Getting the Most Out of Your Old Forks

I came across a good article published by MXA recently titled "How to fix your forks with fork oil height tricks". It reminded me of some of the science behind tuning your forks that I'd long forgotten about. Whilst the article mainly described how to add and remove oil for a cartridge type fork, it did have a good section on the effects of changing the oil height.

I wanted to write about how to get the most from your old-school damper-rod type forks (the one's most common on bikes up until the late '80's). Whilst the MXA article described a cartridge type fork, the principles of changing your fork oil height are the same, regardless of the type of fork. 

How do forks work?

Let's start with some basics. Regardless of whether your forks are upside-down or conventional, cartridge or a damper-rod type, the principle of how a fork works is the same. The spring does most of the work; when a spring compresses, the energy is absorbed by the spring. Air is also compressed inside the fork.

How does the oil come into it?

The thing about springs is, they behave according to a principle of physics called called conservation of energy. The spring will oscillate back and forth until the effects of gravity make it still again. The oil absorbs the energy and "dampens" the bouncing effect thus making for a smoother, more consistent spring action. Energy is effectively transferred to the oil in the form of heat.

What are damper-rod forks and how do they work?

Damping rod forks are the most basic form of motorcycle fork suspension; a spring with oil to damp the recoil. When the fork compresses, oil passes through tiny holes in the damper rod and when it extends the opposite happens; it gets sucked through the tiny holes. The oil does a lot of work trying to squeeze its way back and forward through these holes and this it what allows the oil to absorb the spring energy.

Image: Damper Rod

Most damping-rod forks are in fact so simple, that they don't have adjustable rebound or compression "clickers" that you see on most bikes today. These adjusters change the hole sizes that the oils moves through. The cartridge system effectively allows for two separate oil circuits so that the oil will pass through different sets of holes depending on whether the fork is in compression or rebound.

How to tune your forks

Now this is where it gets interesting; although the damper-rod type fork is fairly basic, there is actually a fair amount of adjustability in them. There is a series of variables that you can change to alter the characteristics of your fork's behaviour: 1) The stiffness of your springs, 2) the oil viscosity and 3) the oil height.

Step 1: Start with your spring. Pick a spring that is most suited to your weight, riding type and skill level. The correct spring will have the right amount of static sag (how much the bike compresses under it's own weight). For this, refer to our separate blog on how to set your "sag".

Step 2: Put new oil in your forks. At this point, it doesn't matter the oil viscosity or the height so long as we know what the starting point is. New oil is important too as old oil will not provide a reliable, measurable starting point. Now, we know you aren't a pro mechanic and you probably don't have money to burn trying different oil weights in your forks, so we'll give you a quick short-cut rule-of-thumb that we use on new bikes. It's fairly simple; If you have installed a heavier spring than stock, then use oil that is 1 graduation higher in weight than stock. Why? Because the heavier spring will have more kinetic energy when it rebounds and will need more damping. But be careful, it is easy to go too far with rebound which is why we recommend only 1 graduation at a time. For example, if the manufacturer recommends 5wt oil, use7.5wt oil for a heavier spring and 2.5wt oil for lighter spring. Most bikes from the '80's are under-sprung so you will most likely by going stiffer. Plus, you aren't getting any lighter!

Step 3: Check your rebound damping and if necessary, change out the oil. You can do this by altering the viscosity of the oil. This is what pro mechanics do - over and over and over - however you are most likely to have nailed the rebound setting in the previous step.

How do you know if your rebound is set correctly?

This part is a little subjective and really comes down to the opinion of the rider. When you bounce up and down on the forks, a correctly set rebound will have the forks return to neutral with minimal oscillation (bounce). Remember, the measure of the fork's rebound is the oil's ability to absorb the spring energy. Bounce will occur if the spring's energy was not completely dissipated when it rebounds.

Here are some other signs of poorly set rebound:

Rebound Setting	Character
Too much rebound (not enough damping)	Bike feels dull and lifeless and the front wants to "knife" in corners or is twitchy and hard to balance. You will know this if it happens. - the bike will want to stick it's handlebars into your stomach.
Not enough rebound (too much damping)	Bike chatters in corners, or under-steers, feels "bouncy" and lively. Rider's will have less confidence in this condition.

 

Step 4: Set the compression damping. The compression damping was already partially adjusted during the rebound setting stage (the oil viscosity effects compression too) however there is still an adjustment you can make that can alter only the compression damping without changing the rebound damping.

Rebound damping is a function of the oil viscosity and the size of the holes in the damper rod that the oil is sucked back through when the forks rebounds. Compression however is set by four variables; in addition to oil viscosity and the damper rod hole size, compression is also affected by the spring rate and the oil volume.

Oil volume you say? Yup. The volume of the oil in the forks does not effect the rebound but it does in compression. Why? Because adding oil to your forks reduces the air volume. Since air is compressible, a smaller airspace is harder to compress than a larger airspace, which results in a stiffer fork. Since the compression of the airspace is gradual, lessening the airspace will be felt by the rider from around the middle of the fork’s stroke to the point of bottoming. The oil height doesn't effect rebound because the rebound damping is solely achieved from the oil squeezing through the holes in the damper rod.

How the Oil Works Inside Your Forks 

A spring is linear in its function - it has the same resistance from the start to the end of the stroke. Adding oil however, changes this relationship to one that is parabolic rather than linear.

Oil viscosity is constant in both the rebound and compression condition whereas changing the oil height can change the curve as shown below:

Notice how changing the viscosity doesn't change the behaviour of the fork but rather its starting point. Increasing the oil height steepens the curve in the latter 2/3rds of the stroke. In essence, adding oil to your forks makes them stiffer from the 1/3rd point on. The obvious corollary is that when you take oil out of your forks, you make them softer from the 1/3rd point on. If a fork has 12 inches of travel, then adding or subtracting oil therefore has a negligible effect on the first four inches of travel.

Now why would I want my forks to be stiffer in the latter part of the stroke? Simple; to soak up the big bumps and landings off large jumps without compromising the fork's performance on the small stuff! Cool huh?

How to Set Your Compression Damping

So, to set your compression correctly, simply add or remove 10cc of oil at a time to your stock oil height. I'll discuss how to do this below but first you need to understand how much oil to add or take out; put a cable tie on your inner fork tube then go for a ride making sure to take some big bumps and a landing of a big jump. Check the position of the cable tie. If it is all the way up to the top of the fork travel, then your fork is bottoming and you need to add more oil. Continue to do this until the fork stops hitting bottom. Conversely, if the cable tie is not reaching bottom yet, take some oil out so that the fork may travel further in its stroke. More stroke means more usability and a more versatile fork. It will work better on large bumps and also on the small stuff. If your fork is set too stiff, your range of usable travel will diminish and you will have an overall harsh ride.

How to Add and Remove Oil From Your Forks

There are a number of ways to do this, with or without removing the fork from the bike. Adding oil is easy and can be done with the forks on the bike. Simply remove the cap, the air bleed screw or the Schrader valve and squirt oil in using a graduated syringe.

To remove oil, first remove the fork caps, then use a syringe and tube to extract the correct amount of oil. 

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So, it all starts with the spring. The correct spring for your weight, riding style and skill level is key. You can further tune your fork's potential by playing with the oil viscosity and height. The viscosity can be changed to improve your fork's rebound characteristics and oil height can be changed to improve your fork's compression resistance in the latter part of the stroke. Tuning your forks however is one small part of setting a bike up to handle good. See our other blog on how to set the "sag" to get the full picture. All part of the joy in owning a dirt bike! Have fun!