Using Data Acquisition

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.

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.

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.

However, when we trail-brake into or accelerate out of a turn, the longitudinal and lateral forces must be balanced so as not to overtax the tires. The vector sum of the two accelerations-taking into consideration both their magnitude and direction-must not exceed a maximum value. This is drawn as a traction circle, a graph that shows how much of one force can be applied given a value for the other. Taken to the extreme, the quickest way around a racetrack is by keeping those added forces at their maximum value all the time. In other words the motorcycle is always accelerating, braking or turning, or some combination of those.

Graphically we can plot the total vector acceleration over the course of a lap by summing the squares of lateral and longitudinal G forces-which is, incidentally, the graphical formula for drawing a circle.

Total G = vlateral G2 + longitudinal G2

Ideally this value would be consistently at the maximum over the course of the lap, indicating the rider is on the outer circumference of the traction circle all the time. Some exceptions should be noted, however: On longer straights, acceleration-and total G force-will taper off as the motorcycle reaches its top speed, at which point it will be zero. And at the end of each straight when the rider switches from acceleration to braking, total G will also momentarily dip to zero. This plot is useful for quickly finding areas the rider needs to address, whether it be braking later into a certain turn, accelerating earlier on an exit or squirting between two corners rather than keeping a constant speed between them.