Suspension fluids are a topic of interest for me for two reasons. First I like the fluids for their simplicity. They are straightforward to design, yet there is a lot of opportunity to fine tune and optimize a fluid for specific suspension designs. The second reason is that no matter how much I learn about suspensions I seem to be constantly finding new ideas and information that I had not discovered before. Since forks and shocks are something I am able to constantly learn about, they remain a constant source of intrigue to me.
Suspension fluids are basically a type of hydraulic fluid. Their purpose is to absorb and dampen the forces exerted on the suspension under pressure. They absorb some of the downward force in conjunction with the spring to lessen the shock on the down-stroke, but where these fluids really shine is on the up-stroke. This is because the fork or shock fluid’s main purpose is to dampen the spring rebound following compression. By slowing the up-stroke and absorbing some of that energy, the oil eliminates suspension the shock of the spring rebound and stabilizes the suspension for smoother operation.
So knowing what fork and shock oils do gives and idea of how they need to perform. They need to:
- Provide a consistent damping force to avoid suspension fade, or in other words, to keep fork dive and suspension rebound steady throughout:
- A wide temperature range
- The entire service life of the oil
- Reduce friction and wear between the moving parts
- Prevent corrosion
- Be compatible with seal materials
- DON’T FOAM!
Providing consistency in the damping force is accomplished in the short term through a high viscosity index. This measures the change in viscosity caused by temperature changes. Viscosity index can be an important characteristic for both fork oils and shock oils, but it is common for rear suspension oils in particular to have extremely high viscosity indexes. This is because rear shocks generate significantly more heat than front forks do; so the operating temperature ranges of a rear shock is very broad.
For long term damping consistency, things like stabilizing additives and shear stable base fluids keep the oil performing like it did fresh out of the bottle. These components reduce the oxidation rate, minimize the effects of contaminants and resist permanent changes to the oil viscosity.
Preventing friction and wear are important in these applications for a number of reasons. If wear starts to become excessive, it can result in increased static friction and failing parts. Reducing friction moderates heat generation and minimizes surface friction as a tuning variable; making suspension tuning easier as well.
Seal compatibility is fairly simple but very important. Blowing out seals is no fun and it can be avoided in most cases by selecting quality fork and shock fluids. There are exceptions, but seal compatibility is almost exclusively a function of the base fluids. Seals can be made from a variety of materials but a vast majority of suspension seals are made from nitrile rubber (Also known as buna-n, NBR, butadiene). In general, nitrile seals are affected by oils in the following ways:
An additive people commonly look for is known as a seal swell additive. While they can certainly be useful, the mere existence of one of these additives in a formula does not make it good. Swelling a seal without counteracting that swelling at least to some degree can result in a failed seal. This is because the seals are meant to fit into a precise space and if they swell too much, they will grow too large for that space and can blow out. The ideal situation is to have a neutrally balanced oil that does not shrink or swell the seals. Since none of the oils are completely neutral, balancing a formula means using both seal swelling and seal shrinking components in the correct proportions to precisely control the seal condition.
Foaming is a big concern in suspension fluids, but one that is easily dealt with. Foam prevention is crucial to maintain performance because forks and shocks are basically oil churners. This means the potential for foaming is high at all times. When a certain volume of the liquid turns into foam, that head of foam offers no damping performance. It also alters the oil height at the same time, and these two conditions prevent the fork or shock from functioning properly. Anti-foam or defoamer additives address the initial formation of bubbles and the ability for those bubbles to persist and build into a head of foam. They do this by weakening the surface tension of the liquid which pops the bubbles before they can build.
These properties are what go into fork and shock oils, but there are considerations to make regarding each one when actually selecting a suspension fluid for your bike.
Selecting Suspension Fluid –
A very common idea that I come across when speaking with people about suspension fluids is that viscosities are standardized. Fork and shock fluids are in fact completely unregulated. Neither the SAE nor anyone else has a standard viscosity specification for suspension fluids. Fork and shock oils that are labeled as SAE grades are simply mislabeled. The reason they are used is SAE grades for engine oils had been ingrained into people’s minds for so many years that lubricant manufacturers decided that relating suspension fluids to those weights would be the easiest way to convey their viscosity to customers. Since then it has merely become the convention used to indicate suspension oil viscosity despite its technical inaccuracy. Because of this we now have grades from 2.5W to 30W and everything in between and it is not likely to change any time soon.
Oils labeled as the same grade from different companies will not necessarily be equal in viscosity. Within the same brand however, the weights generally correlate to one another and you can use them as a broad guide to differentiate between grades. When looking at multiple brands though, I wouldn’t recommend going by the weights listed on the bottle.
This table illustrates my point that different companies measure their viscosity grades according to different standards. Many of them are fairly similar, but there are outlying products that may make a difference for users who are unaware of the discrepancies. The weight system is not perfect, but it is at least relatively useful when comparing different grades within the same brand, and in some cases comparisons between some of the different brands do turn out to be valid.
Besides just the raw viscosity data, there are other considerations to make when selecting a fork oil. Front forks as opposed to rear shocks do not produce a lot of heat, so the viscosity to focus on for fork oil is the kinematic viscosity at 40°C, and although a high viscosity index is good, it is not critical since they will not normally reach temperatures higher than 40ºC.
Another aspect to consider is anti-wear performance. Front forks experience more side load than shocks due to their angle, because there are two of them rather than a centralized shock absorber and because they absorb most of the initial impact of obstacles. Ideally, obstacles are hit dead center and the two front forks each absorb the impact equally. However, obstacles like rocks, logs and potholes are often hit at angles or they can easily twist the handlebars which unbalances the load on the front suspension. These things cause side loads on the forks. Damper rod forks, which are the most common, utilize two fork sleeves in which one slides inside of the other and these side loads cause wear on those surfaces. This characteristic of the fork application makes anti-wear protection significant in selecting fork oil.
In rear shocks, the kinematic viscosities at both 40°C and 100°C are important. This is due to the very wide operating temperature range mentioned earlier. Viscosity index which measures this change is an important characteristic to evaluate shock oils, but even that needs some consideration. In certain instances, viscosity index can be misleading when it comes to determining stability over a temperature range.
For instance, let’s say we have two oils:
- Oil A has viscosities of 5 cSt and 15 cSt at 100°C and 40°C
- Oil B has viscosities of 4.5 cSt and 14 cSt at 100°C and 40°C
- The viscosity indexes of the two oils are
- A = 310
- B = 276
Despite the viscosity index of Oil B being 34 points worse, I would argue that Oil B is the more stable of the two. The reason for this is when you look at the actual change in viscosity for both oils: Oil A’s change is 10 cSt and Oil B’s is 9.5 cSt. It may seem small, but that is 5% of the total differential.
Multi-application Criteria –
Friction reduction is important for both types of suspension fluids and the best way to evaluate a product for frictional characteristics is to obtain coefficient of friction comparisons of different fluids. The unfortunate thing about this property is that it can be measured in many different ways, and the only way to compare oils will be if they are tested under the exact same conditions. However, if you are able to locate reliable frictional characteristics, it can be very useful in selecting a product. Lower friction means less heat generation and simpler suspension tuning.
Oxidation stability is another test that can help to differentiate oils. However, like the coefficient of friction, the testing would need to be carried out identically and it may be difficult to find such a comparison for motorcycle suspension fluids. The results of that testing would give an indication of how long the oil can remain in service before oxidation of the oil becomes excessive.
Low temperature viscosity and pour point can be relevant in some applications, but for a large majority of riders, those values will not provide any useful input in the selection process. However, for those of you who do ride through the winter months, pay attention to the pour point of your suspension fluids.
The few properties I listed for each application are, in my opinion, the most important considerations to keep in mind when selecting suspension fluids. Viscosity data and wear test results will be the easiest information to find regarding these types of products. Those properties I mentioned earlier that were not mentioned again will typically be very comparable from one oil to another. Foam prevention, corrosion prevention and seal compatibility are rarely problems and most products will avoid them with little difficulty.