As a motorcycle rider since the early 70s, what I have experienced with my various rides is that motorcycles come with modern Ceriani-designed front damping rod-type hydraulic forks that actually need very little maintenance when it comes to changing or replacing fork oils.
So, in this guide on motorcycle fork oil, let’s understand:
- What is fork oil and its importance in motorcycles
- When to change fork oil
- How to select fork oil for the motorcycle
- Do’s and don’ts of fork oil and other related questions
Without further ado, let’s dive right in.
What is fork oil?
The fork oil is the oil that goes in the front forks of a motorcycle and acts as the shock absorber.
Fork oil is not the same as engine oil.
Engine oil is mainly used for lubricating and cooling down the engine components. Fork oil, on the other hand, is used as a shock absorber.
Sure, fork oil acts as a lubricant, but the main purpose of fork oil is to absorb the shock.
As for how they do it and what is their importance, let’s discuss this in the next section.
Importance of fork oil in a motorcycle
Front fork oil in a motorcycle acts as a dampener to the vibrations coming from the front wheel and provides a smooth riding experience to the motorcycle rider.
Fork oils are different in composition from engine or brake oils and use chemicals with anti-friction properties. Several friction modifiers are used to modify lubrication qualities as well.
And hence, fork oil should be exclusively used in motorcycle forks.
The most important component of the oil is the viscosity stabilizer that provides long-lasting and consistent performance and response, even as the suspension heats up during motorcycle use.
Unlike engine oils that tend to foam when heated, fork oils are designed to remain cool and eliminate foaming and air entrainment properties.
The fork oil is necessary for all motorcycles irrespective of what they have been designed to do — be it on-road commuters, sports bikes, off-road, adventure, trails, or motocross.
The fork oil ensures top ride quality by absorbing shocks, and road imperfections before they reach the handlebar and in a way also contributes to the stability of the motorcycle.
The fork oil works directly with the valves to provide consistent damping capabilities to the fork. It is vital for the fork oil to consistently control or check the energy generated by springs and absorb the unwanted heat. When the fork oil is new, it plays its role perfectly.
Since the fork movements are dynamic when the motorcycle is on the move, obviously there’s degradation over a period of time.
And as road conditions vary, the duration related to the efficiency of the fork oil will also get impacted. How soon or late should be evident to the rider who may begin to feel the roughness of the front suspension when the motorcycle is ridden.
When to change the fork oil in the motorcycle
Rule of the thumb is that if you consider changing fork oil every 10,000 miles, then it’s an excellent decision.
If roads are good in your area, you can extend the duration to 15,000 miles which will still be considered good for the machine.
To know more, here is a detailed post on the fork oil change.
By the time your motorcycle has done 20,000 miles, fork oil has lost most of its performance properties. At this juncture, there’s a good chance that the oil seals will perish and cause damage to the barrels.
Fork oil seal leaks cause friction marks on the exposed hard chromed barrel that can’t be repaired.
Do remember, you don’t have to wait for the oil seal to perish to change the fork oil. The quantity is usually less than 200cc per shock absorber and is affordable.
How to select the fork oil
Depending on what type of motorcycle you are riding, and how you are riding it, there are a few options to choose from for replacement fork oil.
With several product options available that usually relate to viscosity — the first confusion that pops up is whether should one opt for a thicker or a lighter fork oil.
This may be a trick question, but it’s no rocket science if you can understand some of the fundamental principles.
Let’s try and understand why fork oil viscosity is so important in the performance of front suspension in a motorcycle.
Viscosity is a measure of resistance to flow. It can also be described as the internal friction of a moving fluid.
The higher the viscosity of the oil, the higher the resistance to its motion due to its molecular breakup providing a lot of internal friction. The oil with high viscosity won’t flow easily. Similarly, low viscosity in the oil means low resistance and the oil will flow easily.
So, choosing the fork oil boils down to what level of viscosity you want in the fork oil.
But, there is a problem.
Fork oil grades are NOT reliable
There is a problem when it comes to viscosity grading in fork oil. Unlike engine oil grades, fork oil grades are not uniform across different manufacturers.
The SAE grading which works great for engine oil does not work the best for fork oil.
The oil grading and weight specifications vary from manufacturer to manufacturer.
A 7W fork oil from one manufacturer can actually be lighter weighted than a 5W fork oil from another manufacturer.
For engine oil, the viscosity grading and weight is uniform and is measured in centistokes at 100 degree Celsius. This makes sense for engine oil since the engine components get really hot and the oil can reach such high temperatures.
But, for fork oil, it doesn’t make sense. The fork oil rarely if ever reaches such high temperatures and usually operates at moderate (but still higher than the room) temperatures.
So, what’s the solution then? What measure to check for viscosity?
Check for viscosity at 40 degree Celsius
Along with the viscosity grades and viscosity level at 100 degrees Celsius, look for the kinematic viscosity at 40 degrees Celsius measured in centistokes (cSt).
The viscosity at 40 degrees Celsius should be the measure for differentiating different fork oils.
Depending upon your need as a rider and that of the motorcycle model, you can pick and choose the fork oil based on its viscosity levels.
As a thumb rule, go for fork oil with low viscosity for off-road motorcycles and fork oil with high viscosity for on-road motorcycles.
Here is a chart comparing kinematic viscosity at 40 degrees Celsius for different brands and weights.
Kinematic viscosity at 40C (centiStokes [cSt])
|Brand/ Wt||2.5 or 3||5||10||15||20|
To know more, here is a fork oil viscosity chart for different fork oils.
Other measures to select fork oil
If you explore the market and online resources you will discover in general, there are two types of replacement fork oils — light and medium viscosity.
No need to get worried or confused about deciding on options. While, both types offer excellent thermal protection and damping, but do offer slightly different performance characteristics.
For the uninitiated, light viscosity fork oil is designed for more rapid compression and frequently experiences a rebound of the front fork suspension. So, it’s perfect for off-road riders who jump motorcycles and need safe control upon landing.
In addition, off-road riders need a quick response to every bump, log, and jump that’s encountered. Here, the mileage is not a factor, rather it’s the duration and its usage.
On the other hand, medium viscosity fork oil is thicker and better suited to riding that requires slower compression and rebound. This quality is applicable for cruisers, street usage, mild cross-country rides, or track days on a circuit.
In the market, fork oils are available from the manufacturer who sell them with the correct quantity and viscosity for their branded machines as OEM supplies.
Independent oil companies also offer fork oils for 2-wheelers like Bel-ray, Shell, Motul, Waxpol, Deemol, and Maxima among others.
How much oil to fill in a motorcycle fork?
How much fork oil is to be filled in the forks can be checked from your motorcycle owner’s manual.
That’s the best source since the front fork design varies from model and model and the amount of fork oil to be filled varies as well. There is no one measure fits all here.
Another way to go about it is to fill the new fork oil with as much or slightly less than the amount of old oil drained.
If you are going by this approach then it is better to measure the amount of old oil drained with a measuring cup. Because whatever you do, do not put excessive new fork oil in the forks.
Yes, do not overfill.
Overfilling the fork oil will cause problems. Just like overfilling engine oil is not good, so is fork oil overfilling.
Too much fork oil in the front forks, the oil will start leaking from the oil seals and can even damage the seals.
In addition, too much oil can also lead to fork oil rather than dampening the road vibrations can actually cause the handlebar to rise and the tire to lose contact.
So always ensure that you or the dealer is not overfilling the fork oil.
Fork oil for advanced suspension systems
In high-end markets, there are more advanced fork systems also known as active or semi-active suspension systems in motorcycles that can be customized.
These systems allow for the damping and compression ratios to be tailored according to the riders’ specific requirements.
So, are there any options for fork oil here or does one have to stick to just one formulation for forks in advanced motorcycles that have semi-active or active systems?
Motorcycles like the Ducati Multistrada come with Ducati Skyhook Suspension (DSS) and the BMW HP4 with Dynamic Damping Control (DDC). Actually, these represent the first production motorcycles with advanced semi-active suspension systems.
Active suspension technology gets its origins from the automotive world and its application in high-end 2-wheelers is a recent phenomenon.
Due to its superior performance qualities, the idea is catching up worldwide with several innovative motorcycle brands.
These motorcycles use magnetorheological damper systems which change their performance qualities when current is passed through them. These highly specialized oils are best sourced from the original equipment manufacturer.
For the professional user, it would still be wise and practical to continue with the existing brand. For the technical minded it may make sense to compare shock fluid characteristics at 40ºC of various brands.
End of the day, whatever brand you opt for, your motorcycle should be up for your individual riding style. Factor in your body weight and any extra accessories you may have on the motorcycle.
Do’s and Dont’s of fork oil
- Always avoid homemade or locally formulated fork oils. I have seen situations where mechanics have mixed engine oil with kerosene or mineral oil and fleeced gullible customers.
- Do not mix fork oils of different weights or viscosity levels. And do not mix fork oils of different brands either. Even if the weights are the same, different brands can have different viscosity levels.
- Do not overfill the front forks of the motorcycle with fork oil. Too much oil can be counterproductive and can cause the tire to lose contact with the road, and can possibly damage the fork oil seals.
- Fork oil is different from engine oil, brake oil, and even shock oil (albeit slightly). So do not use them interchangeably. Each of them has different viscosity levels, different compositions, and different additives.
Is there a difference between fork oil and shock oil?
Although few use them interchangeably, fork oil and shock oil are NOT the same.
Fork oil usually has lower viscosity levels when compared to shock oil. Shock oil has to deal with far higher temperatures than fork oil and hence tend to have higher viscosity levels.
In addition, shock oil typically has far more additives and detergents than that of fork oil.
Lastly, fork oil is used in motorcycle front forks whereas shock oil is used in shock absorbers.
In short, while there are lots of similarities between fork oil and shock oil, they are different and should not be used interchangeably.
Can you mix different weight fork oil?
Mixing different weight fork oil or a different brand of oil is NOT advisable.
Fork oil with different weights will have different viscosity levels. So, mixing them together does not work great for the forks.
Similarly, different brand oils even with the same weights are not recommended.
Although the weight may be the same, different brands will have different kinds of additives with different proportions. Also, as discussed earlier, for the same weight different brands can have different viscosity levels.
So, it is always recommended to not mix different weight oils and different brand oils.
Only mix the oil when both the brand and the weight are the same.