspective of preventing premature wear and providing a low coefficient of friction. A substance's lubricity can't be directly measured, rather it is quantified within a specific system and environment based on how much wear a surface undergoes in a given amount of time. Suffice to say that a lubricious (ie. having a low co-efficient of friction) suspension oil is critical to minimize wear, and will also create a molecular protective boundary between parts (we also discussed these boundaries and hydrodynamic states in the Two Minute Expert article about lubes).

WPL suspension oil

Suspension oils need to be optimized for seal & rubber compatibility

Typical suspension oils are petroleum products. Like with most chain lubes, the reasons for this vary from low production cost (and high margins), good performance across different conditions, and availability of materials to work with - but all this comes at the expense of harm done to the environment during the extraction and refinement processes, and disposal challenges.

It's important to make sure a suspension oil fulfills its three main roles (above) but also that it doesn't break down the nitrile rubber seals and components inside your suspension. Some petroleum-based suspension oils aren't nitrile compatible, meaning that over time they will contribute to premature wear of seals and other components, introducing a whole other set of problems.

WPL's suspension oils replace petroleum with natural materials (extracted from a seed) which rely on single digit percentages of additives, and formulated so that the main ingredient does most of the work. There are mineral-based suspension oils, but they generally have higher percentages of additives (up to 20%) and it's important to remember that despite the fact that 'mineral oil' sounds benign, it doesn't mean they're not toxic or harmful to the environment.

It isn't just compromised seals that impact the longevity of your suspension oil. When it changes colour, what you're seeing is oxidation of the additives in the oil - again, a lower percentage of additives can be helpful here. Some of WPL's testing shows competitors' petroleum-based suspension oils evaporated and degraded faster than the all-natural formula they've developed, in addition to showing more oxidation and deposits.

A Final Note on Handling and Disposing of Suspension Oils

By now this may be obvious, but mineral or synthetic oils with toxic additives must be disposed of in the correct way, otherwise they can end up in our ecosystems. Always check with your shop if you're unsure of how to safely dispose of used oils. And of course, regular users of those products should be wearing gloves and avoid inhaling the fumes because they are carcinogenic. 

Further reading & sources

Photos Deniz Merdano