In the first three installments of this series we looked at data gathered by various sensors attached to the acquisition system: speed, rpm and throttle position. In this segment we'll deal with channels derived from position data collected by GPS-based systems that don't require any additional sensors. GPS setups, which calculate ground speed based on the change in position over time, can use that data to calculate acceleration forces in any direction, a helpful addition when it comes to evaluating performance.
To take full advantage of acceleration data it's important to understand the concept in terms of both force and direction. We're all familiar with straight-line acceleration: If a bike can go from zero to 60 mph in three seconds, it has an average acceleration of 20 mph per second. Likewise, braking from 60 to zero in two seconds is an average deceleration of 30 mph per second-deceleration is also referred to as acceleration with a negative result. Because the units can get cumbersome, acceleration data is usually referred to in units of G, or the force due to gravity. One G is equal to about 32 ft/s2, or approximately 22 mph per second. Because we've used some realistic numbers in our example, you can see that a bike is easily capable of about 1 G of acceleration or deceleration.
Buttonwillow Raceway Park...
Buttonwillow Raceway Park
Lateral G (red) and longitudinal...
Lateral G (red) and longitudinal G (blue) forces are plotted here for a lap of Buttonwillow Raceway's west loop, recorded by our Racepak G2X system. A positive value for lateral G indicates a right-hand turn, a negative value the opposite. The top trace (black) is speed, shown here for reference. Note how longitudinal G forces change sharply from positive to negative at the end of each straight as the rider goes from accelerating to braking.
This graph shows absolute...
This graph shows absolute values (all negative numbers are changed to positive) for lateral and longitudinal G over the same full lap, in the same color scheme. Note that on the entrance to each corner, longitudinal G tapers off as the rider releases the brakes and enters the corner, while lateral G increases as the turn is initiated. Where the two lines cross is an indication of how late the rider is trail-braking into the turn. Likewise, on the exit of each corner the traces cross as the rider accelerates (adding longitudinal G) and straightens the bike out (reducing lateral G); the intersection point shows how aggressive the rider is getting on the gas exiting the turn.
Cornering acceleration is felt as centrifugal force-for example, when a car goes around a corner at speed you're forced toward the outside of the turn. The actual acceleration of the car, however, is referred to as acting toward the inside of the turn and is a measure of how hard the car is deviating from a straight line. Most data acquisition systems have internal accelerometers that can record these forces, and in car applications they give accurate data for both longitudinal (straight-line) and lateral (cornering) forces. However, on a motorcycle that leans in corners, the forces acting in the lateral direction are always counteracted by the force of gravity acting toward the ground. These sum to a value close to zero when recorded, and it's very difficult to gather accurate lateral data from accelerometers.
GPS-based systems, however, can calculate accurate acceleration data from position data, and channels for longitudinal and lateral acceleration are usually included in the software. These graphs on their own will give you a general idea of how hard the rider is braking, how quickly the motorcycle can accelerate and how fast a particular corner is being taken in reference to its radius.
A traction circle shows lateral...
A traction circle shows lateral forces on the horizontal axis and longitudinal forces on the vertical axis. Right turns are on the right, left turns on the left; acceleration is toward the top, braking toward the bottom. Using this layout the total acceleration can be plotted at any given time. For example, the point shown in the top left quadrant represents accelerating from a left-hand bend. Likewise, trail braking into a right-hand corner would generate data in the bottom right quadrant. More total force shows as a point closer to the outside of the circle; less force is toward the center. The characteristics of the tire and pavement dictate the maximum total acceleration available, shown visually as a circle.
This data is most useful for times when you visit a new track for which you don't have any data, as a particular corner or braking zone can be compared with a different section of the track for reference. For instance, if the rider is braking with a deceleration of 1.5 G in one corner, given the same conditions and the same pavement over the entire track he should be able to brake at the same value elsewhere. A similar comparison can be made for corners: If a certain G value is seen in one corner it should be consistent at other corners on the same track under similar conditions. Of course there are many influences that can affect this comparison, such as bumps or camber, and these must be taken into account.
The lateral and longitudinal plots can be manipulated with math channels and used to observe certain aspects of the rider's performance entering and exiting turns. Here we are concerned with what is commonly referred to as the traction circle. The tires on your motorcycle (and the conditions you are riding in) determine the total amount of force, or acceleration, that it is capable of. This force can be applied all in one direction-braking in a straight line, for example-and in these cases the acceleration can be maximized. You can see on the plots that longitudinal G generally reaches a maximum value fairly early in each braking zone and is consistent over the entire zone. Alternatively that maximum force can be applied entirely to cornering, and in a long, sweeping bend the lateral G is fairly consistent at a specific value.