Riding Skills Series: The Traction Circle

On the graph, traction is shown as the accelerative force in every direction: What we normally call acceleration is a positive longitudinal force (longitudinal being along the direction of travel of the motorcycle), shown on the graph as a point on the upper half. Braking is a negative longitudinal force, shown on the bottom half of the graph. Cornering generates lateral forces, generally felt as centrifugal force when you're in a car, for instance. On the traction circle, points on the right-hand side represent a right-hand turn, and vice versa. We've covered both axis of the graph so far, but what about points away from an axis? Say you were accelerating out of a right-hand turn, generating forces in both the lateral and longitudinal directions. This would show as a point in the upper right-hand quadrant of the graph. Likewise, trail braking into a left-hand turn is displayed as a point in the bottom left-hand quadrant.

The tires on your motorcycle are capable of providing a certain level of grip in any one direction; let's say the maximum accelerative force is 1 G, a realistic number. This force can be applied all in one direction, such as braking or turning, or in a combination of directions-braking and turning. Since we're dealing with vector forces that are summed with a value as well as a direction, the graph of maximum grip values is shown as a circle: the traction circle with a radius of 1 G.

Looking at the circle, some things become apparent. Say you are at maximum lean in a right-hand turn. On the traction circle, this puts you at a point on the far right. Here, it's unwise to accelerate or brake, leaving you on the horizontal axis with no longitudinal acceleration. But what if you want to change that? Moving around the circle representing maximum available grip, it's clear that to add some longitudinal force in the form of accelerating or braking requires that lateral force be reduced to maintain a maximum total G force less than one. As we exit the turn, more acceleration force is added, requiring that less lateral force be applied: the bike must be straightened to accomplish this. Continuing on, eventually we arrive at the vertical axis of the graph with full acceleration and no turning forces.

In a similar manner, you can see what has to happen entering a corner. Where are you on the graph at maximum braking? What do you have to do when you initiate a turn while the brakes are still applied? In any situation, you should consider the total forces-and hence the total traction-rather than acceleration, braking and cornering individually. For example, if your bike is vertical, with no cornering forces involved, where does that put you on the circle? What does this allow you to do as far as the longitudinal forces are concerned?

The traction circle can also tell you where your concentration should be directed at any given time. In midcorner the forces are almost exclusively lateral, with very little longitudinal. You should be concentrating mostly on side grip at this point, feeling for traction and alert for a slide. Likewise, under heavy braking you are at the very bottom of the traction circle; your sense of traction should be directed almost entirely here until the point of turn in. Once you mix the forces and venture off the axis of the graph, you should similarly devote your concentration proportionately. Note that just as the overall traction cannot be exceeded in any combination of directions, you also can only allocate so much concentration. In his timeless book Twist of the Wrist, Keith Code outlined a system where the rider has ten dollars of attention to spend. No matter how you spend it-a certain amount to side traction and a certain amount to acceleration or braking traction-you only have so much attention money available. For example, exiting a turn you should gradually pay less attention to side grip and more to acceleration grip as the bike becomes more upright. Spend your attention allowance wisely!

Ideally, a complete lap of the racetrack is spent at the very edge of the traction circle. Any time the sum of the lateral and longitudinal G forces is less than the maximum is time spent coasting. Evaluate a lap of your favorite track in terms of these forces. Is there any point at which, for example, your bike is straight up and down (no lateral forces) and yet you are not on the throttle or brakes (no longitudinal forces)? Are you maximizing total traction on corner entry and exit for every turn? While all this data is available from many data acquisition systems, you can evaluate your own performance in this respect to find places that improvements can be made.