Fork Oil...What Is It And Why Change It?

Ottawa, Ontario, Canada

Fork Oil Change?

I was telling a friend that one of my plans for my week of spring vacation was to spend a day changing the fork oil in my BMW and Honda CBR Endurance racer. The puzzled look on his face resulted in a basic description of how a front fork works and what fork oil actually does. After my description, Sean suggested this would be a topic to write about for EatSleepRIDE. So here it is.

This piece applies to the conventional front fork of a motorcycle. It doesn't apply to a USD or upside down style of fork (slider is at the bottom) or the BMW Tele-lever fork.

Lift up the front end so it can’t fall. We used a car engine hoist as there are a lot of these around to be borrowed for a day or two

Secure the rear wheel, we used a front wheel chock sold at most bike shops and Princess Auto

The Function of the Fork

Remember, I am trying to keep this simple. The front fork is the two long tubular things that hold the front wheel on the bike. That is but one function of the fork. They also move up and down to soak up bumps in the road surface like a car shock absorber does. The spring allows the fork leg to compress over a bump and then "spring back ".

This makes the ride better and improves traction. Each of the front fork tubes for most motorcycles contain a spring and oil. There are other parts but let's talk about the springs and the oil.

In the early 1950s, fork legs were just a spring inside a tube. When compressed, the spring would have the front end of the bike bouncing around after a good sized bump or pot hole. Progress was adding a system to dampen or slow down this up and down springing bounce motion. The system was to add fork oil. Most common is a tube inside each fork leg with holes and chambers that modulate the movement of the oil. Your owner’s manual or shop manual will have information on how much oil is in each leg and what grade or weight of oil should be used. Let’s focus on oil now.

The OEM manufacturer specifies the amount and grade of oil. They base it on an 'average' owner. So do I change the grade or weight of oil or the amount of oil in each leg? There is lots of advice on line about suggested changes. Maybe what you use the bike for or your weight or if you carry a passenger would make you want to change from stock settings. If you plan to make a change, change only one thing (oil grade or oil amount).

Front wheel one fork leg removed, ready for the forks oil change

Tips for Changing the Fork Oil

How do I do it? Unfortunately, I can't do a full step by step explanation as it would be too long. However, I will cover some important points. The photos attached show removing the fork legs and draining them by holding them upside down. This is the more complicated option and should be done only in a decently equipped home shop/garage.

Here's a typical fork leg (tastes like chicken!)

Instead of three pages or so of instructions, some quick notes:

Don't do this without a safe and secure way to hold the bike as you remove and re-install the fork legs. We used a car engine hoist and a stand to hold the rear wheel from moving.
Fork oil is a specific kind of product. It isn't motor oil. This oil is designed to do just one function. Fork oil comes in a variety of 'weight' or thickness grades. It is sold as fork oil in all motorcycle shops and even Canadian Tire.
Most modern forks do not have drain screws in the bottom of the fork. Without drains, you either suck the old oil out with a syringe or remove the fork legs and drain by holding them upside down (gravity).
Start by loosening all the bolts while there is still weight on the front end. Just start them! Lift the bike’s front end and remove the front wheel.
It is a good time to check the brake pads after you have removed the front wheel.
The fork tube caps can be under pressure, beware the cap flying off!
Take notes on all the parts that come out of each fork leg and the order they are in. Are the spring coils the same or tighter coiled at one end? Ensure the springs go back in the same way they came out.
The amount of fork oil must be measured precisely!! The amount should be the same in each fork leg. I use a kitchen measuring cup. The other measurement uses a vernier caliper to check how far the fluid is from the top of the tube. A good metal ruler will also work.
Do one side at a time so you don't miss anything.
Most forks will have one or two very thin washers on top of the spring. They are easy to miss and lose. Watch for them.
Most bikes have a very specific measurement of how much of the fork tube extends above the top triple clamp. Measure it so you can replicate it when you put the forks back into the triple clamps. This measurement is most often in millimetres.
The last bolts to be tightened are the one or two at the very bottom of the fork leg on each side. Leave them to last. Bounce the front end up and down before finally tightening these bolts to factory specs.