From Trevor’s book:
Your bike's chassis performance and its steering manners are largely determined by rake and trail, two critical dimensions of the front end. Rake is defined as the angle of the steering head re ve to a line perpendicular to the ground and is between 20 and 25 degrees. On most bikes a fixed dimension, because the steering head is rigidly fixed to the frame. On others, such as Ducacis 999 series, rake is adjustable through inserts in the steering head. Trail refers to the dimension from where the steering axis intersects the ground to the center of the front tire's contact patch. Imagine a line drawn through the steering head, which in most cases is parallel to the fork tubes. That line will pass just behind the front axle and meet the ground a few inches in front of the tire's contact patch. The measurement from that point back to the contact patch (which is directly below the axle) is the trail. Trail is determined by— and can be calculated from rake, the offset of the triple clamps, and tire diameter, and generally falls between 80mm and 110mm.
Trail provides a self-centering force to the front wheel, giving the motorcycle its stability. When the handlebar is turned, the tire's contact patch ares to the side of the bike's centerline, slightly forward of its original position. The friction force of the tire rolling over the ground constantly tries to push the tire to its rearmost position—in-line with the bike. In general, a steeper rake will give less trail, and this makes a bike steer quicker at the expense of stability. Sportbikes have relatively little trail for good han-dling, whereas cruisers have lots of trail for stability. While we may not be able to alter rake or trail directly, we can change rake and, in turn, trail slightly by raising or lowering one end of the bike relative to the other. On most stock bikes the only way to accomplish this is to raise or lower the fork tubes in the triple clamps, but some bikes have a ride-height adjuster incorporated into either the shock or the linkage to raise or lower the rear end. Ride heights and geometry are the subject of chapter 4.
Another important aspect of rear-end geometry to consider is the angle of the swingarm relative to the ground and frame. As bikes become more and more powerful, the engine exerts enough force on the rear suspension through the chain to affect geometry and handling. How this can be used to your advantage, by "tuning" the amount of force transmitted to the suspension and creating anti-squat, is explained in chapter 7.
How a bike steers plays such an important part in its handling and setup that it can almost define a particular model. For instance, when we think of the early generations of Yamaha's R6, what comes to mind is an ultra-nimble middleweight with laser-precise steering that borders on twitchy. That made for a dream ride on Smooth pavement, but the little Yamaha could also become a handful on rough roads—more than once I sported bruised wrists thanks to a tank-slapping R6. Likewise, Ducatis have long been known for slower, heavier steering but rock-solid stability. As mentioned earlier, steering quickness generally comes at the expense of stability, and these two characteristics can leave a big impression on riders of all experience levels. For novice riders, easy steering is important to build confidence: A bike with light, neutral steering will allow a beginning rider to more easily learn good habits rather than constantly fighting his or her bike. Likewise, nothing will discourage a new rider more than a twitchy, unstable motorcycle that seems to have a mind of its own even when ridden in a straight line.
As you gain riding experience, you may find that you prefer a specific front-end feel. Perhaps you want quicker steering and have learned to use your body position or other inputs to limit instabilities in certain corners. Or you may grow to appreciate stability and feedback and be willing to work harder in fast transitions to turn your bike from side to side.
You also should consider the differences between the street and the track: A setup that gives great steering characteristics and works fine in controlled conditions at the track could easily turn into a wobbling mess on a rain-grooved freeway.
The geometry of a particular model is generally defined by its rake and trail. We will discuss other critical dimensions in detail in later chapters, but rake and especially trail are the most important. Changing rake and trail can alter the steering characteristics of your bike. Those characteristics can most easily be classified in terms of steering quickness and stability.
Steep geometry, with low rake and trail numbers, will give light, easy steering, especially at lower speeds. The trade-off for that quick steering is stability. With steep, aggressive geometry your bike may want to shake its head over bumps or when accelerating out of a turn. Relaxed geometry—high rake and trail numbers—generally results in heavier steering but more stability. At lower speeds especially, your bike will have a trucklike feel and be very reluctant to turn. Rake and trail affect other handling traits, but steering and stability are the two most important factors to consider when adjusting your suspension.
This is the first hands on chapter in this book, not only because steering stability are some of the first handling traits you should be making suspension changes for, but also because practically every adjustment you make will affect geometry and steering in some way. It's something you must always be aware of when working toward your ideal setup: even a small change to preload or damping can affect steering. Quite often riders will misinterpret the results of such a suspension change, crediting a new steering or stability character to what they adjusted rather than considering how geometry changed when they made that particular adjustment. That said, read through chapter 5—and make sure your bike's sag numbers are reasonable before making changes so that you fully understand the important relationship between preload and geometry.
The easiest way to feel how your particular bike reacts to a geometry change is to conduct a back-to-back test for yourself. This is a theme repeated throughout this book: Experimentation is the best way to find for yourself what a specific suspension change affects. At the race track, or on a quiet stretch of road that you are familiar with and repreeanis the type of riding you do, make laps or passes until you are comfortable with the speed you are going and with how your bike is working. You should not ride at a pace that taxes your riding abilities, but rather at a speed that leaves you spare concentration to think about what your suspension is doing.
In this case, you want to experiment with changing rake and trail by adjusting ride height. Few sportbikes have provisions for adjusting geometry directly, but we can change effective rake by raising or lowering the front end of the motorcycle. This is accomplished by sliding the fork tubes up or down in the triple clamps. Raising the tubes in the triple clamps lowers the front end, decreasing rake and trail numbers for steeper, more-aggressive geometry.
Lowering the tubes in the clamps raises the brat end, increasing rake and trail for more-relaxed geometry. See the sidebar on page 39 for a detailer explan tion of how to adjust your fork tube height.
A change in fork tube height of 4mm generally equates to 1mm of trail, and experienced riders can easily feel that small of a change in geometry.
If your bike has a rear ride-height adjuster, it's easier to make quick changes using the adjuster rather than moving the forks in the triple clamps. Most aftermarket shocks have a threaded collar at the bottom of the shock, and this can be extended or shortened by loosening the lock nut and turning the adjuster. Check the owner's manual for your bike or aftermarket shock for guidance. Many Ducatis have a rod in the rear suspension's linkage that can be lengthened or shortened to adjust ride height.
Loosen both lock nuts—one is a reverse thread and turn the rod to adjust the rod's length—again, your owner's manual will have detailed instructions and restrictions for the maximum length.
Extending the length of the shock or linkage will raise the rear end of the bike, decreasing rake and trail for steeper geometry. Shortening the shock or rod will lower the rear of the bike, increasing rake and trail for relaxed geometry. Because the magnitude of the actual change in ride height depends on the ratio of the linkage, you should make some measurements to find that ratio. With a friend lifting on the rear of your bike, measure from the rear axle to a solid point on the subframe or seat directly above. Change the ride-height adjustment by one turn in the desired direction, and measure the change in rear ride height. In general, you'll find that a one-turn change equates to between 2mm and 4mm of ride height. As with the front end, a 4mm ride-height adjustment works out to roughly a Imm change in trail. Experiment in one-turn increments, being careful not to exceed the maximum length specified for the adjuster.
Another possibility for altering rear ride height is to place a spacer between the top shock clevis and frame, effectively lengthening the shock and raising the rear end. Kawasaki offers a set of shims for most years of the ZX-6R and ZX-10R that slips in without requiring that the clevis be dropped completely from the frame, and the nut on the clevis is easily accessed. Aftermarket companies manufacture shims for Su zuki GSX-R models and Yamaha's R series. Recent Hondas, with their unique Unit Pro-Link setup, do nor have a top frame mount or clevis, and hence, no ons for an easy shim insert.
However you have decided to alter ride height, make a single change that sharpens your bike's geometry. That means lowering the front end by raising the fork tubes in the triple clamps by 4mm, extending the shock's ride-height adjuster by one turn, or adding a 1mm or 2mm shim under the shock's clevis—whichever is available or easiest for you to adjust. Certainly, raising the rear of the bike is not an identical adjustment to lowering the front, and in later chapters we will discuss the implications of each adjustment. If you are an experienced track-day rider or racer, you will want to experiment separately for raising and lowering the front separately from the rear. For most street riders and beginning track-day riders, however, we are concentrating on the change in trail brought about by altering ride height. That change is far more apparent than the other effects of a ride-height adjustment, and for now you should just make whatever adjustment is easiest.
With the new setting, ride the same track or route as previously, at the same pace, so that you can concentrate on how your bike feels different. After the session, jot down your thoughts, detailing the setting change and describing how your bike reacted.
Specifically, note any changes to stability and steering: Typically, sharpening geometry will make steering lighter at the expense of stability; the bike will be easier to turn, but may feel flighty or less stable... you felt the change was for the better, experim with another change in the same direction. If not, try adjusting ride height in the other direction.
Should you reach a point at which the fork tubes are flush with the top of the top triple clamp, don't be tempted to push the fork tubes down into the triple clamp. Likewise, don't go beyond the shock's adjustment limits or have the fork tubes extruded so far that the front tire is in danger of hitting the fairing or radiator at full compression. Note that it is possible to raise or lower the entire bike by raising or lowering the rear ride height and changing the fork tube height appropriately. This keeps rake and trail the same, and you can use this to your advantage should you reach the end of an adjustment range. This also raises or lowers the center of gravity relative to the ground, and the troubleshooting guide found in chapter 12 outlines some scenarios where this particular adjustment could be useful.
We'll aiso look at how rake and trail are affected dynamically by damping and weight transfer. While the height of the fork tubes and the length of the shock set geometry with the fork and shock fully extended, your bike's rake and trail are constantly changing with every movement of the suspension. For example, under braking the fork is compressed and the shock is extended, significantly reducing rake and trail. How much the fork compresses and how far the shock extends in that circumstance characteristics we can change by making other adjustments will determine the dynamic geometry and how your bike behaves. For now, continue experimenting until you feel you've reached the best compromise of stering and stability for that particular road or track, and you are familiar with how a geometry adjustment—in either direction changes how your bike feels.
