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Cornering: Terminology
This page is a supplement to the tip of the week on cornering skills. When discussing cornering skills, it's useful to have precise terms for the various parts and attributes of a corner. The following terms are in common use: Attributes of a Curve Radius If you imagine that a curve is formed by cutting a segment out of a circle, the radius of the curve is the radius of the circle. It is a measure of how "sharp" the corner is. A large-radius curve is a long, gentle curve (often called a "sweeper") while a small-radius curve is very tight. Constant-Radius 
Curves that are, indeed, segments of a circle are called
"constant-radius" curves. City planners try to achieve
constant-radius curves on city streets because they are easy to drive.
On country roads, however, the road path is often determined by
geography, obstacles, farmland, etc., and constant-radius curves are
rare. They are also rare on racetracks, because racetrack designers
deliberately try to design more challenging roads for the racers to
negotiate.
Changing-Radius Most curves you will encounter, then, are changing-radius curves. The curvature changes as you pass through the curve, sometimes more than once. The simplest changing-radius curves are those where the curvature changes only once, becoming either more sharp or less sharp as the curve progresses.
Parts of Curve Entry & Exit The Entry of a curve is the point where the curvature of the road begins to change. (Where it changes from straight to curved in most cases.) It may be the point where you begin steering your bike to follow the curve, although you might start your steering input slightly before or after the entry. The Exit is the point where the road has straightened out and, under most circumstances, where you will have your bike upright and travelling in a straight line, finished with turning. Outside & Inside Imagine, for a moment, that a curve is formed by cutting a segment out of a circle. The inside of the curve is that part of the lane closest to the center of the circle. This is the right side of the lane for a right-hand curve, and the left side of the lane for a left-hand curve. The outside of the curve is that part of the lane farthest from the center of the circle. This is the left side of the lane for a right-hand curve, and the right side of the lane for a left-hand curve. Apex The apex of the curve is the point at which half of the curvature is "used up". This may seem like a strange definition - isn't the apex just of the middle of the curve? The apex is, indeed, the middle for a constant-radius curve. However, the apex will be beyond the middle for a decreasing-radius curve, and before the middle for an increasing-radius curve. A useful definition for riding is that the apex is the point at which, looking ahead, you can first see a clear path to the exit. 
Camber Camber is a measure of the "Slope" of the road surface. Zero-Camber or Neutral-Camber means the road is perfectly flat. Positive Camber means the road is tilted, with the lower side close to the center of the curve. Positive Camber curves are easy to ride because the positive camber helps fight the sideways forces on the tires, and reduces the amount the bike has to lean. Traditional oval-shaped automobile racetrack curves, like Indianapolis, usually have Positive Camber ("banked") curves. Negative Camber means the road is tilted away from the center of the curve. Negative Camber curves are difficult to ride because the slope increases the sideways pressure on the tires and increases the lean angle of the bike. Many riders feel quite uncomfortable on Negative Camber curves. |
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Increasing-Radius:
In an increasing-radius curve, the curvature opens out as you proceed
through the curve. It's as though you are driving out from the center
of a spiral toward the outside. Increasing radius curves are easy to
ride, and you can accelerate as you progress through the curve. For
this reason, entrance ramps onto freeways are often increasing-radius
curves, allowing you to accelerate to freeway speeds as you negotiate
the ramp.
Decreasing-Radius:
In a decreasing-radius curve, the curvature gets tighter as you
proceed through the curve, as though you were driving in toward the
center of a spiral. Decreasing-radius curves are challenging to ride
[and to draw!].
You must continuously increase the amount of your steering input
(increase the pressure on the inside bar when countersteering)
throughout the curve. Many riders get into trouble in
decreasing-radius curves by entering too fast and then being surprised
by the increased turn and lean requirements. Because
decreasing-radius curves force traffic to slow, they are often used on
freeway exit ramps. 
