Discussion for this post is what happens inside the front fork tubes. Knowing this has helped me understand what I want to do to the suspension on my '85 Limited Edition.
To understand what is happening, I had to look at and understand the fork oil flow between the three oil chambers. Let's look at the internals of the front forks:
You will notice there are three fork oil chambers. Fork oil chamber A is the fork oil reservoir for compression and rebound damping. For compression, fork oil chamber B allows the front forks to absorb road irregularities and such, nothing too drastic, and fork oil chamber C allows the front forks to absorb large size bumps and holes.
Regarding the front fork suspension, it is the fork slider that is attached to the wheel that is moving, not the upper fork tube. I mention this because the two part front fork, upper fork tube and lower fork slider operates much the same as a person skiing. Quiet upper body, quiet upper fork tube. The upper fork tube goes down the road not moving and level. The fork slider on the other hand is very active, much like your legs when skiing. Moving up and down absorbing the terrain irregularities, and at a speed that keeps the skis/wheel in contact with the terrain/road at all times.
A different analogy for the above is that fork oil chamber B is for riding down a highway, or a well maintained road. Small irregularities are absorbed because oil is flowing in and out of fork oil chamber B. This fork oil chamber allows for road irregularities such as tar snake and paving bumps and the likes. Not very exciting action going on.
Fork oil chamber C is for the nasty road issues such as speed bumps, lots of compression needed to absorb these nicely, or the potholes that our cities continue to neglect. Need a lot of front fork action to absorb these, hence a lot of fork oil movement.
What happens? When the front fork spring compresses for whatever reason, the oil is forced out of the lower fork oil cavity "A" into fork oil chamber "B" that is between the upper fork tube and the lower damping rod. The speed at which this chamber fills or empties is dependent on oil flow orifice size and oil viscosity. There is very little pressure differential between these two fork oil chambers, so very little oil flows between these two chambers, but enough to ensure an enjoyable ride.
Since the fork oil chamber "B" is only so big, fork oil chambers "A" and "B" can quickly become a combined fork oil chamber with the same pressure being exerted on both chambers when the size of the road irregularities increase. To control further upward movement of the fork slider, pressure increases in fork oil chamber "A" and fork oil is forced into the centre of the damping rod and up into fork oil chamber "C" through holes in the damping rod that are sized to give a specific fork oil flow rate based on the size of the holes and the oil viscosity. The design of the damping rod and the fork oil viscosity resists the compression of the fork spring. Change the oil viscosity, you change the compression stroke - fast/slow. Change the size and number of holes, you change the compression stroke - fast/slow.
Front fork rebound is much simpler to envisage. This is the return of the fork spring to the set ride height by return of the fork oil to the reservoir, fork oil chamber "A". How fast or slow this happens is, again, dependent on the size of the orifices, holes in the damping rod, fork oil viscosity, and the fork spring rate. The spring rate assists in oil flow because as the fork spring returns to the set ride height, a vacuum is developed in fork oil chamber "A". Fork oil chamber "A" increases in size, and the volume difference between the compressed volume and the ride height volume needs to be replenished, otherwise the front fork would collapse. Makes sense to me.
There is a lot going on regarding the front suspension with the simplistic, but quite effective front fork design. If this design is not up to your standard(s), what can be done to improve the front suspension?
The first item on my list would be to renew the front fork springs and fork components. I recommend this because an investment of $100.00 for a set of new OEM equivalent springs, go for the higher spring rates of 1.1/1.2 kg/mm - same cost, with the recommended manufacturers preload setting and oil viscosity should make immediately improve the ride quality and performance.
I would not get too involved in determining whether the front fork springs should be a straight rate, or progressive rate spring. Most of us will not be able to feel the difference between the two. Let your budget and the higher spring rates be the deciding factors.
If the spring is a straight rate spring and the spring rate is 10 kg/mm for example, this spring rate applies throughout the travel of the spring. 10 kgs to compress the spring 1 mm.
If the spring is a progressive rate spring and the spring rate is 10 kg/mm, the spring rate as I understand it is generally the most firm rate of the spring. My understanding of a progressive rate spring is that initial compression is soft, then progresses to medium soft, then medium, them medium hard, then hard, and returns the same way. Something like buying a pillow.
This should result in an immediate new and better front suspension feel.
Have to eventually get to this point, determine what the front suspension sag is. In or out of spec using the 25 to 30 percent of suspension travel rule.
If the front suspension sag is within the 25 to 30 percent rule, good start. If the suspension sag is more than 30 percent, you can try a different preload spacer, increasing the length of preload spacer in 2-3 mm increments. I would recommend only doing this twice, after which you should look into replacing the fork spring size. Fork spring size is generally rated in lbs, noting that spring rate does not affect suspension sag.
If the front suspension sag is less than 25 percent, shortening the preload spacer can be done. This should be done in small increments as well, but you have more flexibility in that you want the preload set to the minimum necessary to achieve the required suspension sag. In this case you can do more preload spacer changes to meet the requirement.
You could try installing an aftermarket preload fork cap. An inexpensive option that allows you to adjust the preload without taking the front forks apart.
Last on the list is changing the oil viscosity. This is primarily for rebound damping. The thicker the fork oil, the slower it flows, the slower the rebound damping, the slower the fork spring returns to the set ride height, the more firm the feel of the front suspension. The thinner the fork oil, the faster the the fork oil flows, the faster the rebound damping, the faster the fork spring returns to the set ride height, the more plush, softer the ride.
All of the above is just my understanding and opinion. Suspension is not a black art, but takes a while to understand and for you to come up with a game plan.