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.
This scatter graph shows the...
This scatter graph shows the same data from Buttonwillow, with acceleration data plotted in a traction-circle format. Note the heart-shaped pattern, which is typical for a motorcycle. Maximum braking occurs with the motorcycle vertical, but maximum acceleration occurs with the bike just off vertical. This is due to a number of factors, including a larger tire contact patch, lower center of gravity and shorter gearing when the bike is leaned over.
Continuing to utilize the...
Continuing to utilize the same data, this plot shows the vector sum of lateral and longitudinal G forces and represents how much of the available traction the rider is using at any given time-in terms of the traction circle this graph represents how close the rider is to the outer rim of the circle. Ideally this trace would be a straight line at the maximum value, dipping to zero only at the transition from acceleration to braking or when the machine reaches top speed. Any other segments where total G goes to zero is an indication that the rider is coasting where he should be accelerating, braking or turning.
This data shows total G forces...
This data shows total G forces for Robertino Pietri on his Roadracingworld.com Suzuki GSX-R1000 Superstock bike at Daytona and is an almost perfect trace, consistently near the maximum value over the entire lap with crisp, short dips to zero at the braking markers and transitions. This indicates Pietri is at any given time either accelerating, braking or turning, and making full use of the available traction. We left the scales identical to the previous graph so you can see the additional speeds and G forces generated by a racebike at Daytona.
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.