This picture of Casey Stoner at last year's Spanish Grand Prix at Catalunya shows the Australian using every possible centimetre of the track mid-corner. It illustrates, quite graphically, the importance of riding as closely as possible to the apex in each turn. Other pictures show Stoner even riding right over the curb at the apex of some corners. Keeping such a tight apex in a corner shortens the overall distance, even if only by a small amount, and in turn saves time that can add up to a significant amount over the course of a lap. To a certain extent, missing an apex and taking a wider radius around a corner is not that critical, because you can carry more speed on that bigger radius. That extra speed, however, cannot be enough to offset the extra distance travelled unless lateral acceleration (and lean angle) also increases.
A look at the relationships between speed, time, distance and lateral acceleration shows that this is especially important in slower corners. Turn 5B at Mosport—now Canadian Tire Motorsport Park—has a radius of approximately 10 metres and covers a 90-degree arc. Miss your apex by just one metre here and the extra distance will cost you close to a tenth of a second, even if you increase your speed accordingly. Because speeds are higher in faster corners, the time lost is not as great. But there is another aspect to consider: Miss an apex in any turn, and you are essentially extending the straights before and after the corner. For example, in a 180-degree turn such as the turn 4 carousel at Shannonville, if you are one metre wide at the apex of the turn you are adding two metres in distance travelled—one on the preceding straight and one on the succeeding straight. You can't offset that distance no matter how much additional speed you can carry in the corner; it is simply extra distance that must be made up.
The reality is that most trackday riders and many racers don't use that last metre of pavement at the apex. And when they do use those wide lines, they don't increase their corner speed accordingly. This ends up making the fast corners as critical as the slow turns. Obviously, the absolute shortest distance around the track is not the quickest, but it's important to not add any unnecessary distance. It may sound trifling to be worrying about a metre here and there, but it adds up over a full lap. With GPS-based data acquisition, it's possible to measure accurately the actual distance a rider travels on a lap of the track, and the results can be quite surprising. Even with expert-level riders, I have seen variances up to 15 metres from lap to lap on a typical track, which translates to as much as half a second. Races are won and lost on much less than that.
Another benefit of monitoring distance around the track is that it gives a measure of consistency. When I worked with WERA racer Javelin Broderick last summer, I had him think about distance and making his apexes each lap for an entire track day. Over the course of the day, his variance in distance from lap to lap dropped from 20 metres in the first session to just three metres in the last session of the day, a significant reduction. Lap times likewise came down and became more consistent.
Casey Stoner and other MotoGP riders are obviously working hard to minimize unnecessary distance around the track to save every last tenth of a second in lap time, but even club racers and trackday riders can realize significant benefits from some attention in this area.