Proficient Motorcycling. David L. Hough

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       Controlling your motorcycle in turns is more than just avoiding embarrassment. Crossing the centerline is an invitation to a collision.

      It also helps to look where you want to go. If you don’t want to hit a pothole, focus on the road to one side or the other. If you don’t want to cross that centerline, look ahead down your lane; don’t gawk at the line. Even novice riders who haven’t mastered countersteering often gain considerable control by just getting their eyes up and looking where they want the bike to go.

      Those are the two big secrets for the average situation: countersteer and look where you want to go. Now, go out and play.

      Before you thumb the starter, though, let’s note that there are lots of hazardous situations out there that demand more skill than the average situation. For example, let’s say Beemer Bob zooms out of a tunnel in the mountains, smack into a 50-knot crosswind gusting from his right. The gust slams into the bike, pushing it toward the centerline. What should he do?

      Bob needs to push hard on the upwind grip to lean the bike over and maintain enough muscle on the grip to hold the bike leaned over into the wind, but in a straight line. To counter that gust from the right, he needs to push aggressively on the right grip to lean the bike upwind. With the bike leaned over but not turning, steering isn’t going to feel normal, so Bob needs to apply pressure on the grips to make the motorcycle go where he wants it to go, and not just think “lean.”

      Such situations remind us that balancing isn’t just a simple matter of nudging on the low grip. To prepare for a wide variety of situations, it might be helpful to look a little deeper into the dynamics of how two-wheelers balance and steer. If you get confused with any of this, I suggest you go out to the garage and try the experiments on your motorcycle.

      And, as we get started on balancing dynamics, you should be aware that not everyone agrees about how it works any more than everyone agrees about love or war. From time to time, even the experts get into arm-waving arguments about small details, pens hastily scribbling diagrams on lunchroom napkins. What I’m going to offer here is the opinion of one aging moto-journalist/instructor, based on forty years of arm-waving discussions and napkin scribblings. And note that for what follows in this chapter, motorcycle means a two-wheeler, not a rigid sidecar rig or trike.

       Two-Wheeler Stability

      Occasionally, you’ll see a rider let go of the grips and lean back in the saddle at freeway speed. You may marvel at the naiveté of a rider willing to ignore such hazards as a groove in the pavement that might instantly yank the front end into a tank-slapper, but hands-off riding is a great demonstration of the unique stability of a motorcycle. The front-end geometry automatically stabilizes the bike in a straight line, self-correcting for minor changes in lean angle.

      The simplistic suggestion is that this self-centering action is just a result of the castering effect of the front tire trailing behind the steering pivot axis, similar to the front wheels of a shopping cart at the grocery store. But two-wheelers are quite a bit more complex than shopping carts because they lean into turns. The self-balancing action of a motorcycle front end is a result of the combined effects of a number of details, including rake, trail, steering head rise and fall, mass shift, contact patch location, and tire profiles.

       Rake/Trail

      When test riders refer to heavy steering, they are talking about a machine that is so stable in a straight-ahead situation that it requires a lot of muscle to get it leaned over and held into a turn. What they mean by a flickable machine is one that is relatively unstable, that can be easily leaned over or straightened up with very little effort on the grips. This is a delicate balance, and sometimes the engineers have to walk a tightrope between low-effort (flickable) cornering and bad manners, such as the front wheel suddenly steering itself toward the curve (tucking), uncontrollable oscillations (speed wobbles), or falling into turns at slower speeds.

      The behavior of a bike is related to its steering geometry. If you stand off to one side of your motorcycle and observe the angle of the front forks, you’ll notice that the top of the fork tubes are angled (raked) back. And if you look closer, you’ll see that the fork tubes aren’t exactly in line with the steering head on the frame. While you are standing off to one side of the bike, imagine a laser beam passing through the steering head until it strikes the ground. The laser beam represents the pivot center, or steering axis of the whole front end. When we talk about rake angle, we’re talking about how far the steering head is angled back from vertical. Today’s cruisers typically have around 30 degrees of rake, while flickable sportbikes have rake of 24 degrees or less. In general, greater rake produces greater straight-ahead stability at speed, and steeper rake produces low-effort steering. But rake is only part of the equation.

      The steering axis intersects the ground somewhere ahead of the contact patch where the front tire is sitting on the surface. The distance between the contact patch and the steering axis is called trail (the contact patch trails behind the steering axis). Typically, street bikes have trail somewhere in the three- to six-inch range. In general, longer trail results in a machine that resists leaning into corners, and shorter trail results in quicker, easier steering, or perhaps even a machine that wants to fall into corners. Since rake and trail are interdependent, the figures in bike reviews are usually given as rake/trail.

       A motorcycle like this one with greater rake and trail typically has very stable straight-ahead steering but requires more effort to roll into or out of turns.

       A motorcycle like this with steep rake and short trail has very easy steering, making it very flickable from side to side.

       Trail is the distance from which the front wheel contact patch follows behind the steering axis.

       Because of rake, the steering head actually rises and falls when the front end is steered.

       Steering Head Rise and Fall

      One of the interesting results of rake/trail is that the steering head falls slightly as the front end is pivoted from center to either side, and it rises as the front wheel is turned back toward center. The greater the rake, the more the rise or fall. You can see this for yourself. Straddle your bike, get it balanced vertically, and observe the elevation of the top of the steering head as you turn the handlebars from straight ahead to either side and back to center.

      Now, remember, gravity is pulling down on the bike, which is supporting perhaps half its weight on the steering head. So gravity actually helps turn the front end away from center and resists the front end returning to center. That’s not СКАЧАТЬ