This article will be a guide to the mysterious mechanism that is the motorcycle telescopic fork. We are going to cover its characteristic and how each affects the overall handling of the bike. Then we will illustrate how not to, as well as how to properly lower the front end of an old motorcycle.
First lets begin by going over the most common characteristics of the front suspension.
Rake is the angle that the the head stock axis makes with the vertical line. This angle affects how leaned over the entire fork is, and in tern modulates the wheel base of the motorcycle. Generally speaking a higher rake angle like the ones found on custom choppers provides a high speed stability benefit perfect for cruising. But that comes at the cost of slow speed maneuverability, making the motorcycle quite difficult to steer. Now on the other end of the spectrum we have steeper rake angles found on most super sport motorcycles. These short angles provide for shorter wheel bases, higher maneuverability and low speeds, higher responsiveness at higher speeds. Of course high speed stability is lost as well.
The fork length is considered the uncompressed length of the fork as measured from the top of the top yoke of the tripple tree down to the mounting location of the front axle. On some bikes you would find the axle being mounted on the very end of the fork. On others its actually mounted on the top/bottom side of the fork. This offset has additional effects on the trail of the bike. The fork length can impact the rake of the bike, and therefore its handling, and is usually used by manufacturers to achieve a desired balance on the motorcycle. The other characteristic that is affected by the fork length would be the fork travel. The shorter the fork the shorter the maximum travel could be.
The fork travel is simply the delta measure between the uncompressed state of the fork and the fully compressed. The distance the springs inside allow for compression. This characteristic along with the stiffness of the spring will dictate how good the fork actually is at its job, a.k.a. smoothing out the ride over bumpy roads at various speeds. Having lots of travel and softer spring raters results in a bike that is meant for big deviation in the surface, or in other words an off-road or adventure motorcycle. Shorter travel and stiffer spring rates conversely would be better suited for the track. Stiffer suspension is generally more predictable at higher speeds and leaning angles. One important bit to mention here is that there is such thing as too little fork travel, as we will find out later on.
Fork Clamp Offset
The clamp offset is essentially the measure between the center of the fork head to the center line drawn between the mounting holes for the fork tubes in the top yoke. This offset essentially moves the fork tubes further out or closer to the fork head. This offset has a very direct impact to the resulting trail of the motorcycle. Manufacturers usually use these to ensure that the front tire is always away from the frame and the engine. For example if you were to put a modern front end with upside down forks and a 16" wheel (as suppose to the 18" or 19" stock wheel) on of the classic Honda's from the 70s or 80s you would notice this problem right away. Since the modern suspension is shorter, and the front tire is smaller, the rake of the motorcycle will be effected making the bike lean forward. And since the travel is likely shorter then the old forks you would be left with a setup the allows for the front tire to hit the frame or engine under full fork compression. To remidiate that problem you would get aftermarket triple trees with a larger offset, pushing the entire configuration forward and allowing for the clearance.
Last but not least we get to the characteristic of the front suspension that is literally effected by the changes in all the other ones. The trail of the front suspension setup is the resulting measure between the point on the floor the straight line through the fork head makes, to the point on the floor where the tire makes contact with the surface (in other words straight down from the axle). This small distance is usually measured out between 3.5" - 6" on most manufacturer motorcycles. Generally speaking the bigger the trail the more high speed stability, and less responsiveness, and visa versa for shorter trail. Too short of a trail and ... well think grocery store shopping cart. Too long of a trail and you have an 18 wheeler. So that range really is the safe zone that the trail should be kept in.
Since a picture is worth a thousand words, here is a few diagrams showing the characteristics above.
The stock Sabre looked something like this.
The 1983 model with the shortened springs would look like this.
The 1982 model with the pulled up forks through the tripple trees like this.
I know its difficult to see the differences so here is a super imposed version of the stock and dropped forks setups.
As you will see later in the actual photos it is hard to notice the difference, but if you look at the top of the frame where the tank sits. That is where you will notice the angle change. The tank sits more horizontally, and the bottom of the frame actually dips slightly lower.
So what does all that mean for the current project? Lets take a look at the parts bike (1983) and what I have done to it by cutting the springs and shortening the forks.
After i had already made the changes to the forks I had measured out the front end characteristics and these are the numbers I had computed:
What I had thought to be a pretty sick setup initially turned out to be a death trap after I did some research on it. The rake and trail that I had as a result of shortening the forks by 2" were actually not bad. They mimic the setup of a more sportier BMW motorcycle. The problem however was that those 2" were essentially taken off of the stock travel of the forks. Which at 5.5" one would think would leave plenty. Yes but no! The 3-3.5" of travel left on there is definitely not enough to sustain safe operation of the motorcycle with the fairly soft 35 year old springs. What this meant was that after full assembly and filled with fluids, with a person on top the bike would sag down another 1-1.5". Leaving 1.5"-2" of travel. Which is not at all enough. You might as well be riding with a hard front end. The bike would simply bottom out right away on the slightest of deceleration. Not fun.
So lesson learned I took the project bike and started stripping the front, in order to lower the front end in a much safer, far more scientific manner.
Before i began changing things i took a base measurement to know where I am starting from.
That out of the way I raised the front end and started to slide the forks up through the triple trees. At about 1.5" I figured I'd stop before I start running into clearance issues under compression. Then I measured everything again.
Now that seems like it should be quite a bit better. Since I have the leveling issue with my driveway I decided to recompute them by accounting for the 2.1° of deviation.
So things are now looking like the handling of the bike would be quite different. A bit more responsive, a bit shorter, and definitely more fun to ride.
So the moral of the story is do your research, and always consider safety. Here is the before and after pictures. The change is very subtle but when striped from the borderline ape hangers, and setup with clip-ons it will be more noticeable.