As acceleration increases at the exit of a corner, the motorcycle must be brought more upright to cope. This relationship is just as defined as the connection between braking and turning on the entrance to the corner.
You can “cheat” the lean angle/acceleration...
You can “cheat” the lean angle/acceleration relationship somewhat by lifting the bike up at the corner exit as demonstrated here. This puts more of the rear tire on the pavement, increasing contact patch and, in turn, available traction for the combined acceleration and cornering forces.
Even though electronics are rapidly taking throttle control out of the hand of the rider, we are still a long way from having production systems that are better than the human touch. Unless you are riding on a full factory MotoGP or Superbike team, throttle control is still a vital skill that you can use to ride faster on the racetrack and safer on the street. Here we will discuss two aspects that are often misunderstood: the relationship of the initial throttle opening to actual acceleration, and how lean angle must change depending on acceleration.
We have covered the initial application of the throttle in some detail in a previous RSS (May '10), but how the initial opening affects acceleration is the important part here. Figure 1 shows some data from Willow Springs International Raceway for a rider aboard a GSX-R1000 in turn 2, a long, slightly uphill corner taken at more than 90 mph. Speed is shown in blue, with throttle position in red and acceleration in grey. Note that for almost the entire turn, from approximately 14 seconds to 23 seconds, the rider must use between 15 and 25 percent throttle opening just to hold a constant speed-even on a powerful literbike. This is an extreme example, as the turn is slightly uphill and taken at high speed; because there is such a load on the motorcycle (from aerodynamics and going uphill), a lot of power is required to keep speed constant. It's not until the throttle is opened to 30 percent that the motorcycle even begins to accelerate at the exit of the corner.
When the motorcycle is at...
When the motorcycle is at full lean, the ideal situation is to have the throttle open just enough to apply the power needed to keep speed constant. From here, the motorcycle must be raised from full lean — even if just a small amount — before acceleration can begin.
Even in slower turns where aerodynamics are not a factor, the throttle must be open a certain amount-typically at least a few percent-before the motorcycle actually accelerates. With this in mind, it makes sense that the throttle should be open at or before the point in the corner where you expect to make the apex. Opening the throttle is necessary to hold a constant speed, and with even that slight amount of power applied the suspension is nicely settled and the rear-end anti-squat properties can work as they were designed to. Before the apex, you will not want to actually accelerate, and don't want too much power applied; in a downhill corner, for example, even two or three percent throttle may be too much. This is where being smooth on the throttle is crucial; you want just enough power to keep your speed constant, but not so much that you begin to accelerate too early. The relationship between throttle position and actual acceleration determines at what point in the corner you open the throttle, and by how much.
The data in figure 2 shows a much slower corner, turn 2 at the Streets of Willow. Again, speed is shown in blue and acceleration in grey; the yellow trace is lean angle. The vertical cursor on the graph marks the apex of the turn; note at this point speed is at its lowest, lean angle is at its highest, and the motorcycle is just beginning to accelerate. The important detail here is that lean angle decreases as acceleration increases and the rider exits the corner. Granted, lean angle does not change very much and the rider is carrying a lot of acceleration at a significant lean angle, but the connection is definitely noticeable. Further along the apex, at the 12-second mark, the rider brings the bike more rapidly vertical as acceleration steadily increases.
This data shows speed (blue), throttle position (red, scale at left) and acceleration (grey, scale at right) for a rider in turn 2 at Willow Springs International Raceway. Note how much throttle is required to hold a constant speed in the corner and how much before the bike actually accelerates-marked here by the vertical cursor.
Here, data for turn 2 at the Streets of Willow shows speed (blue), lean angle (yellow, scale at left) and acceleration (grey, scale at right). The apex of the corner, at approximately 10.5 seconds, is marked with a vertical cursor. From this point, speed and acceleration increase, and lean angle must correspondingly decrease.
Most riders are aware that the motorcycle must be more vertical to accelerate harder, but many-to their detriment-don't consider what this means at the extreme limit: You cannot accelerate when the motorcycle is at full lean. The situation is very similar to trail braking at the entrance to a corner, where you must gradually release the brakes when you begin arcing in toward the apex and have the brakes fully released by the time you reach full lean. Just as you cannot apply the brake when the motorcycle is at maximum lean, you can't expect to accelerate. All too often we have seen a rider overshoot a corner, and then attempt to accelerate at the usual point while still trying to make the usual apex; often this results in the classic highside. If you overshoot a turn, it's a far better option to sacrifice some mid-turn speed, make a new apex and then accelerate out of the corner, rather than attempting to return as quickly as possible to your regular line and use your usual acceleration marker.
Understanding how your actual throttle position differs from acceleration, and how that acceleration in turn affects how much lean angle you can use at the exit of a corner, will not only help you go faster at the racetrack, it also can go a long way toward saving you from a highside crash on the street or track.